% C-Tutorial für IBM kompatible % kein ANSI, aber systemübergreifend einsetzbar % \______________________________________________/ % >>> gesetzt von Roland Jesse unter Benutzung <<< % >>> des PasTeX-Pakets von Georg Hessmann <<< % /~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\ \documentstyle[twoside,a4]{report} \title{C Tutorial} \author{Coronado Enterprises\\12501 Coronado Ave. NE\\ Albuquerque, NM 87122} \date{June 30, 1986} \pagestyle{headings} \begin{document} \maketitle \begin{abstract} Just a C-Tutor for IBM and compatible PCs. Because C is much device independent you can use this Tutorial for other machines, too. \end{abstract} \tableofcontents \chapter[Introduction]{Introduction to the C Tutorial} The programming language C, was originally developed by Dennis Ritchie of Bell Laboratories and was designed to run on a PDP-11 with a UNIX operating system. Although it was originally intended to run under UNIX, there has been a great interest in running it under the MS-DOS operating system and specifically on the IBM PC and compatibles. It is an excellent language for this environment because of the simplicity of expression, the compactness of the code, and the wide range of applicability. It is not a good ``beginning'' language because it is somewhat cryptic in nature. It allows the programmer a wide range of operations from high level down to a very low level approaching the level of assembly language. There seems to be no limit to the flexibility available. One experienced C programmer made the statement, ``You can program anything in C'', and the statement is well supported by my own experience with the language. Along with the resulting freedom however, you take on a great deal of responsibility because it is very easy to write a program that destroys itself due to the silly little errors that the Pascal compiler will flag and call a fatal error. In C, you are very much on your own as you will soon find. Since C is not a beginners language, I will assume you are not a beginning programmer, and I will not attempt to bore you by defining a constant and a variable. You will be expected to know these basic concepts. You will, however, be expected to know nothing of the C programming language. I will begin with the most basic concepts of C and take you up to the highest level of C programming including the usually intimidating concepts of pointers, structures, and dynamic allocation. To fully understand these concepts, it will take a good bit of time and work on your part because they not particularly easy to grasp, but they are very powerful tools. Enough said about that, you will see their power when we get there, just don't allow yourself to worry about them yet. Programming in C is a tremendous asset in those areas where you may want to use Assembly Language but would rather keep it a simple to write and easy to maintain program. It has been said that a program written in C will pay a premium of a 50 to 100\% increase in runtime because no language is as compact or fast as Assembly Language. However, the time saved in coding can be tremendous, making it the most desirable language for many programming chores. In addition, since most programs spend 90 percent of their operating time in only 10 percent or less of the code, it is possible to write a program in C, then rewrite a small portion of the code in Assembly Language and approach the execution speed of the same program if it were written entirely in Assembly Language. Approximately 75 percent of all new commercial programs introduced for the IBM PC have been written in C, and the percentage is probably growing. Microsoft recently introduced a new Macro Assembler, version 4.0, and they said that it was written in C. There are probably a few routines coded in Assembly Language, but the majority was written in C. Since C was designed essentially by one person, and not by a committee, it is a very usable language but not too well defined. There is no standard for the C language, but the American National Standards Association (ANSI) is developing a standard for the language at which time it will follow rigid rules. It is interesting to note, however, that even though it does not have a standard, the differences between implementations are very small. This is probably due to the fact that the original unofficial definition was so well thought out and carefully planned that extensions to the language are not needed. Pascal, which has a rigorous definition, has many extensions by compiler writers and every extension is different. This leads to a real problem when transporting a Pascal program from one computer to another. Even though the C language enjoys a good record when programs are transported from one implementation to another, there are differences in compilers as you will find anytime you try to use another compiler. Most of the differences become apparent when you use nonstandard extensions such as calls to the DOS BIOS, but even these differences can be minimized by careful choice of programming means. Your first problem will not be how to program in C, but how to use your particular compiler. Since there are over 20 good compilers available, there is no way I can cover the operation of all compilers. Notes about a few of the better known compilers are given in the ``COMPILER.DOC'' file on the distribution diskette. Read the documentation that came with your compiler to learn how to compile and run a program. One last note about compilers. I wrote a moderately large program in C that was composed of about 1200 lines of source code contained in 4 separately compiled files. I was initially using a very inexpensive compiler from MIX Software of Richardson, Texas that sells for \$39.95. This compiler did everything I ever asked it to do and did it well, including floating point numbers. In addition, the compile times were extremely short and there were many extensions to the basic language as defined by Kernigan \& Ritchie. In short, the compiler was a good implementation. Later, I switched over to a Lattice C compiler that sells for \$500.00. It took a bit of work because the Lattice compiler did not have as many extensions as the MIX compiler. The Lattice compiler also took considerably longer to compile, probably 2 to 3 times as long. The big difference in the two compilers was in the execution time of the program which read in a file, did a lot of searching in memory, and displayed the results on the monitor. The final MIX program took 95 seconds to complete all operations, and the Lattice compiled program took only 10 seconds to complete. I should add that the MIX compiler has a speedup utility that increases the speed by a factor of about 8, according to one independent review, getting the speed of the MIX program in the range of the Lattice program. (I did not try the speedup program on this particular file.) The MIX compiler missed several subtle type errors during compile that were flagged as warnings by the Lattice compiler. Due to the nature of that particular program, either run-time would be acceptable and therefore either compiler would be acceptable. The above paragraph was given only to aid you in selecting a compiler. The Lattice compiler is very difficult to use for an inexperienced programmer, but has very few limitations. The MIX compiler, on the other hand, was very easy to set up and begin using, and would be very appropriate for most ``hobby'' computing. Depending on your application, the most expensive is probably not the best. In this case, the MIX compiler would be great for learning the language and for many applications. \chapter{Getting Started} \section{WHAT IS AN IDENTIFIER?} Before you can do anything in any language, you must at least know how you name an identifier. An identifier is used for any variable, function, data definition, etc. In the programming language C, an identifier is a combination of alphanumeric characters, the first being a letter of the alphabet or an underline, and the remaining being any letter of the alphabet, any numeric digit, or the underline. Two rules must be kept in mind when naming identifiers. \begin{enumerate} \item The case of alphabetic characters is significant. Using {\em INDEX\/} for a variable is not the same as using {\em index\/} and neither of them is the same as using {\em InDeX\/} for a variable. All three refer to different variables. \item As C is defined, up to eight significant characters can be used and will be considered significant. If more than eight are used, they may be ignored by the compiler. This may or may not be true of your compiler. You should check your reference manual to find out how many characters are significant for your compiler. \end{enumerate} It should be pointed out that some C compilers allow use of a dollar sign in an identifier name, but since it is not universal, it will not be used anywhere in this tutorial. Check your documentation to see if it is permissible for your particular compiler. \section{WHAT ABOUT THE UNDERLINE?} Even though the underline can be used as part of a variable name, it seems to be used very little by experienced C programmers. It adds greatly to the readability of a program to use descriptive names for variables and it would be to your advantage to do so. Pascal programmers tend to use long descriptive names, but most C programmers tend to use short cryptic names. Most of the example programs in this tutorial use very short names for that reason. Any computer program has two entities to consider, the data, and the program. They are highly dependent on one another and careful planning of both will lead to a well planned and well written program. Unfortunately, it is not possible to study either completely without a good working knowledge of the other. For this reason, this tutorial will jump back and forth between teaching methods of program writing and methods of data definition. Simply follow along and you will have a good understanding of both. Keep in mind that, even though it seems expedient to sometimes jump right into the program coding, time spent planning the data structures will be well spent and the final program will reflect the original planning. \section{HOW THIS TUTORIAL IS WRITTEN} As you go through the example programs, you will find that every program is complete. There are no program fragments that could be confusing. This allows you to see every requirement that is needed to use any of the features of C as they are presented. Some tutorials I have seen give very few, and very complex examples. They really serve more to confuse the student. This tutorial is the complete opposite because it strives to cover each new aspect of programming in as simple a context as possible. This method, however, leads to a lack of knowledge in how the various parts are combined. For that reason, the last chapter is devoted entirely to using the features taught in the earlier chapters. It will illustrate how to put the various features together to create a usable program. They are given for your study, and are not completely explained. Enough details of their operation are given to allow you to understand how they work after you have completed all of the previous lessons. \chapter{Getting started in C} \section{YOUR FIRST C PROGRAM} The best way to get started with C is to actually look at a program, so load the file named ``TRIVIAL.C'' into your editor and display it on the monitor. You are looking at the simplest possible C program. There is no way to simplify this program or to leave anything out. Unfortunately, the program doesn't do anything. The word {\em main\/} is very important, and must appear once, and only once in every C program. This is the point where execution is begun when the program is run. We will see later that this does not have to be the first statement in the program but it must exist as the entry point. Following the {\em main\/} program name is a pair of parentheses which are an indication to the compiler that this is a function. We will cover exactly what a function is in due time. For now, I suggest that you simply include the pair of parentheses. The two curly brackets, properly called braces, are used to define the limits of the program itself. The actual program statements go between the two braces and in this case, there are no statements because the program does absolutely nothing. You can compile and run this program, but since it has no executable statements, it does nothing. Keep in mind however, that it is a valid C program. \section{A PROGRAM THAT DOES SOMETHING} For a much more interesting program, load the program named ``WRTSOME.C'' and display it on your monitor. It is the same as the previous program except that it has one executable statement between the braces. The executable statement is another function. Once again, we will not worry about what a function is, but only how to use this one. In order to output text to the monitor, it is put within the function parentheses and bounded by quotation marks. The end result is that whatever is included between the quotation marks will be displayed on the monitor when the program is run. Notice the semi--colon at the end of the line. C uses a semi--colon as a statement terminator, so the semi--colon is required as a signal to the compiler that this line is complete. This program is also executable, so you can compile and run it to see if it does what you think it should. \section{A PROGRAM WITH MORE OUTPUT} Load the program ``WRTMORE.C'' and display it on your monitor for an example of more output and another small but important concept. You will see that there are four program statements in this program, each one being a {\em printf\/} function statement. The top line will be executed first, then the next, and so on, until the fourth line is complete. The statements are executed in order from top to bottom. Notice the funny character near the end of the first line, namely the backslash. The backslash is used in the printf statement to indicate a special control character is following. In this case, the {\em n\/} indicates that a {\em newline\/} is requested. This is an indication to return the cursor to the left side of the monitor and move down one line. It is commonly referred to as a carriage return/line feed. Any place within text that you desire, you can put a newline character and start a new line. You could even put it in the middle of a word and split the word between two lines. The C compiler considers the combination of the backslash and letter n as one character. A complete description of this program is now possible. The first printf outputs a line of text and returns the carriage. The second printf outputs a line but does not return the carriage so the third line is appended to that of the second, then followed by two carriage returns, resulting in a blank line. Finally the fourth printf outputs a line followed by a carriage return and the program is complete. Compile and run this program to see if it does what you expect it to do. It would be a good idea at this time for you to experiment by adding additional lines of printout to see if you understand how the statements really work. \section{LETS PRINT SOME NUMBERS} Load the file named ONEINT.C and display it on the monitor for our first example of how to work with data in a C program. The entry point {\em main\/} should be clear to you by now as well as the beginning brace. The first new thing we encounter is the line containing {\em int index;\/}, which is used to define an integer variable named {\em index\/}. The {\em int\/} is a reserved word in C, and can therefore not be used for anything else. It defines a variable that can have a value from $-$32768 to 32767 on most microcomputer implementations of C. Consult your users manual for the exact definition for your compiler. The variable name, {\em index\/}, can be any name that follows the rules for an identifier and is not one of the reserved words for C. Consult your manual for an exact definition of an identifier for your compiler. The final character on the line, the semi--colon, is the statement terminator used in C. We will see in a later chapter that additional integers could also be defined on the same line, but we will not complicate the present situation. Observing the main body of the program, you will notice that there are three statements that assign a value to the variable {\em index\/}, but only one at a time. The first one assigns the value of 13 to {\em index\/}, and its value is printed out. (We will see how shortly.) Later, the value of 27 is assigned to {\em index\/}, and finally 10 is assigned to it, each value being printed out. It should be intuitively clear that {\em index\/} is indeed a variable and can store many different values. Please note that many times the words {\em printed out\/} are used to mean {\em displayed on the monitor\/}. You will find that in many cases experienced programmers take this liberty, probably due to the {\em printf\/} function being used for monitor display. section{HOW DO WE PRINT NUMBERS} To keep our promise, let's return to the {\em printf\/} statements for a definition of how they work. Notice that they are all identical and that they all begin just like the {\em printf\/} statements we have seen before. The first difference occurs when we come to the \% character. This is a special character that signals the output routine to stop copying characters to the output and do something different, namely output a variable. The \% sign is used to signal the start of many different types of variables, but we will restrict ourselves to only one for this example. The character following the \% sign is a $d$, which signals the output routine to get a decimal value and output it. Where the decimal value comes from will be covered shortly. After the $d$, we find the familiar $\backslash$ n, which is a signal to return the video {\em carriage\/}, and the closing quotation mark. All of the characters between the quotation marks define the pattern of data to be output by this statement, and after the pattern, there is a comma followed by the variable name {\em index\/}. This is where the {\em printf\/} statement gets the decimal value which it will output because of the {\em \%d\/} we saw earlier. We could add more {\em \%d\/} output field descriptors within the brackets and more variables following the description to cause more data to be printed with one statement. Keep in mind however, that it is important that the number of field descriptors and the number of variable definitions must be the same or the runtime system will get confused and probably quit with a runtime error. Much more will be covered at a later time on all aspects of input and output formatting. A reasonably good grasp of this topic is necessary in order to understand the following lessons. It is not necessary to understand everything about output formatting at this time, only a fair understanding of the basics. Compile and run ONEINT.C and observe the output. \section{HOW DO WE ADD COMMENTS IN C} Load the file COMMENTS.C and observe it on your monitor for an example of how comments can be added to a C program. Comments are added to make a program more readable to you but the compiler must ignore the comments. The slash star combination is used in C for comment delimiters. They are illustrated in the program at hand. Please note that the program does not illustrate good commenting practice, but is intended to illustrate where comments can go in a program. It is a very sloppy looking program. The first slash star combination introduces the first comment and the star slash at the end of the first line terminates this comment. Note that this comment is prior to the beginning of the program illustrating that a comment can precede the program itself. Good programming practice would include a comment prior to the program with a short introductory description of the program. The next comment is after the {\em main()\/} program entry point and prior to the opening brace for the program code itself. The third comment starts after the first executable statement and continues for four lines. This is perfectly legal because a comment can continue for as many lines as desired until it is terminated. Note carefully that if anything were included in the blank spaces to the left of the three continuation lines of the comment, it would be part of the comment and would not be compiled. The last comment is located following the completion of the program, illustrating that comments can go nearly anywhere in a C program. Experiment with this program by adding comments in other places to see what will happen. Comment out one of the printf statements by putting comment delimiters both before and after it and see that it does not get printed out. Comments are very important in any programming language because you will soon forget what you did and why you did it. It will be much easier to modify or fix a well commented program a year from now than one with few or no comments. You will very quickly develop your own personal style of commenting. Some compilers allow you to {\em nest\/} comments which can be very handy if you need to {\em comment out\/} a section of code during debugging. Check your compiler documentation for the availability of this feature with you particular compiler. Compile and run COMMENTS.C at this time. section{GOOD FORMATTING STYLE} Load the file GOODFORM.C and observe it on your monitor. It is an example of a well formatted program. Even though it is very short and therefore does very little, it is very easy to see at a glance what it does. With the experience you have already gained in this tutorial, you should be able to very quickly grasp the meaning of the program in it's entirety. Your C compiler ignores all extra spaces and all carriage returns giving you considerable freedom concerning how you format your program. Indenting and adding spaces is entirely up to you and is a matter of personal taste. Compile and run the program to see if it does what you expect it to do. Now load and display the program UGLYFORM.C and observe it. How long will it take you to figure out what this program will do? It doesn't matter to the compiler which format style you use, but it will matter to you when you try to debug your program. Compile this program and run it. You may be surprised to find that it is the same program as the last one, except for the formatting. Don't get too worried about formatting style yet. You will have plenty of time to develop a style of your own as you learn the language. Be observant of styles as you see C programs in magazines, books, and other publications. This should pretty well cover the basic concepts of programming in C, but as there are many other things to learn, we will forge ahead to additional program structure. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Write a program to display your name on the monitor. \item Modify the program to display your address and phone number on separate lines by adding two additional {\em printf\/} statements. \end{enumerate} \chapter{Program Control} \section{THE WHILE LOOP} The C programming language has several structures for looping and conditional branching. We will cover them all in this chapter and we will begin with the while loop. The while loop continues to loop while some condition is true. When the condition becomes false, the looping is discontinued. It therefore does just what it says it does, the name of the loop being very descriptive. Load the program WHILE.C and display it for an example of a while loop. We begin with a comment and the program name, then go on to define an integer variable {\em count\/} within the body of the program. The variable is set to zero and we come to the while loop itself. The syntax of a while loop is just as shown here. The keyword {\em while\/} is followed by an expression of something in parentheses, followed by a compound statement bracketed by braces. As long as the expression in parenthesis is true, all statements within the braces will be executed. In this case, since the variable count is incremented by one every time the statements are executed, it will eventually reach 6, the statement will not be executed, and the loop will be terminated. The program control will resume at the statement following the statements in braces. We will cover the compare expression, the one in parentheses, in the next chapter. Until then, simply accept the expressions for what you think they should do and you will probably be correct. Several things must be pointed out regarding the while loop. First, if the variable count were initially set to any number greater than 5, the statements within the loop would not be executed at all, so it is possible to have a while loop that never is executed. Secondly, if the variable were not incremented in the loop, then in this case, the loop would never terminate, and the program would never complete. Finally, if there is only one statement to be executed within the loop, it does not need braces but can stand alone. Compile and run this program. \section{THE DO-WHILE LOOP} A variation of the while loop is illustrated in the program DOWHILE.C, which you should load and display. This program is nearly identical to the last one except that the loop begins with the reserved word {\em do\/}, followed by a compound statement in braces, then the reserved word {\em while\/}, and finally an expression in parentheses. The statements in the braces are executed repeatedly as long as the expression in parentheses is true. When the expression in parentheses becomes false, execution is terminated, and control passes to the statements following this statement. Several things must be pointed out regarding this statement. Since the test is done at the end of the loop, the statements in the braces will always be executed at least once. Secondly, if {\em i\/} were not changed within the loop, the loop would never terminate, and hence the program would never terminate. Finally, just like for the while loop, if only one statement will be executed within the loop, no braces are required. Compile and run this program to see if it does what you think it should do. It should come as no surprise to you that these loops can be nested. That is, one loop can be included within the compound statement of another loop, and the nesting level has no limit. \section{THE FOR LOOP} The {\em for\/} loop is really nothing new, it is simply a new way to describe the {\em while\/} loop. Load and edit the file named FORLOOP.C for an example of a program with a {\em for\/} loop. The {\em for\/} loop consists of the reserved word {\em for\/} followed by a rather large expression in parentheses. This expression is really composed of three fields separated by semi--colons. The first field contains the expression {\em index = 0\/} and is an initializing field. Any expressions in this field are executed prior to the first pass through the loop. There is essentially no limit as to what can go here, but good programming practice would require it to be kept simple. Several initializing statements can be placed in this field, separated by commas. The second field, in this case containing {\em index $<$ 6\/}, is the test which is done at the beginning of each loop through the program. It can be any expression which will evaluate to a true or false. (More will be said about the actual value of true and false in the next chapter.) The expression contained in the third field is executed each time the loop is executed but it is not executed until after those statements in the main body of the loop are executed. This field, like the first, can also be composed of several operations separated by commas. Following the for() expression is any single or compound statement which will be executed as the body of the loop. A compound statement is any group of valid C statements enclosed in braces. In nearly any context in C, a simple statement can be replaced by a compound statement that will be treated as if it were a single statement as far as program control goes. Compile and run this program. \section{THE IF STATEMENT} Load and display the file IFELSE.C for an example of our first conditional branching statement, the {\em if\/}. Notice first, that there is a {\em for\/} loop with a compound statement as its executable part containing two {\em if\/} statements. This is an example of how statements can be nested. It should be clear to you that each of the {\em if\/} statements will be executed 10 times. Consider the first {\em if\/} statement. It starts with the keyword {\em if\/} followed by an expression in parentheses. If the expression is evaluated and found to be true, the single statement following the {\em if\/} is executed, and if false, the following statement is skipped. Here too, the single statement can be replaced by a compound statement composed of several statements bounded by braces. The expression {\em data $==$ 2\/} is simply asking if the value of data is equal to 2, this will be explained in detail in the next chapter. (Simply suffice for now that if {\em data = 2\/} were used in this context, it would mean a completely different thing.) \section{NOW FOR THE IF--ELSE} The second {\em if\/} is similar to the first with the addition of a new reserved word, the {\em else\/} following the first printf statement. This simply says that if the expression in the parentheses evaluates as true, the first expression is executed, otherwise the expression following the {\em else\/} is executed. Thus, one of the two expressions will always be executed, whereas in the first example the single expression was either executed or skipped. Both will find many uses in your C programming efforts. Compile and run this program to see if it does what you expect. \section{THE BREAK AND CONTINUE} Load the file named BREAKCON.C for an example of two new statements. Notice that in the first ``for'', there is an if statement that calls a break if xx equals 8. The break will jump out of the loop you are in and begin executing statements following the loop, effectively terminating the loop. This is a valuable statement when you need to jump out of a loop depending on the value of some results calculated in the loop. In this case, when xx reaches 8, the loop is terminated and the last value printed will be the previous value, namely 7. The next {\em for\/} loop, contains a continue statement which does not cause termination of the loop but jumps out of the present iteration. When the value of xx reaches 8 in this case, the program will jump to the end of the loop and continue executing the loop, effectively eliminating the printf statement during the pass through the loop when xx is eight. Compile and run the program to see if it does what you expect. \section{THE SWITCH STATEMENT} Load and display the file SWITCH.C for an example of the biggest construct yet in the C language, the switch. The switch is not difficult, so don't let it intimidate you. It begins with the keyword {\em switch\/} followed by a variable in parentheses which is the switching variable, in this case {\em truck\/}. As many cases as desired are then enclosed within a pair of braces. The reserved word {\em case\/} is used to begin each case entered followed by the value of the variable, then a colon, and the statements to be executed. In this example, if the variable {\em truck\/} contains the value 3 during this pass of the switch statement, the printf will cause {\em The value is three\/} to be displayed, and the {\em break\/} statement will cause us to jump out of the switch. Once an entry point is found, statements will be executed until a {\em break\/} is found or until the program drops through the bottom of the switch braces. If the variable has the value 5, the statements will begin executing where {\em case 5 :\/} is found, but the first statements found are where the case 8 statements are. These are executed and the break statement in the {\em case 8\/} portion will direct the execution out the bottom of the switch. The various case values can be in any order and if a value is not found, the default portion of the switch will be executed. It should be clear that any of the above constructs can be nested within each other or placed in succession, depending on the needs of the particular programming project at hand. Compile and run SWITCH.C to see if it does what you expect it to after this discussion. Load and display the file GOTOEX.C for an example of a file with some {\em goto\/} statements in it. To use a {\em goto\/} statement, you simply use the reserved word {\em goto\/} followed by the symbolic name to which you wish to jump. The name is then placed anywhere in the program followed by a colon. You are not allowed to jump into any loop, but you are allowed to jump out of a loop. Also, you are not allowed to jump out of any function into another. These attempts will be flagged by your compiler as an error if you attempt any of them. This particular program is really a mess but it is a good example of why software writers are trying to eliminate the use of the {\em goto\/} statement as much as possible. The only place in this program where it is reasonable to use the {\em goto\/} is the one in line 17 where the program jumps out of the three nested loops in one jump. In this case it would be rather messy to set up a variable and jump successively out of all three loops but one {\em goto\/} statement gets you out of all three. Some persons say the {\em goto\/} statement should never be used under any circumstances but this is rather narrow minded thinking. If there is a place where a {\em goto\/} will clearly do a neater control flow than some other construct, feel free to use it. It should not be abused however, as it is in the rest of the program on your monitor. Entire books are written on {\em gotoless\/} programming, better known as Structured Programming. These will be left to your study. One point of reference is the Visual Calculater described in Chapter 14 of this tutorial. This program is contained in four separately compiled programs and is a rather large complex program. If you spend some time studying the source code, you will find that there is not a single {\em goto\/} statement anywhere in it. Compile and run GOTOEX.C and study its output. It would be a good exercise to rewrite it and see how much more readable it is when the statements are listed in order. \section{FINALLY, A MEANINGFUL PROGRAM} Load the file named TEMPCONV.C for an example of a useful, even though somewhat limited program. This is a program that generates a list of centigrade and farenheit temperatures and prints a message out at the freezing point of water and another at the boiling point of water. Of particular importance is the formatting. The header is simply several lines of comments describing what the program does in a manner that catches the readers attention and is still pleasing to the eye. You will eventually develop your own formatting style, but this is a good way to start. Also if you observe the for loop, you will notice that all of the contents of the compound statement are indented 3 spaces to the right of the {\em for\/} reserved word, and the closing brace is lined up under the {\em f\/} in {\em for\/}. This makes debugging a bit easier because the construction becomes very obvious. You will also notice that the {\em printf\/} statements that are in the {\em if\/} statements within the big {\em for\/} loop are indented three additional spaces because they are part of another construct. This is the first program in which we used more than one variable. The three variables are simply defined on three different lines and are used in the same manner as a single variable was used in previous programs. By defining them on different lines, we have an opportunity to define each with a comment. \section{ANOTHER POOR PROGRAMMING EXAMPLE} Recalling UGLYFORM.C from the last chapter, you saw a very poorly formatted program. If you load and display DUMBCONV.C you will have an example of poor formatting which is much closer to what you will actually find in practice. This is the same program as TEMPCONV.C with the comments removed and the variable names changed to remove the descriptive aspect of the names. Although this program does exactly the same as the last one, it is much more difficult to read and understand. You should begin to develop good programming practices now. Compile and run this program to see that it does exactly what the last one did. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Write a program that writes your name on the monitor ten times. Write this program three times, once with each looping method. \item Write a program that counts from one to ten, prints the values on a separate line for each, and includes a message of your choice when the count is 3 and a different message when the count is 7. \end{enumerate} \chapter{Assignment \& Logical compares} \section{INTEGER ASSIGNMENT STATEMENTS} Load the file INTASIGN.C and display it for an example of assignment statements. Three variables are defined for use in the program and the rest of the program is merely a series of illustrations of various assignments. The first two lines of the assignment statements assign numerical values to $a$ and $b$, and the next four lines illustrate the five basic arithmetic functions and how to use them. The fifth is the modulo operator and gives the remainder if the two variables were divided. It can only be applied to {\em int\/} or {\em char\/} type variables, and of course {\em int\/} extensions such as {\em long\/}, {\em short\/}, etc. Following these, there are two lines illustrating how to combine some of the variables in some complex math expressions. All of the above examples should require no comment except to say that none of the equations are meant to be particularly useful except as illustrations. The next two expressions are perfectly acceptable as given, but we will see later in this chapter that there is another way to write these for more compact code. This leaves us with the last two lines which may appear to you as being very strange. The C compiler scans the assignment statement from right to left, (which may seem a bit odd since we do not read that way), resulting in a very useful construct, namely the one given here. The compiler finds the value 20, assigns it to $c$, then continues to the left finding that the latest result of a calculation should be assigned to $b$. Thinking that the latest calculation resulted in a 20, it assigns it to $b$ also, and continues the leftward scan assigning the value 20 to $a$ also. This is a very useful construct when you are initializing a group of variables. The last statement illustrates that it is possible to actually do some calculations to arrive at the value which will be assigned to all three variables. The program has no output so compiling and executing this program will be very uninteresting. Since you have already learned how to display some integer results using the $printf$ function, it would be to your advantage to add some output statements to this program to see if the various statements do what you think they should do. This would be a good time for a preliminary definition of a rule to be followed in C. The data definitions are always given before any executable statements in any program block. This is why the variables are defined first in this program and in any C program. If you try to define a new variable after executing some statements, the compiler will issue an error. \section{ADDITIONAL DATA TYPES} Loading and editing MORTYPES.C will illustrate how some additional data types can be used. Once again we have defined a few integer type variables which you should be fairly familiar with by now, but we have added two new types, the {\em char\/}, and the {\em float\/}. The {\em char\/} type of data is nearly the same as the integer except that it can only be assigned values between zero and $255$, since it is stored in only one byte of memory. The {\em char\/} type of data is usually used for ASCII data, more commonly known as text. The text you are reading was originally written on a computer with a word processor that stored the words in the computer one character per byte. In contrast, the integer data type is stored in two bytes of computer memory on most microcomputers. \section{DATA TYPE MIXING} It would be profitable at this time to discuss the way C handles the two types {\em char\/} and {\em int\/}. Most functions in C that are designed to operate with integer type variables will work equally well with character type variables because they are a form of an integer variable. Those functions, when called on to use a {\em char\/} type variable, will actually promote the {\em char\/} data into integer data before using it. For this reason, it is possible to mix {\em char\/} and {\em int\/} type variables in nearly any way you desire. The compiler will not get confused, but you might. It is good not to rely on this too much, but to carefully use only the proper types of data where they should be used. The second new data type is the {\em float\/} type of data, commonly called floating point data. This is a data type which usually has a very large range, a large number of significant digits, and a large number of computer words are required to store it. The {\em float\/} data type has a decimal point associated with it and, on most computers, has an allowable range of from 10~$^{-38}$ to 10~$^{+38}$. Not all compilers have the same available range, so check your reference manual for the limits on your compiler. \section{HOW TO USE THE NEW DATA TYPES} The first three lines of the program assign values to all nine of the defined variables so we can manipulate some of the data between the different types. Since, as mentioned above, a {\em char\/} data type is in reality an {\em integer\/} data type, no special considerations need be taken to promote a {\em char\/} to an {\em int\/}, and a {\em char\/} type data field can be assigned to an {\em int\/} variable. When going the other way, there is no standard, so you may simply get garbage if the value of the integer variable is outside the range of the {\em char\/} type variable. It will translate correctly if the value is within the range of zero to 255. In the second line therefore, when attempting to set $x$ (a char) to $-27$, you may or may not get a well defined answer, it depends on your particular implementation of C. The third line illustrates the simplicity of translating an integer into a {\em float\/}, simply assign it the new value and the system will do the proper conversion. When going the other way however, there is an added complication. Since there may be a fractional part of the floating point number, the system must decide what to do with it. By definitions , it will truncate it. This program produces no output, and we haven't covered a way to print out {\em char\/} and {\em float\/} type variables, so you can't really get in to this program and play with the results, but the next program will cover this for you. \section{LOTS OF VARIABLE TYPES} Load the file LOTTYPES.C and display it on your screen. This file contains every standard simple data type available in the programming language C. There are other types, but they are the compound types that we will cover in due time. Observe the file. First we define a simple {\em int\/}, followed by a {\em long int\/} and a {\em short int\/}. Consult your reference manual for an exact definition of these for your compiler, because they are not consistent from implementation to implementation. The {\em unsigned\/} is next and is defined as the same size as the {\em int\/} but with no sign. The {\em unsigned\/} then will cover a range of 0 to 65535 on most microcomputers. It should be pointed out that when the {\em long\/}, {\em short\/}, or {\em unsigned\/} is desired, the {\em int\/} is optional and is left out by most experienced programmers. We have already covered the {\em char\/} and the {\em float\/}, which leaves only the {\em double\/}. The {\em double\/} usually covers a greater range than the {\em float\/} and has more significant digits for more precise calculations. It also requires more memory to store a value than the simple {\em float\/}. Consult your reference manual for the range and accuracy of the {\em double\/}. Another diversion is in order at this point. Most compilers have no provisions for floating point math, but only double floating point math. They will promote a {\em float\/} to a {\em double\/} before doing calculations and therefore only one math library will be needed. Of course, this is totally transparent to you, so you don't need to worry about it. You may think that it would be best to simply define every floating point variable as double, since they are promoted before use in any calculations, but that may not be a good idea. A {\em float\/} variable requires 4 bytes of storage and a {\em double\/} requires 8 bytes of storage, so if you have a large volume of floating point data to store, the {\em double\/} will obviously require much more memory. Your compiler may require a different number of bytes than 4 or 8. Consult your reference manual for the correct number of bytes used by your compiler. After defining the data types, a numerical value is assigned to each of the defined variables in order to demonstrate the means of outputting each to the monitor. \section{THE CONVERSION CHARACTERS} Following is a list of the conversion characters and the way they are usedin the {\em printf\/} statement. \begin{description} \item[d] decimal notation \item[o] octal notation \item[x] hexadecimal notation \item[u] unsigned notation \item[c] character notation \item[s] string notation \item[f] floating point notation \end{description} Each of these is used following a percent sign to indicate the type of output conversion, and between those two characters, the following fields may be added. \begin{description} \item[-] left justification in its field \item[(n)] a number specifying minimum field width \item[.] to separate n from m \item[(m)] significant fractional digits for a float \item[l] to indicate a {\em long\/} \end{description} These are all used in the examples which are included in the program presently displayed on your monitor, with the exception of the string notation which will be covered later in this tutorial. Compile and run this program to see what effect the various fields have on the output. You now have the ability to display any of the data fields in the previous programs and it would be to your advantage to go back and see if you can display any of the fields anyway you desire. \section{LOGICAL COMPARES} Load and view the file named COMPARES.C for many examples of compare statements in C. We begin by defining and initializing nine variables to use in the following compare statements. This initialization is new to you and can be used to initialize variables while they are defined. The first group of compare statements represents the simplest kinds of compares since they simply compare two variables. Either variable could be replaced with a constant and still be a valid compare, but two variables is the general case. The first compare checks to see if $x$ is equal to $y$ and it uses the double equal sign for the comparison. A single equal sign could be used here but it would have a different meaning as we will see shortly. The second comparison checks to see if $x$ is greater than $z$. The third introduces the {\em NOT\/} operator, the exclamation, which can be used to invert the result of any logical compare. The fourth checks for $b$ less than or equal to $c$, and the last checks for $r$ not equal to $s$. As we learned in the last chapter, if the result of the compare is true, the statement following the {\em if\/} clause will be executed and the results are given in the comments. Note that {\em less than\/} and {\em greater than or equal to\/} are also available, but are not illustrated here. It would be well to mention the different format used for the {\em if\/} statement in this example program. A carriage return is not required as a statement separator and by putting the conditional clause on the same line as the {\em if\/}, it adds to the readability of the overall program. \section{MORE COMPARES} The compares in the second group are a bit more involved. Starting with the first compare, we find a rather strange looking set of conditions in the parentheses. To understand this we must understand just what a {\em true\/} or {\em false\/} is in the C language. A {\em false\/} is defined as a value of zero, and {\em true\/} is defined as a non--zero value. Any integer or char type of variable can be used for the result of a true/false test, or the result can be an implied integer or char. Look at the first compare of the second group of compare statements. The expression ``$r != s$'' will evaluate as a {\em true\/} since $r$ was set to $0.0$ above, so the result will be a non--zero value, probably 1. Even though the two variables that are compared are {\em float\/} variables, the result will be of type {\em integer\/}. There is no explicit variable to which it will be assigned so the result of the compare is an implied integer. Finally the resulting number, 1 in this case, is assigned to the integer variable $x$. If double equal signs were used, the phantom value, namely 1, would be compared to the value of $x$, but since the single equal sign is used, the value 1 is simply assigned to $x$, as though the statement were not in parentheses. Finally, since the result of the assignment in the parentheses was non--zero, the entire expression is evaluated as {\em true\/}, and $z$ is assigned the value of 1000. Thus we accomplished two things in this statement, we assigned $x$ a new value, probably 1, and we assigned $z$ the value of 1000. We covered a lot in this statement so you may wish to review it before going on. The important things to remember are the values that define {\em true\/} and {\em false\/}, and the fact that several things can be assigned in a conditional statement. The value assigned to $x$ was probably a 1 but different compilers may assign a different value as long as it is non--zero. The next example should help clear up some of the above in your mind. In this example, $x$ is assigned the value of $y$, and since the result is 11, the condition is non--zero, which is true, and the variable $z$ is therefore assigned 222. The third example, in the second group, compares $x$ to zero. If the result is true, meaning that if $x$ is not zero, then $z$ is assigned the value of 333, which it will be. The last example in this group illustrates the same concept, since the result will be true if $x$ is non--zero. The compare to zero is not actually needed and the result of the compare is true. The third and fourth examples of this group are therefore identical. \section{ADDITIONAL COMPARE CONCEPTS} The third group of compares will introduce some additional concepts, namely the logical {\em AND\/} and the logical {\em OR\/}. We assign the value of 77 to the three integer variables simply to get started again with some defined values. The first compare of the third group contains the new control {\em \&\&\/}, which is the logical {\em AND\/}. The entire statement reads, if $x$ equals $y$ AND if $x$ equals 77 then the result is {\em true\/}. Since this is true, the variable $z$ is set equal to 33. The next compare in this group introduces the {\em $\mid\mid$\/} operator which is the {\em OR\/}. The statement reads, if $x$ is greater than $y$ OR if $z$ is greater than 12 then the result is true. Since $z$ is greater than 12, it doesn't matter if $x$ is greater than $y$ or not, because only one of the two conditions must be true for the result to be true. The result is true, so therefore $z$ will be assigned the value of 22. \section{LOGICAL EVALUATION} When a compound expression is evaluated, the evaluation proceeds from left to right and as soon as the result of the outcome is assured, evaluation stops. Namely, in the case of an {\em AND\/} evaluation, when one of the terms evaluates to {\em false\/}, evaluation is discontinued because additional true terms cannot make the result ever become {\em true\/}. In the case of an {\em OR\/} evaluation, if any of the terms is found to be {\em true\/}, evaluation stops because it will be impossible for additional terms to cause the result to be {\em false\/}. In the case of additionally nested terms, the above rules will be applied to each of the nested levels. \section{PRECEDENCE OF OPERATORS} The question will come up concerning the precedence of operators. Which operators are evaluated first and which last? There are many rules about this topic, which your compiler will define completely, but I would suggest that you don't worry about it at this point. Instead, use lots of parentheses to group variables, constants, and operators in a way meaningful to you. Parentheses always have the highest priority and will remove any question of which operations will be done first in any particular statements. Going on to the next example in group three, we find three simple variables used in the conditional part of the compare. Since all three are non--zero, all three are {\em true\/}, and therefore the {\em AND\/} of the three variables are true, leading to the result being {\em true\/}, and $z$ being assigned the value of 11. Note that since the variables, $r$, $s$, and $t$ are {\em float\/} type variables, they could not be used this way, but they could each be compared to zero and the same type of expression could be used. Continuing on to the fourth example of the third group we find three assignment statements in the compare part of the {\em if\/} statement. If you understood the above discussion, you should have no difficulty understanding that the three variables are assigned their respective new values, and the result of all three are non-zero, leading to a resulting value of {\em TRUE\/}. \section{THIS IS A TRICK, BE CAREFUL} The last example of the third group contains a bit of a trick, but since we have covered it above, it is nothing new to you. Notice that the first part of the compare evaluates to {\em FALSE\/}. The remaining parts of the compare are not evaluated, because it is an {\em AND\/} and it will definitely be resolved as a {\em FALSE\/} because the first term is false. If the program was dependent on the value of $y$ being set to 3 in the next part of the compare, it will fail because evaluation will cease following the {\em FALSE\/} found in the first term. Likewise, $z$ will not be set to 4, and the variable $r$ will not be changed. \section{POTENTIAL PROBLEM AREAS} The last group of compares illustrate three possibilities for getting into a bit of trouble. All three have the common result that $z$ will not get set to the desired value, but for different reasons. In the case of the first one, the compare evaluates as {\em true\/}, but the semicolon following the second parentheses terminates the {\em if\/} clause, and the assignment statement involving $z$ is always executed as the next statement. The {\em if\/} therefore has no effect because of the misplaced semicolon. The second statement is much more straightforward because $x$ will always be equal to itself, therefore the inequality will never be true, and the entire statement will never do a thing, but is wasted effort. The last statement will always assign 0 to $x$ and the compare will therefore always be {\em false\/}, never executing the conditional part of the {\em if\/} statement. The conditional statement is extremely important and must be thoroughly understood to write efficient C programs. If any part of this discussion is unclear in your mind, restudy it until you are confident that you understand it thoroughly before proceeding onward. \section{THE CRYPTIC PART OF C} There are three constructs used in C that make no sense at all when first encountered because they are not intuitive, but they greatly increase the efficiency of the compiled code and are used extensively by experienced C programmers. You should therefore be exposed to them and learn to use them because they will appear in most, if not all, of the programs you see in the publications. Load and examine the file named CRYPTIC.C for examples of the three new constructs. In this program, some variables are defined and initialized in the same statements for use below. The first executable statement simply adds 1 to the value of $x$, and should come as no surprise to you. The next two statements also add one to the value of $x$, but it is not intuitive that this is what happens. It is simply by definition that this is true. Therefore, by definition of the C language, a double plus sign either before or after a variable increments that variable by 1. Additionally, if the plus signs are before the variable, the variable is incremented before it is used, and if the plus signs are after the variable, the variable is used, then incremented. In the next statement, the value of $y$ is assigned to the variable $z$, then $y$ is incremented because the plus signs are after the variable $y$. In the last statement of the incrementing group of example statements, the value of $y$ is incremented then its value is assigned to the variable $z$. The next group of statements illustrate decrementing a variable by one. The definition works exactly the same way for decrementing as it does for incrementing. If the minus signs are before the variable, the variable is decremented, then used, and if the minus signs are after the variable, the variable is used, then decremented. \section{THE CRYPTIC ARITHMETIC OPERATOR} Another useful but cryptic operator is the arithmetic operator. This operator is used to modify any variable by some constant value. The first statement of the {\em arithmetic operator\/} group of statements simply adds 12 to the value of the variable $a$. The second statement does the same, but once again, it is not intuitive that they are the same. Any of the four basic functions of arithmetic, $+$, $-$, $*$, or $/$, can be handled in this way, by putting the function desired in front of the equal sign and eliminating the second reference to the variable name. It should be noted that the expression on the right side of the arithmetic operator can be any valid expression, the examples are kept simple for your introduction to this new operator. Just like the incrementing and decrementing operators, the arithmetic operator is used extensively by experienced C programmers and it would pay you well to understand it. \section{THE CONDITIONAL EXPRESSION} The conditional expression is just as cryptic as the last two, but once again it can be very useful so it would pay you to understand it. It consists of three expressions within parentheses separated by a question mark and a colon. The expression prior to the question mark is evaluated to determine if it is {\em true\/} or {\em false\/}. If it is true, the expression between the question mark and the colon is evaluated, and if it is not true, the expression following the colon is evaluated. The result of the evaluation is used for the assignment. The final result is identical to that of an {\em if\/} statement with an {\em else\/} clause. This is illustrated by the second example in this group. The conditional expression has the added advantage of more compact code that will compile to fewer machine instructions in the final program. The final two lines of this example program are given to illustrate a very compact way to assign the greater of two variables $a$ or $b$ to $c$, and to assign the lessor of the same two variables to $c$. Notice how efficient the code is in these two examples. \section{TO BE CRYPTIC OR NOT TO BE CRYPTIC} Several students of C have stated that they didn't like these three cryptic constructs and that they would simply never use them. This would be fine if they never have to read anybody else's program, or use any other programs within their own. I have found many functions that I wished to use within a program but needed a small modification to use it, requiring me to understand another person's code. It would therefore be to your advantage to learn these new constructs, and use them. They will be used in the remainder of this tutorial, so you will be constantly exposed to them. This has been a long chapter but it contained important material to get you started in using C. In the next chapter, we will go on to the building blocks of C, the functions. At that point, you will have enough of the basic materials to allow you to begin writing meaningful programs. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Write a program that will count from 1 to 12 and print the count, and its square, for each count. \begin{tabular}{ccr} 1 & 1 \\ 2 & 4 \\ 3 & 9 & etc. \end{tabular} \item Write a program that counts from 1 to 12 and prints the count and its inversion to 5 decimal places for each count. This will require a floating point number. \begin{tabular}{cc} 1 & 1.00000 \\ 2 & .50000 \\ 3 & .33333 \\ 4 & .25000 \\ etc. \end{tabular} \item Write a program that will count from 1 to 100 and print only those values between 32 and 39, one to a line. \end{enumerate} \chapter{Functions and variables} \section{OUR FIRST USER DEFINED FUNCTION} Load and examine the file SUMSQRES.C for an example of a C program with functions. Actually this is not the first function we have encountered because the {\em main\/} program we have been using all along is technically a function, as is the {\em printf\/} function. The {\em printf\/} function is a library function that was supplied with your compiler. Notice the executable part of this program. It begins with a line that simply says {\em header()\/}, which is the way to call any function. The parentheses are required because the C compiler uses them to determine that it is a function call and not simply a misplaced variable. When the program comes to this line of code, the function named {\em header\/} is called, its statements are executed, and control returns to the statement following this call. Continuing on we come to a {\em for\/} loop which will be executed 7 times and which calls another function named {\em square\/} each time through the loop, and finally a function named {\em ending\/} will be called and executed. For the moment ignore the {\em index\/} in the parentheses of the call to {\em square\/}. We have seen that this program therefore calls a header, 7 square calls, and an ending. Now we need to define the functions. \section{DEFINING THE FUNCTIONS} Following the main program you will see another program that follows all of the rules set forth so far for a {\em main\/} program except that it is named {\em header()\/}. This is the function which is called from within the main program. Each of these statements are executed, and when they are all complete, control returns to the main program. The first statement sets the variable {\em sum\/} equal to zero because we will use it to accumulate a sum of squares. Since the variable {\em sum\/} is defined as an integer type variable prior to the main program, it is available to be used in any of the following functions. It is called a {\em global\/} variable, and it's scope is the entire program and all functions. More will be said about the scope of variables at the end of this chapter. The next statement outputs a header message to the monitor. Program control then returns to the main program since there are no additional statements to execute in this function. It should be clear to you that the two executable lines from this function could be moved to the main program, replacing the header call, and the program would do exactly the same thing that it does as it is now written. This does not minimize the value of functions, it merely illustrates the operation of this simple function in a simple way. You will find functions to be very valuable in C programming. \section{PASSING A VALUE TO A FUNCTION} Going back to the main program, and the {\em for\/} loop specifically, we find the new construct from the end of the last lesson used in the last part of the for loop, namely the {\em index++\/}. You should get used to seeing this, as you will see it a lot in C programs. In the call to the function {\em square\/}, we have an added feature, namely the variable {\em index\/} within the parentheses. This is an indication to the compiler that when you go to the function, you wish to take along the value of index to use in the execution of that function. Looking ahead at the function {\em square\/}, we find that another variable name is enclosed in its parentheses, namely the variable {\em number\/}. This is the name we prefer to call the variable passed to the function when we are in the function. We can call it anything we wish as long as it follows the rules of naming an identifier. Since the function must know what type the variable is, it is defined following the function name but before the opening brace of the function itself. Thus, the line containing {\em int number;\/} tells the function that the value passed to it will be an integer type variable. With all of that out of the way, we now have the value of index from the main program passed to the function {\em square\/}, but renamed {\em number\/}, and available for use within the function. Following the opening brace of the function, we define another variable {\em numsq\/} for use only within the function itself, (more about that later) and proceed with the required calculations. We set {\em numsq\/} equal to the square of number, then add numsq to the current total stored in {\em sum\/}. Remember that $sum += numsq$ is the same as $sum = sum + numsq$ from the last lesson. We print the number and its square, and return to the main program. \section{MORE ABOUT PASSING A VALUE TO A FUNCTION} When we passed the value of {\em index\/} to the function, a little more happened than meets the eye. We did not actually pass the value of index to the function, we actually passed a copy of the value. In this way the original value is protected from accidental corruption by a called function. We could have modified the variable {\em number\/} in any way we wished in the function {\em square\/}, and when we returned to the main program, {\em index\/} would not have been modified. We thus protect the value of a variable in the main program from being accidentally corrupted, but we cannot return a value to the main program from a function using this technique. We will find a well defined method of returning values to the main program or to any calling function when we get to arrays and another method when we get to pointers. Until then the only way you will be able to communicate back to the calling function will be with global variables. We have already hinted at global variables above, and will discuss them in detail later in this chapter. Continuing in the main program, we come to the last function call, the call to {\em ending\/}. This call simply calls the last function which has no local variables defined. It prints out a message with the value of {em sum\/} contained in it to end the program. The program ends by returning to the main program and finding nothing else to do. Compile and run this program and observe the output. \section{NOW TO CONFESS A LITTLE LIE} I told you a short time ago that the only way to get a value back to the main program was through use of a global variable, but there is another way which we will discuss after you load and display the file named SQUARES.C. In this file we will see that it is simple to return a single value from a called function to the calling function. But once again, it is true that to return more than one value, we will need to study either arrays or pointers. In the main program, we define two integers and begin a {\em for\/} loop which will be executed 8 times. The first statement of the for loop is {\em y = squ(x);\/}, which is a new and rather strange looking construct. From past experience, we should have no trouble understanding that the {\em squ(x)\/} portion of the statement is a call to the {\em squ\/} function taking along the value of $x$ as a variable. Looking ahead to the function itself we find that the function prefers to call the variable {\em in\/} and it proceeds to square the value of {\em in\/} and call the result {\em square\/}. Finally, a new kind of a statement appears, the {\em return\/} statement. The value within the parentheses is assigned to the function itself and is returned as a usable value in the main program. Thus, the function call {\em squ(x)\/} is assigned the value of the square and returned to the main program such that $y$ is then set equal to that value. If $x$ were therefore assigned the value 4 prior to this call, $y$ would then be set to 16 as a result of this line of code. Another way to think of this is to consider the grouping of characters {\em squ(x)\/} as another variable with a value that is the square of $x$, and this new variable can be used any place it is legal to use a variable of its type. The values of $x$ and $y$ are then printed out. To illustrate that the grouping of {\em squ(x)\/} can be thought of as just another variable, another {em for\/} loop is introduced in which the function call is placed in the print statement rather than assigning it to a new variable. One last point must be made, the type of variable returned must be defined in order to make sense of the data, but the compiler will default the type to integer if none is specified. If any other type is desired, it must be explicitly defined. How to do this will be demonstrated in the next example program. Compile and run this program. \section{FLOATING POINT FUNCTIONS} Load the program FLOATSQ.C for an example of a function with a floating point type of return. It begins by defining a global floating point variable we will use later. Then in the {\em main\/} part of the program, an integer is defined, followed by two floating point variables, and then by two strange looking definitions. The expressions {\em sqr()\/} and {\em glsqr()\/} look like function calls and they are. This is the proper way in C to define that a function will return a value that is not of the type {em int\/}, but of some other type, in this case {em float\/}. This tells the compiler that when a value is returned from either of these two functions, it will be of type {\em float\/}. Now refer to the function {em sqr\/} near the center of the listing and you will see that the function name is preceded by the name {em float\/}. This is an indication to the compiler that this function will return a value of type {em float\/} to any program that calls it. The function is now compatible with the call to it. The line following the function name contains {\em float inval;\/}, which indicates to the compiler that the variable passed to this function from the calling program will be of type {em float\/}. The next function, namely {\em glsqr\/}, will also return a {\em float\/} type variable, but it uses a global variable for input. It also does the squaring right within the return statement and therefore has no need to define a separate variable to store the product. The overall structure of this program should pose no problem and will not be discussed in any further detail. As is customary with all example programs, compile and run this program. There will be times that you will have a need for a function to return a pointer as a result of some calculation. There is a way to define a function so that it does just that. We haven't studied pointers yet, but we will soon. This is just a short preview of things to come. \section{SCOPE OF VARIABLES} Load the next program, SCOPE.C, and display it for a discussion of the scope of variables in a program. The first variable defined is a global variable {\em count\/} which is available to any function in the program since it is defined before any of the functions. In addition, it is always available because it does not come and go as the program is executed. (That will make sense shortly.) Farther down in the program, another global variable named {\em counter\/} is defined which is also global but is not available to the main program since it is defined following the main program. A global variable is any variable that is defined outside of any function. Note that both of these variables are sometimes referred to as external variables because they are external to any functions. Return to the main program and you will see the variable {\em index\/} defined as an integer. Ignore the word {\em register\/} for the moment. This variable is only available within the main program because that is where it is defined. In addition, it is an {\em automatic\/} variable, which means that it only comes into existence when the function in which it is contained is invoked, and ceases to exist when the function is finished. This really means nothing here because the main program is always in operation, even when it gives control to another function. Another integer is defined within the {\em for\/} braces, namely {\em stuff\/}. Any pairing of braces can contain a variable definition which will be valid and available only while the program is executing statements within those braces. The variable will be an {\em automatic\/} variable and will cease to exist when execution leaves the braces. This is convenient to use for a loop counter or some other very localized variable. \section{MORE ON ``AUTOMATIC'' VARIABLES} Observe the function named {\em head1\/}. It contains a variable named {\em index\/}, which has nothing to do with the {\em index\/} of the main program, except that both are automatic variables. When the program is not actually executing statements in this function, this variable named {\em index\/} does not even exist. When {\em head1\/} is called, the variable is generated, and when {\em head1\/} completes its task, the variable {\em index\/} is eliminated completely from existence. Keep in mind however that this does not affect the variable of the same name in the main program, since it is a completely separate entity. Automatic variables therefore, are automatically generated and disposed of when needed. The important thing to remember is that from one call to a function to the next call, the value of an automatic variable is not preserved and must therefore be reinitialized. /section{WHAT ARE STATIC VARIABLES?} An additional variable type must be mentioned at this point, the {\em static\/} variable. By putting the reserved word {\em static\/} in front of a variable declaration within a function, the variable or variables in that declaration are static variables and will stay in existence from call to call of the particular function. By putting the same reserved word in front of an external variable, one outside of any function, it makes the variable private and not accessible to use in any other file. This implies that it is possible to refer to external variables in other separately compiled files, and that is true. Examples of this usage will be given in chapter 14 of this tutorial. \section{USING THE SAME NAME AGAIN} Refer to the function named {\em head2\/}. It contains another definition of the variable named {em count\/}. Even though {\em count\/} has already been defined as a global variable, it is perfectly all right to reuse the name in this function. It is a completely new variable that has nothing to do with the global variable of the same name, and causes the global variable to be unavailable in this function. This allows you to write programs using existing functions without worrying about what names were used for variables in the functions because there can be no conflict. You only need to worry about the variables that interface with the functions. \section{WHAT IS A REGISTER VARIABLE?} Now to fulfill a promise made earlier about what a register variable is. A computer can keep data in a register or in memory. A register is much faster in operation than memory but there are very few registers available for the programmer to use. If there are certain variables that are used extensively in a program, you can designate that those variables are to be stored in a register if possible in order to speed up the execution of the program. Depending on the computer and the compiler, a small number of register variables may be allowed and are designated by putting the word {\em register\/} in front of the desired variable. Check your compiler documentation for the availability of this feature and the number of register variables. Most compilers that do not have any register variables available, will simply ignore the word {\em register\/} and run normally, keeping all variables in memory. Register variables are only available for use with integer and character type variables. This may or may not include some of the other integer-like variables such as unsigned, long, or short. Check the documentation for your compiler. \section{WHERE DO I DEFINE VARIABLES?} Now for a refinement on a general rule stated earlier. When you have variables brought to a function as arguments to the function, they are defined immediately after the function name and prior to the opening brace for the program. Other variables used in the function are defined at the beginning of the function, immediately following the opening brace of the function, and before any executable statements. \section{STANDARD FUNCTION LIBRARIES} Every compiler comes with some standard predefined functions which are available for your use. These are mostly input/output functions, character and string manipulation functions, and math functions. We will cover most of these in subsequent chapters. In addition, most compilers have additional functions predefined that are not standard but allow the programmer to get the most out of his particular computer. In the case of the IBM-PC and compatibles, most of these functions allow the programmer to use the BIOS services available in the operating system, or to write directly to the video monitor or to any place in memory. These will not be covered in any detail as you will be able to study the unique aspects of your compiler on your own. Many of these kinds of functions are used in the example programs in chapter 15. \section{WHAT IS RECURSION?} Recursion is another of those programming techniques that seem very intimidating the first time you come across it, but if you will load and display the example program named RECURSON.C, we will take all of the mystery out of it. This is probably the simplest recursive program that it is possible to write and it is therefore a stupid program in actual practice, but for purposes of illustration, it is excellent. Recursion is nothing more than a function that calls itself. It is therefore in a loop which must have a way of terminating. In the program on your monitor, the variable {\em index\/} is set to 8, and is used as the argument to the function {\em count\_dn}. The function simply decrements the variable, prints it out in a message, and if the variable is not zero, it calls itself, where it decrements it again, prints it, etc. etc. etc. Finally, the variable will reach zero, and the function will not call itself again. Instead, it will return to the prior time it called itself, and return again, until finally it will return to the main program and will return to DOS. For purposes of understanding you can think of it as having 8 copies of the function {\em count\_dn\/} available and it simply called all of them one at a time, keeping track of which copy it was in at any given time. That is not what actually happened, but it is a reasonable illustration for you to begin understanding what it was really doing. \section{WHAT DID IT DO?} A better explanation of what actually happened is in order. When you called the function from itself, it stored all of the variables and all of the internal flags it needs to complete the function in a block somewhere. The next time it called itself, it did the same thing, creating and storing another block of everything it needed to complete that function call. It continued making these blocks and storing them away until it reached the last function when it started retrieving the blocks of data, and using them to complete each function call. The blocks were stored on an internal part of the computer called the {\em stack\/}. This is a part of memory carefully organized to store data just as described above. It is beyond the scope of this tutorial to describe the stack in detail, but it would be good for your programming experience to read some material describing the stack. A stack is used in nearly all modern computers for internal housekeeping chores. In using recursion, you may desire to write a program with indirect recursion as opposed to the direct recursion described above. Indirect recursion would be when a function $A$ calls the function $B$, which in turn calls $A$, etc. This is entirely permissible, the system will take care of putting the necessary things on the stack and retrieving them when needed again. There is no reason why you could not have three functions calling each other in a circle, or four, or five, etc. The C compiler will take care of all of the details for you. The thing you must remember about recursion is that at some point, something must go to zero, or reach some predefined point to terminate the loop. If not, you will have an infinite loop, and the stack will fill up and overflow, giving you an error and stopping the program rather abruptly. \section{ANOTHER EXAMPLE OF RECURSION} The program named BACKWARD.C is another example of recursion, so load it and display it on your screen. This program is similar to the last one except that it uses a character array. Each successive call to the function named {\em forward\_and\_backward\/} causes one character of the message to be printed. Additionally, each time the function ends, one of the characters is printed again, this time backwards as the string of recursive function calls is retraced. Don't worry about the character array defined in line 3 or the other new material presented here. After you complete chapter 7 of this tutorial, this program will make sense. It was felt that introducing a second example of recursion was important so this file is included here. One additional feature is built into this program. If you observe the two calls to the function, and the function itself, you will see that the function name is spelled three different ways in the last few characters. The compiler doesn't care how they are spelled because it only uses the first 8 characters of the function name so as far as it is concerned, the function is named {\em forward\_\/}. The remaining characters are simply ignored. If your compiler uses more that 8 characters as being significant, you will need to change two of the names so that all three names are identical. Compile and run this program and observe the results. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Rewrite TEMPCONV.C, from an earlier chapter, and move the temperature calculation to a function. \item Write a program that writes your name on the monitor 10 times by calling a function to do the writing. Move the called function ahead of the {\em main\/} function to see if your compiler will allow it. \end{enumerate} \chapter{Defines and Macros} \section[DEFINES AND MACROS]{DEFINES AND MACROS ARE AIDS TO CLEAR PROGRAMMING} Load and display the file named DEFINE.C for your first look at some defines and macros. Notice the first four lines of the program each starting with the word {\em \#define\/}. This is the way all defines and macros are defined. Before the actual compilation starts, the compiler goes through a preprocessor pass to resolve all of the defines. In the present case, it will find every place in the program where the combination {\em START\/} is found and it will simply replace it with the 0 since that is the definition. The compiler itself will never see the word {\em START\/}, so as far as the compiler is concerned, the zeros were always there. It should be clear to you by now that putting the word {\em START\/} in your program instead of the numeral 0 is only a convenience to you and actually acts like a comment since the word {\em START\/} helps you to understand what the zero is used for. In the case of a very small program, such as that before you, it doesn't really matter what you use. If, however, you had a 2000 line program before you with 27 references to the START, it would be a completely different matter. If you wanted to change all of the STARTs in the program to a new number, it would be simple to change the one {\em \#define/}, but difficult to find and change all of the references to it manually, and possibly disastrous if you missed one or two of the references. In the same manner, the preprocessor will find all occurrences of the word {\em ENDING\/} and change them to 9, then the compiler will operate on the changed file with no knowledge that {\em ENDING\/} ever existed. It is a fairly common practice in C programming to use all capital letters for a symbolic constant such as {\em START\/} and {\em ENDING\/} and use all lower case letters for variable names. You can use any method you choose since it is mostly a matter of personal taste. \section{IS THIS REALLY USEFUL?} When we get to the chapters discussing input and output, we will need an indicator to tell us when we reach the end--of--file of an input file. Since different compilers use different numerical values for this, although most use either a zero or a minus 1, we will write the program with a {\em define\/} to define the EOF used by our particular compiler. If at some later date, we change to a new compiler, it is a simple matter to change this one {\em define\/} to fix the entire program. End--of--line is another indicator that is not universal. This will make more sense when we get to the chapters on input and output. \section{WHAT IS A MACRO?} A macro is nothing more than another define, but since it is capable of at least appearing to perform some logical decisions or some math functions, it has a unique name. Consider the third line of the program on your screen for an example of a macro. In this case, anytime the preprocessor finds the word {\em MAX\/} followed by a group in parentheses, it expects to find two terms in the parentheses and will do a replacement of the terms into the second definition. Thus the first term will replace every $A$ in the second definition and the second term will replace every $B$ in the second definition. When line 12 of the program is reached, {\em index\/} will be substituted for every $A$, and {\em count\/} will be substituted for every $B$. Remembering the cryptic construct we studied a couple of chapters ago will reveal that $mx$ will receive the maximum value of {\em index\/} or {\em count\/}. In like manner, the {\em MIN\/} macro will result in {\em mn\/} receiving the minimum value of {\em index\/} or {\em count\/}. The results are then printed out. There are a lot of seemingly extra parentheses in the macro definition but they are not extra, they are essential. We will discuss the extra parentheses in our next program. Compile and run DEFINE.C. \section{LETS LOOK AT A WRONG MACRO} Load the file named MACRO.C and display it on your screen for a better look at a macro and its use. The first line defines a macro named {\em WRONG\/} that appears to get the cube of $A$, and indeed it does in some cases, but it fails miserably in others. The second macro named {\em CUBE\/} actually does get the cube in all cases. Consider the program itself where the CUBE of i+offset is calculated. If i is 1, which it is the first time through, then we will be looking for the cube of $1+5 = 6$, which will result in $216$. When using {\em CUBE\/}, we group the values like this, $(1+5)*(1+5)*(1+5) = 6*6*6 = 216$. However, when we use {\em WRONG\/}, we group them as $1+5*1+5*1+5 = 1+5+5+5 = 16$ which is a wrong answer. The parentheses are therefore required to properly group the variables together. It should be clear to you that either {\em CUBE\/} or {\em WRONG\/} would arrive at a correct answer for a single term replacement such as we did in the last program. The correct values of the cube and the square of the numbers are printed out as well as the wrong values for your inspection. The remainder of the program is simple and will be left to your inspection and understanding. \section{PROGRAMMING EXERCISE} \begin{enumerate} \item Write a program to count from 7 to -5 by counting down. Use {\em \#define\/} statements to define the limits. (Hint, you will need to use a decrementing variable in the third part of the {\em for\/} loop control. \end{enumerate} \chapter{Strings and Arrays} \section{WHAT IS A STRING?} A string is a group of characters, usually letters of the alphabet. In order to format your printout in such a way that it looks nice, has meaningful titles and names, and is esthetically pleasing to you and the people using the output of your program, you need the ability to output text data. Actually you have already been using strings, because the second program in this tutorial, way back in Chapter 2, output a message that was handled internally as a string. A complete definition is a series of {\em char\/} type data terminated by a NULL character, which is a zero. When C is going to use a string of data in some way, either to compare it with another, output it, copy it to another string, or whatever, the functions are set up to do what they are called to do until a NULL, which is a zero, is detected. \section{WHAT IS AN ARRAY?} An array is a series of homogeneous pieces of data that are all identical in type, but the type can be quite complex as we will see when we get to the chapter of this tutorial discussing structures. A string is simply a special case of an array. The best way to see these principles is by use of an example, so load the program CHRSTRG.C and display it on your monitor. The first thing new is the line that defines a {\em char\/} type of data entity. The square brackets define an array subscript in C, and in the case of the data definition statement, the 5 in the brackets defines 5 data fields of type {\em char\/} all defined as the variable {\em name}. In the C language, all subscripts start at 0 and increase by 1 each step up to the maximum which in this case is 4. We therefore have 5 "char" type variables named, {\em name[0]\/}, {\em name[1]}, {\em name[2]}, {\em name[3]}, and {\em name[4]}. You must keep in mind that in C, the subscripts actually go from 0 to one less than the number defined in the definition statement. \section{HOW DO WE USE THE STRING?} The variable {\em name\/} is therefore a string which can hold up to 5 characters, but since we need room for the NULL character, there are actually only four useful characters. To load something useful into the string, we have 5 statements, each of which assigns one alphabetical character to one of the string characters. Finally, the last place in the string is filled with the numeral 0 as the end indicator and the string is complete. (A {\em define\/} would allow us to use ``NULL'' instead of a zero, and this would add greatly to the clarity of the program. It would be very obvious that this was a NULL and not simply a zero for some other purpose.) Now that we have the string, we will simply print it out with some other string data in the output statement. The \%s is the output definition to output a string and the system will output characters starting with the first one in {\em name\/} until it comes to the NULL character, and it will quit. Notice that in the {\em printf\/} statement, only the variable name {\em name\/} needs to be given, with no subscript since we are interested in starting at the beginning. (There is actually another reason that only the variable name is given without brackets. The discussion of that topic will be given in the next chapter.) \section{OUTPUTTING PART OF A STRING} The next {\em printf\/} illustrates that we can output any single character of the string by using the {\em \%c\/} and naming the particular character of {\em name\/} we want by including the subscript. The last {\em printf\/} illustrates how we can output part of the string by stating the starting point by using a subscript. The \& specifies the address of {\em name[1]\/}. We will study this in the next chapter but I thought you would benefit from a little glimpse ahead. This example may make you feel that strings are rather cumbersome to use since you have to set up each character one at a time. That is an incorrect conclusion because strings are very easy to use as we will see in the next example program. Compile and run this program. \section{SOME STRING SUBROUTINES} Load the example program STRINGS.C for an example of some ways to use strings. First we define four strings. Next we come to a new function that you will find very useful, the {\em strcpy\/} function, or string copy. It copies from one string to another until it comes to the NULL character. It is easy to remember which one gets copied to which if you think of them like an assignment statement. Thus if you were to say, for example, {\em $x = 23;$\/}, the data is copied from the right entity to the left one. In the {\em strcpy\/} function, the data is also copied from the right entity to the left, so that after execution of the first statement, name1 will contain the string ``Rosalinda'', but without the double quotes, they are the compiler's way of knowing that you are defining a string. Likewise, ``Zeke'' is copied into {\em name2\/} by the second statement, then the {\em title\/} is copied. The title and both names are then printed out. Note that it is not necessary for the defined string to be exactly the same size as the string it will be called upon to store, only that it is at least as long as the string plus one more character for the NULL. \section{ALPHABETICAL SORTING OF STRINGS} The next function we will look at is the {\em strcmp\/} or the string compare function. It will return a 1 if the first string is larger than the second, zero if they are the same length and have the same characters, and $-1$ if the first string is smaller than the second. One of the strings, depending on the result of the compare is copied into the variable {\em mixed\/}, and the largest name alphabetically is printed out. It should come as no surprise to you that ``Zeke'' wins because it is alphabetically larger, length doesn't matter, only the alphabet. It might be wise to mention that the result would also depend on whether the letters were upper or lower case. There are functions available with your C compiler to change the case of a string to all upper or all lower case if you desire. These will be used in an example program later in this tutorial. \section{COMBINING STRINGS} The last four statements have another new feature, the {\em strcat\/}, or string concatenation function. This function simply adds the characters from one string onto the end of another string taking care to adjust the NULL so everything is still all right. In this case, {\em name1\/} is copied into {\em mixed\/}, then two blanks are concatenated to {\em mixed\/}, and finally {\em name2\/} is concatenated to the combination. The result is printed out with both names in the one variable {\em mixed}. Strings are not difficult and are extremely useful. You should spend some time getting familiar with them before proceeding on to the next topic. Compile and run this program and observe the results for compliance with this definition. \section{AN ARRAY OF INTEGERS} Load the file INTARRAY.C and display it on your monitor for an example of an array of integers. Notice that the array is defined in much the same way we defined an array of char in order to do the string manipulations in the last section. We have 12 integer variables to work with not counting the one named {\em index\/}. The names of the variables are {\em values[0]\/}, {\em values[1]\/}, ... , and {\em values[11]}. Next we have a loop to assign nonsense, but well defined, data to each of the 12 variables, then print all 12 out. You should have no trouble following this program, but be sure you understand it. Compile and run it to see if it does what you expect it to do. \section{AN ARRAY OF FLOATING POINT DATA} Load and display the program named BIGARRAY.C for an example of a program with an array of {\em float\/} type data. This program has an extra feature to illustrate how strings can be initialized. The first line of the program illustrates to you how to initialize a string of characters. Notice that the square brackets are empty leaving it up to the compiler to count the characters and allocate enough space for our string. Another string is initialized in the body of the program but it must be declared {\em static\/} here. This prevents it from being allocated as an {\em automatic\/} variable and allows it to retain the string once the program is started. There is nothing else new here, the variables are assigned nonsense data and the results of all the nonsense are printed out along with a header. This program should also be easy for you to follow, so study it until you are sure of what it is doing before going on to the next topic. \section{GETTING DATA BACK FROM A FUNCTION} Back in chapter 6 when we studied functions, I hinted to you that there was a way to get data back from a function by using an array, and that is true. Load the program PASSBACK.C for an example of doing that. In this program, we define an array of 20 variables named {\em matrix}, then assign some nonsense data to the variables, and print out the first five. Then we call the function {\em dosome\/} taking along the entire array by putting the name of the array in the parentheses. The function {\em dosome\/} has a name in its parentheses also but it prefers to call the array {\em list}. The function needs to be told that it is really getting an array passed to it and that the array is of type {\em int}. The following line, prior to the bracket which starts the program, does that by defining {\em list\/} as an integer type variable and including the square brackets to indicate an array. It is not necessary to tell the function how many elements are in the array, but you could if you so desired. Generally a function works with an array until some end--of--data marker is found, such as a NULL for a string, or some other previously defined data or pattern. Many times, another piece of data is passed to the function with a count of how many elements to work with. In our present illustration, we will use a fixed number of elements to keep it simple. So far nothing is different from the previous functions we have called except that we have passed more data points to the function this time than we ever have before, having passed 20 integer values. We print out the first 5 again to see if they did indeed get passed here. Then we add ten to each of the elements and print out the new values. Finally we return to the main program and print out the same 5 data points. We find that we have indeed modified the data in the function, and when we returned to the main program, we brought the changes back. Compile and run this program to verify this conclusion. \section{ARRAYS PASS DATA BOTH WAYS} We stated during our study of functions that when we passed data to a function, the system made a copy to use in the function which was thrown away when we returned. This is not the case with arrays. The actual array is passed to the function and the function can modify it any way it wishes to. The result of the modifications will be available back in the calling program. This may seem strange to you that arrays are handled differently from single point data, but they are. It really does make sense, but you will have to wait until we get to pointers to understand it. \section{A HINT AT A FUTURE LESSON} Another way of getting data back from a function to the calling program is by using pointers which we will cover in the next chapter. When we get there we will find that an array is in reality a pointer to a list of values. Don't let that worry you now, it will make sense when we get there. In the meantime concentrate on arrays and understand the basics of them because when we get to the study of structures we will be able to define some pretty elaborate arrays. \section{MULTIPLY DIMENSIONED ARRAYS} Load and display the file named MULTIARY.C for an example of a program with doubly dimensioned arrays. The variable {\em big\/} is an 8 by 8 array that contains 8 times 8 or 64 elements total. The first element is {\em big[0][0]\/}, and the last is {\em big[7][7]}. Another array named {\em huge\/} is also defined which is not square to illustrate that the array need not be square. Both are filled up with data, one representing a multiplication table and the other being formed into an addition table. To illustrate that individual elements can be modified at will, one of the elements of {\em big\/} is assigned the value from one of the elements of {\em huge\/} after being multiplied by 22. Next {\em big[2][2]\/} is assigned the arbitrary value of 5, and this value is used for the subscripts of the next assignment statement. The third assignment statement is in reality {\em $big[5][5] = 177$\/} because each of the subscripts contain the value 5. This is only done to illustrate that any valid expression can be used for a subscript. It must only meet two conditions, it must be an integer (although a {\em char\/} will work just as well), and it must be within the range of the subscript it is being used for. The entire matrix variable {\em big\/} is printed out in a square form so you can check the values to see if they did get set the way you expected them to. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Write a program with three short strings, about 6 characters each, and use {\em strcpy\/} to copy {em one\/}, {\em two\/}, and {\em three\/} into them. Concatenate the three strings into one string and print the result out 10 times. \item Define two integer arrays, each 10 elements long, called {\em array1\/} and {\em array2\/}. Using a loop, put some kind of nonsense data in each and add them term for term into another 10 element array named {\em arrays}. Finally, print all results in a table with an index number. \begin{tabular}{|ccccccr|} \hline 1 & 2 & + & 10 & = & 12 \\ 2 & 4 & + & 20 & = & 24 \\ 3 & 6 & + & 30 & = & 36 & etc. \\ \hline \end{tabular} Hint: The print statement will be similar to; {\em printf(``\%4d \%4d $+$ \%4d $=$ %4d\n'',index,array1[index],\\ array2[index],arrays[index]);\/} \end{enumerate} \chapter{Pointers} \section{WHAT IS A POINTER?} Simply stated, a pointer is an address. Instead of being a variable, it is a pointer to a variable stored somewhere in the address space of the program. It is always best to use an example so load the file named POINTER.C and display it on your monitor for an example of a program with some pointers in it. For the moment, ignore the data declaration statement where we define {\em index\/} and two other fields beginning with a star. It is properly called an asterisk, but for reasons we will see later, let's agree to call it a star. If you observe the first statement, it should be clear that we assign the value of 39 to the variable {\em index}. This is no surprise, we have been doing it for several programs now. The next statement however, says to assign to {\em pt1\/} a strange looking value, namely the variable {\em index\/} with an ampersand in front of it. In this example, pt1 and pt2 are pointers, and the variable {\em index\/} is a simple variable. Now we have a problem. We need to learn how to use pointers in a program, but to do so requires that first we define the means of using the pointers in the program. The following two rules will be somewhat confusing to you at first but we need to state the definitions before we can use them. Take your time, and the whole thing will clear up very quickly. \section{TWO VERY IMPORTANT RULES} The following two rules are very important when using pointers and must be thoroughly understood. \begin{enumerate} \item A variable name with an ampersand in front of it defines the address of the variable and therefore points to the variable. You can therefore read line six as ``pt1 is assigned the value of the address of index''. \item A pointer with a {\em star\/} in front of it refers to the value of the variable pointed to by the pointer. Line nine of the program can be read as ``The stored (starred) value to which the pointer {\em pt1\/} points is assigned the value 13''. Now you can see why it is convenient to think of the asterisk as a star, it sort of sounds like the word store. \end{enumerate} \section{MEMORY AIDS} \begin{enumerate} \item Think of \& as an address. \item Think of * as a star referring to stored. \end{enumerate} Assume for the moment that {\em pt1\/} and {\em pt2\/} are pointers (we will see how to define them shortly). As pointers, they do not contain a variable value but an address of a variable and can be used to point to a variable. Line six of the program assigns the pointer {\em pt1\/} to point to the variable we have already defined as {\em index\/} because we have assigned the address of {\em index\/} to {\em pt1\/}. Since we have a pointer to {\em index\/}, we can manipulate the value of {\em index\/} by using either the variable name itself, or the pointer. Line nine modifies the value by using the pointer. Since the pointer {\em pt1\/} points to the variable {\em index\/}, then putting a star in front of the pointer name refers to the memory location to which it is pointing. Line nine therefore assigns to {\em index\/} the value of 13. Anyplace in the program where it is permissible to use the variable name {\em index\/}, it is also permissible to use the name {\em *pt1\/} since they are identical in meaning until the pointer is reassigned to some other variable. \section{ANOTHER POINTER} Just to add a little intrigue to the system, we have another pointer defined in this program, {\em pt2}. Since {\em pt2\/} has not been assigned a value prior to statement seven, it doesn't point to anything, it contains garbage. Of course, that is also true of any variable until a value is assigned to it. Statement seven assigns {\em pt2\/} the same address as {\em pt1}, so that now {\em pt2\/} also points to the variable {\em index}. So to continue the definition from the last paragraph, anyplace in the program where it is permissible to use the variable {\em index}, it is also permissible to use the name {\em *pt2\/} because they are identical in meaning. This fact is illustrated in the first {\em printf\/} statement since this statement uses the three means of identifying the same variable to print out the same variable three times. \section{THERE IS ONLY ONE VARIABLE} Note carefully that, even though it appears that there are three variables, there is really only one variable. The two pointers point to the single variable. This is illustrated in the next statement which assigns the value of 13 to the variable {\em index}, because that is where the pointer {\em pt1\/} is pointing. The next {\em printf\/} statement causes the new value of 13 to be printed out three times. Keep in mind that there is really only one variable to be changed, not three. This is admittedly a very difficult concept, but since it is used extensively in all but the most trivial C programs, it is well worth your time to stay with this material until you understand it thoroughly. \section[DECLARE A POINTER]{HOW DO YOU DECLARE A POINTER?} Now to keep a promise and tell you how to declare a pointer. Refer to the third line of the program and you will see our old familiar way of defining the variable {\em index}, followed by two more definitions. The second definition can be read as ``the storage location to which {\em pt1\/} points will be an int type variable''. Therefore, {\em pt1\/} is a pointer to an int type variable. Likewise, {\em pt2\/} is another pointer to an int type variable. A pointer must be defined to point to some type of variable. Following a proper definition, it cannot be used to point to any other type of variable or it will result in a {\em type incompatibility\/} error. In the same manner that a {\em float\/} type of variable cannot be added to an {\em int\/} type variable, a pointer to a {\em float\/} variable cannot be used to point to an integer variable. Compile and run this program and observe that there is only one variable and the single statement in line 9 changes the one variable which is displayed three times. \section{THE SECOND PROGRAM WITH POINTERS} In these few pages so far on pointers, we have covered a lot of territory, but it is important territory. We still have a lot of material to cover so stay in tune as we continue this important aspect of C. Load the next file named POINTER2.C and display it on your monitor so we can continue our study. In this program we have defined several variables and two pointers. The first pointer named ``there'' is a pointer to a {\em char\/} type variable and the second named ``pt'' points to an {\em int\/} type variable. Notice also that we have defined two array variables named ``strg'' and ``list''. We will use them to show the correspondence between pointers and array names. \section[STRING VARIABLES ARE POINTERS]{A STRING VARIABLE IS ACTUALLY A POINTER} In the programming language C, a string variable is defined to be simply a pointer to the beginning of a string. This will take some explaining. Refer to the example program on your monitor. You will notice that first we assign a string constant to the string variable named ``strg'' so we will have some data to work with. Next, we assign the value of the first element to the variable ``one'', a simple {\em char\/} variable. Next, since the string name is a pointer by definition of the C language, we can assign the same value to ``two'' by using the star and the string name. The result of the two assignments are such that ``one'' now has the same value as ``two'', and both contain the character ``T'', the first character in the string. Note that it would be incorrect to write the ninth line as ``two $=$ *strg[0];'' because the star takes the place of the square brackets. For all practical purposes, ``strg'' is a pointer. It does, however, have one restriction that a true pointer does not have. It cannot be changed like a variable, but must always contain the initial value and therefore always points to its string. It could be thought of as a pointer constant, and in some applications you may desire a pointer that cannot be corrupted in any way. Even though it cannot be changed, it can be used to refer to other values than the one it is defined to point to, as we will see in the next section of the program. Moving ahead to line 12, the variable ``one'' is assigned the value of the ninth variable (since the indexing starts at zero) and ``two'' is assigned the same value because we are allowed to index a pointer to get to values farther ahead in the string. Both variables now contain the character ``a''. The C programming language takes care of indexing for us automatically by adjusting the indexing for the type of variable the pointer is pointing to. In this case, the index of 8 is simply added to the pointer value before looking up the desired result because a {\em char\/} type variable is one byte long. If we were using a pointer to an {\em int\/} type variable, the index would be doubled and added to the pointer before looking up the value because an {\em int\/} type variable uses two bytes per value stored. When we get to the chapter on structures, we will see that a variable can have many, even into the hundreds or thousands, of characters per variable, but the indexing will be handled automatically for us by the system. Since ``there'' is already a pointer, it can be assigned the value of the eleventh element of ``strg'' by the statement in line 16 of the program. Remember that since ``there'' is a true pointer, it can be assigned any value as long as that value represents a {\em char\/} type of address. It should be clear that the pointers must be {\em typed\/} in order to allow the pointer arithmetic described in the last paragraph to be done properly. The third and fourth outputs will be the same, namely the letter ``c''. \section{POINTER ARITHMETIC} Not all forms of arithmetic are permissible on a pointer. Only those things that make sense, considering that a pointer is an address somewhere in the computer. It would make sense to add a constant to an address, thereby moving it ahead in memory that number of places. Likewise, subtraction is permissible, moving it back some number of locations. Adding two pointers together would not make sense because absolute memory addresses are not additive. Pointer multiplication is also not allowed, as that would be a funny number. If you think about what you are actually doing, it will make sense to you what is allowed, and what is not. \subsection[INTEGER POINTER]{NOW FOR AN INTEGER POINTER} The array named ``list'' is assigned a series of values from 100 to 199 in order to have some data to work with. Next we assign the pointer ``pt'' the value of the 28th element of the list and print out the same value both ways to illustrate that the system truly will adjust the index for the {\em int\/} type variable. You should spend some time in this program until you feel you fairly well understand these lessons on pointers. Compile and run POINTER2.C and study the output. You may recall that back in the lesson on functions we mentioned that there were two ways to get variable data back from a function. One way is through use of the array, and you should be right on the verge of guessing the other way. If your guess is through use of a pointer, you are correct. Load and display the program named TWOWAY.C for an example of this. \section{FUNCTION DATA RETURN WITH A POINTER} In TWOWAY.C, there are two variables defined in the main program ``pecans'' and ``apples''. Notice that neither of these is defined as a pointer. We assign values to both of these and print them out, then call the function {\em fixup\/} taking with us both of these values. The variable ``pecans'' is simply sent to the function, but the address of the variable ``apples'' is sent to the function. Now we have a problem. The two arguments are not the same, the second is a pointer to a variable. We must somehow alert the function to the fact that it is supposed to receive an integer variable and a pointer to an integer variable. This turns out to be very simple. Notice that the parameter definitions in the function define ``nuts'' as an {\em integer}, and ``fruit'' as a {\em pointer to an integer}. The call in the main program therefore is now in agreement with the function heading and the program interface will work just fine. In the body of the function, we print the two values sent to the function, then modify them and print the new values out. This should be perfectly clear to you by now. The surprise occurs when we return to the main program and print out the two values again. We will find that the value of pecans will be restored to its value before the function call because the C language makes a copy of the item in question and takes the copy to the called function, leaving the original intact. In the case of the variable ``apples'', we made a copy of a pointer to the variable and took the copy of the pointer to the function. Since we had a pointer to the original variable, even though the pointer was a copy, we had access to the original variable and could change it in the function. When we returned to the main program, we found a changed value in ``apples'' when we printed it out. By using a pointer in a function call, we can have access to the data in the function and change it in such a way that when we return to the calling program, we have a changed value of data. It must be pointed out however, that if you modify the value of the pointer itself in the function, you will have a restored pointer when you return because the pointer you use in the function is a copy of the original. In this example, there was no pointer in the main program because we simply sent the address to the function, but in many programs you will use pointers in function calls. One of the places you will find need for pointers in function calls will be when you request data input using standard input/output routines. These will be covered in the next two chapters. Compile and run TWOWAY.C and observe the output. \section{POINTERS ARE VALUABLE} Even though you are probably somewhat intimidated at this point by the use of pointers, you will find that after you gain experience, you will use them profusely in many ways. You will also use pointers in every program you write other than the most trivial because they are so useful. You should probably go over this material carefully several times until you feel comfortable with it because it is very important in the area of input/output which is next on the agenda. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Define a character array and use {\em strcpy\/} to copy a string into it. Print the string out by using a loop with a pointer to print out one character at a time. Initialize the pointer to the first element and use the double plus sign to increment the pointer. Use a separate integer variable to count the characters to print. \item Modify the program to print out the string backwards by pointing to the end and using a decrementing pointer. \end{enumerate} \chapter{Standard Input/Output} \section{THE STDIO.H HEADER FILE} Load the file SIMPLEIO.C for our first look at a file with standard I/O. Standard I/O refers to the most usual places where data is either read from, the keyboard, or written to, the video monitor. Since they are used so much, they are used as the default I/O devices and do not need to be named in the Input/Output instructions. This will make more sense when we actually start to use them so lets look at the file in front of you. The first thing you will notice is the first line of the file, the {\bf \#include ``stdio.h''} line. This is very much like the {\bf \#define} we have already studied, except that instead of a simple substitution, an entire file is read in at this point. The system will find the file named ``stdio.h'' and read its entire contents in, replacing this statement. Obviously then, the file named ``stdio.h'' must contain valid C source statements that can be compiled as part of a program. This particular file is composed of several standard {\bf \#defines} to define some of the standard I/O operations. The file is called a header file and you will find several different header files on the source disks that came with your compiler. Each of the header files has a specific purpose and any or all of them can be included in any program. Most C compilers use the double quote marks to indicate that the {\em include\/} file will be found in the current directory. A few use the ``less than'' and ``greater than'' ($<$ \& $>$) signs to indicate that the file will be found in a standard header file. Nearly all MSDOS C compilers use the double quotes, and most require the {\em include\/} file to be in the default directory. All of the programs in this tutorial have the double quotes in the {\em include\/} statements. If your compiler uses the other notation, you will have to change them before compiling. \section[INPUT \& OUTPUT]{INPUT/OUTPUT OPERATIONS IN C} Actually the C programming language has no input or output operations defined as part of the language, they must be user defined. Since everybody does not want to reinvent his own input and output operations, the compiler writers have done a lot of this for us and supplied us with several input functions and several output functions to aid in our program development. The functions have become a standard, and you will find the same functions available in nearly every compiler. In fact, the industry standard of the C language definition has become the book written by Kernigan and Ritchie, and they have included these functions in their definition. You will often, when reading literature about C, find a reference to K \& R. This refers to the book written by Kernigan and Ritchie. You would be advised to purchase a copy for reference. You should print out the file named ``stdio.h'' and spend some time studying it. There will be a lot that you will not understand about it, but parts of it will look familiar. The name ``stdio.h'' is sort of cryptic for ``standard input/output header'', because that is exactly what it does. It defines the standard input and output functions in the form of {\bf \#defines} and macros. Don't worry too much about the details of this now. You can always return to this topic later for more study if it interests you, but you will really have no need to completely understand the ``stdio.h'' file. You will have a tremendous need to use it however, so these comments on its use and purpose are necessary. \section{OTHER INCLUDE FILES} When you begin writing larger programs and splitting them up into separately compiled portions, you will have occasion to use some statements common to each of the portions. It would be to your advantage to make a separate file containing the statements and use the {\em \#include\/} to insert it into each of the files. If you want to change any of the common statements, you will only need to change one file and you will be assured of having all of the common statements agree. This is getting a little ahead of ourselves but you now have an idea how the {\bf \#include} directive can be used. \section{BACK TO THE FILE NAMED ``SIMPLEIO.C''} Lets continue our tour of the file in question. The one variable $c$ is defined and a message is printed out with the familiar {\em printf\/} function. We then find ourselves in a continuous loop as long as $c$ is not equal to capital {\bf X}. If there is any question in your mind about the loop control, you should review chapter 3 before continuing. The two new functions within the loop are of paramount interest in this program since they are the new functions. These are functions to read a character from the keyboard and display it on the monitor one character at a time. The function {\em getchar()\/} reads a single character from the standard input device, the keyboard being assumed because that is the standard input device, and assigns it to the variable $c$. The next function {\em putchar(c)\/}, uses the standard output device, the video monitor, and outputs the character contained in the variable $c$. The character is output at the current cursor location and the cursor is advanced one space for the next character. The system is therefore taking care of a lot of the overhead for us. The loop continues reading and displaying characters until we type a capital {\bf X} which terminates the loop. Compile and run this program for a few surprises. When you type on the keyboard, you will notice that what you type is displayed faithfully on the screen, and when you hit the return key, the entire line is repeated. In fact, we only told it to output each character once but it seems to be saving the characters up and redisplaying them. A short explanation is in order. \section[DOS IS HELPING US OUT]{DOS IS HELPING US OUT (OR GETTING IN THE WAY)} We need to understand a little bit about how DOS works to understand what is happening here. When data is read from the keyboard, under DOS control, the characters are stored in a buffer until a carriage return is entered at which time the entire string of characters is given to the program. When the characters are being typed, however, the characters are displayed one at a time on the monitor. This is called echo, and happens in many of the applications you run. With the above paragraph in mind, it should be clear that when you are typing a line of data into ``SIMPLEIO'', the characters are being echoed by DOS, and when you return the carriage, the characters are given to the program. As each character is given to the program, it displays it on the screen resulting in a repeat of the line typed in. To better illustrate this, type a line with a capital {\bf X} somewhere in the middle of the line. You can type as many characters as you like following the {\bf X} and they will all display because the characters are being read in under DOS, echoed to the monitor, and placed in the DOS input buffer. DOS doesn't think there is anything special about a capital {\bf X}. When the string is given to the program, however, the characters are accepted by the program one at a time and sent to the monitor one at a time, until a capital {\bf X} is encountered. After the capital {\bf X} is displayed, the loop is terminated, and the program is terminated. The characters on the input line following the capital {\bf X} are not displayed because the capital {\bf X} signalled program termination. Compile and run SIMPLEIO.C. After running the program several times and feeling confidant that you understand the above explanation, we will go on to another program. Don't get discouraged by the above seemingly weird behavior of the I/O system. It is strange, but there are other ways to get data into the computer. You will actually find the above method useful for many applications, and you will probably find some of the following useful also. \section{ANOTHER STRANGE I/O METHOD} Load the file named SINGLEIO.C and display it on your monitor for another method of character I/O. Once again, we start with the standard I/O header file, we define a variable named $c$, and we print a welcoming message. Like the last program, we are in a loop that will continue to execute until we type a capital {\bf X}, but the action is a little different here. The {\em getch()\/} is a new function that is a {\em get character\/} function. It differs from {\em getchar()\/} in that it does not get tied up in DOS. It reads the character in without echo, and puts it directly into the program where it is operated on immediately. This function then reads a character, immediately displays it on the screen, and continues the operation until a capital {\bf X} is typed. When you compile and run this program, you will find that there is no repeat of the lines when you hit a carriage return, and when you hit the capital {\bf X}, the program terminates immediately. No carriage return is needed to get it to accept the line with the {\bf X} in it. We do have another problem here, there is no linefeed with the carriage return. \subsection[LINE FEED]{NOW WE NEED A LINE FEED} It is not apparent to you in most application programs but when you hit the enter key, the program supplies a linefeed to go with the carriage return. You need to return to the left side of the monitor and you also need to drop down a line. The linefeed is not automatic. We need to improve our program to do this also. If you will load and display the program named BETTERIN.C, you will find a change to incorporate this feature. In BETTERIN.C, we have two additional statements at the beginning that will define the character codes for the linefeed (LF), and the carriage return (CR). If you look at any ASCII table you will find that the codes 10 and 13 are exactly as defined here. In the main program, after outputting the character, we compare it to CR, and if it is equal to CR, we also output a linefeed which is the LF. We could have just as well have left out the two \#define statements and used ``\verb|if (c == 13) putchar(10);|'' but it would not be very descriptive of what we are doing here. The method used in the program represents better programming practice. Compile and run BETTERIN.C to see if it does what we have said it should do. It should display exactly what you type in, including a linefeed with each carriage return, and should stop immediately when you type a capital {\bf X}. If you are using a nonstandard compiler, it may not find a ``CR'' because your system returns a ``LF'' character to indicate end-of-line. It will be up to you to determine what method your compiler uses. The quickest way is to add a {\em printf\/} statement that prints the input character in decimal format. \section{WHICH METHOD IS BEST?} We have examined two methods of reading characters into a C program, and are faced with a choice of which one we should use. It really depends on the application because each method has advantages and disadvantages. Lets take a look at each. When using the first method, DOS is actually doing all of the work for us by storing the characters in an input buffer and signalling us when a full line has been entered. We could write a program that, for example, did a lot of calculations, then went to get some input. While we were doing the calculations, DOS would be accumulating a line of characters for us, and they would be there when we were ready for them. However, we could not read in single keystrokes because DOS would not report a buffer of characters to us until it recognized a carriage return. The second method, used in BETTERIN.C, allows us to get a single character, and act on it immediately. We do not have to wait until DOS decides we can have a line of characters. We cannot do anything else while we are waiting for a character because we are waiting for the input keystroke and tying up the entire machine. This method is useful for highly interactive types of program interfaces. It is up to you as the programmer to decide which is best for your needs. I should mention at this point that there is also an {\em ungetch\/} function that works with the {\em getch\/} function. If you {\em getch\/} a character and find that you have gone one too far, you can {\em ungetch\/} it back to the input device. This simplifies some programs because you don't know that you don't want the character until you get it. You can only {\em ungetch\/} one character back to the input device, but that is sufficient to accomplish the task this function was designed for. It is difficult to demonstrate this function in a simple program so its use will be up to you to study when you need it. The discussion so far in this chapter, should be a good indication that, while the C programming language is very flexible, it does put a lot of responsibility on you as the programmer to keep many details in mind. \section{NOW TO READ IN SOME INTEGERS} Load and display the file named INTIN.C for an example of reading in some formatted data. The structure of this program is very similar to the last three except that we define an {\em int\/} type variable and loop until the variable somehow acquires the value of 100. Instead of reading in a character at a time, as we have in the last three files, we read in an entire integer value with one call using the function named {\em scanf\/}. This function is very similar to the {\em printf\/} that you have been using for quite some time by now except that it is used for input instead of output. Examine the line with the {\em scanf\/} and you will notice that it does not ask for the variable ``valin'' directly, but gives the address of the variable since it expects to have a value returned from the function. Recall that a function must have the address of a variable in order to return the value to the calling program. Failing to supply a pointer in the {\em scanf\/} function is probably the most common problem encountered in using this function. The function {\em scanf\/} scans the input line until it finds the first data field. It ignores leading blanks and in this case, it reads integer characters until it finds a blank or an invalid decimal character, at which time it stops reading and returns the value. Remembering our discussion above about the way the DOS input buffer works, it should be clear that nothing is actually acted on until a complete line is entered and it is terminated by a carriage return. At this time, the buffer is input, and our program will search across the line reading all integer values it can find until the line is completely scanned. This is because we are in a loop and we tell it to find a value, print it, find another, print it, etc. If you enter several values on one line, it will read each one in succession and display the values. Entering the value of 100 will cause the program to terminate, and entering the value 100 with other values following, will cause termination before the following values are considered. \section{IT MAKES WRONG ANSWERS SOMETIMES} If you enter a number up to and including $32767$, it will display correctly, but if you enter a larger number, it will appear to make an error. For example, if you enter the value $32768$, it will display the value of $-32768$, entering the value $65536$ will display as a zero. These are not errors but are caused by the way an integer is defined. The most significant bit of the 16 bit pattern available for the integer variable is the sign bit, so there are only 15 bits left for the value. The variable can therefore only have the values from $-32768$ to $32767$, any other values are outside the range of integer variables. This is up to you to take care of in your programs. It is another example of the increased responsibility you must assume using C rather than a higher level language such as Pascal, Modula-2, etc. The above paragraph is true for most MS-DOS C compilers. There is a very small possibility that your compiler uses an integer value other than 16 bits. If that is the case, the same principles will be true but at different limits than those given above. Compile and run this program, entering several numbers on a line to see the results, and with varying numbers of blanks between the numbers. Try entering numbers that are too big to see what happens, and finally enter some invalid characters to see what the system does with nondecimal characters. \section{CHARACTER STRING INPUT} Load and display the file named STRINGIN.C for an example of reading a string variable. This program is identical to the last one except that instead of an integer variable, we have defined a string variable with an upper limit of 24 characters (remember that a string variable must have a null character at the end). The variable in the {\em scanf\/} does not need an \& because ``big'' is an array variable and by definition it is already a pointer. This program should require no additional explanation. Compile and run it to see if it works the way you expect. You probably got a surprise when you ran it because it separated your sentence into separate words. When used in the string mode of input, {\em scanf\/} reads characters into the string until it comes to either the end of a line or a blank character. Therefore, it reads a word, finds the blank following it, and displays the result. Since we are in a loop, this program continues to read words until it exhausts the DOS input buffer. We have written this program to stop whenever it finds a capital {\bf X} in column 1, but since the sentence is split up into individual words, it will stop anytime a word begins with capital {\bf X}. Try entering a 5 word sentence with a capital {\bf X} as the first character in the third word. You should get the first three words displayed, and the last two simply ignored when the program stops. Try entering more than 24 characters to see what the program does. It should generate an error, but that will be highly dependent on the system you are using. In an actual program, it is your responsibility to count characters and stop when the input buffer is full. You may be getting the feeling that a lot of responsibility is placed on you when writing in C. It is, but you also get a lot of flexibility in the bargain too. \section[INPUT/OUTPUT PROGRAMMIG]{INPUT/OUTPUT PROGRAMMING IN C} C was not designed to be used as a language for lots of input and output, but as a systems language where a lot of internal operations are required. You would do well to use another language for I/O intensive programming, but C could be used if you desire. The keyboard input is very flexible, allowing you to get at the data in a very low level way, but very little help is given you. It is therefore up to you to take care of all of the bookkeeping chores associated with your required I/O operations. This may seem like a real pain in the neck, but in any given program, you only need to define your input routines once and then use them as needed. Don't let this worry you. As you gain experience with C, you will easily handle your I/O requirements. One final point must be made about these I/O functions. It is perfectly permissible to intermix {\em scanf\/} and {\em getchar\/} functions during read operations. In the same manner, it is also fine to intermix the output functions, {\em printf\/} and {\em putchar}. \section{IN MEMORY I/O} The next operation may seem a little strange at first, but you will probably see lots of uses for it as you gain experience. Load the file named INMEM.C and display it for another type of I/O, one that never accesses the outside world, but stays in the computer. In INMEM.C, we define a few variables, then assign some values to the ones named ``numbers'' for illustrative purposes and then use a {\em sprintf\/} function. The function acts just like a normal {\em printf\/} function except that instead of printing the line of output to a device, it prints the line of formatted output to a character string in memory. In this case the string goes to the string variable ``line'', because that is the string name we inserted as the first argument in the {\em sprintf\/} function. The spaces after the 2nd \%d were put there to illustrate that the next function will search properly across the line. We print the resulting string and find that the output is identical to what it would have been by using a {\em printf\/} instead of the {\em sprintf\/} in the first place. You will see that when you compile and run the program shortly. Since the generated string is still in memory, we can now read it with the function {\em sscanf\/}. We tell the function in its first argument that ``line'' is the string to use for its input, and the remaining parts of the line are exactly what we would use if we were going to use the {\em scanf\/} function and read data from outside the computer. Note that it is essential that we use pointers to the data because we want to return data from a function. Just to illustrate that there are many ways to declare a pointer several methods are used, but all are pointers. The first two simply declare the address of the elements of the array, while the last three use the fact that ``result'', without the accompanying subscript, is a pointer. Just to keep it interesting, the values are read back in reverse order. Finally the values are displayed on the monitor. \bigskip \hspace*{20em}{\LARGE IS THAT REALLY USEFUL?} \medskip It seems sort of silly to read input data from within the computer but it does have a real purpose. It is possible to read data in using any of the standard functions and then do a format conversion in memory. You could read in a line of data, look at a few significant characters, then use these formatted input routines to reduce the line of data to internal representation. That would sure beat writing your own data formatting routines. \section{STANDARD ERROR OUTPUT} Sometimes it is desirable to redirect the output from the standard output device to a file. However, you may still want the error messages to go to the standard output device, in our case the monitor. This next function allows you to do that. Load and display SPECIAL.C for an example of this new function. The program consists of a loop with two messages output, one to the standard output device and the other to the standard error device. The message to the standard error device is output with the function {\em fprintf\/} and includes the device name {\em stderr\/} as the first argument. Other than those two small changes, it is the same as our standard {\em printf\/} function. (You will see more of the {\em fprintf\/} function in the next chapter, but its operation fit in better as a part of this chapter.) Ignore the line with the {\em exit\/} for the moment, we will return to it. Compile and run this program, and you will find 12 lines of output on the monitor. To see the difference, run the program again with redirected output to a file named ``STUFF'' by entering the following line at the Dos prompt; \begin{verbatim} A> special >stuff \end{verbatim} More information about I/O redirection can be found in your DOS manual. This time you will only get the 6 lines output to the standard error device, and if you look in your directory, you will find the file named ``STUFF'' containing the other 6 lines, those to the standard output device. You can use I/O redirection with any of the programs we have run so far, and as you may guess, you can also read from a file using I/O redirection but we will study a better way to read from a file in the next chapter. \section[THE exit() STATEMENT]{WHAT ABOUT THE exit(4) STATEMENT?} Now to keep our promise about the exit(4) statement. Redisplay the file named SPECIAL.C on your monitor. The last statement simply exits the program and returns the value of 4 to DOS. Any number from 0 to 9 can be used in the parentheses for DOS communication. If you are operating in a BATCH file, this number can be tested with the {\em ERRORLEVEL\/} command. Most compilers that operate in several passes return a 1 with this mechanism to indicate that a fatal error has occurred and it would be a waste of time to go on to another pass resulting in even more errors. It is therefore wise to use a batch file for compiling programs and testing the returned value for errors. A check of the documentation for my COMPAQ, resulted in a minimal and confusing documentation of the {\em errorlevel\/} command, so a brief description of it is given in this file. \section{PROGRAMMING EXERCISE} \begin{enumerate} \item Write a program to read in a character using a loop, and display the character in its normal {\em char\/} form. Also display it as a decimal number. Check for a dollar sign to use as the stop character. Use the {\em getch\/} form of input so it will print immediately. Hit some of the special keys, such as function keys, when you run the program for some surprises. You will get two inputs from the special keys, the first being a zero which is the indication to the system that a special key was hit. \end{enumerate} \chapter{File Input/Output} \section{OUTPUT TO A FILE} Load and display the file named FORMOUT.C for your first example of writing data to a file. We begin as before with the {\em include\/} statement for ``stdio.h'', then define some variables for use in the example including a rather strange looking new type. The type ``FILE'' is used for a file variable and is defined in the ``stdio.h'' file. It is used to define a file pointer for use in file operations. The definition of C contains the requirement for a pointer to a ``FILE'', and as usual, the name can be any valid variable name. \section{OPENING A FILE} Before we can write to a file, we must open it. What this really means is that we must tell the system that we want to write to a file and what the filename is. We do this with the {\em fopen\/} function illustrated in the first line of the program. The file pointer, ``fp'' in our case, points to the file and two arguments are required in the parentheses, the filename first, followed by the file type. The filename is any valid DOS filename, and can be expressed in upper or lower case letters, or even mixed if you so desire. It is enclosed in double quotes. For this example we have chosen the name TENLINES.TXT. This file should not exist on your disk at this time. If you have a file with this name, you should change its name or move it because when we execute this program, its contents will be erased. If you don't have a file by this name, that is good because we will create one and put some data into it. \subsection{READING (``r'')} The second parameter is the file attribute and can be any of three letters, ``r'', ``w'', or ``a'', and must be lower case. When an ``r'' is used, the file is opened for reading, a ``w'' is used to indicate a file to be used for writing, and an ``a'' indicates that you desire to append additional data to the data already in an existing file. Opening a file for reading requires that the file already exist. If it does not exist, the file pointer will be set to NULL and can be checked by the program. \subsection{WRITING (``w'')} When a file is opened for writing, it will be created if it does not already exist and it will be reset if it does resulting in deletion of any data already there. \subsection{APPENDING (``a'')} When a file is opened for appending, it will be created if it does not already exist and it will be initially empty. If it does exist, the data input point will be the end of the present data so that any new data will be added to any data that already exists in the file. \section{OUTPUTTING TO THE FILE} The job of actually outputting to the file is nearly identical to the outputting we have already done to the standard output device. The only real differences are the new function names and the addition of the file pointer as one of the function arguments. In the example program, {\em fprintf\/} replaces our familiar {\em printf\/} function name, and the file pointer defined earlier is the first argument within the parentheses. The remainder of the statement looks like, and in fact is identical to, the {\em printf\/} statement. \section{CLOSING A FILE} To close a file, you simply use the function {\em fclose\/} with the file pointer in the parentheses. Actually, in this simple program, it is not necessary to close the file because the system will close all open files before returning to DOS. It would be good programming practice for you to get in the habit of closing all files in spite of the fact that they will be closed automatically, because that would act as a reminder to you of what files are open at the end of each program. You can open a file for writing, close it, and reopen it for reading, then close it, and open it again for appending, etc. Each time you open it, you could use the same file pointer, or you could use a different one. The file pointer is simply a tool that you use to point to a file and you decide what file it will point to. Compile and run this program. When you run it, you will not get any output to the monitor because it doesn't generate any. After running it, look at your directory for a file named TENLINES.TXT and {\em type\/} it. That is where your output will be. Compare the output with that specified in the program. It should agree. Do not erase the file named TENLINES.TXT yet. We will use it in some of the other examples in this chapter. \section[SINGLE CHARACTER OUTPUT]{OUTPUTTING A SINGLE CHARACTER AT A TIME} Load the next example file, CHAROUT.C, and display it on your monitor. This program will illustrate how to output a single character at a time. The program begins with the {\em include\/} statement, then defines some variables including a file pointer. We have called the file pointer ``point'' this time, but we could have used any other valid variable name. We then define a string of characters to use in the output function using a {\em strcpy\/} function. We are ready to open the file for appending and we do so in the {\em fopen\/} function, except this time we use the lower cases for the filename. This is done simply to illustrate that DOS doesn't care about the case of the filename. Notice that the file will be opened for appending so we will add to the lines inserted during the last program. The program is actually two nested {\tt for} loops. The outer loop is simply a count to ten so that we will go through the inner loop ten times. The inner loop calls the function {\em putc\/} repeatedly until a character in ``others'' is detected to be a zero. \section{THE ``putc'' FUNCTION} The part of the program we are interested in is the {\em putc\/} function. It outputs one character at a time, the character being the first argument in the parentheses and the file pointer being the second and last argument. Why the designer of C made the pointer first in the {\em fprintf\/} function, and last in the {\em putc\/} function is a good question for which there may be no answer. It seems like this would have been a good place to have used some consistency. When the textline ``others'' is exhausted, a newline is needed because a newline was not included in the definition above. A single {\em putc\/} is then executed which outputs the ``$\backslash$n'' character to return the carriage and do a linefeed. When the outer loop has been executed ten times, the program closes the file and terminates. Compile and run this program but once again there will be no output to the monitor. Following execution of the program, {\bf type} the file named TENLINES.TXT and you will see that the 10 new lines were added to the end of the 10 that already existed. If you run it again, yet another 10 lines will be added. Once again, do not erase this file because we are still not finished with it. \section{READING A FILE} Load the file named READCHAR.C and display it on your monitor. This is our first program to read a file. This program begins with the familiar {\em include}, some data definitions, and the file opening statement which should require no explanation except for the fact that an ``r'' is used here because we want to read it. In this program, we check to see that the file exists, and if it does, we execute the main body of the program. If it doesn't, we print a message and quit. If the file does not exist, the system will set the pointer equal to NULL which we can test. The main body of the program is one {\em do while\/} loop in which a single character is read from the file and output to the monitor until an EOF (end of file) is detected from the input file. The file is then closed and the program is terminated. \bigskip \hspace*{15em}{\LARGE CAUTION CAUTION CAUTION} \medskip At this point, we have the potential for one of the most common and most perplexing problems of programming in C. The variable returned from the {\em getc\/} function is a character, so we could use a {\em char\/} variable for this purpose. There is a problem with that however, because on some, if not most, implementations of C, the EOF returns a minus one which a {\em char\/} type variable is not capable of containing. A {\em char\/} type variable can only have the values of zero to $255$, so it will return a $255$ for a minus one on those compilers that use a minus one for EOF. This is a very frustrating problem to try to find because no diagnostic is given. The program simply can never find the EOF and will therefore never terminate the loop. This is easy to prevent, always use an {\em int\/} type variable for use in returning an EOF. You can tell what your compiler uses for EOF by looking at the ``stdio.h'' file where EOF is defined. That is the standard place to define such values. There is another problem with this program but we will worry about it when we get to the next program and solve it with the one following that. After you compile and run this program and are satisfied with the results, it would be a good exercise to change the name of ``TENLINES.TXT'' and run the program again to see that the NULL test actually works as stated. Be sure to change the name back because we are still not finished with ``TENLINES.TXT''. \section[READING A WORD]{READING A WORD AT A TIME} Load and display the file named READTEXT.C for an example of how to read a word at a time. This program is nearly identical as the last except that this program uses the {\em fscanf\/} function to read in a string at a time. Because the {\em fscanf\/} function stops reading when it finds a space or a newline character, it will read a word at a time, and display the results one word to a line. You will see this when you compile and run it, but first we must examine a programming problem. \bigskip \hspace{15em}{\LARGE THIS IS A PROBLEM} \medskip Inspection of the program will reveal that when we read data in and detect the EOF, we print out something before we check for the EOF resulting in an extra line of printout. What we usually print out is the same thing printed on the prior pass through the loop because it is still in the buffer ``oneword''. We therefore must check for EOF before we execute the {\em printf\/} function. This has been done in READGOOD.C, which you will shortly examine, compile, and execute. Compile and execute the original program we have been studying, READTEXT.C and observe the output. If you haven't changed TENLINES.TXT you will end up with ``Additional'' and ``lines.'' on two separate lines with an extra ``lines.'' displayed because of the {\em printf\/} before checking for EOF. Compile and execute READGOOD.C and observe that the extra ``lines.'' does not get displayed because of the extra check for the EOF in the middle of the loop. This was also the problem referred to when we looked at READCHAR.C, but I chose not to expound on it there because the error in the output was not so obvious. \section[READING A FULL LINE]{FINALLY, WE READ A FULL LINE} Load and display the file READLINE.C for an example of reading a complete line. This program is very similar to those we have been studying except for the addition of a new quantity, the NULL. We are using {\em fgets\/} which reads in an entire line, including the newline character into a buffer. The buffer to be read into is the first argument in the function call, and the maximum number of characters to read is the second argument, followed by the file pointer. This function will read characters into the input buffer until it either finds a newline character, or it reads the maximum number of characters allowed minus one. It leaves one character for the end of string NULL character. In addition, if it finds an EOF, it will return a value of NULL. In our example, when the EOF is found, the pointer ``c'' will be assigned the value of NULL. NULL is defined as zero in your ``stdio.h'' file. When we find that ``c'' has been assigned the value of NULL, we can stop processing data, but we must check before we print just like in the last program. Last of course, we close the file. \section{HOW TO USE A VARIABLE FILENAME} Load and display the file ANYFILE.C for an example of reading from any file. This program asks the user for the filename desired, reads in the filename and opens that file for reading. The entire file is then read and displayed on the monitor. It should pose no problems to your understanding so no additional comments will be made. Compile and run this program. When it requests a filename, enter the name and extension of any text file available, even one of the example C programs. \section{HOW DO WE PRINT?} Load the last example file in this chapter, the one named PRINTDAT.C for an example of how to print. This program should not present any surprises to you so we will move very quickly through it. Once again, we open TENLINES.TXT for reading and we open PRN for writing. Printing is identical to writing data to a disk file except that we use a standard name for the filename. There are no definite standards as far as the name or names to be used for the printer, but some of the usual names are, ``PRN'', ``LPT'', ``LPT1'', and ``LPT2''. Check your documentation for your particular implementation. Some of the newest compilers use a predefined file pointer such as ``stdprn'' for the print file. Once again, check your documentation. The program is simply a loop in which a character is read, and if it is not the EOF, it is displayed and printed. When the EOF is found, the input file and the printer output files are both closed. You can now erase TENLINES.TXT from your disk. We will not be using it in any of the later chapters. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Write a program that will prompt for a filename for a read file, prompt for a filename for a write file, and open both plus a file to the printer. Enter a loop that will read a character, and output it to the file, the printer, and the monitor. Stop at EOF. \item Prompt for a filename to read. Read the file a line at a time and display it on the monitor with line numbers. \end{enumerate} \chapter{Structures and Unions} \section{WHAT IS A STRUCTURE?} A structure is a user defined data type. You have the ability to define a new type of data considerably more complex than the types we have been using. A structure is a combination of several different previously defined data types, including other structures we have defined. An easy to understand definition is, a structure is a grouping of related data in a way convenient to the programmer or user of the program. The best way to understand a structure is to look at an example, so if you will load and display STRUCT1.C, we will do just that. The program begins with a structure definition. The key word {\em struct\/} is followed by some simple variables between the braces, which are the components of the structure. After the closing brace, you will find two variables listed, namely ``boy'', and ``girl''. According to the definition of a structure, ``boy'' is now a variable composed of three elements, ``initial'', ``age'', and ``grade''. Each of the three fields are associated with ``boy'', and each can store a variable of its respective type. The variable ``girl'' is also a variable containing three fields with the same names as those of ``boy'' but are actually different variables. We have therefore defined 6 simple variables. \subsection{A SINGLE COMPOUND VARIABLE} Lets examine the variable ``boy'' more closely. As stated above, each of the three elements of ``boy'' are simple variables and can be used anywhere in a C program where a variable of their type can be used. For example, the ``age'' element is an integer variable and can therefore be used anywhere in a C program where it is legal to use an integer variable, in calculations, as a counter, in I/O operations, etc. The only problem we have is defining how to use the simple variable ``age'' which is a part of the compound variable ``boy''. We use both names with a decimal point between them with the major name first. Thus {\em boy.age\/} is the complete variable name for the ``age'' field of ``boy''. This construct can be used anywhere in a C program that it is desired to refer to this field. In fact, it is illegal to use the name ``boy'' or ``age'' alone because they are only partial definitions of the complete field. Alone, the names refer to nothing. \subsubsection{ASSIGNING VALUES TO THE VARIABLES} Using the above definition, we can assign a value to each of the three fields of ``boy`` and each of the three fields of ``girl''. Note carefully that {\em boy.initial\/} is actually a {\em char\/} type variable, because it was assigned that in the structure, so it must be assigned a character of data. Notice that {\em boy.initial\/} is assigned the character 'R' in agreement with the above rules. The remaining two fields of ``boy'' are assigned values in accordance with their respective types. Finally the three fields of girl are assigned values but in a different order to illustrate that the order of assignment is not critical. \subsection{HOW DO WE USE THE RESULTING DATA?} Now that we have assigned values to the six simple variables, we can do anything we desire with them. In order to keep this first example simple, we will simply print out the values to see if they really do exist as assigned. If you carefully inspect the "printf" statements, you will see that there is nothing special about them. The compound name of each variable is specified because that is the only valid name by which we can refer to these variables. Structures are a very useful method of grouping data together in order to make a program easier to write and understand. This first example is too simple to give you even a hint of the value of using structures, but continue on through these lessons and eventually you will see the value of using structures. Compile and run STRUCT1.C and observe the output. \section{AN ARRAY OF STRUCTURES} Load and display the next program named STRUCT2.C. This program contains the same structure definition as before but this time we define an array of $12$ variables named ``kids''. This program therefore contains $12$ times $3 = 36$ simple variables, each of which can store one item of data provided that it is of the correct type. We also define a simple variable named ``index'' for use in the for loops. In order to assign each of the fields a value, we use a for loop and each pass through the loop results in assigning a value to three of the fields. One pass through the loop assigns all of the values for one of the ``kids''. This would not be a very useful way to assign data in a real situation, but a loop could read the data in from a file and store it in the correct fields. You might consider this the crude beginning of a data base, which it is. In the next few instructions of the program we assign new values to some of the fields to illustrate the method used to accomplish this. It should be self explanatory, so no additional comments will be given. \bigskip \hspace*{15em}{\LARGE A NOTE TO PASCAL PROGRAMMERS} \medskip Pascal allows you to copy an entire RECORD with one statement. This is not possible in C. You must copy each element of a structure one at a time. As improvements to the language are defined, this will be one of the refinements. In fact, some of the newer compilers already allow structure assignment. Check your compiler documentation to see if your compiler has this feature yet. \medskip \hspace*{15em}{\large WE FINALLY DISPLAY ALL OF THE RESULTS} \smallskip The last few statements contain a {\em for\/} loop in which all of the generated values are displayed in a formatted list. Compile and run the program to see if it does what you expect it to do. \section{USING POINTERS AND STRUCTURES TOGETHER} Load and display the file named STRUCT3.C for an example of using pointers with structures. This program is identical to the last program except that it uses pointers for some of the operations. The first difference shows up in the definition of variables following the structure definition. In this program we define a pointer named "point" which is defined as a pointer that points to the structure. It would be illegal to try to use this pointer to point to any other variable type. There is a very definite reason for this restriction in C as we have alluded to earlier and will review in the next few paragraphs. The next difference is in the for loop where we use the pointer for accessing the data fields. Since ``kids'' is a pointer variable that points to the structure, we can define ``point'' in terms of ``kids''. The variable ``kids'' is a constant so it cannot be changed in value, but ``point'' is a pointer variable and can be assigned any value consistent with its being required to point to the structure. If we assign the value of ``kids'' to ``point'' then it should be clear that it will point to the first element of the array, a structure containing three fields. \section{POINTER ARITHMETIC} Adding 1 to ``point'' will now cause it to point to the second field of the array because of the way pointers are handled in C. The system knows that the structure contains three variables and it knows how many memory elements are required to store the complete structure. Therefore if we tell it to add one to the pointer, it will actually add the number of memory elements required to get to the next element of the array. If, for example, we were to add 4 to the pointer, it would advance the value of the pointer 4 times the size of the structure, resulting in it pointing 4 elements farther along the array. This is the reason a pointer cannot be used to point to any data type other than the one for which it was defined. Now to return to the program displayed on your monitor. It should be clear from the previous discussion that as we go through the loop, the pointer will point to the beginning of one of the array elements each time. We can therefore use the pointer to reference the various elements of the structure. Referring to the elements of a structure with a pointer occurs so often in C that a special method of doing that was devised. Using {\em point$->$initial\/} is the same as using {\em (*point).initial\/} which is really the way we did it in the last two programs. Remember that {\em *point\/} is the data to which the pointer points and the construct should be clear. The ``$->$'' is made up of the minus sign and the greater than sign. Since the pointer points to the structure, we must once again define which of the elements we wish to refer to each time we use one of the elements of the structure. There are, as we have seen, several different methods of referring to the members of the structure, and in the for loop used for output at the end of the program, we use three different methods. This would be considered very poor programming practice, but is done this way here to illustrate to you that they all lead to the same result. This program will probably require some study on your part to fully understand, but it will be worth your time and effort to grasp these principles. Compile and run this program. \section{NESTED AND NAMED STRUCTURES} Load and display the file named NESTED.C for an example of a nested structure. The structures we have seen so far have been very simple, although useful. It is possible to define structures containing dozens and even hundreds or thousands of elements but it would be to the programmers advantage not to define all of the elements at one pass but rather to use a hierarchical structure of definition. This will be illustrated with the program on your monitor. The first structure contains three elements but is followed by no variable name. We therefore have not defined any variables only a structure, but since we have included a name at the beginning of the structure, the structure is named ``person''. The name ``person'' can be used to refer to the structure but not to any variable of this structure type. It is therefore a new type that we have defined, and we can use the new type in nearly the same way we use {\em int\/}, {\em char\/}, or any other types that exist in C. The only restriction is that this new name must always be associated with the reserved word {\em struct}. The next structure definition contains three fields with the middle field being the previously defined structure which we named ``person''. The variable which has the type of ``person'' is named ``descrip''. So the new structure contains two simple variables, ``grade'' and a string named ``lunch[25]'', and the structure named ``descrip''. Since ``descrip'' contains three variables, the new structure actually contains 5 variables. This structure is also given a name ``alldat'', which is another type definition. Finally we define an array of 53 variables each with the structure defined by ``alldat'', and each with the name ``student''. If that is clear, you will see that we have defined a total of $53$ times $5$ variables, each of which is capable of storing a value. \bigskip \hspace*{15em}{\large TWO MORE VARIABLES} \medskip Since we have a new type definition we can use it to define two more variables. The variables ``teacher'' and ``sub'' are defined in the next statement to be variables of the type ``alldat'', so that each of these two variables contain 5 fields which can store data. \section{NOW TO USE SOME OF THE FIELDS} In the next five lines of the program, we will assign values to each of the fields of ``teacher''. The first field is the ``grade'' field and is handled just like the other structures we have studied because it is not part of the nested structure. Next we wish to assign a value to her age which is part of the nested structure. To address this field we start with the variable name ``teacher'' to which we append the name of the group ``descrip'', and then we must define which field of the nested structure we are interested in, so we append the name ``age''. The teachers status is handled in exactly the same manner as her age, but the last two fields are assigned strings using the string copy {\em strcpy\/} function which must be used for string assignment. Notice that the variable names in the {\em strcpy\/} function are still variable names even though they are made up of several parts each. The variable ``sub'' is assigned nonsense values in much the same way, but in a different order since they do not have to occur in any required order. Finally, a few of the ``student'' variables are assigned values for illustrative purposes and the program ends. None of the values are printed for illustration since several were printed in the last examples. Compile and run this program, but when you run it you may get a ``stack overflow'' error. C uses it's own internal stack to store the automatic variables on but most C compilers use only a 2048 byte stack as a default. This program has more than that in the defined structures so it will be necessary for you to increase the stack size. The method for doing this for some compilers is given in the accompanying COMPILER.DOC file with this tutorial. Consult your compiler documentation for details about your compiler. There is another way around this problem, and that is to move the structure definitions outside of the program where they will be external variables and therefore static. The result is that they will not be kept on the internal stack and the stack will therefore not overflow. It would be good for you to try both methods of fixing this problem. \section{MORE ABOUT STRUCTURES} It is possible to continue nesting structures until you get totally confused. If you define them properly, the computer will not get confused because there is no stated limit as to how many levels of nesting are allowed. There is probably a practical limit of three beyond which you will get confused, but the language has no limit. In addition to nesting, you can include as many structures as you desire in any level of structures, such as defining another structure prior to ``alldat'' and using it in ``alldat'' in addition to using ``person''. The structure named ``person'' could be included in ``alldat'' two or more times if desired, as could pointers to it. Structures can contain arrays of other structures which in turn can contain arrays of simple types or other structures. It can go on and on until you lose all reason to continue. I am only trying to illustrate to you that structures are very valuable and you will find them great aids to programming if you use them wisely. Be conservative at first, and get bolder as you gain experience. More complex structures will not be illustrated here, but you will find examples of additional structures in the example programs included in the last chapter of this tutorial. For example, see the ``\#include'' file ``STRUCT.DEF''. \section{WHAT ARE UNIONS?} Load the file named UNION1.C for an example of a union. Simply stated, a union allows you a way to look at the same data with different types, or to use the same data with different names. Examine the program on your monitor. In this example we have two elements to the union, the first part being the integer named ``value'', which is stored as a two byte variable somewhere in the computers memory. The second element is made up of two character variables named ``first'' and ``second''. These two variables are stored in the same storage locations that ``value'' is stored in, because that is what a union does. A union allows you to store different types of data in the same physical storage locations. In this case, you could put an integer number in ``value'', then retrieve it in its two halves by getting each half using the two names ``first'' and ``second''. This technique is often used to pack data bytes together when you are, for example, combining bytes to be used in the registers of the microprocessor. Accessing the fields of the union are very similar to accessing the fields of a structure and will be left to you to determine by studying the example. One additional note must be given here about the program. When it is run using most compilers, the data will be displayed with two leading f's due to the hexadecimal output promoting the char type variables to int and extending the sign bit to the left. Converting the char type data fields to int type fields prior to display should remove the leading f's from your display. This will involve defining two new int type variables and assigning the char type variables to them. This will be left as an exercise for you. Note that the same problem will come up in a few of the later files also. Compile and run this program and observe that the data is read out as an {\em int\/} and as two {\em char\/} variables. The {\em char\/} variables are reversed in order because of the way an {\em int\/} variable is stored internally in your computer. Don't worry about this. It is not a problem but it can be a very interesting area of study if you are so inclined. \bigskip \hspace*{15em}{\LARGE ANOTHER UNION EXAMPLE} \medskip Load and display the file named UNION2.C for another example of a union, one which is much more common. Suppose you wished to build a large database including information on many types of vehicles. It would be silly to include the number of propellers on a car, or the number of tires on a boat. In order to keep all pertinent data, however, you would need those data points for their proper types of vehicles. In order to build an efficient data base, you would need several different types of data for each vehicle, some of which would be common, and some of which would be different. That is exactly what we are doing in the example program on your monitor. In this program, we will define a complete structure, then decide which of the various types can go into it. We will start at the top and work our way down. First, we define a few constants with the \#defines, and begin the program itself. We define a structure named ``automobile'' containing several fields which you should have no trouble recognizing, but we define no variables at this time. \section[THE TYPEDEF-CONCEPT]{A NEW CONCEPT, THE TYPEDEF} Next we define a new type of data with a {\em typedef}. This defines a complete new type that can be used in the same way that {\em int\/} or {\em char\/} can be used. Notice that the structure has no name, but at the end where there would normally be a variable name there is the name ``BOATDEF''. We now have a new type, ``BOATDEF'', that can be used to define a structure anyplace we would like to. Notice that this does not define any variables, only a new type definition. Capitalizing the name is a personal preference only and is not a C standard. It makes the {\em typedef\/} look different from a variable name. We finally come to the big structure that defines our data using the building blocks already defined above. The structure is composed of 5 parts, two simple variables named ``vehicle'' and ``weight'', followed by the union, and finally the last two simple variables named ``value'' and ``owner''. Of course the union is what we need to look at carefully here, so focus on it for the moment. You will notice that it is composed of four parts, the first part being the variable ``car'' which is a structure that we defined previously. The second part is a variable named ``boat'' which is a structure of the type ``BOATDEF'' previously defined. The third part of the union is the variable ``airplane'' which is a structure defined in place in the union. Finally we come to the last part of the union, the variable named ``ship'' which is another structure of the type ``BOATDEF''. I hope it is obvious to you that all four could have been defined in any of the three ways shown, but the three different methods were used to show you that any could be used. In practice, the clearest definition would probably have occurred by using the ``typedef'' for each of the parts. \section[SUMMARY]{WHAT DO WE HAVE NOW?} We now have a structure that can be used to store any of four different kinds of data structures. The size of every record will be the size of that record containing the largest union. In this case part 1 is the largest union because it is composed of three integers, the others being composed of an integer and a character each. The first member of this union would therefore determine the size of all structures of this type. The resulting structure can be used to store any of the four types of data, but it is up to the programmer to keep track of what is stored in each variable of this type. The variable "vehicle" was designed into this structure to keep track of the type of vehicle stored here. The four defines at the top of the page were designed to be used as indicators to be stored in the variable ``vehicle''. A few examples of how to use the resulting structure are given in the next few lines of the program. Some of the variables are defined and a few of them are printed out for illustrative purposes. The union is not used too frequently, and almost never by beginning programmers. You will encounter it occasionally so it is worth your effort to at least know what it is. You do not need to know the details of it at this time, so don't spend too much time studying it. When you do have a need for a variant structure, a union, you can learn it at that time. For your own benefit, however, do not slight the structure. You should use the structure often. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Define a named structure containing a string field for a name, an integer for feet, and another for arms. Use the new type to define an array of about 6 items. Fill the fields with data and print them out as follows. \begin{flushleft} A human being has 2 legs and 2 arms. \\ A dog has 4 legs and 0 arms. \\ A television set has 4 legs and 0 arms. \\ A chair has 4 legs and 2 arms. \\ etc. \end{flushleft} \item Rewrite exercise 1 using a pointer to print the data out. \end{enumerate} \chapter{Dynamic Allocation} \section[DYNAMIC ALLOCATION]{WHAT IS DYNAMIC ALLOCATION?} Dynamic allocation is very intimidating to a person the first time he comes across it, but that need not be. Simply relax and read this chapter carefully and you will have a good grounding in a very valuable programming resource. All of the variables in every program up to this point have been static variables as far as we are concerned. (Actually, some of them have been {\tt automatic} and were dynamically allocated for you by the system, but it was transparent to you.) In this chapter, we will study some dynamically allocated variables. They are simply variables that do not exist when the program is loaded, but are created dynamically as they are needed. It is possible, using these techniques, to create as many variables as needed, use them, and deallocate their space for use by other variables. As usual, the best teacher is an example, so load and display the program named DYNLIST.C. We begin by defining a named structure ``animal'' with a few fields pertaining to dogs. We do not define any variables of this type, only three pointers. If you search through the remainder of the program, you will find no variables defined so we have nothing to store data in. All we have to work with are three pointers, each of which point to the defined structure. In order to do anything, we need some variables, so we will create some dynamically. \section{DYNAMIC VARIABLE CREATION} The first program statement, which assigns something to the pointer ``pet1'' will create a dynamic structure containing three variables. The heart of the statement is the {\em malloc\/} function buried in the middle of the statement. This is a {\tt memory allocate} function that needs the other things to completely define it. The {\em malloc\/} function, by default, will allocate a piece of memory on a {\tt heap} that is {\tt n} characters in length and will be of type character. The {\tt n} must be specified as the only argument to the function. We will discuss {\tt n} shortly, but first we need to define a {\tt heap}. \subsection{WHAT IS A HEAP?} Every compiler has a set of limitations on it as to how big the executable file can be, how many variables can be used, how long the source file can be, etc. One limitation placed on users by many compilers for the IBM-PC and compatibles is a limit of 64K for the executable code. This is because the IBM-PC uses a microprocessor with a 64K segment size, and it requires special calls to use data outside of a single segment. In order to keep the program small and efficient, these calls are not used, and the program is limited but still adequate for most programs. A heap is an area outside of this 64K boundary which can be accessed by the program to store data and variables. The data and variables are put on the {\tt heap} by the system as calls to {\em malloc\/} are made. The system keeps track of where the data is stored. Data and variables can be deallocated as desired leading to holes in the heap. The system knows where the holes are and will use them for additional data storage as more {\em malloc\/} calls are made. The structure of the heap is therefore a very dynamic entity, changing constantly. \subsection{MORE ABOUT SEGMENTS} Some of the more expensive compilers give the user a choice of memory models to use. Examples are Lattice and Microsoft, which allow the programmer a choice of using a model with a 64K limitation on program size but more efficient running, or using a model with a 640K limitation and requiring longer address calls leading to less efficient addressing. Using the larger address space requires inter segment addressing resulting in the slightly slower running time. The time is probably insignificant in most programs, but there are other considerations. If a program uses no more than 64K bytes for the total of its code and memory and if it doesn't use a stack, it can be made into a .COM file. Since a .COM file is already in a memory image format, it can be loaded very quickly whereas a file in a .EXE format must have its addresses relocated as it is loaded. Therefore a small memory model can generate a program that loads faster than one generated with a larger memory model. Don't let this worry you, it is a fine point that few programmers worry about. Using dynamic allocation, it is possible to store the data on the {\tt heap} and that may be enough to allow you to use the small memory model. Of course, you wouldn't store local variables such as counters and indexes on the heap, only very large arrays or structures. Even more important than the need to stay within the small memory model is the need to stay within the computer. If you had a program that used several large data storage areas, but not at the same time, you could load one block storing it dynamically, then get rid of it and reuse the space for the next large block of data. Dynamically storing each block of data in succession, and using the same storage for each block may allow you to run your entire program in the computer without breaking it up into smaller programs. \subsection[THE ``MALLOC'' FUNCTION]{BACK TO THE ``MALLOC'' FUNCTION} Hopefully the above description of the {\tt heap} and the overall plan for dynamic allocation helped you to understand what we are doing with the {\em malloc\/} function. It simply asks the system for a block of memory of the size specified, and gets the block with the pointer pointing to the first element of the block. The only argument in the parentheses is the size of the block desired and in our present case, we desire a block that will hold one of the structures we defined at the beginning of the program. The {\em sizeof\/} is a new function, new to us at least, that returns the size in bytes of the argument within its parentheses. It therefore, returns the size of the structure named animal, in bytes, and that number is sent to the system with the {\em malloc\/} call. At the completion of that call, we have a block on the heap allocated to us, with pet1 pointing to the first byte of the block. \subsubsection{WHAT IS A CAST?} We still have a funny looking construct at the beginning of the {\em malloc\/} function call. That is called a {\em cast\/}. The {\em malloc\/} function returns a block with the pointer pointing to it being a pointer of type {\em char\/} by default. Many times, if not most, you do not want a pointer to a {\em char\/} type variable, but to some other type. You can define the pointer type with the construct given on the example line. In this case we want the pointer to point to a structure of type ``animal'', so we tell the compiler with this strange looking construct. Even if you omit the cast, most compilers will return a pointer correctly, give you a warning, and go on to produce a working program. It is better programming practice to provide the compiler with the cast to prevent getting the warning message. \section{USING THE DYNAMICALLY ALLOCATED MEMORY BLOCK} If you remember our studies of structures and pointers, you will recall that if we have a structure with a pointer pointing to it, we can access any of the variables within the structure. In the next three lines of the program, we assign some silly data to the structure for illustration. It should come as no surprise to you that these assignment statements look just like assignments to statically defined variables. In the next statement, we assign the value of ``pet1'' to ``pet2'' also. This creates no new data, we simply have two pointers to the same object. Since ``pet2'' is pointing to the structure we created above, ``pet1'' can be reused to get another dynamically allocated structure which is just what we do next. Keep in mind that ``pet2'' could have just as easily been used for the new allocation. The new structure is filled with silly data for illustration. Finally, we allocate another block on the heap using the pointer ``pet3'', and fill its block with illustrative data. Printing the data out should pose no problem to you since there is nothing new in the three print statements. It is left for you to study. \section{GETTING RID OF THE DYNAMICALLY ALLOCATED DATA} Another new function is used to get rid of the data and free up the space on the heap for reuse, the function {\em free}. To use it, you simply call it with the pointer to the block as the only argument, and the block is deallocated. In order to illustrate another aspect of the dynamic allocation and deallocation of data, an additional step is included in the program on your monitor. The pointer ``pet1'' is assigned the value of ``pet3''. In doing this, the block that ``pet1'' was pointing to is effectively lost since there is no pointer that is now pointing to that block. It can therefore never again be referred to, changed, or disposed of. That memory, which is a block on the heap, is wasted from this point on. This is not something that you would ever purposely do in a program. It is only done here for illustration. The first {\em free\/} function call removes the block of data that ``pet1'' and ``pet3'' were pointing to, and the second {\em free\/} call removes the block of data that ``pet2'' was pointing to. We therefore have lost access to all of our data generated earlier. There is still one block of data that is on the heap but there is no pointer to it since we lost the address to it. Trying to {\em free\/} the data pointed to by ``pet1'' would result in an error because it has already been {\em freed\/} by the use of ``pet3''. There is no need to worry, when we return to DOS, the entire heap will be disposed of with no regard to what we have put on it. The point does need to made that losing a pointer to a block of the heap, forever removes that block of data storage from our program and we may need that storage later. Compile and run the program to see if it does what you think it should do based on this discussion. \bigskip \hspace*{15em}{\LARGE THAT WAS A LOT OF DISCUSSION} \medskip It took some pages to get through the discussion of the last program but it was time well spent. It should be somewhat exciting to you to know that there is nothing else to learn about dynamic allocation, the last pages covered it all. Of course, there is a lot to learn about the technique of using dynamic allocation, and for that reason, there are two more files to study. But the fact remains, there is nothing more to learn about dynamic allocation than what was given so far in this chapter. \section{AN ARRAY OF POINTERS} Load and display the file BIGDYNL.C for another example of dynamic allocation. This program is very similar to the last one since we use the same structure, but this time we define an array of pointers to illustrate the means by which you could build a large database using an array of pointers rather than a single pointer to each element. To keep it simple we define 12 elements in the array and another working pointer named ``point''. The ``*pet[12]'' is new to you so a few words would be in order. What we have defined is an array of $12$ pointers, the first being ``pet[0]'', and the last ``pet[11]''. Actually, since an array is itself a pointer, the name ``pet'' by itself is a pointer to a pointer. This is valid in C, and in fact you can go farther if needed but you will get quickly confused. I know of no limit as to how many levels of pointing are possible, so a definition such as ``int ****pt'' is legal as a pointer to a pointer to a pointer to a pointer to an integer type variable, if I counted right. Such usage is discouraged until you gain considerable experience. Now that we have $12$ pointers which can be used like any other pointer, it is a simple matter to write a loop to allocate a data block dynamically for each and to fill the respective fields with any data desirable. In this case, the fields are filled with simple data for illustrative purposes, but we could be reading in a database, readings from some test equipment, or any other source of data. A few fields are randomly picked to receive other data to illustrate that simple assignments can be used, and the data is printed out to the monitor. The pointer ``point'' is used in the printout loop only to serve as an illustration, the data could have been easily printed using the ``pet[$n$]'' means of definition. Finally, all 12 blocks of data are freed before terminating the program. Compile and run this program to aid in understanding this technique. As stated earlier, there was nothing new here about dynamic allocation, only about an array of pointers. \section{A LINKED LIST} We finally come to the grandaddy of all programming techniques as far as being intimidating. Load the program DYNLINK.C for an example of a dynamically allocated linked list. It sounds terrible, but after a little time spent with it, you will see that it is simply another programming technique made up of simple components that can be a powerful tool. In order to set your mind at ease, consider the linked list you used when you were a child. Your sister gave you your birthday present, and when you opened it, you found a note that said, ``Look in the hall closet.'' You went to the hall closet, and found another note that said, ``Look behind the TV set.'' Behind the TV you found another note that said, ``Look under the coffee pot.'' You continued this search, and finally you found your pair of socks under the dogs feeding dish. What you actually did was to execute a linked list, the starting point being the wrapped present and the ending point being under the dogs feeding dish. The list ended at the dogs feeding dish since there were no more notes. In the program DYNLINK.C, we will be doing the same thing as your sister forced you to do. We will however, do it much faster and we will leave a little pile of data at each of the intermediate points along the way. We will also have the capability to return to the beginning and retraverse the entire list again and again if we so desire. \subsection{THE DATA DEFINITIONS} This program starts similarly to the last two with the addition of the definition of a constant to be used later. The structure is nearly the same as that used in the last two programs except for the addition of another field within the structure, the pointer. This pointer is a pointer to another structure of this same type and will be used to point to the next structure in order. To continue the above analogy, this pointer will point to the next note, which in turn will contain a pointer to the next note after that. We define three pointers to this structure for use in the program, and one integer to be used as a counter, and we are ready to begin using the defined structure for whatever purpose we desire. In this case, we will once again generate nonsense data for illustrative purposes. \subsection{THE FIRST FIELD} Using the {\em malloc\/} function, we request a block of storage on the {\em heap\/} and fill it with data. The additional field in this example, the pointer, is assigned the value of NULL, which is only used to indicate that this is the end of the list. We will leave the pointer ``start'' at this structure, so that it will always point to the first structure of the list. We also assign ``prior'' the value of ``start'' for reasons we will see soon. Keep in mind that the end points of a linked list will always have to be handled differently than those in the middle of a list. We have a single element of our list now and it is filled with representative data. \subsection{FILLING ADDITIONAL STRUCTURES} The next group of assignments and control statements are included within a {\em for\/} loop so we can build our list fast once it is defined. We will go through the loop a number of times equal to the constant ``RECORDS'' defined at the beginning of our program. Each time through, we allocate memory, fill the first three fields with nonsense, and fill the pointers. The pointer in the last record is given the address of this new record because the ``prior'' pointer is pointing to the prior record. Thus ``prior$->$next'' is given the address of the new record we have just filled. The pointer in the new record is assigned the value ``NULL'', and the pointer ``prior'' is given the address of this new record because the next time we create a record, this one will be the prior one at that time. That may sound confusing but it really does make sense if you spend some time studying it. When we have gone through the {\em for\/} loop 6 times, we will have a list of 7 structures including the one we generated prior to the loop. The list will have the following characteristics. \begin{enumerate} \item ``start'' points to the first structure in the list. \item Each structure contains a pointer to the next structure. \item The last structure has a pointer that points to NULL and can be used to detect the end. \begin{verbatim} start->struct1 This diagram should aid in name understanding the structure of breed the data at this point. age point->struct2 name breed age point->struct3 name breed age point-> . . . . struct7 name breed age point->NULL \end{verbatim} \end{enumerate} It should be clear to you, if you understand the above structure, that it is not possible to simply jump into the middle of the structure and change a few values. The only way to get to the third structure is by starting at the beginning and working your way down through the structure one record at a time. Although this may seem like a large price to pay for the convenience of putting so much data outside of the program area, it is actually a very good way to store some kinds of data. A word processor would be a good application for this type of data structure because you would never need to have random access to the data. In actual practice, this is the basic type of storage used for the text in a word processor with one line of text per record. Actually, a program with any degree of sophistication would use a doubly linked list. This would be a list with two pointers per record, one pointing down to the next record, and the other pointing up to the record just prior to the one in question. Using this kind of a record structure would allow traversing the data in either direction. \subsection{PRINTING THE DATA OUT} To print the data out, a similar method is used as that used to generate the data. The pointers are initialized and are then used to go from record to record reading and displaying each record one at a time. Printing is terminated when the NULL on the last record is found, so the program doesn't even need to know how many records are in the list. Finally, the entire list is deleted to make room in memory for any additional data that may be needed, in this case, none. Care must be taken to assure that the last record is not deleted before the NULL is checked. Once the data is gone, it is impossible to know if you are finished yet. \section[SOMETHING MORE]{MORE ABOUT DYNAMIC ALLOCATION AND LINKED LISTS} It is not difficult, and it is not trivial, to add elements into the middle of a linked lists. It is necessary to create the new record, fill it with data, and point its pointer to the record it is desired to precede. If the new record is to be installed between the 3rd and 4th, for example, it is necessary for the new record to point to the 4th record, and the pointer in the 3rd record must point to the new one. Adding a new record to the beginning or end of a list are each special cases. Consider what must be done to add a new record in a doubly linked list. Entire books are written describing different types of linked lists and how to use them, so no further detail will be given. The amount of detail given should be sufficient for a beginning understanding of C and its capabilities. \section{ANOTHER NEW FUNCTION - CALLOC} One more function must be mentioned, the {\em calloc\/} function. This function allocates a block of memory and clears it to all zeros which may be useful in some circumstances. It is similar to {\em malloc\/} and will be left as an exercise for you to read about and use {\em calloc\/} if you desire. \section{PROGRAMMING EXERCISES} \begin{enumerate} \item Rewrite the example program STRUCT1.C from chapter 11 to dynamically allocate the two structures. \item Rewrite the example program STRUCT2.C from chapter 11 to dynamically allocate the 12 structures. \end{enumerate} \chapter{Character and Bit Manipulation} \section{UPPER AND LOWER CASE} Load and display the program UPLOW.C for an example of a program that does lots of character manipulation. More specifically, it changes the case of alphabetic characters around. It illustrates the use of four functions that have to do with case. It should be no problem for you to study this program on your own and understand how it works. The four functions on display in this program are all within the user written function, {\em mix\_up\_the\_line\/}. Compile and run the program with the file of your choice. The four functions are; \begin{tabular}{|rl|} \hline isupper(); & Is the character upper case? \\ islower(); & Is the character lower case? \\ toupper(); & Make the character upper case. \\ tolower(); & Make the character lower case. \\ \hline \end{tabular} \section{CLASSIFICATION OF CHARACTERS} Load and display the next program, CHARCLAS.C for an example of character counting. We have repeatedly used the backslash n character representing a new line. There are several others that are commonly used, so they are defined in the following table; \begin{tabular}{|rl|} \hline \verb|\n| & Newline \\ \verb|\t| & Tab \\ \verb|\b| & Backspace \\ \verb|\"| & Double quote \\ \verb|\\| & Backslash \\ \verb|\0| & NULL (zero) \\ \hline \end{tabular} By preceding each of the above characters with the backslash character, the character can be included in a line of text for display, or printing. In the same way that it is perfectly all right to use the letter ``n'' in a line of text as a part of someone's name, and as an end--of--line, the other characters can be used as parts of text or for their particular functions. The program on your screen uses the functions that can determine the class of a character, and counts the characters in each class. The number of each class is displayed along with the line itself. The three functions are as follows; \begin{tabular}{|rl|} \hline isalpha(); & Is the character alphabetic? \\ isdigit(); & Is the character a numeral? \\ isspace(); & Is the character any of, \verb|\n|, \verb|\t|, \\ & or blank? \\ \hline \end{tabular} This program should be simple for you to find your way through so no explanation will be given. It was necessary to give an example with these functions used. Compile and run this program with any file you choose. \section{THE LOGICAL FUNCTIONS} Load and display the program BITOPS.C. The functions in this group of functions are used to do bitwise operations, meaning that the operations are performed on the bits as though they were individual bits. No carry from bit to bit is performed as would be done with a binary addition. Even though the operations are performed on a single bit basis, an entire byte or integer variable can be operated on in one instruction. The operators and the operations they perform are given in the following table; \begin{tabular}{|rl|} \hline \& & Logical AND, if both bits are 1, the result is 1. \\ $|$ & Logical OR, if either bit is one, the result is 1. \\ \verb|^| & Logical XOR, (exclusive OR), if one and only one \\ & bit is 1, the result is 1. \\ $\widetilde{ }$ & Logical invert, if the bit is 1, the result is 0, \\ & and if the bit is 0, teh result is 1. \\ \hline \end{tabular} The example program uses several fields that are combined in each of the ways given above. The data is in hexadecimal format. It will be assumed that you already know hexadecimal format if you need to use these operations. If you don't, you will need to study it on your own. Teaching the hexadecimal format of numbers is beyond the scope of this tutorial. Run the program and observe the output. \section{THE SHIFT INSTRUCTIONS} The last two operations to be covered in this chapter are the left shift and the right shift instructions. Load the example program SHIFTER.C for an example using these two instructions. The two operations use the following operators; \begin{tabular}{rl} $<<$ n & Left shift n places. \\ $>>$ n & Right shift n places. \end{tabular} Once again the operations are carried out and displayed using the hexadecimal format. The program should be simple for you to understand on your own, there is no tricky code. \chapter{Example Programs} \section{WHY THIS CHAPTER?} Although every program in this tutorial has been a complete program, each one has also been a very small program intended to teach you some principle of programming in C. It would do you a disservice to leave you at that point without introducing you to a few larger programs to illustrate how to put together the constructs you have learned to create a major program. This chapter contains four programs of increasing complexity, each designed to take you into a higher plateau of programming, and each designed to be useful to you in some way. DOSEX will illustrate how to make DOS system calls and will teach you, through self--study, how the system responds to the keyboard. WHATNEXT reads commands input on the command line and will aid you in setting up a variable batch file, one that requests an operator input and responds to the input by branching to a different part of the batch file. LIST is the source code for the program you used to print out the C source files when you began studying C with the aid of this tutorial. Finally we come to VC, the Visual Calculator, which you should find to be a useful program even if you don't study its source code. VC uses most of the programming techniques we have studied in this course and a few that we never even mentioned such as separately compiled subroutines. We will take a look at the example programs one at a time but without a complete explanation of any of them because you have been studying C for some time now and should be able to read and understand most of these programs on your own. One other thing must be mentioned, these programs use lots of nonstandard constructs and you will probably need to modify some of them to get them to compile with your particular compiler. That will be left as an exercise for you. \section{DOSEX.C - The DOS Example Program} The copy of DOS that you received with your IBM-PC or compatible has about 50 internal DOS calls that you can use as a programmer to control your peripheral devices and read information or status from them. Some of the earlier IBM DOS manuals, DOS 2.0 and earlier, have these calls listed in the back of the manual along with how to use them. Most of the manuals supplied with compatible computers make no mention of these calls even though they are extremely useful. These calls can be accessed from nearly any programming language but they do require some initial study to learn how to use them. This program is intended to aid you in this study. Display the program on your monitor or print it out for reference. It is merely a loop watching for a keyboard input or a change in the time. If either happens, it reacts accordingly. In line 23, the function {\em kbhit()\/} returns a value of 1 if a key has been hit but not yet read from the input buffer by the program. This is a nonstandard function and may require a name change for your particular compiler. There will probably be several similar calls that will need changed for your compiler in order to compile and run the programs in chapter 15. Look at the function named {\em get\_time\/} for an example of a DOS call. An interrupt 21(hex) is called after setting the AH register to 2C(hex) $=$ 44(decimal). The time is returned in the CH, CL, and DH registers. Refer to the DOS call definitions in your copy of DOS. If the definitions are not included there, Peter Nortons book, ``Programmers Guide to the IBM PC'' is recommended as a good reference manual for these calls and many other programming techniques. Another useful function is the {\em pos\_cursor()\/} function that positions the cursor anywhere on the monitor that you desire by using a DOS interrupt. In this case, the interrupt used is 10(hex) which is the general monitor interrupt. This particular service is number 2 of about 10 different monitor services available. This particular function may not be needed by your compiler because some compilers have a cursor positioning function predefined for your use. This function is included here as another example to you. The next function, service number 6 of interrupt 10(hex) is the window scroll service. It should be self explanatory. In this program, the cursor is positioned and some data is output to the monitor, then the cursor is ``hidden'' by moving it to line 26 which is not displayed. After you compile and run the program, you will notice that the cursor is not visible on the monitor. This is possible in any program, but be sure to put the cursor in view before returning to DOS because DOS does not like to have a ``hidden'' cursor and may do some strange things. Some time spent studying this program will be valuable to you as it will reveal how the keyboard data is input to the computer. Especially of importance is how the special keys such as function keys, arrows, etc. are handled. \section{WHATNEXT.C - The Batch File Interrogator} This is an example of how to read the data on the command line following the function call. Notice that there are two variables listed within the parentheses following the main() call. The first variable is a count of words in the entire command line including the command itself and the second variable is a pointer to an array of pointers defining the actual words on the command line. First the question on the command line, made up of some number of words, is displayed on the monitor and the program waits for the operator to hit a key. If the key hit is one of those in the last ``word'' of the group of words on the command line, the number of the character within the group is returned to the program where it can be tested with the ``errorlevel'' command in the batch file. You could use this technique to create a variable AUTOEXEC.BAT file or any other batch file can use this for a many way branch. Compile and run this file with TEST.BAT for an example of how it works in practice. You may find this technique useful in one of your batch files and you will almost certainly need to read in the command line parameters someday. An interesting alternative would be for you to write a program named ``WOULD.C'' that would return a 1 if a {\bf Y} or {\bf y} were typed and a zero if any other key were hit. Then your batch file could have a line such as; \medskip {\Large WOULD YOU LIKE TO USE THE ALTERNATIVE METHOD (Y/N)} \medskip Dos would use ``WOULD'' as the program name, ignore the rest of the statement except for displaying it on the screen. You would then respond to the question on the monitor with a single keyhit. Your batch file would then respond to the 1 or 0 returned and either run the alternative part of the batch file or the primary part whatever each part was. \begin{verbatim} WOULD YOU LIKE PRIMARY (Y/N) IF ERRORLEVEL 1 GOTO PRIMARY (secondary commands) GOTO DONE :PRIMARY (primary commands) :DONE \end{verbatim} \section{LIST.C - The Program Lister} This program is actually composed of two files, LIST.C and LISTF.C that must be separately compiled and linked together with your linker. There is nothing new here and you should have no trouble compiling and linking this program by reading the documentation supplied with your compiler. The only thing that is new in this program is the inclusion of three {em extern\/} variables in the LISTF.C listing. The only purpose for this is to tie these global variables to the main program and tell the compiler that these are not new variables. The compiler will therefore not generate any new storage space for them but simply use their names during the compile process. At link time, the linker will get their actual storage locations from the LIST.OBJ file and use those locations for the variables in the LISTF part of the memory map also. The variables of those names in both files are therefore the same identical variables and can be used just as any other global variables could be used if both parts of the program were in one file. There is no reason why the variables couldn't have been defined in the LISTF.C part of the program and declared as {\em extern\/} in the LIST.C part. Some of the variables could have been defined in one and some in the other. It is merely a matter of personal taste. Carried to an extreme, all of the variables could have been defined in a third file and named {\em extern\/} in both of these files. The third file would then be compiled and included in the linking process. It would be to your advantage to compile, link, and run this program to prepare you for the next program which is composed of 5 separate files which must all work together. \section{VC.C - The Visual Calculator} This program finally ties nearly everything together because it uses nearly every concept covered in the entire tutorial. It is so big that I will not even try to cover the finer points of its operation. Only a few of the more important points will be discussed. The first thing you should do is go through the tutorial for VC included in the file VC.DOC. There are several dozen steps for you to execute, with each step illustrating some aspect of the Visual Calculator. You will get a good feel for what it is capable of doing and make your study of the source code very profitable. In addition, you will probably find many ways to use the Visual Calculator to solve problems involving calculations where the simplicity of the problem at hand does not warrant writing a program. Notice that the structure definitions, used in all of the separate parts of the program, are defined in the file STRUCT.DEF. During program development, when it became necessary to change one of the structures slightly, it was not necessary to change it in all of the files, only one file required modification which was then {\em included\/} in the source files. Notice that the transcript data is stored in a doubly linked list with the data itself being stored in a separate dynamically allocated char string. This line is pointed to by the pointer ``lineloc''. For ease of development, the similar functions were grouped together and compiled separately. Thus, all of the functions involving the monitor were included in the file named VIDEO.C, and all of the functions involving the data storage were grouped into the FILE.C collection. Dividing your program in a way similar to this should simplify debugging and future modifications. Of special interest is the {\em monitor()\/} function. This function examines the video mode through use of a DOS command and if it is a 7, it assumes it is a monochrome monitor, otherwise it assumes a color monitor. The colors of the various fields are established at this time and used throughout the program. Most of the data is written directly to the video memory, but some is written through the standard BIOS routines. The file DEFIN.C is simply a catalogue of the functions to aid in finding the functions. This file was generated as one of the first files and was maintained and updated for use during the entire design and coding lifetime. Feel free, after understanding this code, to modify it in any way you desire for your own use. \end{document}