INTRODUCTORY PROGRAMMING  by  David Perkovic  Ed: Check out Tutes_&_CLI this issue for a hint on how to run AmigaBasic on Workbench 2. This is the beginning of a series on Basic, so if you like it, please let the author know.  <> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <>  Hi there and welcome to the introductory programming column. I hope this column will become a regular part of MegaDisc. The aim of this column is to provide the basics of programming to the users out there who would like to learn but do not know where to begin. I will first start with Basic and then move on to C depending upon your response, so write in and tell me what you would like.  About Basic  BASIC (Beginners All purpose Instruction Code) was developed in 1964 by John Kemeny and Thomas Kurtz at the Dartmouth College in America. The intention of Kemeny and Kurtz was to create a computer language that is easy to learn, use and teach. Due to its simplistic nature Basic has become very widespread and is available on most home computers. Basic is an interpreted language meaning that each time the program is run each instruction is converted to machine language and then executed. Due to this, Basic programs run fairly slowly as each time the program is run the conversion to machine code has to take place. Most other computer languages are compiled rather than interpreted. Compiled languages first convert their particular code to a separate machine language program and then each time the program is run the converted program is executed.  Why Basic?  The reason why I chose Basic is that nearly every Amiga owner already has a copy of Basic on their Extras disk (if you have a version of Workbench greater than 2.04 you won't, as I believe Commodore exchanged Basic it for the Arexx language). This means hat you don't have to fork out a couple of hundred of dollars on a interpreter or compiler as you would have to for another language. The speed of Basic will not worry us as the programs you will be writing are reasonably small and thus will be executed at a reasonable speed. Basic has been criticised due to its nature allowing the use of badly structured code (i.e. code that is hard to understand and modify). In this column I will try to encourage people to write structured code that is easy to understand and not introduce statements that can lead to badly written programs.  A First Course in Basic  As this is an introductory programming tutorial I will presume that you already know about Workbench, particularly how to open and load programs, if not have a read of the manuals that came with your Amiga. AmigaBasic is located on your Workbench extras disk so first load up Workbench and then place your extras disk in your drive and load the AmigaBasic interpreter. Once AmigaBasic has loaded you should see two windows, one labelled "Basic" the other "List". These two windows behave like ordinary Workbench windows, as windows are selected by clicking anywhere with in the desired window. As mentioned above Basic is an interpreted language and the Basic window is the interpreter. Whatever you type in the Basic window is interpreted and then if the command is valid, executed. Whereas the List window is where a set of instructions (called a program) is temporarily stored. From the List window the program can then be executed, stored on disk, edited etc. In todays column I will be concentrating on the basic window, next time I will introduce the list window and show you how to create, load and save programs. The simplest basic command that produces anything useful by itself is the print statement. The print statement allows the user to display text on the screen. To use the print statement first select the basic window by clicking your mouse button anywhere within the basic window. To display the words 'Hello world' the following command is entered:  print "Hello world"  after pressing return the computer will respond by displaying 'Hello world'. The print statement displays on the screen whatever you type between the quotes("). Play around with the print statement and see what happens when you place different text within the quotes. If the above print statement is typed without the quotes (eg print Hello world) the compiler will assume that Hello and world are variables. Variables are a combination of characters that point to a place in memory where a certain value is stored. Since we have not defined "Hello" and "world" to be variables the computer will respond with two zeros when the command is entered. To define a variable the following is entered into the basic window:  a = 1  b = 2  The above statements told the computer to set the variable 'a' to equal 1 and the variable 'b' to equal 2. If now we instruct the interpreter to display 'a' and 'b' by typing  print a b  the computer will respond with 1 and 2. To declare a string variable we have to include the dollar sign($) as the last character in the variable name, for example:  a$ = "this is a string variable".  In the above command we have declared a$ to be the string 'this is a string variable' so now if we type in:  print a$  the string that was declared to be a$(i.e 'this is a string variable' will be displayed. As all string variables end with a dollar sign and all number integers don't an error will occur if we try to declare a number as a string or a string variable as a number. Note that you can declare a number as a string variable if you include the number inside quotes as anything inside quotes is taken to be a string eg:  a$ = "123".  The print statement will also allow mathematical computations, for example to calculate 1 plus 3 the following command is entered:  print 1 + 3  Once return has been pressed the computer will respond with the answer of 1 + 3 which is 4. If you would like the actual computation that was carried out to be displayed you should type in the following:  print "1 + 3 =" 1 + 3  If you typed the above command in correctly the computer will respond with:  1 + 3 = 4  Experiment around with other mathematical computations. The symbols for addition, subtraction, multiplication and division are +, -, * and / respectively. In an attempt to make life easier for Basic programmers the designers have allowed programmers to use the question mark in place of the print statement eg.  print "hello world"  is equivalent to  ? "hello world"  I have mentioned this shortcut mainly as a time saving device for people typing in programs with many print statements. My advice though, is not to use it as it is not a standard basic command and it decreases the readability of the program as a whole. Now that you have learnt how to program the computer to output text to the screen I will show you how to program the computer to accept input from the keyboard. To demonstrate the input command type the following command:  input test$  After you have typed in the command and pressed return the interpreter will display a question mark and pause until the user has entered some text. The interpreter will not continue until the user has pressed return. Once the above input command has been successfully executed what ever you typed in, the computer has stored in the variable test$. If we now print out the contents of test$ by typing the following:  print test$  the computer will display what you had just typed in. Well that's it for today. To keep you busy till next time I suggest you experiment around with the print and input commands and have a read of the Basic manual. Remember the best way to learn a programming language is to play around with it yourself and learn from your mistakes. In the next edition I will explain how to write, save, load and edit your first basic program and show you how your Amiga can make decisions.  <> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <%> 34 <>