/************************************************************************ * * * Home Work Exercise from Part Two * * ================================ * * * * Copyright © 1993 by Mark Little, All Rights Reserved * * * * This source and program are provided as part of the OZAmiga Cover * * Disk and may not be used in any other publication or training course * * without the written permission of the Author. * * * ************************************************************************ * * * Function * * ======== * * * * This program demonstrates how structure pointers can be set. * * * ************************************************************************ * * * Source File: HomeWork.c * * Source Version: 1.0 * * Destination File: HomeWork * * * * Source Computer: Amiga 3000 * * Operating System: AmigaDOS * * O/S Version: 2.04 * * Compiler: SAS C * * Version: 6.2 * * Destination Computer: Any Amiga * * * ************************************************************************ * * * Revision History * * ================ * * * * 07-Mar-93 * * --------- * * * * Original Version * * * ************************************************************************/ /************************************************************************ * INCLUDE Files * ************************************************************************ These files usually define external functions which will be called by the program. They can be functions supplied with the compiler, or they can be functions you have written. File names enclosed in angle brackets will be looked for along the search path, while file names enclosed in double quotes (") will be looked for in the current directory only. ************************************************************************/ #include #include /************************************************************************ * Definations * ************************************************************************ These lines define structures which will be used in the program. This allows changes to be made in many places by only changing one line. ************************************************************************/ struct Employee { char Name[80]; int EmployeeNo; int PayRate; /* Use int so we don't need Floating Point Numbers! */ }; struct MyStruct { struct Employee AnEmployee; struct MyStruct *NextEmployee; struct MyStruct *PreviousEmployee; }; /************************************************************************ * GLOBAL Variables * ************************************************************************ Variables defined outside of the program modules can be accessed by all program modules in this file and by other modules which are linked to this file. Variables defined inside the opening braces of a program module are only accessable by that module. ************************************************************************/ char Reply[10]; struct MyStruct Item1, Item2, Item3, Item4, *Pointer; /*======================================================================* * Force a Larger WorkBench Window * *======================================================================* *** CAUTION *** This following statements are designed for the SAS C compiler version 6.2 and defines the type of window to be opened if this program is run from WorkBench instead of the CLI. To stop this from being compiled by other compilers, it is enclosed in a conditional clause. My compiler defines __SASC_60 which means this statement will be compiled into the program. You should also remember that some compilers may produce programs which may crah if they are run from the WorkBench when they should be run from the CLI - Rebuild this program and see what happens! *======================================================================*/ #ifdef __SASC_60 char __stdiowin[] = "CON:0/0/640/200/Learning C - HomeWork"; #endif /*======================================================================* * Program Variables * *======================================================================* These statements tell the compiler what the names of the variables used in this program are. They also define what type of information is to be stored in the variables. *======================================================================*/ /************************************************************************ * MAIN PROGRAM * ************************************************************************ The module named "main" is the starting point of execution. Your compiler may open resources such as DOS, EXEC and other libraries before main is called. Consult your compiler documentation for details of what it can do. ************************************************************************/ void main(void) { /*======================================================================* * Initialise Each Item * *======================================================================* In this case, the special value NULL (0) is used to indicate that the pointer does NOT point to a valid structure. This is important for the pointer to the previous entry at the first items and the pointer to the next entry at the last item. If this special value is not set, then the program is likely to crash. *======================================================================*/ Item1.PreviousEmployee = NULL; /* There is no entry prior to this one, so let the computer know */ Item1.NextEmployee = &Item2; /* Point to the ADDRESS of the next entry, so we can move to it */ strcpy(Item1.AnEmployee.Name,"David Reeves"); /* As 'C' is a medium level language, it can't use strings directly, so we use a standard function to copy for us. */ Item1.AnEmployee.PayRate = 100; /* Easy ones here, just set the */ Item1.AnEmployee.EmployeeNo = 1; /* values. We use the "." notation because we are using the structure itself, not a pointer. */ Item2.PreviousEmployee = &Item1; Item2.NextEmployee = &Item3; strcpy(Item2.AnEmployee.Name,"Juliet Way"); Item2.AnEmployee.PayRate = 50; Item2.AnEmployee.EmployeeNo = 2; Item3.PreviousEmployee = &Item2; Item3.NextEmployee = &Item4; strcpy(Item3.AnEmployee.Name,"Chris Leathley"); Item3.AnEmployee.PayRate = 25; Item3.AnEmployee.EmployeeNo = 3; Item4.PreviousEmployee = &Item3; Item4.NextEmployee = NULL; strcpy(Item4.AnEmployee.Name,"Mark Little"); Item4.AnEmployee.PayRate = 20; Item4.AnEmployee.EmployeeNo = 4; /*======================================================================* * Let's Print out the Items Forward * *======================================================================* In this section of code, the Employee's will be printed out from the start to the finish. As I said before, the NULL value will tell the program when it is at the end. This section uses the control statements discussed in this edition's tutorial, so read the article is you have any problems with how this works! ** NOTE ** This do-while loop assumes that you always have a first item. It will fail if you don't. These problems will be addressed in the articles. *======================================================================*/ Pointer = &Item1; /* Point to the First Item */ do { printf("Name:\t%s\n",Pointer->AnEmployee.Name); printf("Number:\t%3d\t\tPay Rate:\t%3d\n\n", Pointer->AnEmployee.EmployeeNo, Pointer->AnEmployee.PayRate); Pointer = Pointer->NextEmployee; } while (Pointer != NULL); /*======================================================================* * Say What's going to Occur * *======================================================================*/ printf("Let's start from the end and work back\n"); printf("We'll print David first again tho' - " "Better not upset the Editor!!\n"); printf("Press RETURN to try again\n"); gets(Reply); printf("Altering the list pointers so David is at the END!\n" "and so will be listed FIRST!\n\n"); /*======================================================================* * Make David the END of the List * *======================================================================* Because the list will be printed out using the PreviousEntry pointers, we will need to put David at the END of the list. This will mean changing so of the pointers. We must make sure that the NULL values are in the right place or we're in trouble. I'll use the Pointer here, rather than just the address to show how its done. *======================================================================*/ Pointer = &Item1; Pointer->NextEmployee = NULL; /* As Item 4 was before */ Pointer->PreviousEmployee = &Item4; /* Point back to Item 4 */ Pointer = &Item4; Pointer->NextEmployee = &Item1; /* No Longer the Last Item */ Pointer = &Item2; Pointer->PreviousEmployee = NULL; /* Now the First Item */ /*======================================================================* * Let's Print out the Items Backward * *======================================================================* In this section of code, the Employee's will be printed out from the finish to the start. *======================================================================*/ Pointer = &Item1; /* Point to the Last Item */ do { printf("Name:\t%s\n",Pointer->AnEmployee.Name); printf("Number:\t%3d\t\tPay Rate:\t%3d\n\n", Pointer->AnEmployee.EmployeeNo, Pointer->AnEmployee.PayRate); Pointer = Pointer->PreviousEmployee; } while (Pointer != NULL); }