Chapter 10 More on Modular Programming and Functions

10.1 INTRODUCTION For a function that must return multiple values. We use parameter passing by pointers.

10.2 POINTER VARIABLES A method in C that allows a function to return multiple values through its parameter list is parameter passing by pointers. A variable name is the symbolic address of a memory location, which contains a value. Pointer variables are variables whose values are memory addresses.

Declaring Pointer Variables Associate it with data type such as int, char or double and use the symbol * as its prefix. int margaret; int *point_to_margaret; int *another_pointer;

Initializing Pointer Variables 1. The value NULL 2. An address 3. The value of another pointer variable point_to_margaret =NULL;

With the exception of NULL and 0, no other constant value can be assigned to pointer variables. margaret = 20; The symbol & is the address operator. int *pointer_to_margaret; pointer_to_margaret=&margaret; int *pointer_to_margaret =&margaret

printf(“%d”, *pointer_to_margaret); The symbol * in this context is an operator, known as the indirection operator. another_pointer= pointer_ to_ margaret; both pointer_ to_ margaret and another_pointer point to margaret.

10.3 PARAMETER PASSING BY POINTERS 1.Declare void called_ function ( int *result); 2.In calling called_ function (&value_ returned); 3.In a function prototype void called_ function( int *x);

Example 10.2 Validates the day part of a date

10.4 ADDITIONAL STYLE CONSIDERATIONS FOR MODULAR PROGRAMS 1.While implementing a module as a function, clearly identify data that it must receive and data that it must return. 2.To send data to functions, use parameter passing by value. 3.If a function has one value to return,you may return it. For functions with two or more return values, it is better to use parameter passing by pointers.

10.5 EXAMPLE PROGRAM 1: A C Program that Generates Calendars

10.6 MODULAR PROGRAMMING WITH PROGRAMMER- DEFINED LIBRARIES A library in C is a collection of reasonably general- purpose and related functions.

Program modularization through Programmer-Defined Libraries We can collect such functions in libraries and implement them, then we may use them in other projects, just as we use standard library functions.

Step 1. Identify the reusable modules For example, the functions compute_number_of_days, determine_week_day, get_ year, get_month, get_ day, get_ week_ day, include these six functions in a programmer -defined library named calendar.

Step 2. A programmer - defined library, have only declarations in header files, and only the definitions of the functions in the implementation file. A header file, named calendar.h, and store it in our default directory.

Step 3. The implementation file, contains the following items: 1.The preprocessor directive #include “calendar.h”,enclosed by double quotations. 2.include preprocessor directives for all standard libraries needed #include 3.The definitions for the functions.

Step 4. A function main will consist of these items: 1.The preprocessor directive #include “calendar.h”. 2.The include preprocessor directives for the standard libraries. 3.The function definitions.

10.1 INTRODUCTION