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C - Using Functions

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A function is a module or block of program code which deals with a particular task. Making functions is a way of isolating one block of code from other independent blocks of code.

Functions serve two purposes.

  • They allow a programmer to say: `this piece of code does a specific job which stands by itself and should not be mixed up with anyting else',

  • Second they make a block of code reusable since a function can be reused in many different contexts without repeating parts of the program text.

A function can take a number of parameters, do required processing and then return a value. There may be a function which does not return any value.

You already have seen couple of built-in functions like printf(); Similar way you can define your own functions in C language.

Consider the following chunk of code

   int total = 10;
   printf("Hello World");
   total = total + l;

To turn it into a function you simply wrap the code in a pair of curly brackets to convert it into a single compound statement and write the name that you want to give it in front of the brackets:

Demo()
{
   int total = 10;
   printf("Hello World");
   total = total + l;
}

curved brackets after the function's name are required. You can pass one or more paramenters to a function as follows:

Demo( int par1, int par2)
{
   int total = 10;
   printf("Hello World");
   total = total + l;
}

By default function does not return anything. But you can make a function to return any value as follows:

int Demo( int par1, int par2)
{
   int total = 10;
   printf("Hello World");
   total = total + l;

   return total;
}

A return keyword is used to return a value and datatype of the returned value is specified before the name of function. In this case function returns total which is int type. If a function does not return a value then void keyword can be used as return value.

Once you have defined your function you can use it within a program:

main()
{
  Demo();
}

Functions and Variables:

Each function behaves the same way as C language standard function main(). So a function will have its own local variables defined. In the above example total variable is local to the function Demo.

A global variable can be accessed in any function in similar way it is accessed in main() function.

Declaration and Definition

When a function is defined at any place in the program then it is called function definition. At the time of definition of a function actual logic is implemented with-in the function.

A function declaration does not have any body and they just have their interfaces.

A function declaration is usually declared at the top of a C source file, or in a separate header file.

A function declaration is sometime called function prototype or function signature. For the above Demo() function which returns an integer, and takes two parameters a function declaration will be as follows:

int Demo( int par1, int par2);

Passing Parameters to a Function

There are two ways to pass parameters to a function:

  • Pass by Value: mechanism is used when you don't want to change the value of passed paramters. When parameters are passed by value then functions in C create copies of the passed in variables and do required processing on these copied variables.

  • Pass by Reference mechanism is used when you want a function to do the changes in passed parameters and reflect those changes back to the calling function. In this case only addresses of the variables are passed to a function so that function can work directly over the addresses.

Here are two programs to understand the difference: First example is for Pass by value:


#include <stdio.h>

/* function declaration goes here.*/
void swap( int p1, int p2 );

int main()
{
   int a = 10;
   int b = 20;

   printf("Before: Value of a = %d and value of b = %d\n", a, b );
   swap( a, b );
   printf("After: Value of a = %d and value of b = %d\n", a, b );
}

void swap( int p1, int p2 )
{
    int t;

    t = p2;
    p2 = p1;
    p1 = t;
   printf("Value of a (p1) = %d and value of b(p2) = %d\n", p1, p2 );
}

Here is the result produced by the above example. Here the values of a and b remain unchanged before calling swap function and after calling swap function.

Before: Value of a = 10 and value of b = 20
Value of a (p1) = 20 and value of b(p2) = 10
After: Value of a = 10 and value of b = 20

Following is the example which demonstrate the concept of pass by reference

#include <stdio.h>

/* function declaration goes here.*/
void swap( int *p1, int *p2 );

int main()
{
   int a = 10;
   int b = 20;

   printf("Before: Value of a = %d and value of b = %d\n", a, b );
   swap( &a, &b );
   printf("After: Value of a = %d and value of b = %d\n", a, b );
}

void swap( int *p1, int *p2 )
{
    int t;

    t = *p2;
    *p2 = *p1;
    *p1 = t;
   printf("Value of a (p1) = %d and value of b(p2) = %d\n", *p1, *p2 );
}

Here is the result produced by the above example. Here the values of a and b are changes after calling swap function.

Before: Value of a = 10 and value of b = 20
Value of a (p1) = 20 and value of b(p2) = 10
After: Value of a = 20 and value of b = 10

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