C Programming language

Writing functions avoids rewriting the same code over and over.
For example- if you have a section of code in a program which calculates the area of triangle. again you want to calculate the area of different triangle then you would not want to write the same code again and again for triangle then you would prefer to jump a "section of code" which calculate the area of the triangle and then jump back to the place where you left off. that section of code is called 'function'.
using function it becomes easier to write a program and keep track of what they are doing. If the operation of a program can be divided into separate activities, and each activity placed in a different function, then each could be written and checked more or less independently. Separating the code into modular functions also makes the program easier to design and understand.

passing values between functions

The mechanism used to convey information to the function is the ‘argument’. You have unknowingly used the arguments in the printf( ) and scanf( ) functions; the format string and the list of variables used inside the parentheses in these functions are arguments. The arguments are sometimes also called ‘parameters’.

Example

/* Sending and receiving values between functions */
main( )
{
int a, b, c, sum ;
printf ( "\nEnter any three numbers " ) ;
scanf ( "%d %d %d", &a, &b, &c ) ;
sum = calsum ( a, b, c ) ;
printf ( "\nSum = %d", sum ) ;
}
calsum ( x, y, z )
int x, y, z ;
{
int d ;
d = x + y + z ;
return ( d ) ;
}
And here is the output...
Enter any three numbers 10 20 30
Sum = 60

In above program, in main( ) we receive the values of a, b and c through the keyboard and then output the sum of a, b and c. However, the calculation of sum is done in a different function called calsum( ). If sum is to be calculated in calsum( ) and values of a, b and c are received in main( ), then we must pass on these values to calsum( ), and once calsum( ) calculates the sum we must return it from calsum( ) back to main( ).

Here the return statement serves two purposes:

On executing the return statement it immediately transfers the control back to the calling program.
It returns the value present in the parentheses after return, to the calling program. In the above program the value of sum of three numbers is being returned.

Scope rule of function

Let us take an example to understand the scope rule of the function:

main( )
{
int i = 20 ;
display ( i ) ;
}
display ( int j )
{
int k = 35 ;
printf ( "\n%d", j ) ;
printf ( "\n%d", k ) ;
}

In above program it is necessary to pass the value of the variable i to the function display( ). it will not become automatically available to the function display( ) Because by default the scope of avariable is local to the function in which it is defined. The presence of i is known only to the function main( ) and not to any other function. Similarly, the variable k is local to the function display( ) and hence it is not available to main( ). That is why to make the value of i available to display( ) we have to explicitly pass it to display( ). Likewise, if we want k to be available to main( ) we will have to return it to main( ) using the return statement. In general we can say that the scope of a variable is local to the function in which it is defined.

Calling convention

Calling convention indicates the order in which arguments are passed to a function when a function call is encountered. There are two possibilities here:

Arguments might be passed from left to right.
Arguments might be passed from right to left.

C language follows the second order.

Consider the following function call:

fun (a, b, c, d ) ;

In this call it doesn’t matter whether the arguments are passed from left to right or from right to left. However, in some function call the order of passing arguments becomes an important consideration.

For example:

int a = 1 ;
printf ( "%d %d %d", a, ++a, a++ ) ;

It appears that this printf( ) would output 1 2 3.

This however is not the case. Surprisingly, it outputs 3 3 1. This is because C’s calling convention is from right to left. That is, firstly 1 is passed through the expression a++ and then a is incremented to 2. Then result of ++a is passed. That is, a is incremented to 3 and then passed. Finally, latest value of a, i.e. 3, is passed. Thus in right to left order 1, 3, 3 get passed. Once printf( ) collects them it prints them in the order in which we have asked it to get them printed (and not the order in which they were passed). Thus 3 3 1 gets printed.

ADdvance features of function

Function Declaration and Prototypes.
Calling functions by value or by reference.
Recursion.

Function declaration and prototype

Any C function by default returns an int value.More specifically, whenever a call is made to a function, the compiler assumes that this function would return a value of the type int. If we desire that a function should return a value other than an int, then it is necessary to explicitly mention so in the calling function as well as in the called function.

Suppose we want to find out square of a number using a function. then the program will be given as:

main( )
{
float a, b ;
printf ( "\nEnter any number " ) ;
scanf ( "%f", &a ) ;
b = square ( a ) ;
printf ( "\nSquare of %f is %f", a, b ) ;
}
square ( float x )
{
float y ;
y = x * x ;
return ( y ) ;
}

And here are three sample runs of this program...
Enter any number 3
Square of 3 is 9.000000
Enter any number 1.5
Square of 1.5 is 2.000000
Enter any number 2.5
Square of 2.5 is 6.000000