.. _functions_intro:

Functions
=========

Three required pieces of code make up the use of a function in a
program are:


#. The function prototype.

#. The calling of the function.

#. The definition of the function.


Function Prototypes
-------------------

The *function prototype* defines:


-  The name of the function.

-  The type of value returned by the function.

- 
   The number of arguments passed to the function and the data type of
   the arguments passed to the function.


Function prototypes are often placed in included files. (file.h)


Function Definition
-------------------

The *function definition* defines the code used to evaluate the
function.

Return statement
----------------


- 
   The keyword `return` causes the function to exit, passing control
   back to the calling function.

-  A  `return` is not required if the function is of type `void`.

- 
   `return` is the facility for returning a single value to the
   calling function.


Function Invocation - call-by-value
-----------------------------------

Arguments are passed to a function by value only. The name of the
variable in the calling function has no meaning in the called
function. Pointers provide a way to enhance what is passed to a
function.

Function Example
----------------

    ::

        #include <stdio.h>
        
        int min( int, int );   /* function prototype  */
        
        int main(void)
        {
           int val1,val2;
        
           printf("Please enter an integer value: ");
           scanf( "%d", &val1 );
           printf("\nPlease enter another integer value: ");
           scanf( "%d", &val2 );
           printf( "\nThe smaller value is %d\n", min( val1, val2 ));
           return 0;
        }
        
        int min( int x, int y )  /* function definition */
        {
           if( x <= y )
              return x;
           else
              return y;
        }

Now complete :ref:`hw4`.