1. User defined functions
Lecture 7
User defined functions
Department of Computer Science

2. In C language a large program can be divided into a
series of a Individual related programs called modules
called as “functions”.
A function is also sometimes called as a subprogram that
carries out some specific and well defined tasks.

3. Need for user defined functions
When the program size is large/ big ,
the task of debugging, testing and
maintaining becomes difficult.
i.e. if a program is divided into functional parts, then each part may be independently coded and later combined into a single unit, these subprograms are called as functions which are much easier to understand debug and test.

4. Subprocess Box Rectangle shaped, with bars on the sides
One line in, One line out
Used to include a predefined process
Subprocess

5. Elements of user defined functions
The 3 elements that are related to functions are
1.Function definition
2. Function call
3. Function declaration
The function definition is an independent program module that is specially written to implement the requirements of the function.
In order to use this function we need to invoke it at a required place in the program and is known as function call.
The program that calls the function is referred to as the calling program or calling function.
The calling program should declare any function that is to be used later in the program and is referred to as function declaration or function prototype.

6. Definition of functions
Function definition also referred to as function implementation shall possess
the following elements:
function name
function type
list of parameters
local variable declarations
function statements
a return statement
function header
function body

7. General format of a function definition
function_type function name(parameter list) {
local variable declaration;
executable statement1;
executable statement2;
…………………………
return statement; }

8. Return values and their types
The return statement can take up one of the 2 forms
return; or
return (expression);
Note 1: The plain return does not return any value.
if(error)
Note 2: The 2nd form or return with the expression returns value of the
expression
int sum(int x, int y) {
int add;
add=x+y;
return(add); }
return( x+y);

9. Example 1: addition of 2 numbers
start
Input a,b
Values of a, b
int add(int m,int n )
sum=add (a,b)
sum=m+n;
Output
sum
return(sum)
Value of sum returned
Stop

10. sum=add ( a, b) add ( int m, int n)
Actual arguments
m & n get their values from a & b
add ( int m, int n)
Formal arguments

11. /* To find the sum of 2 numbers using functions*/
#include<stdio.h>
#include<conio.h>
void main( ) {
int a,b,sum;
int add(int a, int b);
printf(“\n enter the 2 numbers\n”);
scanf(%d%d”,&a,&b);
sum=add ( a, b);
printf(“\n the sum of 2 numbers is :”,sum); }
int add( int m, int n) {
int sum;
sum=m+n;
return(sum); }

12. Categories of Functions
A function depending on whether arguments are present or not and
whether a value is returned or not, may belong to one of the following
Categories:
Functions with no arguments and no return values
Functions with arguments and no return values
Functions with arguments and one return value
Functions with no arguments but return a value
Functions that return multiple values

13. function 1 ( ) { …………………….. function 2 ( ) ……………………... }
Functions with no arguments and no return values
control
function 1 ( ) {
……………………..
function 2 ( )
……………………... }
function 2 ( ) {
……………………..
…… ……………………..
……………………... }
No input
No output
control

14. Functions with no arguments and no return values
When a function has no arguments, it does not receive any data from the calling function, similarly when it does not return a value, the calling function does not receive any data from the called function
In effect, there is no data transfer between the calling function and the called function, the dotted lines indicate that there is only a transfer of control but not data

15. function 1 ( ) { …………………….. function 2 (a ) ……………………... }
Functions with arguments and no return values
function 1 ( ) {
……………………..
function 2 (a )
……………………... }
function 2 (f ) {
……………………..
…… ……………………..
……………………... }
Values of arguments
No return value

16. Functions with arguments and no return values
Arguments matching between the function call and the called function
main ( ) {
…………………
function 1( a1, a2, a3 ………am); }
function 1( f1, f2, f3 ……… fn)
The function call should have
matching arguments.
2. If the actual arguments are more than the formal arguments, then
the extra actual arguments are discarded.
3. If the actual arguments are less than the formal arguments, the unmatched formal arguments are initialized to some garbage values.
4. A mismatch in the data type will also result in passing garbage values.
Function call
actual arguments
Called Function
formal arguments

17. function 1 ( ) { …………………….. function 2 (a ) ……………………... }
Functions with arguments and one return value
function 1 ( ) {
……………………..
function 2 (a )
……………………... }
function 2 (f ) {
……………………..
…… ……………………..
return (e); }
Values of arguments
Function result