User-defined functions allow programmers to break programs into smaller, reusable parts. There are two types of functions: built-in functions that are predefined in C like printf() and user-defined functions created by the programmer. A function is defined with a return type, name, and parameters. Functions can call other functions and be called from main or other functions. Parameters can be passed by value, where the value is copied, or by reference, where the address is passed so changes to the parameter are reflected in the caller. Functions allow for modularity and code reuse.

1. User-defined Functions
Topics
 Function and it’s advantages
 Form of C Function
 Calling function

2. Function and it’s advantages
 Function: A set of statement(s) to solve
certain kind of problem is called function.
Each function has it’s own name. Ex:
printf(), scanf(), sqrt() etc.
 Advantages of Function in C:
1. The length of source program can be
reduced.
2. It is easy to locate and isolate a faulty
function.
3. A function may be used by many other
function.

3. Classification of Function
 Functions can be classified into two
categories.
1. Built in function: The function which is
build in the C is called built in function. Ex:
printf(), scanf(), getch().
2. User defined function: The functions
which are designed by programmer called
user defined function.

4. Form of C Function
 The form of function:
return_type function_name(argument list)
{
local variable declarations;
statement(s);
return(expression);
}
 Here argument list contains valid variable names
separated by commas. Return_type is the type of
the data returned from function.

5. Continue…
 Example of function:
int add(int a, int b)
{
int sum;
sum= a+b;
return(sum);
}

6. Calling of Function
 We can call C function by mentioning the
name with appropriate argument.
 Example of calling:
main()
{
int x=10, y=20,z;
z = add(x,y);
printf(“Summation = %d”,z);
}

7. Call by Value
 In this case of function calling, the value of actual
parameter is passed to the formal parameter. Here
constant value or variable can be used as actual
parameter. If we change the value of formal parameter then
there is no effect in actual parameter.
 void change(int x)
{
x = x + 10;
}
main()
{ int k = 10;
change(k);
printf(“%d”,k);
}

8. Call by Reference
 Here the address of actual parameter is passed to the
formal parameter. We cannot use address directly. If we
change the value of formal parameter then the value of
actual parameter also changed.
 Void change(int *x)
{
*x = *x+10;
}
main()
{ int k=10;
change(&k);
printf(“%d”,k);
}

9. Nesting of C Function
 C permits nesting of functions freely. Function1 can call function2,
function2 can call function3,……….and so on.
 main()
{
int a,b,c;
function1();
}
function1()
{
…….
function2();
}
function2()
{
………
}

10. Recursion
 When a function calls itself is called
recursion. Simple example-
 main()
{
printf(“This is Recursion”);
main();
}
 When executed, the output like
This is Recursion
This is Recursion
This is

11. Continue..
 A recursive function for factorial:
int factorial(int n)
{
if(n==0)
return(1);
else
return(n*factorial(n-1));
}

12. Function with Array
 Like the values of simple variables, it is also
possible to pass the values of an array to a
function.
 main()
{
static int value[ ]={10,45,25,40};
printf(“Largest value: %d”,maximum(value,4));
}
int maximum(int x[ ], int n)
{ int i, max=0;
for(i=0;i<n;i++)
if(max<x[i])
max=x[i];
return(max);
}