A function is a block of code that performs a specific task. There are two types of functions: 1) built-in/library functions whose definitions are predefined in header files, and 2) user-defined functions that are created by the user to perform desired tasks. User-defined functions provide benefits like modularity, reusability, and clarity. Functions in C can be defined to take parameters and return values, take parameters but not return values, or not take parameters and not return values. Functions are called by their name followed by parentheses containing any parameters.

1. Function
A function is a set of
statements, written in a
particular language that
performs a particular
task ,when ever called.

2. Types of functions
1.Inbuilt or system defined or Library Functions
2.User defined Functions

3. Inbuilt functions
These are the functions whose defintions is
already defined and stored in a respective
header files of c library .
Example of this type of functions are
gets(),puts(),clrscr(),getche(),printf(),scanf().
Etc.

4. User defined functions
 The functions which are created by the user
to, perform the desired task,are called user
defined functions.
User defined functions provide the following
Benefits:1.By using user defined functions,we can divide
a large problem into small independence
modules,each having a defined purpose.the
process is called modulation.

5. benefits
2.Smaller modules are easy to design and
debug.
3.We can create own user defined functions,in
the way,a function can be created once and
used for many times.i.e sharing of the code.
4Modulation provide greater clarity to the
programs.

6. Defining a function
Syntax :<return type><name>(parameterlist)>
{
Statement 1;
Statement 2;
----------------Statement n;
Return (expression);
}

7. Elements of functions
1. Return type :- is datatype of the value ,returned
by the function .void is used when the function
does not return any value,int and float are used
when functions returns integer or floating point
values.
2.Name:-is the name of the function
3.Parameter list:-contain variables carrying
information from the main function.
4 Function call is represented by writing the name
of the function followed by braces and
semicolon;

8. Example describes a function to calc sum of two
no.
Int sum(int a,int b)
{
// function body
Printf(“the sum of no is being calculated”);
Return(a+b);
}
Void main()
{
Int j=0;
Int k=10;
Int m=12;
J=sum(k,m);
//call to sum function
Printf(“the sum is %d”,j);
}

9. Return type of the function is “int”.the name of
the function is “sum” .the parameter are” a”
and “b” of integer type.
C allows us to define user defined function in
various ways.we can define the functions in
following ways:
1. Functions with no argument and no return
type.
2. Function with argument and no return type.
3. Functions with argument and with return type
like int ,float.

10. Functions with no argument
and no return type.
These are the simple function which
do not take any data from calling
functions, in the form of parameters
and do not return anything to calling
functions, after execution.

11. The example describes the use of func.
with no parameter and no return type
Simple function.
#include<stdio.h>
#include<conio.h>
Void main()
{
void punjab();
//function prototyping
Void delhi();
//function prototyping
Printf(“n I m from main”);
Punjab();
// call to punjab()

12. Printf(“nt I m back in main”);
Himachal();
Print f(“nt t I m from main again”);
}
Void punjab()
{
Printf(“n I m from punjab”);
}
Void himachal()
{
Print(“n I m from himachal”);
}

13. output
I m from main
I m from punjab
I m back in main
I m from himchal
I m from main again

14. The example describes the use of func.
with no parameter and return type
Example to calculate the sum of two numbers
using
#include<stdio.h>
#include<conio.h>
Void main()
{
Int sum();
Int result();

15. Clrscr();
Printf(“calculate sum of two numbers “);
Getch();
Result=sum();
call to function sum
//result contains the value entered
Printf(“n the result after adding is number
%d”,result);
Getch();
}
Int sum()
body of the function
{

16. Int a,b;
Printf(“n enter 2 numbers”);
Scanf(“%d%d”,&a,&b);
Return(a+b);
}

17. Out put
Ready to calculate sum of two numbers-press
any key
Enter 2 number 12
23
The result after adding numbers is 35

18. Functions with argu and no return type
 To demonstrate the functions with parameter and
no return
#include<stdio.h>
#include<conio.h>
Void main()
{
Void sum(int,int);
Int a,b;
Printf(“enter 2 numbers”);
Scanf(“%d%”,&a,&b);
Sum(a,b);
Getch();

19. } //end of main
Void sum(int f1,int f2)
{
Int result;
//variable in function
Result=(f1+f2);
Printf(“n the result after addition is%d”,result);
}

20. Out put
Enter 2 numbers 10 10
The result after addition is 20

21. Func with argu and return type
#include<stdio.h>
#include<conio.h>
Void main()
{
Int sum(int,int);
Int a,b,ans;
Printf(“enter 2 numbers”);
Sanf(“%d%d”,&a,&b);
Ans=sum(a,b);
Printf(“n the result after addition is %d”,ans);

22. Getch();
}
Int sum(int f1,int f2);
{
Int result;
Result=f1+f2;
Return(result);
}

23. output
 Enter 2 numbers 12 12
 The result after addition is 24