1. By : Rakesh Kumar D.A.V. Centenary Public School, Chander Nagar
Function
Function is a small sub program , which is designed to perform a particular task in a complete program and
it is designed in this way that it can be coupled with another function.
return type function_name ( argument)
{
statement; function Syntax
}
where
return type : Data type of the value return by the function
function_name : Any Valid identifier
Statement : Any Valid C/C++ statement(s)
Our first Function : To print “hello world” on the screen
void print_message(void) 1. void at the position of return type shows that
{ the function does not return any value to it’s
cout<<”n Hello world”; calling function.
return ; 2. Void at the position of argument shows that
} the function does not accept any argument.
NOTE : return is not compulsory, but good programming skill recommends every function should have a
return.
How to implement this function in a C++ program
Method -1 Method-2
#include<iostream> #include<iostream>
#include<conio.h> #include<conio.h>
using namespace std; using namespace std;
void print_message(void) void print_message(void); // function prototype
{ int main()
cout<<"n Hello world"; {
return; print_message(); // function call
} getch();
int main() return 0;
{ }
print_message();
getch(); void print_message(void)
return 0; {
} cout<<"n Hello world";
return;
}
Function Prototype: A function prototype in C or C++ is a declaration of a function that omits the function
body but does specify the function's name, argument types and return type. While a
function definition specifies what a function does, a function prototype can be
thought of as specifying its interface.

2. By : Rakesh Kumar D.A.V. Centenary Public School, Chander Nagar
TYPE-I
Main function User defined Function
Input Phase
USE UDF Processing Phase
function here
Output Phase
Problem : Write a function to read base and height of a triangle and find out area of a right angle
Triangle ( Area = ½ *b*h). Also implement this function in a C++ program.
Solution
Program Output
#include<iostream>
#include<conio.h>
using namespace std;
void area_triangle(void)
{
int b,h,ar;
system("cls"); // clrscr()
cout<<"n Enter base :";
cin>>b;
cout<<"n Enter height :";
cin>>h;
ar =0.5*b*h;
cout<<"n Area of Triangle :"<<ar;
return;
}
int main()
{
area_triangle(); // function Call
getch();
return 0;
}
NOTE : main( ) is also a function. It has the following feature
without main program can not execute.
A program can have more than one function but can not have more than one main ( )
Program execution begin from main( )
TYPE –II
Main Function User defined function
1. Input
2. Processing
3. result must
return to it’s
Output Here calling
function

3. By : Rakesh Kumar D.A.V. Centenary Public School, Chander Nagar
Type –II Problem : Write a function in C++ to read base and height of a right angle triangle, calculate
area of triangle using formula area = ½*b*h and return it to it’s calling function. Also implement this
function in a C++ program
Solution
Program Output
#include<iostream>
#include<conio.h>
using namespace std;
int area_triangle(void)
{
int b,h,ar;
cout<<"n Enter base :";
cin>>b;
cout<<"n Enter Height :";
cin>>h;
ar =int(0.5*b*h);
return (ar);
}
int main()
{
int res =area_triangle();
cout<<"n Area of Triangle :"<<res;
getch();
return 0;
}
TYPE-III
Main function USER defined function
1. Input 2. Processing
3. Output
Type –III Problem : Define a function in C++ Area_triangle( ) which accept two integer type parameter
(i) int base (ii) int height. This function calculates area of Triangle using formula area = 1/2*base*height and
also display this calculated area on the screen. Also implement this function in C++ program
Solution
Program Output
#include<iostream>
#include<conio.h>
using namespace std;
void area_triangle(int base, int height)
{
int ar;
ar =int(0.5*base*height);
cout<<"n Area of Triangle :"<<ar;
}
int main()
{
area_triangle(10,20);
getch();
return 0;
}

4. By : Rakesh Kumar D.A.V. Centenary Public School, Chander Nagar
Type –IV Input Travels from main to UDF
Main Function User defined Function
1. Input
2. Processing
3. Output Phase
Result Travels from UDF to Main and used inside main
TYPE –IV Problem : Write a user defined function in C++ Area_Triangle( ) which accept two integer type
parameter (i) int base (ii) int height. This function calculates area of triangle using formula area =
½*base*height and return it to it’s calling function. Also implement this function in C++ program
Solution
Problem Output
#include<iostream>
#include<conio.h>
using namespace std;
int area_triangle(int base, int height)
{
int ar;
ar =int(0.5*base*height);
return(ar);
}
int main()
{
int res =area_triangle(10,20);
cout<<"n Area of Triangle :"<<res;
getch();
return 0;
}
Parameter Types
• Call by Value method
• Call By reference method
Call By Value Method: In this method actual parameter make it’s copy and send to formal parameter.
The processing inside function use this copy of actual parameter. So the changes made inside function does
not automatically available to it’s calling function.
Program Output
#include<iostream>
#include<conio.h>
using namespace std;
void add ( int a) // formal Parameter
{
a = a+10;
}
int main()
{
int x=10;
cout<<"nBefore function call x :"<<x;
add(x);
cout<<"nafter function call x :"<<x;
getch();

5. By : Rakesh Kumar D.A.V. Centenary Public School, Chander Nagar
return 0;
}
Call By reference Method : In this method actual parameter pass the address of actual parameter. So the
changes made inside function is automatically available to it’s calling function.
Program output
#include<iostream>
#include<conio.h>
using namespace std;
void add ( int &a) // Call by reference
{
a = a+10;
}
int main()
{
int x=10;
cout<<"nBefore function call x :"<<x;
add(x);
cout<<"nafter function call x :"<<x;
getch();
return 0;
}
NOTE : in case of reference type parameter, actual
parameter must be of variable type
Scope of Variable
• Auto / Local Variable : The variable whose life begins within opening curly braces and it dies at
the position of it’s corresponding curly braces, is called local variable
• Global Variable : The variable whose scope is whole program , and is defined outside function ,is
called global variable
Program Output
#include<iostream>
#include<conio.h>
using namespace std;
int a=20; // Global Variable
int main()
{
int a=10; // Local Variable
{
int a=30;
cout<<"n Value of a :"<<::a;
}
cout<<"n Value of a :"<<a;
getch();
return 0;
}
•Static Variable : These are local variable to that function where it is defind , but does not loose
their values between the function call.
Example Program Output
#include<iostream>
#include<conio.h>
using namespace std;
void show(void)
{
static int a=0;
a++;
cout<<"n Value of a :"<<a;
}
int main()
{

6. By : Rakesh Kumar D.A.V. Centenary Public School, Chander Nagar
show();
show();
show();
getch();
return 0;
}
Parameter with default Values
Exmaple Program Output
#include<iostream>
#include<conio.h>
using namespace std;
void show(int a=10)
{
cout<<"n Value of a :"<<a;
}
int main()
{
show();
show(30);
getch();
return 0;
}
Some Additional Definition_______________________________________________________________
Formal Parameter : The parameter which appears with function prototype is called formal parameter.
Actual parameter : The Parameter which is used at the time of call function , is called actual parameter
Example
#include<iostream>
#include<conio.h>
void show ( int a) // here a is formal parameter
{
………….
………….
}
int main( )
{
int x =10;
Show (x) ; // x is here actual parameter.
………….
…………
}