A function is a collection of
statements used for performing some specific task. Function is a way to achieve modularization.
Modularization is a process of dividing a big task into small subtasks. A
function can be pre-defined or user-defined.
Pre-defined function: A function
whose declaration and definition is already written in header files and library
files. Examples are getch(), clrscr() and so on.
User-defined function: A function
whose declaration and definition is written by user in a program. A user can
write a function for performing addition operation.
FUNCTION DECLARATION
A function must be declared inside
or outside the main function. Function declaration is also known as function
prototype. A function can be declared locally or globally.
When function is declared inside
the main function, it is called local
declaration. For example,
#include<iostream>
using namespace std;
int main()
{
int add(int,int);
int a,b,c;
a=2;
b=3;
c=add(a,b); //calling
function
cout<<"Sum of a and b is
"<<c;
return 0;
}
int add(int x,int y) //defining function
{
return x+y;
}
In the above program, add is a user-defined function. Values
of a and b are copied into x and y. Then, sum of x and y is assigned into c.
When a function is declared outside the main function, it is called
global declaration. For example,
#include<iostream>
using namespace std;
int add(int,int);
int main()
{
int a,b,c;
a=2;
b=3;
c=add(a,b);
cout<<"Sum of a and b is
"<<c;
return 0;
}
int add(int x,int y)
{
return x+y;
}
Formal arguments and Actual arguments
Formal arguments are the arguments used in user-defined function for
storing values of arguments used for calling that user-defined function.
Actual
arguments are used in calling function.
For example,
#include<iostream>
using namespace std;
int add(int,int);
int main()
{
int a,b,c;
a=2;
b=3;
c=add(a,b);
cout<<"Sum of a and b is
"<<c;
return 0;
}
int add(int x,int y)
{
return x+y;
}
Here a and b are actual arguments while x and y are formal arguments.
FUNCTION DEFINITION
Function definition can be written either below or after main()
function.
Syntax:
<return-type> <function-name> (<list of arguments>)
{
//function body
}
Example:
int mul(int a,int b)
{
return a*b;
}
FUNCTION CALLING
In c++, functions can be called be three methods.
Call by value
A function can be called by passing values of actual arguments to formal arguments. Values of actual arguments is copied into formal arguments.
Example:
#include<iostream>
#include<string.h>
using namespace std;
float mul(float,float);
int main()
{
float a,b,c;
cout<<"Enter two
numbers : ";
cin>>a>>b;
c=mul(a,b);
cout<<endl;
cout<<"Multiplication is "<<c;
return 0;
}
float mul(float x,float y)
{
return x*y;
}
OUTPUT:
Enter two numbers : 1
2
Multiplication is 2
Call by address
A function can be called by passing addresses of actual arguments to formal arguments. Pointer variables are used to access the values of actual arguments. You can change the original values of actual arguments using pointers.
Example:
#include<iostream>
#include<string.h>
using namespace std;
float mul(float*,float*);
int main()
{
float a,b,c;
cout<<"Enter two
numbers : ";
cin>>a>>b;
c=mul(&a,&b);
cout<<endl;
cout<<"Multiplication
is "<<c;
return 0;
}
float mul(float *x,float *y)
{
return *x**y;
}
OUTPUT:
Enter two numbers : 1
2
Multiplication is 2
Call by reference
A function can be called by passing references of actual arguments to formal arguments. Reference variables are used to access actual arguments. Any change in reference variable causes change in actual argument.
Example:
#include<iostream>
#include<string.h>
using namespace std;
float mul(float&,float&);
int main()
{
float a,b,c;
cout<<"Enter two
numbers : ";
cin>>a>>b;
c=mul(a,b);
cout<<endl;
cout<<"Multiplication is "<<c;
return 0;
}
float mul(float &x,float &y)
{
return x*y;
}
OUTPUT:
Enter two numbers : 1
2
Multiplication is 2
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