#include keyword is the pre-defined keyword in C Language. That is used for allowing the pre-processor to copy the content of header files in the current source code where the #include keyword is used.

#include <header_file_name>

OR

#include "header_file_name"

Here is the simple example where we are using #include keyword or directory to check the number is perfect number or not.

#include<stdio.h>

#include<conio.h>

Void main()

{

int number, remainder, sum = 0, i;

clrscr(); // use for clear the console window

printf(“Enter Any Number : “);

scanf(“%d”,&number);

for(i=1;i<=number/2;i++)

{

remainder = number % i;

sum = sum + i;

}

if (sum == number)

        printf("Entered Number is perfect number");

   else

        printf("Entered Number is not a perfect number");

}