C Program to Check Whether a Given Number is Prime Number or Not

A Prime Number is a positive integer greater than 1 which is divisible by 1 and itself. In other words, Prime number is a whole number greater than whose factors are 1 and itself. Examples of first few prime numbers are 2, 3, 5, 7, 11, 13..

Note : Number 2 is first and only even prime number.

Program

#include<stdio.h>
#include<conio.h>

int main()
{
	 int number, i, flag=0;
	 clrscr();
	 
	 // Inputs
	 printf("Enter integer number: ");
	 scanf("%d", &number);
	 
	 // Checking for prime
	 for(i=2;i<=number/2; i++)
	 {
		  if(number%i==0)
		  {
			   flag = 1;
			   break;
		  }
	 }
	 
	 // Taking decision
	 if(flag==0 && number>=2)
	 {
	  	printf("%d is PRIME.", number);
	 }
	 else
	 {
	  	printf("%d is NOT PRIME.", number);
	 }
	
	 getch();
	 return(0);
}

Output of above program :

Run 1:
-----------
Enter integer number: 13 ↲
13 is PRIME.

Run 2:
-----------
Enter integer number: 16 ↲
16 is NOT PRIME.

Run 3:
-----------
Enter integer number: 1 ↲
1 is NOT PRIME.

Run 4:
-----------
Enter integer number: 2 ↲
2 is PRIME.

Note: ↲ indicates enter is pressed.