C Program to Check Whether a Number is Prime or Not

A positive integer which is only divisible by 1 and iself is known as prime number. For example: 13 is a prime number because it is only divisible by 1 and 13 but, 15 is not prime number because it is divisible by 1, 3, 5 and 15.

C program to Check Prime Number


/* C program to check whether a number is prime or not. */

#include <stdio.h>
int main()
{
  int n, i, flag=0;
  printf("Enter a positive integer: ");
  scanf("%d",&n);
  for(i=2;i<=n/2;++i)
  {
      if(n%i==0)
      {
          flag=1;
          break;
      }
  }
  if (flag==0)
      printf("%d is a prime number.",n);
  else
      printf("%d is not a prime number.",n);
  return 0;
}

Output

Enter a positive integer: 29
29 is a prime number.

Explanation

This program takes a positive integer from user and stores it in variable n. Then, for loop is executed which checks whether the number entered by user is perfectly divisible by i or not starting with initial value of i equals to 2 and increasing the value of i in each iteration. If the number entered by user is perfectly divisible by i then, flag is set to 1 and that number will not be a prime number but, if the number is not perfectly divisible by i until test condition i<=n/2 is true means, it is only divisible by 1 and that number itself and that number is a prime number.

Visit this page to learn, how you can display all prime numbers between two intervals entered by user.