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

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

Example to check whether an integer (entered by the user) is a prime number or not using for loop and if...else statement.

To understand this example, you should have the knowledge of the following C++ programming topics:


A positive integer which is only divisible by 1 and itself 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.


Example: Check Prime Number

#include <iostream>
using namespace std;

int main() {
   int n, i;
   bool isPrime = true;

   cout << "Enter a positive integer: ";
   cin >> n;

   for (i = 2; i <= n / 2; ++i) {
      if (n % i == 0) {
         isPrime = false;
         break;
      }
   }
   if (isPrime)
      cout << n << " is a prime number";
   else
      cout << n << " is not a prime number";

   return 0;
}

Output

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

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.

The foor loop initiates with an 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, isPrime is set to false and the 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.

So, the given number is a prime number.