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

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Number is Prime or Not
(Last Updated On: September 2, 2018)

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

To understand this example to find Number is Prime or Not, you should have the knowledge of following C++ programming topics:

  • C++ if, if…else and Nested if…else
  • C++ for Loop
  • C++ break and continue Statement

A positive integer which is only divisible by 1 and itself is known as the prime number.

For example, 13 is a prime number because it is only divisible by 1 and 13 but, 15 is not a prime number because it is divisible by 1, 3, 5 and 15.

Program to Check Prime Number in C++

#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 << "This is a prime number";
  else
      cout << "This is not a prime number";

  return 0;
}

Output

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

Enter a positive integer: 12
This is not 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 above given number is a prime number.

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