C++ program to check prime number

In this program, you will learn about C++ program to check prime number in two different ways.

First, let’s be clear about the prime number:

A number is called prime number if it is divisible by 1 and itself only.

For example: 1, 3, 5, …., 91, ….

Example 1: C++ program to check prime number

//C++ program to check for prime number
#include<iostream>
using namespace std;

int main()
{
   int num, i, j = 0;

   cout << "Enter number: ";
   cin >> num;

   //check for prime number
   for (i = 1; i <= num; i++)
   {
      if ((num % i) == 0)
      {
         j++;
      }
   }

   if (j == 2)
      cout << "\n" << num << " is a prime number.\n";
   else
      cout << num << " is not a prime number.\n";

   return 0;
}

Output

Explanation

The logic of the above program is to check the number entered by the user is perfectly divisible by 1 and itself which is accomplished inside for loop.

The iteration continues till the value of i equals the number.

Let’s check out another way of solving the same problem.

Example 2: Prime number program in C++

//C++ program to check for prime number
#include<iostream>
using namespace std;

int main()
{
   int num, i;
   bool checkPrime = true; //value of checkPrime must be true or false

   cout << "Enter number: ";
   cin >> num;

   //check for prime number
   for (i = 2; i <= num / 2; ++i)
   {
      if ((num % i) == 0)
      {
         checkPrime = false;
         break;
      }
   }

   if (checkPrime)
      cout << "\n" << num << " is a prime number.\n";
   else
      cout << "\n"<< num << " is not a prime number.\n";

   return 0;
}

Output

Enter number: 42
42 is not a prime number
Enter number: 29
29 is a prime number

Explanation

This program also checks whether the number entered by the user is perfectly divisible by i or not.

In this case, we have declared and initialized boolean variable checkPrime to true. If the num is perfectly divisible by i then checkPrime is set to false.