# 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`.