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

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
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.