# 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 following C++ programming topics:

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

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 << "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.```

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.