# 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.

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.

Note: 0 and 1 are not prime numbers.

## Example: Check Prime Number

``````
#include <iostream>
using namespace std;

int main(){
int i, n;
bool isPrime = true;

cout << "Enter a positive integer: ";
cin >> n;

// 0 and 1 are not prime numbers
if (n == 0 || n == 1) {
isPrime = false;
}
else {
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 the user and stores it in the variable n.

Also, notice that the boolean variable isPrime is initialized to `true` at the beginning of the program.

Since 0 and 1 are not prime numbers, we first check if the input number is one of those numbers or not. If the input number is either 0 or 1, then the value of isPrime is set to `false`.

Else, the initial value of isPrime is left unchanged and the `for` loop is executed, which checks whether the number entered by the user is perfectly divisible by i or not.

The `for` loop initiates with an initial value of i equals to `2` and increases the value of i by 1 with each iteration.

If the number entered by the user is perfectly divisible by i, then isPrime is set to `false` and the number will not be a prime number.

But if the input number is not perfectly divisible by i throughout the entirety of the loop, then it means that the input number is only divisible by 1 and that number itself.

So, the given number is a prime number.

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

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