C++ Programming Code Examples C++ > Mathematics Code Examples Program to Check Whether a Number is Prime or Not 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. 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. #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; }