Functions that answer questions

A function can return a bool, which makes code read like a sentence: if (isPrime(n)).

n is prime if nothing from 2 up to √n divides it. You only need to check up to √n: if n = a × b, one of a and b is at most √n. In code: loop while i * i <= n.

1bool isPrime(int n) {
2    if (n < 2) return false;
3    for (int i = 2; i * i <= n; i++) {
4        if (n % i == 0) return false;
5    }
6    return true;
7}