Simple prime number check function.

#include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>

bool is_prime(uint64_t pn = { 0ul }) {
    auto isprime = true;
    if (pn < 2ul) {
        isprime = false;
    }
    else {
        for (auto j_ { 2ul }; j_ <= /*pn / 2ul*/ static_cast<uint64_t>(sqrt(pn)); j_++) {
            if ( pn % j_ == 0) {
                isprime = false;
                break;
            }
            std::cout << j_ << std::endl;
        }
    }
    return isprime;
}

int main() {
    // put any number of values in this vector.
    auto nrs = std::vector<uint64_t> {
      2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 18, 20, 21,
    };

    for (auto nr : nrs) {
        if (is_prime(nr)) {
            std::cout << std::setw(5) << nr << " is prime\n";
        }
        else {
            std::cout << std::setw(5) << nr << " is not prime\n";
        }
    }

    return 0;
}