isPrime

#include <cmath>
#include <iostream>

using namespace std;

bool isPrime(const int num) {
    if (num < 2) {
        return false;
    } else if (num < 4) {
        return true;
    } else if (num % 2 == 0) {
        return false;
    }

    int sup = sqrt(num) + 1;
    for (int i = 3; i < sup; i += 2) {
        if (num % i == 0) {
            return false;
        }
    }

    return true;
}

bool Eratosthenes(const int num) {
    if (num < 2) {
        return false;
    } else if (num < 4) {
        return true;
    } else if (num % 2 == 0) {
        return false;
    }

    bool *composite = new bool[num + 1]{false};

    for (int delta = 3; delta < num; delta += 2) {
        if (!composite[delta]) {
            const int step = 2 * delta;
            for (int n = 3 * delta; n <= num; n += step) {
                composite[n] = true;
            }

            if (composite[num]) {
                break;
            }
        }
    }

    bool result = !composite[num];
    delete[] composite;
    return result;
}

int main() {
    for (int i = 1; i <= 100; i++) {
        cout << i << std::boolalpha << "\t" << isPrime(i) << "\t"
             << Eratosthenes(i) << endl;
    }
}