Check prime numbers and list the primes up to 30

Description: Write is_prime(int n) and print the primes from 1 to 30.

Hint:

• Test divisors while i * i <= n.

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Solution

#include <iostream>

bool is_prime(int n) {
    if (n < 2) return false;
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}

int main() {
    for (int n = 1; n <= 30; n++) {
        if (is_prime(n)) std::cout << n << ' ';
    }
    std::cout << '\n';
}

How it works

• If n has a divisor bigger than its square root, it also has a smaller one, so we can stop early.

Read C++ Functions.

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