eratosthenes.cc

#include <iostream>
#include <vector>

const int N = 10000000;
int main() {
    // Let the seive be an array (really, std::vector<bool> is not exactly
    // an array, but that's not important now) of size N with all elements
    // initially set to 'true' istead of 'false'.
    std::vector<bool> sieve(N, true);

    for (int i=2; i <= N / 2; ++i) {
        if (sieve[i]) {
            for (int j=i+i; j < N; j += i)
                sieve[j] = false;
        }
    }

    for (int i=2; i < N; ++i) {
        if (sieve[i])
            std::cout << i << std::endl;
    }
}