Get prime numbers using the Sieve of Eratosthenes. Also, finds the sum of all the primes below one million. Used for Project Euler.

PrimeNumbers.cpp

// Finds the sum of all the primes below one million.
#include <iostream>
#include <bitset>

using namespace std;

int main()
{
    const int NUM = 1000000;
    int count = 1;
    bitset<NUM> Sieve;
    __int64 sum = 0;
    
    Sieve.flip(); // Sets all bits to 1.
    Sieve[0].flip(); // Sets 0 to not prime.
    Sieve[1].flip(); // Sets 1 to not prime.
    
    // Check all numbers from 2 to sqrt(NUM).
    for (long i = 2; i * i < NUM; ++i)
    {
        if (!Sieve[i]) // If marked not prime.
        {
            continue;
        }
        else // If marked prime, set all multiples as not prime.
        {
            for (long j = 2 * i; j < NUM; j += i)
            {
                Sieve[j] = 0;
            }
        }
    }
    
    // Add together all primes less than NUM.
    for (long i = 2; i < NUM; ++i)
    {
        if (Sieve[i])
        {
            count++;
            sum += i;
        }
    }
    
    cout << "Number of primes: " << count << endl;
    cout << "The sum is: " << sum << endl;
}