Question

I have this simple program:

// Include libraries

#include <iostream>
#include <string>
#include <vector>

using namespace std;

// Include locals

// Start

#define NUMBER 600851475143

int main(int argc, const char* argv[])
{
    long long int ans = 0;
    long long int num = NUMBER;

    vector<int> factors;

    do
    {
        // Get lowest factor

        for (int i = 1; i <= num; ++i)
        {
            if (!(num % i))
            {
                factors.push_back(i);

                num /= i;
                break;
            }
        }
    } while (num > 1);

    cout << "Calculated to 1.\n";

    int highestFactor = numeric_limits<int>::min();

    for (int i = 0; i < factors.size(); ++i)
    {
        if (factors[i] > highestFactor)
        {
            highestFactor = factors[i];
        }
    }

    ans = highestFactor;

    cout << ans << endl;

    return EXIT_SUCCESS;
}

compiling with g++ -O2 -c -o prob3.o prob3.cpp proved successful, but when I ran it I saw nothing and it just kept running and I had to Ctrl-C (force-kill) it in the end. When I try to add

int main(int argc, const char* argv[])
{
    cout << "Test\n";

to the program, Test didn’t get printed too. It’s like my program is not executed at all.

Any help or advice is appreciated!

Solution

I forgot prime numbers started at 2. Change for (int i = 1 to for (int i = 2.

Answer 1

Those nested loops are going to loop forever. The inner for loop will only ever execute once because of the break so it will only ever do num /= 1. That means num never decreases and so num > 1 will never be false. I suppose you just need to wait longer!

The reason you’re not seeing “Test” is probably because you haven’t flushed the output. Try:

std::cout << "Test" << std::endl;