Question

I can take a guess that it has something to do with working with the unsigned long long int.

#include <cstdlib>
#include <iostream>
#include <cmath>

using namespace std;

typedef unsigned long long int uint64;

int main(int argc, char *argv[])
{



    uint64 number_in_question = 600851475143LL;

    long double sqrt_in_question = sqrt(number_in_question);
    bool primes_array[number_in_question+1];



    for (uint64 i = 0; i <= number_in_question; i++) {
        primes_array[i] = true;
    }

    for (uint64 i = 2; i <= sqrt_in_question; i++) {
        if(primes_array[i] == true) {
            // for every multiple of this prime, mark it as not prime
            for (uint64 ii = i*2; ii <= number_in_question; ii += i) {
                primes_array[ii] = false;           
            }
        }
    }   

    for (uint64 i = 0; i <= number_in_question; i++) {
        if(primes_array[i] == true)
        cout << i << ", ";
    }


    system("PAUSE");


    return EXIT_SUCCESS;
}

Edit1:
Some background of what I am trying to do:

I am trying to mimic this technique: http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
while I am using an array to store a simple “is it prime” 1 for yes, 0 for no. The end goal is to solve this:

What is the largest prime factor of the number 600851475143 ? Listed here: http://projecteuler.net/problem=3. I am just working on the primes and then will work on the prime factors.

Edit2:

Upon looking at the Wikipedia link I posted, I realized they have puesdocode (skipped over that and came up with what I have) and realized that had this note:
Large ranges may not fit entirely in memory. In these cases it is necessary to use a segmented sieve where only portions of the range are sieved at a time.[14] For ranges so large that the sieving primes could not be held in memory, space-efficient sieves like that of Sorenson are used instead.
Therefore I will have to think of a way to do this using a “segmented sieve” method.

Edit3:

Changed the array to account for the [0] element so the “issue” is only focused on the array memory size being too large for future references; also stored the array as a bool instead of a uint64.

Answer 1

You are trying to allocate an uint64 array of length 600851475143. For 8 byte uint64 that means this array will take up 600851475143*8byte which is roughly 4.5TB of memory. Even if your system can allocate that much memory (unlikely) you are trying trying to put it on the stack which has typically a size bound to only a few MB. Furthermore you are trying to write to index number_in_question, while the last index in the array is number_in_question-1.