Project Euler problem #3, not working for very large numbers.

euler3

#include <stdio.h>
#include <stdbool.h>

int main()
{
    //populate sieve array
    int num=600851475143;
    int num_array[num-1];
    int i, j;
    int factor_mod;
    int highest_prime=0;

    for(i=1; i<num; i++)
    {
        num_array[i] = i+1;
    }

    //get sieve array size
    int div_array[0];
    int array_size = sizeof(num_array)/sizeof(div_array[0]); //why does this work? need to learn about sizeof

    printf ("Sieve array size = %d\n", array_size);

    //print starting sieve array
//    printf("\nStarting sieve array: ");
//    for(i=1; i<num; i++)
//    {
//        if(i<num)
//        {
//            printf("%d, ", num_array[i]);
//        }
//        else
//        {
//            printf("%d\n", num_array[i]);
//        }
//    }

    //eliminate non-primes from num_array
    for(i=2; i<num; i++)//need to find a better way to do this than nested for loops (?)
    {
        for(j=2; j<num; j++)
        {
            if(num_array[j]%i==0 && num_array[j]!=i)
            {
                num_array[j] = 0;
            }
        }
    }

    //print modified sieve array
//    printf("\nModified sieve array: ");
//    for(i=1; i<num; i++)
//    {
//        if(i<num)
//        {
//            printf("%d, ", num_array[i]);
//        }
//        else
//        {
//            printf("%d\n", num_array[i]);
//        }
//    }

    //divide number by non-zero entries in num_array to find highest prime factor
    i = array_size;
    while(highest_prime==0)
    {
        if(num_array[i]!=0 && num%num_array[i]==0) //if number isn't 0 (is a prime), divide with modulus to determine if it is a factor
            {
                highest_prime = num_array[i];
            }
        i--;

    }

    //show result
    printf("\nThe highest prime factor of %d is %d", num, highest_prime);
}


//Junk code
//        for(i=array_size; i>2; i--) //start from highest numbers and decrement until you find the first non-zero entry
//            {
//                printf("\ni = %d, num_array[i] = %d", i, num_array[i]);
//                if(num_array[i]!=0 && num%num_array[i]==0) //if number isn't 0 (is a prime), divide with modulus to determine if it is a factor
//                    {
//                        highest_prime = num_array[i];
//                        printf("\nnum_array[i] = %d, num(mod)num_array[i] = %d, highest_prime = %d", num_array[i], num%num_array[i], highest_prime);
//                    }
//                    printf("\nhighest_prime post-if = %d", highest_prime);
//            }
//            printf("\nhighest_prime post-for = %d", highest_prime);