Question

I wrote up a solution for a puzzle-type website. On XCode with the latest g++, my code compiles fine. On their website (and codepad), my output is negative. Could someone help me understand why, because I’m honestly stumped.

#include <iostream>
#include <cmath>
#include <vector>
#include <map>

using namespace std;

vector<int> getAllPrimes(vector<int> primesArray, int n)
{
    vector<int> numArray (n+1, 1);

    for (int i = 2; i <= n; i++)
    {
        if (numArray[i] == 1)
        {
            primesArray.push_back(i);
            for (int k = i; k <= n; k+= i)
            {
                numArray[k] = 0;
            }
        }
    }

    return primesArray;
}

int main()
{
    long n = 32327;

    if (n == 1)
    {
        printf("%ld\n", n);
        return EXIT_SUCCESS;
    }


    map <int, int> primeMap;
    map <int, int>::iterator itr;
    vector<int> primesArray;

    primesArray = getAllPrimes(primesArray, n);

    while(!primesArray.empty())
    {
        long currPrime = primesArray.back(), curr = currPrime;
        while (currPrime <= n)
        {
            primeMap[curr] += (int)floor(n/currPrime);
            currPrime *= curr;   //multiply currPrime to add another factor of curr. 
        }
        primesArray.pop_back();
    }

    //get the number of divisors of n!
    long numDivisors = 1;
    for (itr=primeMap.begin(); itr != primeMap.end(); itr++)
    {
        numDivisors *= ((*itr).second*2)+1;     //power of each prime + 1, * 2 because you need the number of divisors of the square of n!
        numDivisors = numDivisors % 1000007;
    }

    printf("%ld\n", numDivisors);

    return 0;
}

Ordinarily “long n” is supposed to read an integer between 1 and 1 million from standard input, but I just assigned a value to simulate it.

I have put the code in codepad here: http://codepad.org/RpPFuLzX. As you can see, the output is -596936, whereas on my machine it is 656502 (which is the correct output). What exactly is going on?

Answer 1

Very likely culprit is that CodePad and the other site are compiling on a 32-bit system, where a long is 4 bytes long (http://codepad.org/W00vCFIN). On OS X, everything defaults to 64-bit, and on those systems (but not Windows), a long is 8 bytes long. As such, you are overflowing a computation at some point.

If you depend on a specific integer size, use stdint.h

Here is an adapted version which matches your expected output, using an int64_t: http://codepad.org/Owsl3ClR