CODEWITHSUNDEEP
c++numbers
Nimitz14
Nimitz14

Reputation: 2328

C++ giving weird answers with large numbers (afaik not overflow)

I was writing a small program to compare speed between CPP and Matlab, and I noticed I was getting the wrong answer when working with large numbers.

Here's the code:

#include "stdafx.h"
#include <iostream>
#include "time.h"



int main()
{       

    clock_t t = clock();

    long x = 1000000;

    long sum = 0;    

    for (long i = 1;i <= x;i++) {
        sum = sum + i;
    }

    t = clock() - t;

    printf("It took %d ms, sum equals=",t);
    std::cout << sum << std::endl;

    return 0;
}

100 000 returns the right answer, but above 1000 0000 does not. Anyone have an idea what's going on? I can't imagine the problem being overflow because the numbers involved are simply not that big (correct answer is 5e9).

Upvotes: 0

Views: 1261

Answers (2)

Professor_Dodo
Professor_Dodo

Reputation: 99

I gave your Programm a test run and it returned the right answer (compared with wolfram alpha). So there isn't an error in the code I would say.

EDIT: run on a 64bit machine, so you may use a 32bit and got the overflow like told in the answer above.

Upvotes: 0

Sebastian Redl
Sebastian Redl

Reputation: 71909

The problem is overflow. 5e9 is 5'000'000'000, but the largest number a 32-bit signed integer (long on 32-bit platforms, and on 64-bit Windows; it's 64-bit on 64-bit Linux) can hold is 2'147'483'648.

Upvotes: 3

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