CODEWITHSUNDEEP
c++gccg++doublemodulus
aLearner
aLearner

Reputation: 1047

Why Does fmod Produce Different Results?

I was playing around with the following piece of code

#include <iostream>
#include <cmath>
#include <cstdio>
using namespace std;

int main() {
    double speed = 60.0;
    double distance;
    int a, b, c;
    scanf("%lf %d %d %d", &distance, &a, &b, &c);

    cout << distance <<  " " << a << " " << b << " " << c << endl;
    cout << (fmod((distance*3600)/speed, (a + b + c))) << endl;
    return 0;
}

and noticed that, for the following input

15.1 1 1 1

it produces this (incorrect?) result on my machine 

15.1 1 1 1
3

But it seems to produce this result on ideone (http://ideone.com/mwMalv)

15.1 1 1 1
0

I realize that "0.1" (in the "15) can't be represented accurately as a double

http://www.exploringbinary.com/why-0-point-1-does-not-exist-in-floating-point/

but others that have run this code with gcc 4.8.2 also obtain what's produced on ideone.

I'm running Ubuntu 14.04.2 LTS Desktop (32-bit) on VMware 6.0.5 build-2443746 with gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1).

What gives? Why does fmod behave differently on my machine than it does on others (given the compiler's the same)?

Upvotes: 0

Views: 102

Answers (1)

M.M
M.M

Reputation: 141633

Your calculations come out to approximately fmod(906, 3). However, 906 could be 905.9999999999998532 or something, due to errors with values not being exactly representable.

The result of fmod with the latter than 3 will give 2.999999999999999998532 which will display as 3 when rounded to the default precision of 6.

To see what is going on in detail, do << setprecision(30) in your cout statements.

It also might be illuminating to inspect the assembly output generated in each case. Perhaps the compilers are doing different things; or perhaps they are doing the same thing and the FPU is different.

For example, the result of the 906 might even be done in higher precision than double. Changing your code to store this in a double before going on to fmod may or may not make a difference.

Upvotes: 1

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