From: Alex Gutteridge <alexg@...>
Date: 2007-08-30T10:31:11+09:00
Subject: Re: Bug in % (Float)?

On 30 Aug 2007, at 09:02, Charlie Lehardy wrote:

> So, can anybody tell me what's going on here? Are things really  
> broken, or am I just
> expecting the wrong results?
>
> Thanks,
> Charlie

As Florian says, the problem is inherent with floating point values.  
The Python docs have a good section which explains how 0.1 cannot be  
represented as a binary fraction:

http://docs.python.org/tut/node16.html

Ruby just uses your systems C functions (fmod in this case) to do  
these calculations and they show the same behavior:

[alexg@powerbook]/Users/alexg/Desktop(209): cat test.c
#include <math.h>
#include <stdio.h>

int main(){
   printf("fmod of 1 / 0.1 is %lf\n",fmod(1,0.1));
   return 0;
}
[alexg@powerbook]/Users/alexg/Desktop(210): gcc -lm test.c -o modtest
[alexg@powerbook]/Users/alexg/Desktop(211): ./modtest
fmod of 1 / 0.1 is 0.100000

Alex Gutteridge

Bioinformatics Center
Kyoto University