Subj : Re: Weird Math
To   : Andy Madigan
From : Igor Bukanov
Date : Thu Sep 16 2004 01:24 am

Andy Madigan wrote:
> I've been running some scripts through rhino, and one script kept on
> getting an equation wrong, I finally found the problem, I ran a simple
> test through the Rhino Shell, here's the output:
> 
> js> 100 - 69.1
> 30.900000000000006
> 
> Am I going nuts? Is this something about how javascript does math that
> I don't know? (Note: 100 - 59.1 is correct)

69.1 can not be represented as exact Java double (binary representation 
of 0.1 is infinite fraction) so it is rounded to the nearest 
representable number. When the rounded number is subtracted from 100, 
its closes decimal that when read back preserve the binary presentation 
is 30.900000000000006.

You do not see that with 100 - 59.1 since 69.1. and 59.1 have rather 
different binary presentations (starting from the fact that 69 > 64 and 
59 < 64) so in this case the decimal 40.9 is the best fit.

Note that SpiderMonkey has exactly the same behavior which is not 
surprising since Rhino code is just a translation of SM code into Java.

Regards, Igor

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