From: Rob Biedenharn <Rob@...>
Date: 2008-02-08T07:57:42+09:00
Subject: Re: BigDecimal.new('15.25') == 15.25, false ??

On Feb 7, 2008, at 5:26 PM, Henry Jones wrote:

> Hi,
>
> I was writing some unit tests when I ran into some troubles.. In the  
> end
> I found out the problem :
>
> BigDecimal.new('15.25') == 15.25
>
> returns false !!
>
> BigDecimal.new('15.25').to_f == 15.25.to_f also returns false.
>
> Substracting both values returns 1.7763... e-015
>
> This is on ruby 1.8.6, Win32.
>
> I'm a comp. engineer, so I know it's ultimately related to the binary
> value of 15.25 which cannot be precisely represented (without an
> infinite number of bits) , but I'm still amazed that such a simple
> comparison with a literal value should fail...


15.25 in decimal
is exactly
1111.01 in binary

Seems like a finite number of bits to me ;-)  However, the  
construction of 15.25 as a literal is likely something roughly (1*10^1  
+ 5*10^0) + (2*10^-1 + 5*10^-2) and those intermediate fractional  
terms are problematic.

Hey, this is Ruby!  Roll your own with ===

irb> class Float
irb>   def ===(other,eps=0.000000001)
irb>     (self - other.to_f).abs < eps
irb>     end
irb>   end
=> nil
irb> require 'bigdecimal'
=> true
irb> 15.25 == BigDecimal.new("15.25")
=> false
irb> 15.25 === BigDecimal.new("15.25")
=> true

Of course, you'd have to add a similar BigDecimal#=== to get symmetry.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com