From: Tim Hunter <cyclists@...>
Date: 2006-03-31T09:13:43+09:00
Subject: Re: Very interesting Float comparison!

Ryan Allen wrote:
> Hi Guys (and Gals!)
> 
> This is my first post on the list, so Hello!
> 
> Something very strange is happening (as far as I can tell) with a  float 
> comparison; here is the code in question:
> 
> ("55.59".to_f / 100) == 0.5559    # returns false
> 
> However:
> 
> "55.59".to_f / 100    # returns Float 0.5559
> 0.5559                # returns Float 0.5559
> 0.5559 == 0.5559      # returns true
> 
> And more!:
> 
> ("55.5".to_f / 100)  == 0.555     # returns true
> ("55.53".to_f / 100) == 0.5553    # returns true
> ("55.56".to_f / 100) == 0.5556    # returns true
> ("55.58".to_f / 100) == 0.5558    # returns true
> ("55.59".to_f / 100) == 0.5559    # returns false !!
> ("44.59".to_f / 100) == 0.4449    # returns true  !!
> ("44.59".to_f / 100) == 0.4449    # returns false !!!!
> 
> It doesn't seem to like .59's... What did .59 ever do?
> 
> I picked this up while writing a unit test, I've run this code in irb  
> and I'm not sure what's going on...
> 
> Can anyone shed some light on this?
> 
> I'm using ruby 1.8.4 (2005-12-24) [powerpc-darwin8.5.0]
> 
> Cheers,
> Ryan.
> 
> 

The comparison of floating-point numbers is notoriously problematic 
throughout all computing, not just Ruby. Here's a good description of 
the problem: http://en.wikipedia.org/wiki/Floating_point. You can also 
find many discussions of this same "problem" by searching the archives 
of this list.

Typically you don't test two floating-point numbers for equality, you 
test that the difference is very small. This is what the assert_in_delta 
assertion is for.