From: Mike Stok <mike@...>
Date: 2006-03-31T09:04:55+09:00
Subject: Re: Very interesting Float comparison!


On 30-Mar-06, at 6:58 PM, 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.

You might want to look at http://docs.sun.com/source/806-3568/ 
ncg_goldberg.html and http://support.microsoft.com/kb/q69333/

ratdog:~ mike$ irb
irb(main):001:0> '%.40f' % 0.5559
=> "0.5558999999999999497291014449729118496180"
irb(main):002:0> '%.40f' % (55.59 / 100)
=> "0.5559000000000000607514039074885658919811"
irb(main):003:0> '%.40f' % ("55.59".to_f / 100)
=> "0.5559000000000000607514039074885658919811"


Mike

-- 

Mike Stok <mike@stok.ca>
http://www.stok.ca/~mike/

The "`Stok' disclaimers" apply.