From: Gavin Kistner <gavin@...>
Date: 2005-11-28T07:52:18+09:00
Subject: Re: floating point subtraction question

On Nov 27, 2005, at 3:41 PM, Nicholas Rahn wrote:
> I'm a bit confused with some simple floating point arithmetic:
>
> % ruby -e 'puts "#{(1.2 - 1.0) == 0.2}"'
> false
>
> % ruby -e 'puts "#{(1.5 - 1.0) == 0.5}"'
> true

Slim:~ gavinkistner$ irb
irb(main):001:0> 1.2-1.0
=> 0.2
irb(main):002:0> (1.2-1.0)==0.2
=> false
irb(main):003:0> "%.31f" % [1.2-1.0]
=> "0.1999999999999999555910790149937"
irb(main):004:0> "%.31f" % [0.2]
=> "0.2000000000000000111022302462516"
irb(main):005:0> "%.31f" % [1.2]
=> "1.1999999999999999555910790149937"

Because many digits cannot be precisely represented as powers of 2.

For more info, try a plethora of online resources on floating point  
math, such as:
http://en.wikipedia.org/wiki/Floating_point#Problems_with_floating-point