From: Christopher Dicely <cmdicely@...>
Date: 2009-03-07T12:42:38+09:00
Subject: Re: 2000-1992.2 - is this a known bug?

2009/3/6 S2 <some.r@ndom.mail.invalid.net>:
> ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
>
>>> 2000-1992.2
> => 7.79999999999995
>
>

The use of inexact floating point numbers by default in Ruby (which
follows C and most, but not all, other programming languages) is a
design choice, not a bug. One might dispute whether or not its a good
design choice (personally, I prefer Scheme's numeric tower and exact
numbers by default with conversion to inexact numbers only when an
inexact operation is applied to an exact number or an explicit
designation of an inexact number is given), but its a point about
which people could debate endlessly, and since BigDecimal is at least
in the standard library, its not a matter of whether Ruby can do exact
math, but just how verbose the code is to do it.

ex:

require 'bigdecimal'
require 'bigdecimal/math'

class BigDecimal
  # BigDecimal.to_s is ugly
  def pretty
    digits, magnitude = to_s.split('.')[1].split('E')
    magnitude=magnitude.to_i
    digits.insert(0,'0'*[(1-magnitude),0].max)
    digits.insert([magnitude,1].min,'.')
  end
end

include BigMath

(2000-BigDecimal('1992.2')).pretty
=> "7.8"