From: Mark Hubbart <discordantus@...>
Date: 2005-05-06T02:10:16+09:00
Subject: Re: Float to Rational

On 5/4/05, Luke Galea <lgalea@gmmsolutions.com> wrote:
> Hi All,
> 
> I need to convert a float to a fraction.. So 1.5 to 1 1/2..
> The rational class would represent at least 3/2 well.. but I was surprised to
> find that there is no way to easily go from float to rational..
> 
> Am I missing an easier way?

there is no built-in way (that I know of), but here are two methods I
wrote a while back that should cover all the bases:
----
require 'mathn'

class Float
  def to_r
    n = 1
    n *= 2 until (self*n) % 1 == 0
    (self*n).to_i/n
  end
  
  def round_to_r
    i, d = to_s.split /\./
    i.to_i * 10**d.size + d.to_i / 10**d.size
  end
end
----

#to_r directly converts the float to a rational, and includes any
intrinsic inaccuracies. This will be *exactly* equal to the original
float.

  2.125.to_r
    ==>17/8
  0.2.to_r 
    ==>3602879701896397/18014398509481984

#round_to_r uses the displayed representation of the float to generate
a value that, while not always being the actual value of the float, is
much better for display, or if you know you want it rounded a tiny
bit.

  0.2.round_to_r
    ==>1/5
  0.23.round_to_r
    ==>23/100

It could deal with being a little smarter, for catching repeating
digits and the like.

cheers,
Mark