From: serialhex <serialhex@...>
Date: 2011-05-12T22:54:21+09:00
Subject: Re: Math cube root

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On Thu, May 12, 2011 at 8:25 AM, Josef 'Jupp' Schugt <jupp@gmx.de> wrote:

> On Wed, 11 May 2011 21:45:42 +0200, Sergey Avseyev <
> sergey.avseyev@gmail.com> wrote:
>
>  How can you explain this:
>>
>>    $ irb
>>    1.9.2p180 (main):001:0> 1000 ** (1.0/3)
>>    9.999999999999998
>>    1.9.2p180 (main):002:0> Math.sqrt(100)
>>    10.0
>
>
instead of using floats you can try to use rational numbers in ruby:

ruby-1.9.2-p180 :001 > require 'mathn'
 => true
ruby-1.9.2-p180 :002 > 1000 ** (1.0/3)
 => 9.999999999999998
ruby-1.9.2-p180 :003 > 1000 ** (1/3)
 => 10
ruby-1.9.2-p180 :004 > 1.0/3
 => 0.3333333333333333
ruby-1.9.2-p180 :005 > 1/3
 => (1/3)

you need to require 'mathn' which will change integer division (and make all
the numbers play nicely together) but other than that things will work
better. the only down side to this whole thing is that rationals are slower
than floats (probably significantly so) because it is a pair of integers.
 and some values when you look at them wont seem to make sense as a
rational, like PI:

ruby-1.9.2-p180 :011 > Math::PI
 => 3.141592653589793
ruby-1.9.2-p180 :012 > Math::PI.to_r
 => (884279719003555/281474976710656)

you can always num.to_f back but that also takes time.  one last thing: if
your inputs are floats i'm not sure there is a nice way to convert them to
rationals:

ruby-1.9.2-p180 :016 > (1.0/3).to_r
 => (6004799503160661/18014398509481984)
ruby-1.9.2-p180 :017 > 1/3
 => (1/3)

so this entire e-mail may be moot :P
hex

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