From: Robert Klemme <shortcutter@...>
Date: 2010-07-13T21:36:25+09:00
Subject: Re: Ruby 1.9.2 float precision

2010/7/13 Diogo Almeida <diogo.borges.almeida@gmail.com>:

> The Liquid templating engine has the following tests passing till ruby
> 1.9.2 rc1:
>
> assert_template_result "4.66666666666667", "{{ 14 | divided_by:'3.0' }}"
> #equals to 14/3.0 in irb
> assert_template_result "6.3", "{{ '2.1' | times:3 }}" #equals to 2.1*3
> in irb
>
> However, as far as I could understand, the float precision has been
> changed with RC1 and the first assertion now returns 15 decimal digits
> instead of 14:
> ruby-1.9.2-rc1 > 14/3.0
>  => 4.666666666666667
>
> And the second returns the same 15 decimal digits instead of 1:
> ruby-1.9.2-rc1 > 2.1*3
>  => 6.300000000000001
>
> Even though I realize that the templating engine should not be messing
> with the ruby pre-defined precisions, I would like to ask what would be
> the recommended way to get all ruby versions to behave the same?
>
> For instance, I've noticed that in both 1.9.1 and 1.9.2-rc1 the
> instruction (14/3.0).to_r returns the exact same rational, just
> differing when using the #to_f

I question the validity of the test.  Expecting a particular
formatting of a float number without giving a concrete format is at
least fragile (as you have discovered) - I would even call it wrong.
Different than for integers there is no common agreed way to convert
floats to strings so if you want a particular formatting you need to
make it part of the test.  An alternative would be a regexp match
against \A4\.6+7\z

irb(main):001:0> /\A4\.6+7\z/ =~ (14.0/3).to_s
=> 0

Kind regards

robert

-- 
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