From: Nasir Khan <rubylearner@...>
Date: 2007-06-22T04:50:58+09:00
Subject: Re: Ruby doesn't know how to multiply

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And if you are using test/unit then use the assertion method
assert_in_delta()

- nasir

On 6/21/07, MenTaLguY <mental@rydia.net> wrote:
>
> On Fri, 22 Jun 2007 04:25:04 +0900, "rjprado@gmail.com" <rjprado@gmail.com>
> wrote:
> > Dear friends, today I have stumbled into a really weird problem. Try
> > typing this on irb:
> >
> > 14.95 * 0.6 == 8.97
> >
> > Ruby says it's false!
>
> This happens in all languages that use floating point to represent decimal
> numbers (you will get precisely the same result in C or Javascript or Perl,
> for instance).  Floating-point arithmetic is only approximate, so the result
> does not _exactly_ equal 8.97, even though it is very close.
>
> Because of this, when using floating-point arithmetic, testing for exact
> equality is impractical.  The best you can do is test whether the result is
> within some interval (epsilon) of the expected result:
>
> ( ( 14.95 * 0.6 ) - 8.97 ).abs < 1e-6
>
> (Here, we've chosen 1e-6 as our epsilon, which is arbitrary but probably
> "small enough" in this case.  For mathematically intensive code, you may
> need to be more careful.)
>
> Although floating-point is the default for Ruby, you do have the option of
> using a different representation of numbers for which arithmetic is exact,
> although it will not be as fast.  One such option is the Rational class in
> Ruby's standard library, which represents numbers as fractions rather than
> floating-point numbers.
>
> -mental
>
>
>

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