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