From: Warren Brown <wkb@...>
Date: 2003-06-10T05:22:46+09:00
Subject: Re: array.uniq does not use == ?

Orion,

> My Agent is an object I wish to use in a genetic
> algorithm.  Among other things, it has attributes
> chromosome and fitness.  I have "mixed in" the
> Comparable module and defined my <=> operator so
> I can sort my Agents based on their level of
> fitness.
>
> However, I wish to grab "unique" agents based on
> their chromosomes as well.

    The way Array#uniq works is basically:

class Array
  def uniq
    tmphash = {}
    self.each { |e| tmphash[e] = true }
    return self.dup if self.length == tmphash.length
    tmparray = []
    self.each { |e| tmparray << e if tmphash.delete(e)}
    tmparray
  end
end

    This means that you will want to override Agent#hash.  About the only
requirement for a hash key is that it must respond to #hash with a value
that does not change.  See the Pickaxe book (The Ruby Language/The Basic
Types/Requirements for a Hash Key) for more information.  You could probably
get away with hash returning "#{@chromosome}:#{@fitness}".

    In addition to adding Agent#hash, you will want to override Agent#eql?
to assert your definition of equality when comparing for uniqueness.

    I hope this helps.

    - Warren Brown