From: gga <GGarramuno@...>
Date: 2007-02-08T11:35:11+09:00
Subject: Re: Fetching objects used as hash keys

On 6 feb, 16:04, Andre Nathan <a...@digirati.com.br> wrote:
> Hello
>
> I'm using objects which are instances of a class that has 3 instance
> variables as hash keys, but I'm using the value of only two of those
> instance variables for the hash calculation. It's something like this:
>
> class A
>   attr_reader :x, :y, :z
>
>   def initialize(x, y)
>     @x = x
>     @y = y
>     @z = rand
>   end
>
>   def hash
>     @x.hash ^ @y.hash
>   end
>
>   def eql?(other)
>     @x == other.x and @y == other.y
>   end
> end
>
> a1 = A.new('foo', 'bar')  # say @z = 0.25 here
> a2 = A.new('foo', 'bar')  # say @z = 0.90 here
>
> h = {}
> h[a1] = true
> p h[a2] #=> true
>

Okay, your explanation does not make much sense.  But... if @z is not
needed and you want it to be returned as the hash value, why is it in
the class?   Make *that* the hash value, like:

h = {}
h[a1] = rand
p h[a2]
>> 0.25

should give you what you want.  If you do want it in the class, then
you don't need a hash.  Just doing a1.z should give you back what you
want.
If you need it in both, pass it around when you create it, like:

h = {}
[ a1, a2 ]. each { |key|
  z = h[key] || rand
  h[key] = z
  key.z = z              # make z attr_accessor, not attr_reader of
course
}


Or did I miss something?