From: Tom Willis <tom.willis@...>
Date: 2005-03-07T07:00:32+09:00
Subject: Re: Noob:Objects as key in hash

On Mon, 7 Mar 2005 05:48:47 +0900, Jim Weirich <jim@weirichhouse.org> wrote:
> On Sunday 06 March 2005 01:47 pm, James Britt wrote:
> > Jim Weirich wrote:
> > >   def hash
> > >     @x.object_id + @y.object_id
> > >   end
> >
> > Isn't this the same value as
> >
> >    @y.object_id + @x.object_id
> 
> Yes.  As Joel points out that to some degree you expect things to occasionally
> hash the the same value.  If too many things hash to the same value, then
> your hash array will slow down.
> 
> For small boards, it probably doesn't matter, but not only do [1,2] and [2,1]
> map to the same hash, but so would [0,3] and [3,0].  In fact, all diagnols
> running from the lower left to the upper right have the same hash value on
> all their cells.
> 
> So x+y suggestion, while working, is rather suboptimal.  How about this ...
> 
>   def hash
>     @x + 1000*@y
>   end
> 
> (I dropped the object_id call because it looks like we are mapping from
> fixnums).
> 
> --
> -- Jim Weirich    jim@weirichhouse.org     http://onestepback.org
> -----------------------------------------------------------------
> "Beware of bugs in the above code; I have only proved it correct,
> not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)
> 
> 

LOL I should learn to read. or get a bigger monitor or glasses.

I've been pulling my hair out for the last few hours trying to get it
to work with Jim's suggestions of using eq? only to compose a lengthy
self-deprecating rantish reply.

Only to discover, Jim said the eql? operator. 

So, I'm to new to know the difference just yet. 


But I got it working.

Here's the code in all it's glory for future noobs sake.

#---ruby proggie
class PointID
  attr_reader :x,:y

  def initialize(x,y)
    @x = x
    @y = y
    freeze
  end

  def hash
   return [@x.object_id ,@y.object_id].hash
  end

  def ==(other)
    result = self.hash == other.hash
    #result = (self.x == other.x)and (self.y == other.y)
    return result 
  end

  def eql?(other)
    return self == other
  end 

  def to_s
    return "#{@x},#{@y}"
  end

end
#---end proggie

Thanks again for all the feedback. Hopefully someday I can return the favor.
-- 
Thomas G. Willis
http://paperbackmusic.net