From: Yuri Leikind <YuriLeikind@...>
Date: 2001-09-16T01:18:36+09:00
Subject: [ruby-talk:21211] Re: using my own objects as hash keys - a question

Lyle Johnson wrote:
> 
> > Could someone explain to me why the following
> > does not work as I expect:
> >
> <snip>
> 
> > hash=Hash.new
> >
> > hash[key1]=6
> >
> > puts hash[key1]  #=> 6
> > # Ok, as expected
> >
> > puts hash[key2]  #=> nil
> > # Why?!!  If key1.hash == key2.hash, why doesn't it work?
> >
> > # Same question here
> > puts hash.has_key?(key1) #=> true
> > puts hash.has_key?(key2) #=> false
> >
> >
> > Have I missed something ?
> 
> Well, yes, I guess you've missed how hash tables work. A hash table is
> usually described as a collection of "buckets", one for each hash value.
> Whenever you add a new key to a hash table, the code will compute the hash
> value for that key, find the corresponding bucket for that hash value, and
> place a reference to the key in that bucket. When you look up that key in
> the hash table (e.g. by calling has_key?) the code once again computes the
> hash code for the key, looks in the corresponding bucket -- which may now
> have several different keys, all with the same hash value -- and looks for
> the requested key.
> 
> So even though key1 and key2 have the same hash code (6), you've only put
> key1 in the hash table, and it's still a different instance than key2. If
> you were to also add key2 to the hash table, they would both end up in the
> same "bucket" and the hash table would include both keys.

I got it. Thank you.

Yuri Leikind