From: Thomas Hurst <tom.hurst@...>
Date: 2002-02-17T04:11:11+09:00
Subject: Re: hash as key in hash

* Albert Wagner (alwagner@tcac.net) wrote:

> I don't understand why this doesn't work.  Can anyone help?
>
> hash = { {'a'=> 1, 'b' => 2, 'c' =>3} => 1, {'c' => 1, 'b' => 3} => 2}
> arg_hash = {'c' => 1, 'b' => 3}
> p hash.key?(arg_hash)

Hashes use the numeric result of the hash method call for any object
used as a key:

irb(main):001:0>  {'c' => 1, 'b' => 3}.hash
67528232
irb(main):002:0>  {'c' => 1, 'b' => 3}.hash
67514422
irb(main):003:0>  {'c' => 1, 'b' => 3}.hash
67507872

You're creating new hash objects; their object ID's differ, hence their
hashes differ.

Instead, create one instance and use that everywhere:

key1 = {'a'=> 1, 'b' => 2, 'c' => 3}
key2 = {'c' => 1, 'b' => 3}
hash = {key1 => 1, key2 => 2}
p hash.key?(key2)

Since:

irb(main):001:0> bla = {'c' => 1, 'b' => 3}
{"b"=>3, "c"=>1}
irb(main):002:0> bla.hash
67527642
irb(main):003:0> bla.hash
67527642
irb(main):004:0> bla.hash
67527642

However, I suspect you really want to index by hash content, not hash
objects.  For that, something more like:

hash = { {'a'=> 1, 'b' => 2, 'c' => 3}.to_str => 1,
         {'c' => 1, 'b' => 3}.to_str => 2}
arg_hash = {'c' => 1, 'b' => 3}
p hash.key?(arg_hash.to_str)

Will be more like it.  However, hash ordering is not guaranteed, hence
to_str probably isn't either, so to be more correct you better sort it
first:

hash = { {'a'=> 1, 'b' => 2, 'c' => 3}.sort.to_s => 1,
         {'c' => 1, 'b' => 3}.sort.to_s => 2}
arg_hash = {'c' => 1, 'b' => 3}
p hash.key?(arg_hash.sort.to_s)

You may additionaly want to .join explicitly with a seperator, since
to_s provides none, so you could conceivably have two different arrays
convert to the same string.

After going through all this, you might want to extend the Hash class
and override hash() with these calls:

class HashWhichYouCanReliablyUseAsKeyInAHash < Hash
        def hash
                return sort.to_s
        end
end

And create your hash indexes by creating new instances of the new class.

-- 
Thomas 'Freaky' Hurst  -  freaky@aagh.net  -  http://www.aagh.net/
-
There's no point in being grown up if you can't be childish sometimes.
		-- Dr. Who