From: Jean-Hugues ROBERT <jean_hugues_robert@...>
Date: 2004-04-26T19:23:40+09:00
Subject: Re: IdentityHash, like Hash but with keys using equals? vs eql?

At 10:01 25/04/2004 +0900, you wrote:
>On Thu, 8 Apr 2004 14:00:26 +0900, Jean-Hugues ROBERT wrote:
> > At 07:44 08/04/2004 +0900, you wrote:
> >> Jean-Hugues ROBERT <jean_hugues_robert@yahoo.com> writes:
> >>> BTW: I need that to detect "true" circular paths when walking a
> >>> tree/graph. Rigth now I detect false circulars when nodes' values
> >>> compare equal where in fact it is two different node objects (with
> >>> the same "value").
> >> Can you just hash the ids?
> >> hash[obj.id] = blah
> > Yes !
> > So: an IdentityHash in ruby is just a Hash, but
> > with obj.object_id() in it vs the obj itself.
> > Makes a lot of sense.
> > Thanks George !
> >
> > Jean-Hugues
> > PS: As one said, in Computer Science everything is solvable by adding one
> > indirection level.
>
>class IdentityHash < Hash
>   def [](k)
>     super k.object_id
>   end
>
>   def []=(k, v)
>     super k.object_id, v
>   end
>end
>
>Note that this will, of necessity, prevent this from doing anything
>meaningful:
>
>   i = IdentityHash.new
>   i["abc"] = "def"
>   puts i["abc"] # => nil
>
>-austin

Many thanks austin.
a = "abc"
i = IdentityHash.new
i[a] = "def"
puts i[a] # => "def" !

That is exactly what an IdentityHash should do.

In my case, I use it more like this

visited = IdentityHash.new
def walk( graph, &block )
   for node in graph do
     next if visited[node] # Avoid circular path
     visited[node] = true
     block.yield node
     walk( node, &block) if node.respond_to: each
   end
end

Thanks again Austin.

Jean-Hugues


>--
>austin ziegler    * austin@halostatue.ca * Toronto, ON, Canada
>software designer * pragmatic programmer * 2004.04.24
>                                          * 20.57.28

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