From: Martin DeMello <martindemello@...>
Date: 2006-01-26T23:48:23+09:00
Subject: Re: Hash with array as value type

Robert Klemme <bob.news@gmx.net> wrote:
> Michael McGreevy wrote:
> > Hello all,
> >
> > I would like to store arrays in a hash, indexed by a string key.  I
> > would like to have the hash create an empty array as the default
> > value, when it sees a new key, something like the code involving
> > "hash1" below. However, this code is giving some strange results --
> > it claims that the hash is empty, even though there is an array
> > stored in it, and I can then retrieve that array.
> >
> > I am new to ruby, so maybe I am just doing something stupid (... I am
> > not sure about that "Hash.new( [] )" for example... )
> >
> > Can anyone explain these results?
> 
> Yes.  You ran into the typical Hash pitfal: the default value is the one
> returned if something is not found for the given key.  But it never
> changes the hash and there is just this single instance.  Consider this:

I find it instructive to think of the default as a #key_missing method,
analogous to #method_missing.

martin