From: Mike Fletcher <lemurific+rforum@...>
Date: 2006-01-26T22:19:53+09:00
Subject: Re: Hash with array as value type

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?
> 
> Thanks.
> 
> CODE:
> 
> #!/usr/bin/ruby
> 
> 
> hash1 = Hash.new( [] )

This makes a new Hash with the default value being the Array reference 
you've passed it.  This is only executed *once*, so each new key/value 
pair points to the *same* Array instance.  You want

hash1 = Hash.new { |h,k| Array.new }

That'll make a new Array instance each time it needs a new default value 
for a new key.

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