From: Austin Ziegler <halostatue@...>
Date: 2005-06-26T01:28:08+09:00
Subject: Re: Hash hidden in hash with default object

On 6/25/05, Marcel Molina Jr. <marcel@vernix.org> wrote:
> I observed this odd behavior when setting a hash to have as its default
> value another hash which itself has a default value set:

> >> hash_in_hash = Hash.new(Hash.new([]))
> => {}
> >> hash_in_hash[:a][:b] << :incognito
> => [:incognito]
> >> hash_in_hash
> => {}
> >> hash_in_hash[:a]
> => {}
> >> hash_in_hash.keys
> => []
> >> hash_in_hash[:a][:b]
> => [:incognito]
> >> hash_in_hash[:a].keys
> => []

That's now how you want to do this in any case, because that doesn't
actually *set* the key. Even if it did, it's still probably not what
you want, because all of the secondary hashes will be referencing
the same array as their default value, e.g.:

  hash_in_hash[:a][:c] << :cognito
  => [:incognito, :cognito]

Instead, do:

  hash_in_hash = Hash.new { |h, k|
    h[k] = Hash.new { |h1, k1|
      h1[k1] = []
    }
  }

-austin
-- 
Austin Ziegler * halostatue@gmail.com
               * Alternate: austin@halostatue.ca