From: "Marcel Molina Jr." <marcel@...>
Date: 2005-06-26T01:34:04+09:00
Subject: Re: Hash hidden in hash with default object

On Sun, Jun 26, 2005 at 01:28:08AM +0900, Austin Ziegler wrote:
> 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] = []
>     }
>   }

Thanks for the reply. That's exactly what I came up with except I foolishly
did 

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

(difference is ||= [] vs = []) which causes SystemStackError.

Thanks again.

marcel
-- 
Marcel Molina Jr. <marcel@vernix.org>