From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2011-11-18T02:52:47+09:00
Subject: Re: Problem on hash with default value

On Thu, Nov 17, 2011 at 6:45 PM, Florent G. <florent2@gmail.com> wrote:
> Hi,
>
> I face the following issue with a hash having an empty array as default
> value.
>
> I don't how to describe it in English, here is an illustrative code:
>
> hash = Hash.new Array.new
>
> hash[:a] = [1]
>
> hash[:b] << 2
>
> puts hash[:b]
> # => returns [2]
>
> puts hash.keys
> # => returns [:a], not [:a, :b] ?
>
> hash[:b] is set to [2] but I don't understand why hash.keys does not
> include :b.

The reason is that the Hash constructor you are using uses the object
you pass, in your case, Array.new, to return when you access a
non-existing key. But, it doesn't assign the array to the key. You
have to do it yourself. One way is to use the default proc:

h = Hash.new {|hash, k| hash[k] = Array.new}

This will create a new array *and* assign it to a key, whenever you
access a non-existing key:

ruby-1.8.7-p334 :001 > h = Hash.new {|hash, k| hash[k] = Array.new}
 => {}
ruby-1.8.7-p334 :002 > h[:a]
 => []
ruby-1.8.7-p334 :003 > h
 => {:a=>[]}

Also, keep in mind that the other constructor you were using will
return the same Array instance to all non-existing keys:

ruby-1.8.7-p334 :004 > h = Hash.new Array.new
 => {}
ruby-1.8.7-p334 :005 > h[:a]
 => []
ruby-1.8.7-p334 :006 > h[:a] << 2
 => [2]
ruby-1.8.7-p334 :007 > h[:a]
 => [2]
ruby-1.8.7-p334 :008 > h[:b]
 => [2]


Jesus.