From: Rob Biedenharn <Rob@...>
Date: 2007-06-29T09:16:10+09:00
Subject: Re: newby non/programmer trying to understand classes

On Jun 28, 2007, at 8:05 PM, Igor Sutton Lopes wrote:
> On Jun 28, 2007, at 8:53 PM, Rob Biedenharn wrote:
>
>> # create a Hash where referencing a non-existent
>> # key makes its value an empty Array
>> names = Hash.new { |h,k| h[k] = [] }
>
> Can I assume this is equivalent to perl's autovivification?

Basically.  If you just had:

   names = Hash.new

then referencing non-existent keys gives nil, but they still do not  
exist.

   names['joe'] << 'hello'

would give an error trying to call nil.<<('hello')

By assigning to h[k], that key is created in the 'names' hash with  
the indicated value (an empty Array in this case).

It's about the same as:

  names = Hash.new    # or just names = {}
  (names['joe'] ||= []) << 'hello'

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com