From: Robert Klemme <shortcutter@...>
Date: 2009-03-02T00:53:58+09:00
Subject: Re: Newbie question > Hash workings

On 01.03.2009 12:35, AD60 wrote:
> I see the use with an integer default, I specifically meant the
> Hash.new([]) syntax as problematic, but it probably all depends on
> knowing what your doing.

Ah.  Since you talked about "idiom" I thought you were referring to the 
general pattern.

> Just to detail the difference:
> 
> 
> hav = Hash.new { |h,k| h[k] = []}
> hav[0] += [1]

Actually this is very inefficient.  With this idiom you should rather do

hav[0] << [1]

because otherwise you'll end up creating and deleting Array instances 
all the time without any benefit.

> p hav               # {0=>[1]}
> p hav[1]            # []
> p hav               # {0=>[1], 1=>[]}
> 
> hiv = Hash.new 0
> hiv[0] += 1
> p hiv               # {0=>1}
> p hiv[1]            # 0
> p hiv               # {0=>1}
> 
> it's the difference between "{0=>[1], 1=>[]}" and "{0=>1}" result that
> had me wondering. Anyway thanks again, I understand how to use it and
> that's more than enough for now.

Good.

Kind regards

	robert