From: Adam Gardner <adam.oddfellow@...>
Date: 2009-03-03T03:18:10+09:00
Subject: Re: Newbie question > Hash workings

Robert Klemme wrote:
> 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.
> 
>> 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

The method I would use is this:

hav = Hash.new {[]}

This gives you the benefit of creating a new array when necessary 
(rather than a single default array which can be destructively modified 
in troubling ways), but means that you only get permanent effects if you 
explicitly use assignment, so you don't get this

p hav               # {0=>[1]}
p hav[1]            # []
p hav               # {0=>[1], 1=>[]}

Of course, it means you can't use the << operator to stick values into 
non-present keys, but I rarely use the << operator anyway.

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