From: "David A. Black" <dblack@...>
Date: 2009-09-11T20:45:48+09:00
Subject: Re: populating a hash from an array using inject

Hi --

On Fri, 11 Sep 2009, Glenn Jackman wrote:

> At 2009-09-10 09:16PM, "7stud --" wrote:
>>  7stud -- wrote:
>>> h = Hash.new([])
>>>
>>> h["A"] =  h["A"] << 10  #==>[10]
>>>
>>> h["B"] = h["B"] << 20   #==>[20]
>>>
>>> Look what happens:
>>>
>>>
>>> p h
>>>
>>> {"A"=>[10, 20], "B"=>[10, 20]}
>>>
>>
>>  Yep, ruby hands you a reference to the same array over and over
>>  again...when you try to access non-existent keys.
>
> While I thank you for takking the time, I can't say I'm enlightened by
> your explanation.  What's the real difference between the blocks
>
> bar = products.inject(Hash.new([])) {|h,p| h[p.category] << p; h}
> baz = products.inject(Hash.new([])) {|h,p| h[p.category] += [p]; h}
>
> Is it because += explicitly assigns a new object to the hash key?

Yes. Here's another way to look at it:

   array = []
   hash = Hash.new(array)
   hash[:x] << 1           # equivalent to: array << 1
   hash[:y] << 2           # equivalent to: array << 2
   hash[:z] += [:a,:b,:c]  # equivalent to: h[:z] = array + [:a,:b,:c]


David

-- 
David A. Black / Ruby Power and Light, LLC / http://www.rubypal.com
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