From: Glenn Jackman <glennj@...>
Date: 2009-09-12T04:26:58+09:00
Subject: Re: populating a hash from an array using inject

At 2009-09-11 02:07PM, "7stud --" wrote:
>  Glenn Jackman wrote:
> > 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.  When the key doesn't exist the first line is equivalent to:
>  
>  bar = products.inject(Hash.new([])) {|h,p| [] << p; h}
>  
>  which does nothing to the hash--all it does is append p to an empty 
>  array, and then the empty array is discarded.

As we've seen, it's not discarded:  it's kept for h's reference:

    products = [[1,2],[3,4],[1,5]]
    foo = products.inject(Hash.new([])) {|h,(a,b)| h[a] << b; h} # => {}
    foo[:unknown] # => [2, 4, 5]

-- 
Glenn Jackman
    Write a wise saying and your name will live forever. -- Anonymous