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From: Glenn Jackman on 11 Sep 2009 15:22
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
From: 7stud -- on 11 Sep 2009 17:25
Glenn Jackman wrote:
> At 2009-09-11 02:07PM, "7stud --" wrote:
>>
>> 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]

...and this is a lie too:

> If you access a non-existent key, say h["A"], then that line is
> equivalent to

> baz = products.inject(Hash.new([])) {|h,p| h["A"] = [] + [p]; h}

You asked for lies. I gave them to you.
--
Posted via http://www.ruby-forum.com/.

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