From: 7stud -- <bbxx789_05ss@...>
Date: 2009-09-12T03:07:10+09:00
Subject: Re: populating a hash from an array using inject

Glenn Jackman wrote:
> At 2009-09-10 09:16PM, "7stud --" wrote:
>> > 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.  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.

The second line is equivalent to:

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

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}

or

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

which explicitly sets a key in the hash to the value [p], thereby 
altering the hash.
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