From: Rick DeNatale <rick.denatale@...>
Date: 2009-09-11T21:21:26+09:00
Subject: Re: populating a hash from an array using inject

On Thu, Sep 10, 2009 at 8:42 PM, 7stud -- <bbxx789_05ss@yahoo.com> wrote:
> Glenn Jackman wrote:
>> I was looking at this problem on Stack Overflow (this one:
>> http://stackoverflow.com/questions/1405657/reorganizing-ruby-array-into-hash
>>
>> The question is how to "convert" an array of objects into a hash.
>>
>> Consider this code:
>>
>>     require 'pp'
>>     p RUBY_VERSION
>>
>>     Product = Struct.new(:name, :category)
>>     products = [
>>         ['Apple','Golden Delicious'],
>>         ['Apple','Granny Smith'],
>>         ['Orange','Navel']
>>     ].collect {|cat, name| Product.new(name, cat)}
>>
>>     foo = products.inject({}) {|h,p| h[p.category] ||= []; h[p.category]
>> << p; h}
>>     pp foo
>>
>>     bar = products.inject(Hash.new([])) {|h,p| h[p.category] << p; h}
>>     pp bar
>>
>>     baz = products.inject(Hash.new([])) {|h,p| h[p.category] += p; h}
>>     pp baz
>>
>> It outputs:
>>
>>     "1.8.7"
>>     {"Orange"=>[#<struct Product name="Navel", category="Orange">],
>>      "Apple"=>
>
> Even though it is not very well written, if you read the documentation
> on creating hashes with default values:
>
> $ ri Hash.new
>
> -------------------------------------------------------------- Hash::new
>     Hash.new                          => hash
>     Hash.new(obj)                     => aHash
>     Hash.new {|hash, key| block }     => aHash
> ------------------------------------------------------------------------
>     Returns a new, empty hash. If this hash is subsequently accessed by
>     a key that doesn't correspond to a hash entry, the value returned
>     depends on the style of +new+ used to create the hash. In the first
>     form, the access returns +nil+. If _obj_ is specified, this single
>     object will be used for all _default values_. If a block is
>     specified, it will be called with the hash object and the key, and
>     should return the default value. It is the block's responsibility
>     to store the value in the hash if required.
>
>        h = Hash.new("Go Fish")
>        h["a"] = 100
>        h["b"] = 200
>        h["a"]           #=> 100
>        h["c"]           #=> "Go Fish"
>        # The following alters the single default object
>        h["c"].upcase!   #=> "GO FISH"
>        h["d"]           #=> "GO FISH"
>        h.keys           #=> ["a", "b"]
> -----------------------------------------------------------
>
>
> The key line is:
>
> ***8It is the block's responsibility to store the value in the hash if
> required.***
>
> Because your block does not assign the array to a hash key, the array is
> discarded.


The thread has moved on a bit, but.

He actually isn't USING the block form of Hash.new, the only blocks
are arguments to inject.

And Hash.new {|h,k| ... } is what I tend to reach for in a situation like this.

   foo = products.inject(Hash.new {|h,k| h[k] = []}) {|h,p|
h[p.category] << p; h}


-- 
Rick DeNatale

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