From: Glenn Jackman <glennj@...>
Date: 2009-09-11T08:30:07+09:00
Subject: populating a hash from an array using inject

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"=>
      [#<struct Product name="Golden Delicious", category="Apple">,
       #<struct Product name="Granny Smith", category="Apple">]}
    {}
    {"Orange"=>[#<struct Product name="Navel", category="Orange">],
     "Apple"=>
      [#<struct Product name="Golden Delicious", category="Apple">,
       #<struct Product name="Granny Smith", category="Apple">]}

My question: why is bar empty?

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