From: Leslie Viljoen <leslieviljoen@...>
Date: 2008-02-23T00:19:35+09:00
Subject: Re: ||= [] idiom

On Fri, Feb 22, 2008 at 2:33 PM, Thufir <hawat.thufir@gmail.com> wrote:
> On Wed, 20 Feb 2008 07:21:15 +0900, MenTaLguY wrote:
>
>  > I sometimes do this sort of thing:
>  >
>  >  ( @categories[resource.tag] ||= [] ) << resource
>  >
>  > Although this might possibly be more readable:
>  >
>  >  category = ( @categories[resource.tag] ||= [] ) category << resource
>  >
>  > Often it can make sense to factor the ||= portion into a separate
>  > method:
>  >
>  >  def category_for(tag)
>  >    @categories[tag] ||= []
>  >  end
>  >
>  >  # ...
>  >
>  >    category_for(resource.tag) << resource
>  >
>  > -mental
>  >
>
>  What's the real-world application of this?



It's for making hashes of arrays:

class Article
	attr_accessor :name, :month

	def initialize(name, month)
		@name = name
		@month = month
	end
end

articles = [
	Article.new("Article1", "Feb"),
	Article.new("Article2", "Feb"),
	Article.new("Article3", "Feb"),
	Article.new("Article4", "Mar"),
	Article.new("Article5", "Mar"),
]

months = {}
articles.each do |a|
	months[a.month] ||= []
	months[a.month] << a.name
end

p months

>> {"Feb"=>["Article1", "Article2", "Article3"], "Mar"=>["Article4",
"Article5"]}

Now that is easy to iterate through and produce a tree:

Feb:
  Article1
  Article2
  Article3

Mar:
  Article4
  Article5


...
This sets an element for key "a.month" (the article's month) to an
empty array if it is currently set to nil:
months[a.month] ||= []

This adds "a.name" (the article's name) to the array that already
exists or has just been
created above:
months[a.month] << a.name




Les