Question

I’m working on a project where I need to associate newspaper articles with the page numbers they appear on in print.

My input data is just a bunch of pairs of article titles and page numbers. I came up with the following code to create a new Hash where the keys are page numbers and the values are arrays of the article titles:

a = ["A1", "title 1"]
b = ["A1", "title 2"]
c = ["A2", "title 3"]
hash = {}
articles = [a,b,c]
articles.each do |a|
  if hash.has_key?(a[0])
    hash[a[0]] << a[1]
  else
    hash.merge!({a[0] => [a[1]]})
  end
end

The code works well enough, but I’m wondering if there is a cleaner way of doing this. I checked the Ruby docs and couldn’t find any built in methods, but I’d like SO’s input on this.

Answer 1

Since Michael Kohl has been reminding of group_by lately:

articles = [
    ["A1", "title 1"],
    ["A1", "title 2"],
    ["A2", "title 3"]
]
page_to_titles = articles.group_by(&:first).each { |k,v| v.map!(&:last) }

Works the same in 1.9.2 and 1.8.7.