For this blog post I’ve decided to demonstrate some cool tricks I’ve either used or seen online regarding hash default values. In all cases a default value is defined by either passing an argument (example 1) or code block (examples 2 and 3) to the Hash contructor. As you’ll see, changing the default value to something other than nil can provide for some very interesting and powerful tools.
1. Create histograms
words_histogram = Hash.new(0)
=> {}
sentence = "How much wood could a woodchuck chuck if a woodchuck could chuck wood?"
=> "How much wood could a woodchuck chuck if a woodchuck could chuck wood?"
words = sentence.gsub(/[,.?!]/,"").split(" ").each do |word|
words_histogram[word.downcase] += 1
end
=> ["How", "much", "wood", "could", "a", "woodchuck", "chuck", "if", "a", "woodchuck", "could", "chuck", "wood"]
words_histogram
=> {"how"=>1, "much"=>1, "wood"=>2, "could"=>2, "a"=>2, "woodchuck"=>2, "chuck"=>2, "if"=>1}
For reference some one-liners that count duplicate array elements are shown below.
(1) histogram = array.uniq.map{|elem| [elem, array.count(elem)]}.to_h
(2) histogram = array.each_with_object(Hash.new(0)){|elem,h| h[elem] += 1}
2. Generate lookup tables
square_roots = Hash.new do |h,k|
h[k] = Math.sqrt(k)
end
=> {}
square_roots[2]
=> 1.4142135623730951
square_roots[4]
=> 2.0
square_roots[9]
=> 3.0
square_roots
=> {2=>1.4142135623730951, 4=>2.0, 9=>3.0}
3. Cache recursive functions
factorial = Hash.new do |h,k|
k > 1 ? h[k] = h[k-1] * k : h[k] = 1
end
=> {}
factorial[6]
=> 720
factorial
=> {1=>1, 2=>2, 3=>6, 4=>24, 5=>120, 6=>720}