Hash invert in Ruby?

Question

I\'ve got a hash of the format:

{key1 => [a, b, c], key2 => [d, e, f]}

and I want to end up with:

{ a => key1, b =         


        

Answer 1

Lots of other good answers. Just wanted to toss this one in too for Ruby 2.0 and 1.9.3:

hash = {apple: [1, 14], orange: [7, 12, 8, 13]}

Hash[hash.flat_map{ |k, v| v.map{ |i| [i, k] } }]
# => {1=>:apple, 14=>:apple, 7=>:orange, 12=>:orange, 8=>:orange, 13=>:orange}

This is leveraging: Hash::[] and Enumerable#flat_map

Also in these new versions there is Enumerable::each_with_object which is very similar to Enumerable::inject/Enumerable::reduce:

hash.each_with_object(Hash.new){ |(k, v), inverse|
  v.each{ |e| inverse[e] = k }
}

Performing a quick benchmark (Ruby 2.0.0p0; 2012 Macbook Air) using an original hash with 100 keys, each with 100 distinct values:

Hash::[] w/ Enumerable#flat_map
            155.7 (±9.0%) i/s -        780 in   5.066286s
Enumerable#each_with_object w/ Enumerable#each
            199.7 (±21.0%) i/s -        940 in   5.068926s

Shows that the each_with_object variant is faster for that data set.

Answer 2

One way to achieve what you're looking for:

arr = [{["k1"] => ["a", "b", "c"]}, {["k2"] => ["d", "e", "f"]}]

results_arr = []
arr.each do |hsh|
  hsh.values.flatten.each do |val|
    results_arr << { [val] => hsh.keys.first }···
  end
end


Result: [{["a"]=>["k1"]}, {["b"]=>["k1"]}, {["c"]=>["k1"]}, {["d"]=>["k2"]}, {["e"]=>["k2"]}, {["f"]=>["k2"]}]