Ruby/Hash/assoc

h = { :a => 1, :b => 2}   # Start with a hash
a = h.to_a                # => [[:b,2], [:a,1]]: associative array
a.assoc(:a)               # => [:a,1]: subarray for key :a
a.assoc(:b).last          # => 2: value for key :b
a.rassoc(1)               # => [:a,1]: subarray for value 1
a.rassoc(2).first         # => :b: key for value 2
a.assoc(:c)               # => nil
a.transpose               # => [[:a, :b], [1, 2]]: swap rows and cols