Ruby - mapping an array to hashmap

Note that since Ruby 2.1.0 you can also use Array#to_h, like this:

[1,2,3,4].map{ |x| [x, f(x)] }.to_h

Ruby 2.6.0 enables passing a block to the to_h-method. This enables an even shorter syntax for creating a hash from an array:

[1, 2, 3, 4].to_h { |x| [x, f(x)] }