Ruby - mapping an array to hashmap

Solution 1:

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)] }

You could also define the function as the hash's default value:

hash = Hash.new {|hash, key| hash[key] = f(key) }

Then when you lookup a value, the hash will calculate and store it on the fly.

hash[10]
hash.inspect #=> { 10 => whatever_the_result_is }

You need each_with_object.

def f x
  x * 2
end

t = [1, 2, 3, 4].each_with_object({}) do |x, memo|
  memo[x] = f(x)
end

t # => {1=>2, 2=>4, 3=>6, 4=>8}

Another one:

t2 = [1, 2, 3, 4].map{|x| [x, f(x)]}
Hash[t2] # => {1=>2, 2=>4, 3=>6, 4=>8}