Ruby. From Array to Hash

array_to_hash.rb

# Given an array
arr = [1, 2, 3]

# I want to build an hash starting from an array
# like { 1 => 1, 2 => 4, 3 => 9} (where the values are the square of keys)

# long form:
hash = {}
arr.each do |e|
  hash[e] = e**2
end


# compact form:
hash = Hash[arr.map{|e| [e, e**2]}]

# Pay attention to brackets!
# Hash[[1,1],[2,4],[3,9]]    # <= wrong!
# Hash[[[1,1],[2,4],[3,9]]] # <= correct