cesar.rb

def encrypt(text)
  alphabet = ("A".."Z").to_a

  text.split(' ').map do |word|
    word.split('').map do |letter|
      position = alphabet.index(letter)
      new_position = position - 3
      alphabet[new_position]
    end.join('')
   end.join(' ')
end

p encrypt("THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG")
p encrypt("THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG") == "QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD"