Rotating letters in a string so that each letter is shifted to another letter by n places

Question

I have been tasked with coming up with a way of encoding a string. Among other things, I need to shift each letter by a given number but the transformed letter must be a let

Answer 1

Arrays have a rotate method:

def play_pass(str,n)
  abc = ("a".."z").to_a.join
  abc_rot = abc.chars.rotate(n).join
  str.tr(abc, abc_rot)
end

p play_pass("abcdefghijklmnopqrstuvwxyz", 2)
# => "cdefghijklmnopqrstuvwxyzab"

A negative n rotates the other way.