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

Your mistake is that you just blindly shift characters and don't use your letters array to wrap around.

def play_pass(str, n)
  letters = ('a'..'z').to_a

  str.chars.map do |x| 
    if letters.include?(x.downcase)
      idx = letters.index(x)
      new_idx = (idx + n) % letters.length
      letters[new_idx]
    else
      x
    end
  end.join

end

play_pass('ay1', 2) # => "ca1"

The key to success here is the modulo operator (%). We use to get an index of a substitution character from letters array. It guarantees that the index will always be within bounds of the array. Instead of going past the end, it wraps around.

Read up on it. It's useful in many places.