Caesar's cipher in Ruby

Question

I'm working on a Caesar's cipher problem with a left shift of 1 but having a difficulty getting started. The prompt asks me to look at the String method #ord and the Integer method #chr in the Ruby Documentation. And letter 'a' has to be shifted to 'z'...

Below is my work..

def solve_cipher(string, n)
letters = ['a'..'z']
string.map {|x| letters.include?(x.ord + n).chr : x}.join

Thanks for any advice...!

Answer 1

You need to split your word into the individual characters. Then you need to convert the characters to ints with ord so you can do arithmetic. The arithmetic operation is to add an offset relative to 'a', modulo 26, so that the result maps to a different offset from 'a' which will result in a new character. Change the result back to character with chr, and join the characters back together to form a string.

Here's one possible implementation which accommodates both upper and lower case letters:

def shift_char(c, base, offset)
  (((c.ord - base) + offset) % 26 + base).chr
end

def cipher(s, offset)
  s.chars.map do |c|
    case c
    when 'a'..'z'
      shift_char(c, 'a'.ord, offset)
    when 'A'..'Z'
      shift_char(c, 'A'.ord, offset)
    else
      c
    end
  end.join
end

cipher_text = cipher('Now is the time for all good men...', 13)
p cipher_text       # "Abj vf gur gvzr sbe nyy tbbq zra..."
original_text = cipher(cipher_text, 13)
p original_text     # "Now is the time for all good men..."