Ceaser cypher exercise on Exercism


I am trying to do the typical Ceaser cypher exercise in Elixir. The description of the exercise is as follows:

Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.

The Caesar cipher is a simple shift cipher that relies on transposing all the letters in the alphabet using an integer key between 0 and 26 . Using a key of 0 or 26 will always yield the same output due to modular arithmetic. The letter is shifted for as many values as the value of the key.

The general notation for rotational ciphers is ROT + <key> . The most commonly used rotational cipher is ROT13 .


To achieve this I made some research and came up with the following solution which doesn’t work:

defmodule RotationalCipher do
  @doc """
  Given a plaintext and amount to shift by, return a rotated string.

  iex> RotationalCipher.rotate("Attack at dawn", 13)
  "Nggnpx ng qnja"
  @spec rotate(text :: String.t(), shift :: integer) :: String.t()
  def rotate(text, shift) do
    |> String.to_charlist()
    |> Enum.map( fn char -> char < 97 || 97 + rem( char - 71 - shift, 26 ) end )
    |> to_string()

For example, if I call RotationalCipher.rotate("a", 1) instead of b, I get z.

Now since the objective here is to work with strings, I am fairly confident I am not converting the string correctly to it’s bitstring equivalent ( https://elixir-lang.org/getting-started/binaries-strings-and-char-lists.html ).

I have read the documentation and searched for several functions but I am clearly missing something.


Which bitstring function should I be using ?

I think you have bitstrings and charlists confused. Bitstrings look at binary data at the bit and byte level. What you really want to deal with are charlists, which you are.

I think your bug is that you should add your shift value instead of subtracting it.


Ups, you’re correct. A miss calculation from my side :stuck_out_tongue:
Thanks for noticing !

FYI: I had some fun golfing this problem a few years ago https://gist.github.com/gvaughn/b295e69b4eb302a4fab5


Just out of curiosity, were you the winner of this challenge ?

I took some inspiration from their top solutions, though my solution is still not working xD

No I was not the winner. I ended up in 2nd place. But there was no prize involved so it was very casual and we were tweeting our solutions. The one in first place was basically a combination of my approach with that of one other person.

Here’s the link to a binary pattern matching approach: How to improve this code?