Caesar Cypher

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.9", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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Caesar Cypher

You’re going to create a Caesar Cypher. A cypher alters characters in order to make secret messages. The Caesar cypher is a substitution cypher named after the Roman emperor Julius Caesar.

A Caesar cypher shifts characters by some key. For example, if the key is 1 then each character shifts over by one, so "a" becomes "b", "b" becomes "c" and so on.

Create a CaesarCypher.encode/1 function which shifts each letter in a string by a key. The default key should be 1.

CaesarCypher.encode("abcdefghijklmnopqrstuvwxyz")
"bcdefghijklmnopqrstuvwxyza"

You only need to handle lowercase letters.

Any Key

Create a CaesarCypher.encode/2 function which encodes the string by a provided key. For example, if the key is 2 then each letter shifts over by two, so "a" becomes "c", "b" becomes "d" and so on.

CaesarCypher.encode("abcdefghijklmnopqrstuvwxyz", 2)
"cdefghijklmnopqrstuvwxyzab"

Example Solution

defmodule CaesarCypher do
 def encode(string) do
    encode(string, 1)
  end

  def encode(string, key) do
    String.to_charlist(string)
    |> Enum.map(fn char ->
      overflow = char + key > ?z
      if overflow do
        ?a + rem(char + key, ?z) - 1
      else
        char + key
      end
    end)
    |> List.to_string()
  end
end

Implement the CaesarCypher module as documented.

defmodule CaesarCypher do
  @moduledoc """
  Documentation for `CaesarCypher`
  """

  @doc """
  Encode a string using the caesar cypher.
  Shift every letter over by one.

  ## Examples

    iex> CaesarCypher.encode("abcdefghijklmnopqrstuvwxyz")
    "bcdefghijklmnopqrstuvwxyza"

  Encoding should work on any string

    iex> CaesarCypher.encode("hello")
    "ifmmp"
  """
  def encode(string), do: encode(string, 1)

  @doc """
  Encode a string using the caesar cypher and a provided key.
  Shift every letter over by the provided key.

  ## Examples

    iex> CaesarCypher.encode("abcdefghijklmnopqrstuvwxyz", 1)
    "bcdefghijklmnopqrstuvwxyza"

    iex> CaesarCypher.encode("abcdefghijklmnopqrstuvwxyz", 2)
    "cdefghijklmnopqrstuvwxyzab"

    iex> CaesarCypher.encode("abcdefghijklmnopqrstuvwxyz", 14)
    "opqrstuvwxyzabcdefghijklmn"

  Encoding should work on any string.

    iex> CaesarCypher.encode("hello", 1)
    "ifmmp"
  """
  def encode(string, key) do
    String.to_charlist(string)
    |> Enum.map(fn char -> shift_char(char, key) end)
    |> List.to_string()
  end

  defp shift_char(c, key) do
    if char + key > ?z do
      ?a + rem(char + key, ?z) - 1
    else
      char + key
    end
  end
end

Commit Your Progress

DockYard Academy now recommends you use the latest Release rather than forking or cloning our repository.

Run git status to ensure there are no undesirable changes. Then run the following in your command line from the curriculum folder to commit your progress.

$ git add .
$ git commit -m "finish Caesar Cypher exercise"
$ git push

We’re proud to offer our open-source curriculum free of charge for anyone to learn from at their own pace.

We also offer a paid course where you can learn from an instructor alongside a cohort of your peers. We will accept applications for the June-August 2023 cohort soon.

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