Today’s problem - Caesar Salad

A Caesar cipher, also known as a shift cipher, is one of the simplest encryption techniques. In a Caesar cipher, each letter in the plaintext is replaced by a letter some fixed number of positions away in the alphabet, effectively “shifting” the alphabet.

Write a function that:

takes a single integer left argument representing the size of the shift
takes a character vector right argument representing the plaintext message
returns a result with the same shape as the right argument representing the encrypted message

Notes:

Use ' ',⎕A as the alphabet
You can assume that the plaintext message will contain only characters found in the alphabet.

Hint: The rotate function ⌽ could be helpful in solving this problem.

Examples

      4 caesar 'HELLO WORLDS'
LIPPSD SVPHW
    
      ¯4 caesar 'HELLO WORLDS' ⍝ negative shifts are okay
DAHHKWSKNH O 

      0 caesar 'HELLO WORLDS' ⍝ no shift is okay
HELLO WORLDS

      27 caesar 'HELLO WORLDS'
HELLO WORLDS

      ¯10 caesar '' ⍝ returns an empty vector

Solution

See the solution

 
{(⍺⌽' ',⎕A)[(' ',⎕A) ⍸⍵]}

Breaking it down:

  • 1. Define the alphabet: ' ',⎕A creates our alphabet consisting of a space followed by the uppercase letters A-Z.
  • 2. Find positions in the alphabet: (' ',⎕A) ⍸⍵ uses the index-of (⍸) function to find the positions of each character in the input string (⍵) within our alphabet.
  • 3. Create the shifted alphabet: ⍺⌽' ',⎕A rotates our alphabet by the shift amount (⍺). The rotate function (⌽) shifts the entire alphabet by the specified number of positions.
  • 4. Map to the new characters: Using the positions found in step 2, we index into the shifted alphabet: (⍺⌽' ',⎕A)[...] to get our encrypted message.