Caesar Cipher (How do I convert for into .map)

Question

It's a Caesar cipher program, I have written this code by myself and want to convert this for loop into .map JavaScript built-in function, I have tried so many times but can't figure out.

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Map

There are many questions on .map on this website but that doesn't work for me.

function rot13(str) {
   var chArray = [];
   str = str.split("");
   
   for(var i in str){
    var char = str[i].charCodeAt(0);
     if(/[A-M]/g.test(str[i])){
       chArray.push(char + 13);
     }
     else if(/[N-Z]/g.test(str[i])){
       chArray.push(char - 13);
     }
     else chArray.push(char);
   }
   str = String.fromCharCode.apply(String,chArray);
  return str;
}

rot13("SERR PBQR PNZC");

score 1 · Answer 1 · answered Aug 04 '17 at 20:46

Below is a simple adaptation of your code to use map (used a variable rotation to avoid duplicated code):

function rot13(str) {
   str = str.split("");

   var encryptedChArray = str.map(char => {
     var rotation = 0;

     if(/[A-M]/g.test(char)){
       rotation = 13;
     }
     else if(/[N-Z]/g.test(char)) {
       rotation = -13;
     }

     return char.charCodeAt(0) + rotation;
   });

   return String.fromCharCode.apply(String, encryptedChArray);
}

console.log(rot13("SERR PBQR PNZC"));

score 1 · Answer 2 · answered Aug 04 '17 at 20:47

You can split the string into an array of characters, and then map over it, performing the transformations, and then at the end, join the array of transformed characters back to a string.

function rot13(str) {

    // get the char code for the character
    const charCode = str.charCodeAt(0);
    
    if(/[A-M]/g.test(str)){
        return String.fromCharCode(charCode + 13);
    }
    else if(/[N-Z]/g.test(str)){
        return String.fromCharCode(charCode - 13);
    }
    else {
        return String.fromCharCode(charCode);
    }
}

// first split the string into an array of single characters
const newString = "SERR PBQR PNZC".split("").map(rot13).join('');

console.log(newString);

score 0 · Answer 3 · 2017-08-04T20:51:46.873

Another option would be to use array.map here.

The mapping method takes in a callback, and that callback takes as parameter each of the elements in the array.

Example:

var arr = [1,2,3,4,5];
arr.map(someFunc);

var someFunc = function(currentItem){
     console.log(currentItem);
     //This will output 1
     //2
     //...
}

Therefore you can define you cypher transformation inside of the someFunc that you pass to map.

Suppose you want to encode the following string

var secretMessage = "hello"
msgArray = secretMessage.split("");

msgArray.map(cypher);

var cypher = function(i){ 
     ///doStuff
}

score 0 · Accepted Answer · answered Aug 04 '17 at 20:49

0

If you want to do this very compactly, you can do the following:

const A = 'A'.charCodeAt(0)
const M = 'M'.charCodeAt(0)
const N = 'N'.charCodeAt(0)
const Z = 'Z'.charCodeAt(0)
function rot13(str) {
   var chArray = [];
   return String.fromCharCode(...(
     str.split('')
       .map(c => c.charCodeAt(0))
       .map(c =>
         (A <= c && c <= M) ? c + 13 :
         (N <= c && c <= Z) ? c - 13 :
         c
       )
   ))
}

console.log(rot13("SERR PBQR PNZC"))

answered Aug 04 '17 at 20:49

csander

1,385
10
11

This is somewhat clever, but I would argue it's less clear what the code is doing (for maintenance) and more importantly is a lot less performant for larger strings (since you're iterating over the character set twice where once would do) – Paul Aug 04 '17 at 21:07

Caesar Cipher (How do I convert for into .map)

4 Answers4