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Posted on Jul 24, 2019

Daily Challenge #23 - Morse Code Decoder

#challenge

Using the power of programming, we've translated Pig Latin and told humans the time. Now for this challenge, you have to write a simple Morse code decoder. Today's challenge comes from user jolaf on CodeWars

Your task is to implement a function that would take the Morse code as input and return a decoded human-readable string.

For example:
decodeMorse('.... . -.-- .--- ..- -.. .')
should return "HEY JUDE"

C++/Go/JavaScript/PHP/Python/Ruby/TypeScript:MORSE_CODE['.--']
C#:MorseCode.Get(".--") (returns string)
Elixir:morse_codes variable
Elm:MorseCodes.get : Dict String String
Haskell:morseCodes ! ".--" (Codes are in a Map String String)
Java:MorseCode.get(".--")
Kotlin:MorseCode[".--"] ?: "" or MorseCode.getOrDefault(".--", "")
Rust:self.morse_code
Scala: morseCodes(".--")

NOTE: For coding purposes, you should use ASCII characters . and -, not Unicode characters.

I always thought it would be really cool to make a full-size Morse code decoder, taking audio as input and reading aloud the output. Perhaps I could start building it as a side project - it would give me an excuse to buy another Raspberry Pi.

Good luck, happy coding!

Thank you to CodeWars, who has licensed redistribution of this challenge under the 2-Clause BSD License!

Want to propose a challenge for a future post? Email yo+challenge@dev.to with your suggestions!

Top comments (15)

La blatte • Jul 24 '19 • Edited

JS does not have any builtin morse library, therefore we need to build it in order to use a simple oneliner:

m = {
  '.-': 'A',
  '-...': 'B',
  '-.-.': 'C',
  '-..': 'D',
  '.': 'E',
  '..-.': 'F',
  '--.': 'G',
  '....': 'H',
  '..': 'I',
  '.---': 'J',
  '-.-': 'K',
  '.-..': 'L',
  '--': 'M',
  '-.': 'N',
  '---': 'O',
  '.--.': 'P',
  '--.-': 'Q',
  '.-.': 'R',
  '...': 'S',
  '-': 'T',
  '..-': 'U',
  '...-': 'V',
  '.--': 'W',
  '-..-': 'X',
  '-.--': 'Y',
  '--..': 'Z',
  '-----': '0',
  '.----': 1,
  '..---': 2,
  '...--': 3,
  '....-': 4,
  '.....': 5,
  '-....': 6,
  '--...': 7,
  '---..': 8,
  '----.': 9
};
decodeMorse=(s)=>s.split(' ').map(e=>m[e]||'?').join('')

And the result is:

decodeMorse('.... . -.-- .--- ..- -.. .')
"HEYJUDE"

// Unknown values are handled
decodeMorse('...... . -.-- .--- ..- -.. .')
"?EYJUDE"

// And it doesn't affect "falsey" values
decodeMorse('...... ----- -.-- .--- ..- -.. .')
"?0YJUDE"

And here goes the encoder, using the same m variable as above

encodeMorse=(s)=>Object.entries(m).map(e=>s=s.replace(RegExp(e[1],'gi'),' '+e[0]))&&s.slice(1)

The encoder produces the following result:

encodeMorse('HEYJUDE')
".... . -.-- .--- ..- -.. ."

encodeMorse('heyjude')
".... . -.-- .--- ..- -.. ."

Corey Alexander • Jul 25 '19

Rust Solution!

Decided to make functions that can go in both directions! So we can both encode and decode worse code!

It's gonna just panic on invalid input at the moment, but it should probably return a Result type but I was feeling lazy in this department

#[macro_use]
extern crate lazy_static;

use std::collections::HashMap;

lazy_static! {
    static ref MORSE_TO_CHAR: HashMap<&'static str, char> = {
        let mut m = HashMap::new();
        m.insert(".-", 'A');
        m.insert("-...", 'B');
        m.insert("-.-.", 'C');
        m.insert("-..", 'D');
        m.insert(".", 'E');
        m.insert("..-.", 'F');
        m.insert("--.", 'G');
        m.insert("....", 'H');
        m.insert("..", 'I');
        m.insert(".---", 'J');
        m.insert("-.-", 'K');
        m.insert(".-..", 'L');
        m.insert("--", 'M');
        m.insert("-.", 'N');
        m.insert("---", 'O');
        m.insert(".--.", 'P');
        m.insert("--.-", 'Q');
        m.insert(".-.", 'R');
        m.insert("...", 'S');
        m.insert("-", 'T');
        m.insert("..-", 'U');
        m.insert("...-", 'V');
        m.insert(".--", 'W');
        m.insert("-..-", 'X');
        m.insert("-.--", 'Y');
        m.insert("--..", 'Z');
        m.insert("-----", '0');
        m.insert(".----", '1');
        m.insert("..---", '2');
        m.insert("...--", '3');
        m.insert("....-", '4');
        m.insert(".....", '5');
        m.insert("-....", '6');
        m.insert("--...", '7');
        m.insert("---..", '8');
        m.insert("----.", '9');
        m
    };
}

lazy_static! {
    static ref CHAR_TO_MORSE: HashMap<char, &'static str> = {
        let mut m = HashMap::new();

        for (morse, c) in MORSE_TO_CHAR.iter() {
            m.insert(*c, *morse);
        }

        m
    };
}

pub fn encode_morse(morse_code: &str) -> String {
    morse_code
        .chars()
        .map(|c| {
            CHAR_TO_MORSE
                .get(&c)
                .expect("Invalid character for morse code")
                .to_string()
        })
        .collect::<Vec<String>>()
        .join(" ")
}

pub fn decode_morse(morse_code: &str) -> String {
    morse_code
        .split_whitespace()
        .map(|word| {
            MORSE_TO_CHAR
                .get(word)
                .expect("Invalid morse code")
                .to_string()
        })
        .collect::<Vec<String>>()
        .join("")
}

#[cfg(test)]
mod tests {
    use crate::*;

    #[test]
    fn it_works_to_decode_example() {
        assert_eq!(
            decode_morse(".... . -.-- .--- ..- -.. ."),
            "HEYJUDE".to_string()
        );
    }

    #[test]
    fn it_works_to_encode_example() {
        assert_eq!(
            encode_morse("HEYJUDE"),
            ".... . -.-- .--- ..- -.. .".to_string()
        );
    }

    #[test]
    fn it_is_reversible() {
        let input = "SOMETESTSTRING";

        assert_eq!(decode_morse(&encode_morse(input)), input.to_string());
    }
}

Corey Alexander • Jul 25 '19

Meta: @thepracticaldev Might want to remove the
Morse code tables are already preloaded for many languages as a dictionary, feel free to use them:
bit since I think that only applies to the CodeWars solution runner, so isn't as relevant here!

Cause for instance Rust definitely does NOT just have a morse code map available at self.morse_code

Michael Tharrington • Jul 25 '19

Great point! Going to make the fix here shortly. Thanks for the heads up!

E. Choroba • Jul 26 '19 • Edited

Solved several years ago in a golf challenge. Perl (newlines added for readability):

@m{tc,cwt,ctct,cw,t,wct,et,ww,w,tec,ctc,tcw,e,ct,ec,tet,etc,
tct,wt,c,wc,wtc,te,cwc,cte,ew,eec,tee,wec,wte,wwc,wwt,cww,
ewt,ecw,eet}=(A..Z,0..9);s/- ?/c/g,s/\. ?/t/g,s/tt/w/g,
s/cc/e/g,s/(\S+) ? ?/$m{$1}/g,y/ //s,print for@ARGV

Dylan Paulus • Jul 25 '19

Similar solution. Nim.

import strutils, sequtils, sugar, tables

const morseCodeMap = {
  ".-": "A",
  "-.": "N",
  "-...": "B",
  "---": "O",
  "-.-.": "C",
  ".--.": "P",
  "-..": "D",
  "--.-": "Q",
  ".": "E",
  ".-.": "R",
  "..-.": "F",
  "...": "S",
  "--.": "G",
  "-": "T",
  "....": "H",
  "..-": "U",
  "..": "I",
  "...-": "V",
  ".---": "J",
  ".--": "W",
  "-.-": "K",
  "-..-": "X",
  ".-..": "L",
  "-.--": "Y",
  "--": "M",
  "--..": "Z",
  ".----": "1",
  "-....": "6",
  "..---": "2",
  "--...": "7",
  "...--": "3",
  "---..": "8",
  "....-": "4",
  "----.": "9",
  ".....": "5",
  "-----": "0",
}.toTable()

proc morseCodes(input: string): string =
  return input.splitWhitespace().map((x) => morseCodeMap.getOrDefault(x,
      "")).join("")

when isMainModule:
  const code = ".--. . . .--."

  echo morseCodes(code)

Praneet Nadkar • Jul 25 '19

Its a one liner in C# once you have the dictionary set up:

var output = input.Select(c => { c = dictionary[c]; return c; }).ToList();

Complete code:

            var dictionary = new Dictionary<string, string>
            {
                { ".-", "A" },
                { "-...", "B"},
                { "-.-.", "C"},
                { "-..", "D"},
                { ".", "E"},
                { "..-.", "F"},
                { "--.", "G"},
                { "....", "H"},
                { "..", "I"},
                { ".---", "J"},
                { "-.-", "K"},
                { ".-..", "L"},
                { "--", "M"},
                { "-.", "N"},
                { "---", "O"},
                { ".--.", "P"},
                { "--.-", "Q"},
                { ".-.", "R"},
                { "...", "S"},
                { "-", "T"},
                { "..-", "U"},
                { "...-", "V"},
                { ".--", "W"},
                { "-..-", "X"},
                { "-.--", "Y"},
                { "--..", "Z"},
                { "-----", "0"},
                { ".----", "1"},
                { "..---", "2"},
                { "...--", "3"},
                { "....-", "4"},
                { ".....", "5"},
                { "-....", "6"},
                { "--...", "7"},
                { "---..", "8"},
                { "----.", "9"}
            };

            var input = Console.ReadLine().Split(' ').ToList();

            var output = input.Select(c => { c = dictionary[c]; return c; }).ToList();

            Console.WriteLine(string.Join("", output));
            Console.ReadKey();

Oleksii Filonenko • Jul 25 '19

Obligatory Elixir:

defmodule Morse do
  @morse_to_char %{
    "-----" => "0",
    ".----" => "1",
    "..---" => "2",
    "...--" => "3",
    "....-" => "4",
    "....." => "5",
    "-...." => "6",
    "--..." => "7",
    "---.." => "8",
    "----." => "9",
    ".-" => "A",
    "-..." => "B",
    "-.-." => "C",
    "-.." => "D",
    "." => "E",
    "..-." => "F",
    "--." => "G",
    "...." => "H",
    ".." => "I",
    ".---" => "J",
    "-.-" => "K",
    ".-.." => "L",
    "--" => "M",
    "-." => "N",
    "---" => "O",
    ".--." => "P",
    "--.-" => "Q",
    ".-." => "R",
    "..." => "S",
    "-" => "T",
    "..-" => "U",
    "...-" => "V",
    ".--" => "W",
    "-..-" => "X",
    "-.--" => "Y",
    "--.." => "Z"
  }

  @spec decode(String.t()) :: String.t()
  def decode(morse) do
    morse
    |> String.split()
    |> Enum.map(&Map.get(@morse_to_char, &1, "?"))
    |> Enum.join()
  end
end

Alvaro Montoro • Jul 25 '19

Am I the only one having trouble with the specified preloaded dictionaries?

Corey Alexander • Jul 25 '19

I'm pretty sure this an aggressive copy pasta from the CodeWars site.

Code wars has a solution runner, and I think these are defined in their specific runner. Cause the Rust example is definitely not something that is just globally available in Rust lol

Here is the Codewars link if you want to try it out. I didn't give it a shot so no idea how well it works: codewars.com/kata/decode-the-morse...

Alvaro Montoro • Jul 25 '19

That's half an hour of my life I won't get back.
I should have thought about that sooner :-/

Michael Lang • Aug 11 '19

Ruby Language

The problem definition was initially tripping because it appeared to somehow render "HEY JUDE" without some sort of demarcation that indicated a space between words since HTML rendering removes extra spacing. However, opening and viewing the HTML source revealed there was indeed multiple spaces between the words. In this implementation, two or more spaces represents a "pause" which is the morse standard for recognizing word breaks.

Code with Specs

MORSE_CODE = Hash[*%w/
  A .-    B -...  C -.-.  D -..   E .     F ..-.
  G --.   H ....  I ..    J .---  K -.-   L .-..
  M --    N -.    O ---   P .--.  Q --.-  R .-.
  S ...   T -     U ..-   V ...-  W .--   X -..-
  Y -.--  Z --..  1 .---- 2 ..--- 3 ...-- 4 ....-
  5 ..... 6 -.... 7 --... 8 ---.. 9 ----. 0 -----
/].invert.freeze

def decode_morse str
  words = str.to_s.split(/\s{2,}/)
  words.map{|w| w.split(" ").map{|mc| MORSE_CODE[mc]}.join}.join " "
end

require "spec"

describe "#decode_morse" do
  it { expect(decode_morse nil).to eq "" }
  it { expect(decode_morse "").to eq "" }
  it { expect(decode_morse ".").to eq "E" }
  it { expect(decode_morse ". ").to eq "E" }
  it { expect(decode_morse "... --- ...").to eq "SOS" }
  it { expect(decode_morse "- . ... - .. -. --.  .----  ..---  ...--").to eq "TESTING 1 2 3" }
  it { expect(decode_morse ".... . -.-- .--- ..- -.. .").to eq "HEYJUDE" }
  it { expect(decode_morse ".... . -.--   .--- ..- -.. .").to eq "HEY JUDE" }
  it { expect(decode_morse "-- --- .-. ... .    -.-. --- -.. .").to eq "MORSE CODE" }
end

output

>> rspec morse_code.rb
.........

Finished in 0.00551 seconds (files took 0.15391 seconds to load)
9 examples, 0 failures

Mat-R-Such • Apr 5 '20 • Edited

Python solution:

m = {
  '.-': 'A', '-...': 'B', '-.-.': 'C', '-..': 'D', '.': 'E',
  '..-.': 'F', '--.': 'G', '....': 'H', '..': 'I','.---': 'J',
  '-.-': 'K', '.-..': 'L', '--': 'M', '-.': 'N', '---': 'O',
  '.--.': 'P', '--.-': 'Q', '.-.': 'R', '...': 'S', '-': 'T',
  '..-': 'U', '...-': 'V', '.--': 'W', '-..-': 'X', '-.--': 'Y',
  '--..': 'Z',
  '-----': '0', '.----': 1, '..---': 2, '...--': 3, '....-': 4,
  '.....': 5, '-....': 6, '--...': 7, '---..': 8, '----.': 9
}
def morse_code(a):
    a=a.split(' ')
    return ''.join(m[i]for i in a)

print(morse_code('.... . -.-- .--- ..- -.. .'))

View full discussion (15 comments)

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Daily Challenge #23 - Morse Code Decoder

Top comments (15)

Ruby Language

Code with Specs

output

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