Morse Code

April 28, 2009

There are dozens of ways to solve this problem. We choose an association list, which is simple and good enough. We first build a morse/character table; it is convenient to build the table twice, once in each direction:

(define morse-code '(
  (#\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 "-----")
  (".-"    #\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)
  (#\space "")))

Converting from characters to Morse code is done by looking up each character in the table. The output is built in reverse.

(define (morse str)
  (let loop ((cs (string->list str)) (result '()))
    (if (null? cs) (list->string (cdr (reverse result)))
      (loop (cdr cs) (append (reverse (string->list
            (cadr (assoc (char-upcase (car cs)) morse-code))))
            (list #\space) result)))))

Converting back from Morse code to characters is just the opposite of the previous function:

(define (un-morse str)
  (let loop ((cs (string->list str)) (ms '()) (result '()))
    (cond ((null? cs)
            (if (null? ms)
                (list->string (reverse result))
                (list->string (reverse (cons (cadr (assoc
                    (list->string (reverse ms)) morse-code)) result)))))
          ((char=? (car cs) #\space)
            (if (null? ms)
                (loop (cdr cs) ms (cons #\space result))
                (loop (cdr cs) '() (cons (cadr (assoc (list->string
                    (reverse ms)) morse-code)) result))))
          ((or (char=? (car cs) #\.) (char=? (car cs) #\-))
            (loop (cdr cs) (cons (car cs) ms) result)))))

An example is shown below, and can be run at http://programmingpraxis.codepad.org/7Gf5N1I2.

> (un-morse (morse "Programming Praxis"))
"PROGRAMMING PRAXIS"

Pages: 1 2

Posted by programmingpraxis
Filed in Exercises
11 Comments »

11 Responses to “Morse Code”

  1. Programming Praxis - Morse code « Bonsai Code said
    April 28, 2009 at 10:28 AM

    […] Praxis – Morse code By Remco Niemeijer Today’s Programming Praxis problem is about morse code. We’re supposed to write a program to convert […]

  2. Remco Niemeijer said
    April 28, 2009 at 10:31 AM

    My Haskell solution. For a version with comments, see http://bonsaicode.wordpress.com/2009/04/28/programming-praxis-morse-code/

    import Data.Char
import Data.List
import Data.List.Split
import qualified Data.Map as M

plain :: [Char]
plain = ['A'..'Z'] ++ ['0'..'9']

morse :: [String]
morse = words ".- -... -.-. -.. . ..-. --. .... .. .--- -.- .-.. -- \
              \-. --- .--. --.- .-. ... - ..- ...- .-- -..- -.-- --.. \
              \.---- ..--- ...-- ....- ..... -.... --... ---.. ----. -----"

convert :: Ord a => [a] -> [b] -> a -> b
convert from to x = M.fromList (zip from to) M.! x

toMorse :: String -> String
toMorse = intercalate "  " . map (unwords .
              map (convert plain morse . toUpper)) . words

fromMorse :: String -> String
fromMorse = unwords . map (map (convert morse plain) . words) . splitOn "  "

main :: IO ()
main = do print $ toMorse "Programming Praxis"
          print $ fromMorse ".--. .-. --- --. .-. .- -- -- .. -. --.  \
                            \.--. .-. .- -..- .. ..."
  3. Remco Niemeijer said
    April 28, 2009 at 10:36 AM

    Hm. Evidently this code causes the syntax highlighting plugin to be incredibly slow.

  4. J. V. Toups said
    April 30, 2009 at 3:45 PM

    Here is a factor implementation:
    I wish I could preview this comment before posting it but here goes:

    
        
            .NULL {
color: #000000;
}
.COMMENT1 {
color: #cc0000;
}
.COMMENT2 {
color: #ff8400;
}
.COMMENT3 {
color: #6600cc;
}
.COMMENT4 {
color: #cc6600;
}
.DIGIT {
color: #ff0000;
}
.FUNCTION {
color: #9966ff;
}
.INVALID {
background: #ffffcc;
color: #ff0066;
}
.KEYWORD1 {
color: #006699;
font-weight: bold;
}
.KEYWORD2 {
color: #009966;
font-weight: bold;
}
.KEYWORD3 {
color: #0099ff;
font-weight: bold;
}
.KEYWORD4 {
color: #66ccff;
font-weight: bold;
}
.LABEL {
color: #02b902;
}
.LITERAL1 {
color: #ff00cc;
}
.LITERAL2 {
color: #cc00cc;
}
.LITERAL3 {
color: #9900cc;
}
.LITERAL4 {
color: #6600cc;
}
.MARKUP {
color: #0000ff;
}
.OPERATOR {
color: #000000;
font-weight: bold;
}

            /home/toups/tinywork/praxis/morse.factor
        
        
            
    USING: ascii lists sequences regexp ;

: ch>morse ch>upper ( char -- morse-str )
         { { CHAR: 0 [ "— — — — —" ] }
           { CHAR: 1 [ "• — — — —" ] }
           { CHAR: 2 [ "• • — — —" ] }
           { CHAR: 3 [ "• • • — —" ] }
           { CHAR: 4 [ "• • • • —" ] }
           { CHAR: 5 [ "• • • • •" ] }
           { CHAR: 6 [ "— • • • •" ] }
           { CHAR: 7 [ "— — • • •" ] }
           { CHAR: 8 [ "— — — • •" ] }
           { CHAR: 9 [ "— — — — •" ] }
           { CHAR: A [ "• —" ] } 
           { CHAR: B [ "— • • •" ] }
           { CHAR: C [ "— • — •" ] } 
           { CHAR: D [ "— • •" ] } 
           { CHAR: E [ "• • •" ] }
           { CHAR: F [ "• • — •" ] }
           { CHAR: G [ "— — •" ] }
           { CHAR: H [ "• • • •" ] } 
           { CHAR: I [ "• •" ] } 
           { CHAR: J [ "• — — —" ] } 
           { CHAR: K [ "— • —" ] } 
           { CHAR: L [ "• — • •" ] } 
           { CHAR: M [ "— —" ] } 
           { CHAR: N [ "— •" ] } 
           { CHAR: O [ "— — —" ] } 
           { CHAR: P [ "• — — •" ] } 
           { CHAR: Q [ "— — • —" ] } 
           { CHAR: R [ "• — •" ] } 
           { CHAR: S [ "• • •" ] } 
           { CHAR: T [ "—" ] } 
           { CHAR: U [ "• • —" ] } 
           { CHAR: V [ "• • • —" ] } 
           { CHAR: W [ "• — —" ] } 
           { CHAR: X [ "— • • —" ] }  
           { CHAR: Y [ "— • — —" ] }  
           { CHAR: Z [ "— — • •" ] } [ drop " " ] } case ;

: morse>ch ( morse-substring -- char )
         { { "— — — — —" [ CHAR: 0 ] }
           { "• — — — —" [ CHAR: 1 ] }
           { "• • — — —" [ CHAR: 2 ] }
           { "• • • — —" [ CHAR: 3 ] }
           { "• • • • —" [ CHAR: 4 ] }
           { "• • • • •" [ CHAR: 5 ] }
           { "— • • • •" [ CHAR: 6 ] }
           { "— — • • •" [ CHAR: 7 ] }
           { "— — — • •" [ CHAR: 8 ] }
           { "— — — — •" [ CHAR: 9 ] }
           { "• —" [ CHAR: A ] } 
           { "— • • •" [ CHAR: B ] }
           { "— • — •" [ CHAR: C ] } 
           { "— • •" [ CHAR: D ] } 
           { "• • •" [ CHAR: E ] }
           { "• • — •" [ CHAR: F ] }
           { "— — •" [ CHAR: G ] }
           { "• • • •" [ CHAR: H ] } 
           { "• •" [ CHAR: I ] } 
           { "• — — —" [ CHAR: J ] } 
           { "— • —" [ CHAR: K ] } 
           { "• — • •" [ CHAR: L ] } 
           { "— —" [ CHAR: M ] } 
           { "— •" [ CHAR: N ] } 
           { "— — —" [ CHAR: O ] } 
           { "• — — •" [ CHAR: P ] } 
           { "— — • —" [ CHAR: Q ] } 
           { "• — •" [ CHAR: R ] } 
           { "• • •" [ CHAR: S ] } 
           { "—" [ CHAR: T ] } 
           { "• • —" [ CHAR: U ] } 
           { "• • • —" [ CHAR: V ] } 
           { "• — —" [ CHAR: W ] } 
           { "— • • —" [ CHAR: X ] }  
           { "— • — —" [ CHAR: Y ] }  
           { "— — • •" [ CHAR: Z ] }
           [ drop 32 ] } case ;


: string>morse ( string -- morse-string )
         sequence>cons { } [ ch>morse suffix ] foldl
         "  " join ;

: morse>string ( morse-string -- string )
         R/ [ ]{2,}/ re-split
         [ >string morse>ch ] map >string ;

  5. g said
    May 7, 2009 at 9:46 PM

    python

    from string import lowercase, digits
morse = dict((c, (""".- -... -.-. -.. . ..-. --. .... .. .--- -.- .-.. -- -. --- 
                  .--. --.- .-. ... - ..- ...- .-- -..- -.-- --.. 
                  ----- .---- ..--- ...-- ....- ..... -.... --... ---.. ----.""".split() + [" "])[i])
             for i, c in enumerate(lowercase + digits + " "))
             
def to_morse(text):
    return "".join([morse[c] + " " for c in text if c in morse])
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  11. alexthelionx said
    August 14, 2013 at 10:24 PM 
     
class String
	MORSE_CODE = [
		['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', '-----'],
	]

	def to_morse
		self.split(' ').collect { |word| _to_morse word }.join '  '
	end

	def to_ascii
		self.split('  ').collect { |word| _to_ascii word }.join ' '
	end

private
	def _to_morse string
		string.downcase.split(//).collect do |letter| 
			MORSE_CODE.select { |pair| pair[0].eql? letter }
		end.map { |pair| pair[0][1] }.join(' ')
	end

	def _to_ascii string
		string.downcase.split(' ').collect do |code| 
			MORSE_CODE.select { |pair| pair[1].eql? code }
		end.map { |pair| pair[0][0] }.join
	end
end

message = 'Programming Praxis'
puts message.to_morse
puts message.to_morse.to_ascii

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