Double Space

April 4, 2017

Our algorithm makes two passes over the input; the first pass counts spaces, then a new string is allocated, and the second pass copies, adding spaces as it goes:

(define (double-space oldstr)
  (define (space? str idx)
    (char=? (string-ref str idx) #\space))
  (let ((oldlen (string-length oldstr)))
    (do ((oldidx 0 (+ oldidx 1))
         (count 0 (+ count (if (space? oldstr oldidx) 1 0))))
        ((= oldidx oldlen)
          (let* ((newlen (+ oldlen count))
                 (newstr (make-string newlen #\space)))
            (do ((oldidx 0 (+ oldidx 1))
                 (newidx 0 (+ newidx (if (space? newstr newidx) 2 1))))
                ((= oldidx oldlen) newstr)
              (string-set! newstr newidx (string-ref oldstr oldidx))))))))

Here are some examples:

> (double-space "hello")
"hello"
> (double-space "hello hello")
"hello  hello"
> (double-space "hello  hello")
"hello    hello"
> (double-space "hello   hello")
"hello      hello"
> (double-space "hello hello hello")
"hello  hello  hello"
> (double-space " hello ")
"  hello  "
> (double-space "")
""
> (double-space " ")
"  "
> (double-space "  ")
"    "
> (double-space "   ")
"      "

There are other ways to do that. Here we convert the string to a list of characters, make a single pass through the list copying two spaces wherever one is found, then reverse the list and convert back to a string:

(define (double-space str)
  (let loop ((xs (string->list str)) (zs (list)))
    (if (null? xs) (list->string (reverse zs))
      (if (char=? (car xs) #\space)
          (loop (cdr xs) (cons #\space (cons #\space zs)))
          (loop (cdr xs) (cons (car xs) zs))))))

All the test cases produce identical results. You can run the program at http://ideone.com/YoEiw9.

Pages: 1 2

Posted by programmingpraxis
Filed in Exercises
15 Comments »

15 Responses to “Double Space”

  1. James Curtis-Smith said
    April 4, 2017 at 9:11 AM

    As a perl programmer this one is trivial:

    sub ds{shift=~s{ }{  }rg}

    if you don’t want to use s{} could do

    sub ds{join'',map{$_,' 'eq$_?' ':''}split'',shift}
  2. Pavel Tumilovich said
    April 4, 2017 at 9:37 AM

    Clojure:

    (defn dup_space
  [input]
  (apply str
         (map
           (fn [c] (if(= \space c) 
                     "  " 
                     (str c)))
           input)))
  3. Paul said
    April 4, 2017 at 9:41 AM

    Also not too difficult in Python.

    def double_space(txt):
    return txt.replace(" ", "  ")
  4. Devdhar Patel said
    April 4, 2017 at 10:20 AM

    javascript:

    function double_space(txt){
    return txt.replace(/ /g,’ ‘);
    }

  6. Jussi Piitulainen said
    April 4, 2017 at 1:23 PM

    Read-write Scheme.

    (setlocale LC_ALL "") ;sigh (this is in guile 2.0.9)

(define (double-space-port in out)
  (let ((o (read-char in)))
    (or (eof-object? o)
        (begin
          (write-char o out)
          (case o ((#\space) (write-char o out)))
          (double-space-port in out)))))

(define (double-spaced-string s)
  (let ((out (open-output-string)))
    (double-space-port (open-input-string s) out)
    (get-output-string out)))

(write (double-spaced-string "Taas mennään.  Ja äätkin näkyy!"))
(newline)

    Read-write Python.

    import io

def double_space_io(inio, outio):
    while True:
        o = inio.read(1)
        if not o: return
        outio.write(o)
        if o in ' ': outio.write(o)

def double_space_str(s):
    outio = io.StringIO()
    double_space_io(io.StringIO(s), outio)
    return outio.getvalue()

print(repr(double_space_str("Taas mennään.  Ja äätkin näkyy!")))
  7. Rutger said
    April 4, 2017 at 2:49 PM

    Took the Python generator approach. We can feed the double_space generator with characters and they get doubled up. 

    import time

def double_space(g):
	while True:
		x = next(g)
		if x == ' ':
			yield x
		yield x

def text_streamer(text):
	i = 0
	while i < len(text):
		yield(text[i])
		i += 1
		time.sleep(1/10)

text = "Hello  from Python !"
s = text_streamer(text)

for v in double_space(s):
	print(v,)
  8. Steve said
    April 5, 2017 at 12:00 AM

    MUMPS:

    r !,word,! f i=1:1:$l(word,” “) w:i>1 ” ” w $p(word,” “,i)

    hello, world!
    hello, world!

  9. Globules said
    April 5, 2017 at 12:29 AM

    A Haskell version. (Note: cat -e prints a ‘$’ at the end of a line.)

    main :: IO ()
main = interact (concatMap (\c -> if c == ' ' then "  " else [c]))

    $ printf " foo  bar   \nbaz  zot\n" | ./dblspc | cat -e
  foo    bar      $
baz    zot$
  10. Jeff said
    April 5, 2017 at 7:54 AM

    Nothing difficult for Ruby

    `def double_space(string)
    return string.gsub(/\s/, ‘ ‘)
    end

    puts double_space(“hello word”)`

  11. trasterosite said
    April 5, 2017 at 5:01 PM 
    dup (' ':xs) = ' ':' ':dup xs
dup ( x :xs) =      x :dup xs
dup      []  = []
  12. trasterosite said
    April 5, 2017 at 5:16 PM

    for non streaming algorithms the former maybe slow and memory expensive then, we can use ByteStrings (lazy or strict, depends)

    {-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString.Lazy.Char8 as BS

dup' = BS.concatMap d
  where d ' ' = "  "
        d  x  = BS.singleton x
  13. A Simple Programming Praxis Solution in Elisp | Irreal said
    April 7, 2017 at 2:35 PM

    […] how easy your favorite programming language makes it to work with strings. The problem is to double every blank in a string. That’s pretty simple but some languages make you deal with string allocation or perhaps, in […]

  14. jleechpe said
    April 7, 2017 at 6:07 PM

    Powershell makes it easy:

    Function Set-DoubleSpace ($a) {$a-replace” “,” “}

    or from STDIN

    (read-host)-replace” “,” “

  15. Aurelio said
    April 12, 2017 at 6:55 AM

    in Java:
    public String doubleSpaces(String in) {

    return in.replace(” “, ” “);
    }

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