March 25, 2016

We first build the prefix, then perform a single append:

(define (leftpad str len ch)
  (let loop ((n (- len (string-length str)))
             (prefix (list)))
    (if (positive? n)
        (loop (- n 1) (cons ch prefix))
        (string-append (list->string prefix) str))))

> (leftpad "hello" 7 #\@)

You can run the program at http://ideone.com/zPY6S6.

Pages: 1 2

12 Responses to “Leftpad”

  1. d1rge said

    Why not use just use R5RS make-string?

    (define (leftpad str len ch)
      (let ((prefix (- len (string-length str))))
        (if (positive? prefix)
            (string-append (make-string prefix ch) str)
  2. John Cowan said

    Or indeed SRFI 13 string-pad.

  3. Globules said

    A Haskell version. I use a “Maybe Char” to represent the optional character argument.

    leftpad :: String -> Int -> Maybe Char -> String
    leftpad str len ch = let n = len - length str
                             c = maybe ' ' id ch
                         in replicate n c ++ str
    main :: IO ()
    main = do
      putStrLn $ leftpad "foo" 0 (Just 'z')
      putStrLn $ leftpad "foo" 5 (Just 'z')
      putStrLn $ leftpad "foo" 5 Nothing
    $ ./leftpad 
  4. John said

    A version in Factor:

    :: left-pad ( seq n elt — newseq )
    seq n seq length [-] elt <repetition> prepend ;


  5. kernelbob said

    Here’s a Bash implementation. I guess I think we need more satire.


    curl -s "https://api.left-pad.io/?str=$1&len=$2${3+&ch=$3}" 2>/dev/null | \
        sed -Ee 's/^{"[^"]*":"//' -e 's/"[^"]*$//'

    And here it is in action.

    tmp$ ./leftpad.bash test 10
    tmp$ ./leftpad.bash test 10 \~

  6. matthew said

    Here’s an attempt at a better Javascript implementation that is linear (uses Array..prototype.join() to assemble the actual string) and that tries to handle Unicode properly (strings internally are UTF-16 in Javascript, so we need to take account of surrogate pairs):

    function issurrogate(codepoint) {
        return codepoint >= 0xD800 && codepoint < (0xD800 + 0x4000);
    function length16(str) {
        var len = 0;
        // Assumes well-formed UTF-16
        for (var i = 0; i < str.length; i++) {
            if (issurrogate(str.charCodeAt(i))) i++;
        return len;
    function leftpad (str, len, ch) {
        str = String(str);
        len -= length16(str);
        if (len <= 0) return str;
        if (ch == undefined) ch = ' ';
        else ch = String(ch);
        if (length16(ch) != 1) {
            console.log("leftpad: not single UTF-16 character: " + ch);
            return null;
        var pad = []
        for (var i = 0; i < len; i++) pad.push(ch);
        return pad.join("");
    function test() {
        console.log(leftpad('𐌰𐌳𐌴𐌡', 10,'𐎘'));
  7. Jussi Piitulainen said

    Pure Bash solution. I failed to find a really nice way to generate a padding string of given length. This overgenerates in expected cases, and undergenerates (is incorrect) in unexpectedly long cases:

    leftpad () {
        pad=${xs:0:(new > old ? new - old : 0)}
        echo "${pad}$1"

    It also trusts the caller that the padding character actually is a character and not a longer string.


    for k in {0..8}
        echo $k $(leftpad foo $k '$')

    Test result:

    0 foo
    1 foo
    2 foo
    3 foo
    4 $foo
    5 $$foo
    6 $$$foo
    7 $$$$foo
    8 $$$$$foo
  8. Andrei said

    With modern Javascript, this is basically a oneliner.

    function leftpad(str, len, chr = ' ') {
        return chr.repeat(len >= str.length ? len-str.length : 0) + str;
  9. Jos Koot said

    (define (leftpad str len ch)
    (let ((n (string-length str)))
    (string-append (make-string (max 0 (- len n)) ch) str)))

  10. matthew said

    @Andrei – for sure, and ES6 makes the Unicode handling much more straightforward too (though I suppose to do that properly we should handle combining characters & normalization and it all starts to get a bit complicated).

  11. pmarfleet said

    Solution in C#. The .NET String class already provides a PadLeft method. Using that would defeat the purpose of the exercise however, so here is an alternative implementation using an Extension Method:

    public static string LeftPad(this string input, int width, char ch)
                var array = new char[width];
                var offset = width - input.Length;
                int i;
                for (i = 0; i < offset; i++)
                    array[i] = ch;
                for (; i < width; i++)
                    array[i] = input[i - offset];
                return new string(array);
  12. Jason Orendorff said

    JavaScript engines have an optimization that causes repeated string `+=` to take linear time rather than quadratic.

    Still, what leftpad does is not the best way to do it. There’s a neat trick that lets you build the same string with fewer `+=` operations, and Firefox’s implementation of `String.prototype.repeat` uses that trick:


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