[racket] string-trim : an implementation & a question

Actually #rx seems to be much faster than #px (in this case, at any rate),
but it's still slower:

> (test)
cpu time: 1162 real time: 1181 gc time: 40
cpu time: 230 real time: 230 gc time: 0
> (test)
cpu time: 1184 real time: 1198 gc time: 38
cpu time: 258 real time: 259 gc time: 21
> (test)
cpu time: 1220 real time: 1544 gc time: 40
cpu time: 233 real time: 233 gc time: 0


On Sat, Apr 2, 2011 at 5:56 PM, Jon Zeppieri <zeppieri at gmail.com> wrote:

> I was a bit surprised to find that the scanning-by-hand approach really is
> significantly faster than using regexps.
>
> Between these two functions:
>
> (define (string-trim s)
>   (regexp-replace #px"^\\s*([^\\s]*)\\s*$" s "\\1"))
>
> ... and ...
>
> (define (string-trim s)
>   (define-syntax scan
>     (syntax-rules ()
>       ((_ s start end step)
>        (for/first ((i (in-range start end step))
>                    #:when (not (char-whitespace? (string-ref s i))))
>          i))))
>
>   (let* ((len (string-length s))
>          (last-index (sub1 len))
>          (start (or (scan s 0 len 1) 0))
>          (end (or (scan s last-index start -1) last-index)))
>     (substring s start (add1 end))))
>
>
> ... the latter is much faster. On 100000 iterations, using the test string:
>  "                                                      \n  \t foo bar
> baz\n                                    \r   "
> as input, I'm getting numbers like these (where the first time is for the
> regexp function and the second is for the hand-scanning function):
>
> > (test)
> cpu time: 8003 real time: 8008 gc time: 0
> cpu time: 256 real time: 257 gc time: 22
> > (test)
> cpu time: 8028 real time: 8025 gc time: 0
> cpu time: 255 real time: 255 gc time: 22
> > (test)
> cpu time: 8418 real time: 8424 gc time: 0
> cpu time: 260 real time: 260 gc time: 22
> > (test)
> cpu time: 8390 real time: 8401 gc time: 0
> cpu time: 252 real time: 253 gc time: 20
>
>
>
>
> On Sat, Apr 2, 2011 at 5:20 PM, Richard Cleis <rcleis at mac.com> wrote:
>
>> You can use an index to the string to find the location of your goal, then
>> return the substring when you are done.
>>
>> rac
>>
>> On Apr 2, 2011, at 3:08 PM, Charles Hixson wrote:
>>
>> > This seems to be what I want the string-trim to do, but it seems that
>> all the string copying would be expensive.  Is there a way to improve it by
>> avoiding the string copying?
>> >
>> > My original inclination was to use a while loop with a test for
>> non-whitespace, but that appears to not be something scheme supports.
>> >
>> > (define (string-trim s)
>> >    (let ( (l (string-length s) ) )
>> >      (cond
>> >        [ (= l 0) #f]
>> >        [ (char-whitespace? (string-ref s (- l 1) ) )    (string-trim
>> (substring s 0 (- l 1) ) ) ]
>> >        [else s]) ) )
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