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

How about this string?

  " "

Robby

On Sat, Apr 2, 2011 at 5:38 PM, Jon Zeppieri <zeppieri at gmail.com> wrote:
> Ha. But I should have tested the first function, right? Yes, that was also
> broken.
> (define (string-trim s)
>   (regexp-replace #px"^\\s*(.*?)\\s*$" s "\\1"))
> ... passes the test cases and is a lot faster than the broken version; it's
> now a little less than a 2x difference:
>> (test)
> cpu time: 426 real time: 437 gc time: 22
> cpu time: 231 real time: 230 gc time: 0
>> (test)
> cpu time: 422 real time: 431 gc time: 21
> cpu time: 231 real time: 231 gc time: 0
>> (test)
> cpu time: 450 real time: 456 gc time: 21
> cpu time: 237 real time: 261 gc time: 0
>
>
>
>
> On Sat, Apr 2, 2011 at 6:23 PM, Jon Zeppieri <zeppieri at gmail.com> wrote:
>>
>>
>> On Sat, Apr 2, 2011 at 6:06 PM, Robby Findler
>> <robby at eecs.northwestern.edu> wrote:
>>>
>>> (oh, and I meant to add the usual "where are you test cases?!?!"
>>> comment here, but forgot.)
>>>
>>> Robby
>>
>>
>> Ugh, you're right.
>> My understanding is that the function is supposed to return a string equal
>> to the input string with leading and trailing whitespace removed. But I
>> wasn't the one who originally started the discussion; that was Richard
>> Hixson. I just got curious, because he wanted to avoid using regexps for
>> performance reasons (I think), and that made we wonder what the how large
>> the difference was.
>> But back to the function... Yes, that's broken. (Also, it turns out that
>> replacing #px with #rx may make the former function a lot faster, but it
>> doesn't actually work, at all.)
>> In the second function, the end parameter to the second use of 'scan' is
>> wrong, of course, since first non-whitespace character in the string may
>> also be the last.  So, changing the second string-trim function to:
>> (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 (sub1 start) -1) last-index)))
>>     (substring s start (add1 end))))
>> ... works on the the following test cases:
>> > (string-trim "")
>> ""
>> > (string-trim "a")
>> "a"
>> > (string-trim "ab")
>> "ab"
>> > (string-trim " ab")
>> "ab"
>> > (string-trim "   ab")
>> "ab"
>> > (string-trim "   ab   ")
>> "ab"
>> > (string-trim "ab   ")
>> "ab"
>> > (string-trim " s sdf d  ")
>> "s sdf d"
>> ... and the times aren't much altered.
>> -Jon
>>
>>>
>>> On Sat, Apr 2, 2011 at 5:06 PM, Robby Findler
>>> <robby at eecs.northwestern.edu> wrote:
>>> > I've lost track of what the function is supposed to be doing, but your
>>> > two functions don't agree on the input "a ", I don't think. I get
>>> > this:
>>> >
>>> > (define (string-trim.1 s)
>>> >  (regexp-replace #px"^\\s*([^\\s]*)\\s*$" s "\\1"))
>>> >
>>> > (define (string-trim.2 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))))
>>> >
>>> >> (string-trim.2 "a ")
>>> > "a "
>>> >> (string-trim.1 "a ")
>>> > "a"
>>> >
>>> >
>>> > On Sat, Apr 2, 2011 at 5:03 PM, Jon Zeppieri <zeppieri at gmail.com>
>>> > wrote:
>>> >> 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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>>> >>>>
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>>> >>>
>>> >>
>>> >>
>>> >> _________________________________________________
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>>> >>
>>> >
>>
>
>