Actually #rx seems to be much faster than #px (in this case, at any rate), but it's still slower:<div><br></div><div><div>> (test)</div><div>cpu time: 1162 real time: 1181 gc time: 40</div><div>cpu time: 230 real time: 230 gc time: 0</div> <div>> (test)</div><div>cpu time: 1184 real time: 1198 gc time: 38</div><div>cpu time: 258 real time: 259 gc time: 21</div><div>> (test)</div><div>cpu time: 1220 real time: 1544 gc time: 40</div><div>cpu time: 233 real time: 233 gc time: 0</div> <div><br></div><br><div class="gmail_quote">On Sat, Apr 2, 2011 at 5:56 PM, Jon Zeppieri <span dir="ltr"><<a href="mailto:zeppieri@gmail.com">zeppieri@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> I was a bit surprised to find that the scanning-by-hand approach really is significantly faster than using regexps.<div><br></div><div>Between these two functions:</div><div><br></div><div><div>(define (string-trim s)</div> <div> (regexp-replace #px"^\\s*([^\\s]*)\\s*$" s "\\1"))</div><div><br></div><div>... and ...</div><div><br></div><div><div>(define (string-trim s)</div><div> (define-syntax scan</div><div> (syntax-rules ()</div> <div> ((_ s start end step)</div><div> (for/first ((i (in-range start end step)) </div><div> #:when (not (char-whitespace? (string-ref s i))))</div><div> i))))</div><div> </div><div> (let* ((len (string-length s))</div> <div> (last-index (sub1 len))</div><div> (start (or (scan s 0 len 1) 0))</div><div> (end (or (scan s last-index start -1) last-index)))</div><div> (substring s start (add1 end))))</div></div><div> <br></div><div><br></div><div>... the latter is much faster. On 100000 iterations, using the test string:</div><div> " \n \t foo bar baz\n \r "</div> <div>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):</div><div><br></div><div><div>> (test)</div><div>cpu time: 8003 real time: 8008 gc time: 0</div> <div>cpu time: 256 real time: 257 gc time: 22</div><div>> (test)</div><div>cpu time: 8028 real time: 8025 gc time: 0</div><div>cpu time: 255 real time: 255 gc time: 22</div><div>> (test)</div><div>cpu time: 8418 real time: 8424 gc time: 0</div> <div>cpu time: 260 real time: 260 gc time: 22</div><div>> (test)</div><div>cpu time: 8390 real time: 8401 gc time: 0</div><div>cpu time: 252 real time: 253 gc time: 20</div></div><div><div></div><div class="h5"><div><br> </div><div><br></div><div><br> </div><br><div class="gmail_quote">On Sat, Apr 2, 2011 at 5:20 PM, Richard Cleis <span dir="ltr"><<a href="mailto:rcleis@mac.com" target="_blank">rcleis@mac.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> You can use an index to the string to find the location of your goal, then return the substring when you are done.<br> <br> rac<br> <div><div></div><div><br> On Apr 2, 2011, at 3:08 PM, Charles Hixson wrote:<br> <br> > 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?<br> ><br> > My original inclination was to use a while loop with a test for non-whitespace, but that appears to not be something scheme supports.<br> ><br> > (define (string-trim s)<br> > (let ( (l (string-length s) ) )<br> > (cond<br> > [ (= l 0) #f]<br> > [ (char-whitespace? (string-ref s (- l 1) ) ) (string-trim (substring s 0 (- l 1) ) ) ]<br> > [else s]) ) )<br> > _________________________________________________<br> > For list-related administrative tasks:<br> > <a href="http://lists.racket-lang.org/listinfo/users" target="_blank">http://lists.racket-lang.org/listinfo/users</a><br> <br> _________________________________________________<br> For list-related administrative tasks:<br> <a href="http://lists.racket-lang.org/listinfo/users" target="_blank">http://lists.racket-lang.org/listinfo/users</a><br> </div></div></blockquote></div><br></div></div></div> </blockquote></div><br></div>