<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META content="text/html; charset=iso-8859-1" http-equiv=Content-Type><BASE href="x-msg://1444/"> <META name=GENERATOR content="MSHTML 8.00.6001.18928"> <STYLE></STYLE> </HEAD> <BODY style="WORD-WRAP: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space" bgColor=#ffffff> <DIV><FONT size=2 face=Arial>Interesting. How does the pattern matcher do that? I tried comparing two solutions in terms of performance (see below, group 1&4 (naive approach) against 2&3 (matching approach)), and aside from the fact that I get somewhat inconsistent results (when I run the test suite several times after one another, I would expect comparable result every time but don't seem to accomplish those even though calling collect-garbage should get me a "clean state" every time), it appears that neither #2 or #3 (#3 being taken from the docs) appears to be significantly worse than #1 or #4 even though the pattern matching and substitution process is expectedly rather sophisticated (and thus more resource consuming)?</FONT></DIV> <DIV><FONT size=2 face=Arial></FONT> </DIV> <DIV><FONT size=2 face=Arial>P.S. #4 only added for the sake of comparison between the list and '(,...) implementations.</FONT></DIV> <DIV><FONT size=2 face=Arial></FONT> </DIV> <DIV><FONT size=2 face=Arial>(define (switch-one-and-three1 ls)<BR>    (list (caddr ls)(cadr ls)(car ls)))</FONT></DIV> <DIV> </DIV><FONT size=2 face=Arial> <DIV><BR>(define (switch-one-and-three2 ls)<BR>  (match ls<BR>    [`(,one ,two ,three) `(,three ,two ,one)]))</DIV> <DIV> </DIV> <DIV>(define (switch-one-and-three3 ls)<BR>  (match ls<BR>    [(list a b c) (list c b a)]))</DIV> <DIV> </DIV> <DIV>(define (switch-one-and-three4 ls)<BR>    `(,(caddr ls) ,(cadr ls) ,(car ls)))<BR> <BR> <BR> <BR>(define iterate (lambda (ct thunk) <BR>                      (if (zero? ct) <BR>                         #t (begin (thunk) (iterate (- ct 1) thunk)))))</DIV> <DIV> </DIV> <DIV>(define (thunk1) (switch-one-and-three1 '(a b c)))</DIV> <DIV> </DIV> <DIV>(define (thunk2) (switch-one-and-three2 '(a b c)))</DIV> <DIV> </DIV> <DIV>(define (thunk3) (switch-one-and-three3 '(a b c)))</DIV> <DIV> </DIV> <DIV>(define (thunk4) (switch-one-and-three4 '(a b c)))</DIV> <DIV> </DIV> <DIV><BR>(collect-garbage)       <BR>       <BR>(time (iterate 10000000 thunk1))</DIV> <DIV> </DIV> <DIV>(collect-garbage) </DIV> <DIV> </DIV> <DIV>(time (iterate 10000000 thunk2))</DIV> <DIV> </DIV> <DIV>(collect-garbage) </DIV> <DIV> </DIV> <DIV>(time (iterate 10000000 thunk3))</DIV> <DIV> </DIV> <DIV>(collect-garbage) </DIV> <DIV> </DIV> <DIV>(time (iterate 10000000 thunk4))<BR></FONT></DIV><FONT size=2 face=Arial> <DIV></FONT> </DIV> <BLOCKQUOTE style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px"> <DIV style="FONT: 10pt arial">----- Original Message ----- </DIV> <DIV style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B> <A title=matthias@ccs.neu.edu href="mailto:matthias@ccs.neu.edu">Matthias Felleisen</A> </DIV> <DIV style="FONT: 10pt arial"><B>To:</B> <A title=afowler2@broncos.uncfsu.edu href="mailto:afowler2@broncos.uncfsu.edu">Ashley Fowler</A> </DIV> <DIV style="FONT: 10pt arial"><B>Cc:</B> <A title=users@racket-lang.org href="mailto:users@racket-lang.org">users@racket-lang.org</A> </DIV> <DIV style="FONT: 10pt arial"><B>Sent:</B> Wednesday, September 05, 2012 8:29 PM</DIV> <DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [racket] Question</DIV> <DIV><BR></DIV> <DIV> <DIV>#lang racket</DIV> <DIV><BR></DIV> <DIV>(require rackunit)</DIV> <DIV><BR></DIV> <DIV>;; [List X Y Z] -> [List Z Y X]</DIV> <DIV>;; switch items one and three in a 3-element list </DIV> <DIV><BR></DIV> <DIV>(module+ test </DIV> <DIV>  (check-equal? (switch-one-and-three1 '(a b c)) '(c b a))</DIV> <DIV>  (check-equal? (switch-one-and-three1 '(1 2 3)) '(3 2 1)))</DIV> <DIV><BR></DIV> <DIV>(define (switch-one-and-three1 ls)</DIV> <DIV>  (match ls</DIV> <DIV>    [`(,one ,two ,three) `(,three ,two ,one)]))</DIV> <DIV><BR></DIV> <DIV>That's 50$ -- Matthias</DIV></DIV> <DIV><BR></DIV><BR> <DIV> <DIV>On Sep 5, 2012, at 2:26 PM, Ashley Fowler wrote:</DIV><BR class=Apple-interchange-newline> <BLOCKQUOTE type="cite"> <DIV ocsi="0" fpstyle="1"> <DIV style="FONT-FAMILY: Tahoma; DIRECTION: ltr; COLOR: rgb(0,0,0); FONT-SIZE: 10pt"><SPAN style="WIDOWS: 2; TEXT-TRANSFORM: none; TEXT-INDENT: 0px; DISPLAY: inline !important; FONT: 13px Tahoma; WHITE-SPACE: normal; ORPHANS: 2; FLOAT: none; LETTER-SPACING: normal; COLOR: rgb(0,0,0); WORD-SPACING: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px">Can anybody help me with this problem? I have an idea but would like some suggestions on how to start. The problem is below...</SPAN> <DIV style="WIDOWS: 2; TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 13px Tahoma; WHITE-SPACE: normal; ORPHANS: 2; LETTER-SPACING: normal; COLOR: rgb(0,0,0); WORD-SPACING: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px"><BR></DIV> <DIV style="WIDOWS: 2; TEXT-TRANSFORM: none; TEXT-INDENT: 0px; FONT: 13px Tahoma; WHITE-SPACE: normal; ORPHANS: 2; LETTER-SPACING: normal; COLOR: rgb(0,0,0); WORD-SPACING: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px"><PRE style="WORD-WRAP: break-word; WHITE-SPACE: pre-wrap"><B>Write a procedure (switch-one-and-three1 LS) that takes a list of exactly three items as its argument and returns the list with the first and third items switched around. You may assume the input list contains exactly three items</B></PRE></DIV></DIV>____________________<BR> Racket Users list:<BR> <A href="http://lists.racket-lang.org/users">http://lists.racket-lang.org/users</A><BR></DIV></BLOCKQUOTE></DIV><BR> <P> <HR> <P></P>____________________<BR>  Racket Users list:<BR>  http://lists.racket-lang.org/users<BR></BLOCKQUOTE></BODY></HTML>