From rhoekstr@wins.uva.nl Fri Jan 7 12:35:00 2000 Received: from smtp2.a2000.nl (duck.a2000.nl [62.108.1.88]) by swi.psy.uva.nl (8.9.3/8.9.3) with ESMTP id MAA03493 for ; Fri, 7 Jan 2000 12:35:00 +0100 (MET) Received: from node10e0e.a2000.nl ([24.132.14.14] helo=RHINO) by smtp2.a2000.nl with smtp (Exim 2.02 #4) id 126Xfc-0001WU-00; Fri, 7 Jan 2000 12:35:04 +0100 From: "Rinke Hoekstra" To: "Prolog Mailing List" Cc: , Subject: RE: I am having trouble writing replace/4 Date: Fri, 7 Jan 2000 12:42:15 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook IMO, Build 9.0.2416 (9.0.2910.0) In-Reply-To: Importance: Normal X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2919.6600 It could be done a lot quicker: replace_list([],[],List,List). replace_list([HReplacant|TReplacant],[HReplacer|TReplacer],List,NewList):- replace(HReplacant,HReplacer,List,IList), replace_list(TReplacant,TReplacer,IList,NewList). replace(_,_,[],[]). replace(HReplacant,HReplacer,[HReplacant|Tail],[HReplacer|NewTail]):- replace(HReplacant,HReplacer,Tail,NewTail). replace(HReplacant,HReplacer,[Head|Tail],[Head|NewTail]):- replace(HReplacant,HReplacer,Tail,NewTail). This algorithm takes less steps (trace it to find out). Regards, Rinke Hoekstra Amsterdam -----Original Message----- From: Pascal Vaillant [mailto:Pascal.Vaillant@rz.hu-berlin.de] Sent: vrijdag 7 januari 2000 10:13 To: Douglas Miles Cc: prolog@swi.psy.uva.nl Subject: Re: I am having trouble writing replace/4 > replaceList([],[],NothingToDo,NothingToDo):-!. > replaceList([HeadBefore|TailBefore],[HeadAfter|TailAfter],Start,End):-!, > replace(HeadBefore,HeadAfter,Start,Midde), > replaceList(TailBefore,TailAfter,Middle,End). > > replace(_, _, [], []):-!. > replace(A, B, [A|L], [B|R]) :- !, replace(A, B, L, R). > replace(A, B, [C|L], [C|R]) :- replace(A, B, L, R). > > here is my trouble.. > > ?- replaceList([a,b],[A,B],[1,2,3,a,b,c,x,y,z],O). > A = b > B = _G422 > O = [1, 2, 3, _G422, _G422, c, x, y, z] > yes > > I guess the real question is how can I write a replace only once.. or > rewrite replaceList? > > I want it to do this... > > ?- replaceList([a,b],[A,B],[1,2,3,a,b,c,x,y,z],O). > A = _G421 > B = _G422 > O = [1, 2, 3, _G421, _G422, c, x, y, z] > yes > > is copy_term/2 the key? I have tried a few tricks.. > or maybe numbervars can help? > No, it's simply because your test (second clause of replace/4) involves unification, and not only equivalence. Hence, after having replaced a by A, your current list becomes [b, etc.], and you try, at that point, to unify A with b (which it has no reason to refuse :-) This works : replaceList([],[],NothingToDo,NothingToDo). replaceList([HeadBefore|TailBefore],[HeadAfter|TailAfter],Start,End) :- replace(HeadBefore,HeadAfter,Start,Middle), replaceList(TailBefore,TailAfter,Middle,End). replace(_,_,[],[]). replace(A,B,[C|L],[D|R]) :- ((A==C) *-> D=B ; D=C), replace(A,B,L,R). Tschuess, Pascal Vaillant Berlin ---------------- * To UNSUBSCRIBE, please use the HTML form at http://www.swi.psy.uva.nl/projects/SWI-Prolog/index.html#mailinglist or send mail to prolog-request@swi.psy.uva.nl using the Subject: "unsubscribe" (without the quotes) and *no* message body. ** An ARCHIVE of this list is maintained at http://www.swi.psy.uva.nl/projects/SWI-Prolog/mailinglist/archive/