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Posting-Version: version B 2.10.1 6/24/83; site burdvax.UUCP
Path: utzoo!linus!philabs!seismo!presby!burdvax!puder
From: puder@burdvax.UUCP
Newsgroups: net.lang.prolog
Subject: Re: Transitive Closures
Message-ID: <1143@burdvax.UUCP>
Date: Mon, 26-Sep-83 14:52:05 EDT
Article-I.D.: burdvax.1143
Posted: Mon Sep 26 14:52:05 1983
Date-Received: Thu, 29-Sep-83 03:32:38 EDT
References: <11903@sri-arpa.UUCP>
Organization: SDC - a Burroughs Company, Paoli PA
Lines: 69

Here is how I define the transitive closure and avoid infinite loops, etc.

% cprolog
CProlog version 1.3
| ?- [user].
| 
| % The base relation.
| r(a,b).
| r(b,c).
| r(b,d).
| r(f,a).
| r(e,a).
| 
| % The transitive relation.
| tr(X,Y):-r(X,Y).
| % Note that the next clauses call r and tr, they do not call tr twice!
| tr(X,Y):-nonvar(X),!,r(X,Z),tr(Z,Y).
| tr(X,Y):-r(Z,Y),tr(X,Z).
| 
| user consulted 484 bytes 0.2 sec.

yes
| ?- tr(A,B).

A = a
B = b ;

A = b
B = c ;

A = b
B = d ;

A = f
B = a ;

A = e
B = a ;

A = f
B = b ;

A = e
B = b ;

A = a
B = c ;

A = f
B = c ;

A = e
B = c ;

A = a
B = d ;

A = f
B = d ;

A = e
B = d ;

no
| ?- 

-- 

Karl Puder   {sdcrdcf,presby,psuvax}!burdvax!puder   (215)648-7555