Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!mcgill-vision!bloom-beacon!mit-eddie!uw-beaver!ubc-cs!fs1!fs0.ee.ubc.ca!vincel
From: vincel@fs0.ee.ubc.ca (vincent li)
Newsgroups: comp.lang.prolog
Subject: loops in prolog
Keywords: loops
Message-ID: <1032@fs1.ee.ubc.ca>
Date: 8 Feb 90 22:41:15 GMT
Sender: news@fs1.ee.ubc.ca
Lines: 40


Question: how would you code the following loop in (Edingburgh)
Prolog?

loop:
   action_1;
   IF condition_1 THEN
     action_2;
     goto loop
  END IF
  action_3;

Granted this is bad code, but the idea is to perform action_1 and
action_2 within the loop. action_1 would change some data, say, and
condition_1 would check the result. If condition_1 is satisfied, then
action_2 is performed and stays in the loop, otherwise rest of the
program is continued with action_3 WITHOUT doing action_2.

The code I came up with looks something like this:

loop :-
   action_1,
   (condition_1 -> action_2,loop),
   action_3.

This works, but the recursive call eats up memory and I get out of heap
errors. Is there some other way of doing this? A fail driven loop seems
an obvious alternative, but action_1 is a proven-once predicate (ie.
fails on backtrack). So, this doesn't seem to work:

loop :- action_1,condition_1,action_2,fail.
loop :- action_3.

Thanks in advance for any suggestions.
Vince.
--------------------------------------------
vincel@ee.ubc.ca
vinceli@triumfcl.bitnet
vincent@graham.uucp
.