mob@mit-amt.MIT.EDU (Mario O. Bourgoin) (08/04/86)
Hello, I am trying to write a solution program for the Farmer problem. Simply stated: A farmer wants to move himself, a silver fox, a fat goose and some jucy grain across a river. Unfortunately, his boat is so tiny he can only take one of his things across on any trip. Worse yet, an unattended fox will eat a goose, and an unattended goose will eat the grain, so the farmer must not leave the fox alone with the goose or the goose alone with the grain. What is he to do? So far, I have stated the problem in the way outlined below. Can someone help we to get this working? It figures out the first alternative of moving the goose across but then starts looping on what to do for the next move. same(yes,1,1). same(yes,2,2). same(no,1,2). same(no,2,1). legal(state(F,X,G,A)) :- (same(no,X,G);same(yes,F,X)), (same(no,G,A);same(yes,F,G)). linked(state(F1,X1,G1,A1),state(F2,X2,G2,A2)) :- same(no,F1,F2), (same(yes,X1,X2);(same(yes,F1,X1),same(yes,G1,G2),same(yes,A1,A2))), (same(yes,G1,A2);(same(yes,F1,G1),same(yes,X1,X2),same(yes,A1,A2))), (same(yes,A1,A2);(same(yes,F1,A1),same(yes,G1,G2),same(yes,X1,X2))). path(State1,State2) :- legal(State1),legal(State2), (linked(State1,State2);(path(State1,State3),path(State3,State2))).
windes@uicsg.UUCP (08/07/86)
Well, I have a couple of suggestions:
- First, I don't think your 'linked' procedure is doing what it
should. Seems to me that the query 'linked(state(1,1,1,1),X).' should
produce four distinct alternatives.
- Second, you need to rewrite your 'path' procedure because
A) it is left-recursive, and B) you need to keep a record of states already
visited, and exclude them from the possible next states.
hope this helps.
Ed Windes * UofI ...ihnp4!uiucdcs!uicsl!uicsg!windes - until 8/14/86
* AT&T ...ihnp4!ihopa!edw - after 8/14/86
(HELP, I'm being help in a closet and forced to learn prolog!)