Prolog Program 2: A Day at the River

• River Crossing problem - The farmer, wolf, goat, and cabbage
  • A farmer has a wolf, a goat, and a cabbage on the north side of the river, and also a small boat.
    He wants to bring his property to the south side of the river, but the boat can only carry
    himself and at most one of the three. The problem is that if he leaves the wolf with the goat
    unattended then he will have no more goat; also if the goat is left behind with the
    cabbage, then the cabbage will be finished. How can the farmer bring his property to the
    south side of the river?
  • In this problem there are four relevant entries: the farmer, wolf, goat, and cabbage.
    The problem state can be represented by a list of four variables for the farmer, wolf, goat, cabbage.

    [F,W,G,C]

  • Each of these entities can have only two states: "n" or "s" for north or south side of the river.
    The initial state is when all the entities are on the north side, and the final
    state is when they are on the south side.

    
    go :-
        initial_state(X),
    %    boat(X,Y),
    %    safestate(Y),
        rivercross(X,[X],Crossings,1), 
        write('Start: '), write(X),
        write(' to: '), write(Crossings), nl.
    
    rivercross([s,s,s,s],FinalCrossings,FinalCrossings, _).
    rivercross(X,Crossings,FinalCrossings,Count) :-    THIS IS ONLY PARTIALLY DONE
        Count < 20,                           Here "COUNT" is used to stop after
        boat(X,NewState),                   20 moves, if recursion was stack overflowing
        safestate(NewState),
        write('Newstate: '),write(NewState),nl,
        Count2 is Count + 1,
        rivercross([s,s,s,s],Crossings, FinalCrossings,Count2). %THIS IS NOT CORRECT
        
    initial_state([n,n,n,n]).  % each entity is on the north side.
    final_state([s,s,s,s]).  % each is on the south side.
    
    boat([F,W,C,G],[F1,W,C,G]) :- cross(F,F1). % Farmer boats alone
    boat([F,F,C,G],[F1,F1,C,G]) :- cross(F,F1). % Farmer and wolf cross in boat
    boat([F,W,F,G],[F1,W,F1,G]) :- cross(F,F1). % Farmer and cabbage cross in boat
    boat([F,W,C,F],[F1,W,C,F1]) :- cross(F,F1). % Farmer and goat cross in boat
    
    cross(s,n).
    cross(n,s).
    
    safestate([n,n,n,n]).	 % All on one side of river
    safestate([s,s,s,s]).
    safestate([n,_,_,n]).    % Farmer with goat
    safestate([s,_,_,s]).
    safestate([n,n,n,_]).	 % Farmer with wolf and cabbage
    safestate([s,s,s,_]). 	 
    % Other safe states?
    
    member_state(S, [S|_]).
    member_state(X, [_|T]) :- member_state(X,T).
    
    

  • Prolog