Breadth first search in prolog gives me an error

问题

This is my code for breadth first search strategy in prolog :

s(a, b).
s(a, c).
s(b, g).
s(b, f).
s(c, r).
s(c, e).
goal(g).

solve( Start, Solution) :-
    breadthlirst( [ [Start] ], Solution).

breadthfirst( [ [Node | Path] |_], [Node | Path] ) :-
    goal( Node).

breadthfirst( [ [N | Path] | Paths], Solution) :-
    bagof([M,N|Path],
    ( s( N, M), \+ member( M, [N | Path] ) ), NewPaths),
    conc( Paths, NewPaths, Pathsl), !,
    breadthfirs( Pathsl, Solution);
    breadthfirst( Paths, Solution).

But when I run this code it issue an exception as so :

?- solve(a, S).
uncaught exception: error(existence_error(procedure,breadthlirst/2),solve/2)

What's going on here? also, is there any easier version of breadth first search than this one ?

回答1:

For this solution I used SWI-Prolog

What's going on here?

uncaught exception: error(existence_error(procedure,breadthlirst/2),solve/2)

The compiler/interpreter is showing you that in trying to solve your query it started with predicate solve/2 and then tried to find breadthlirst/2 which it could not.

Fixing the typos and changing conc/3 to append/3 results in

s(a, b).
s(a, c).
s(b, g).
s(b, f).
s(c, r).
s(c, e).
goal(g).

solve( Start, Solution) :-
    breadthfirst( [ [Start] ], Solution).

breadthfirst( [ [Node | Path] |_], [Node | Path] ) :-
    goal( Node).

breadthfirst( [ [N | Path] | Paths], Solution) :-
    bagof([M,N|Path],
    ( s( N, M), \+ member( M, [N | Path] ) ), NewPaths),
    %conc( Paths, NewPaths, Pathsl), !,
    append(Paths, NewPaths, Pathsl), !,
    breadthfirst( Pathsl, Solution);
    breadthfirst( Paths, Solution).

Executing the query

?- solve(a,S).
S = [g, b, a] ;

Normally I would expect the goal, in this case g to be a parameter of solve and not hard coded as a fact goal(g).

Is there any easier version of breadth first search than this one ?

When working with breath first search in Prolog beyond trivial cases I prefer to use meta-interpreters.

Here is another version of BFS in Prolog and if you get any descent book on Prolog it should cover BFS.

The Power of Prolog will help you with understanding Prolog better. Be warned that this is more of the advanced stuff and not something to start playing with until you understand the basics.

回答2:

Sorry but no street-cred so no commment for MDG :-(

If you want breadth first maybe best to use queue?? I don't know. I can show code for breadth first with queue but this is not your question and there is so many such questions just search no problem.

来源：https://stackoverflow.com/questions/53236609/breadth-first-search-in-prolog-gives-me-an-error

标签

prolog

breadth-first-search