Breadth First Search in Prolog

Question

I\'m new to Prolog and currently implementing DFS (depth-first search) and BFS (breadth-first search) algorithms. My DFS works fine as the code below, but the BFS is termina

Answer 1

First, the usual notion of s(A,B) is just like your connect2(A,B,_).

You should make your interface predicates explicit:

depth_first( Start, Goal, Path):-
    depth_first( Start, Goal, [Start], Path).

Maintaining a queue in BFS is not complicated at all. Instead of Visited, have VisitedLists queue (pop from front; add at end; thus FIFO):

consed( A, B, [B|A]).

bfs( Goal, [Visited|Rest], Path) :-                     % take one from front
    Visited = [Start|_],            
    Start \== Goal,
    findall( X,
        ( connected2(X, Start, _), \+ member(X, Visited) ),
        [T|Extend]),
    maplist( consed(Visited), [T|Extend], VisitedExtended),      % make many
    append( Rest, VisitedExtended, UpdatedQueue),       % put them at the end
    bfs( Goal, UpdatedQueue, Path ).

When the goal is reached, Path is instantiated:

bfs(Goal, [[Goal|Visited]|_], Path):- 
    reverse([Goal|Visited], Path).

The interface call needs to be adjusted correspondingly. It should be

breadth_first( Start, Goal, Path):- bfs( Goal, [[Start]], Path).

later note: repeated appending of course causes quadratic slowdown, so for efficiency this should be re-written with difference lists (also a straightforward task).