search all paths and the shortest path for a graph - Prolog

问题

I have a problem in my code with turbo prolog which searches all paths and the shortest path in a graph between 2 nodes. The problem that i have is to test if the node is in the list or not (exactly in the clause of member)

           1    ---- b ----   3
           ---       |        ---
        ---          |             -----
      a              |5                  d
        ---          |             -----
            ---      |         ---
             2  ---  |     ---   4
                  -- c  --

for example we have for b--->c 
([b,c],5) , ([b,a,c],3) and ([b,d,c],7) : possible paths.
([b,a,c],3) : the shortest path.

and this is my code :

DOMAINS
    list=Symbol *

PREDICATES
    distance(Symbol, Symbol)
    path1(Symbol, Symbol, list, integer)
    path(Symbol, Symbol,list, list, integer)
    distance(Symbol, list, integer)
    member(Symbol, list)
    shortest(Symbol, Symbol, list, integer)

CLAUSES
    distance(a, b, 1).
    distance(a, c, 2).
    distance(b, d, 3).
    distance(c, d, 4).
    distance(b, c, 5).
    distance(b, a, 1).
    distance(c, a, 2).
    distance(d, b, 3).
    distance(d, c, 4).
    distance(c, b, 5).

    member(X, [X|T]).
    member(X, [Y|T]) :- member(X, T).

    absent(X, L) :-
        member(X, L),
        !,
        fail.
    absent(_, _).

    /* find all paths */
    path1(X, Y, L, C) :- path(X, Y, L, I, C).
    path(X, X, [X], I, C) :- absent(X, I).
    path(X, Y, [X|R], I, C) :-
        distance(X, Z, A),
        absent(Z, I),
        path(Z, Y, R, [X|I], C1),
        C = C1 + A
        .

    /* to find the shortest path */
    shortest(X, Y, L, C) :-
        path(X, Y, L, C),
        path(X, Y, L1, C1),
        C < C1.

回答1:

This shows the shortest path and it's weight:

edge(a,b,6).
edge(a,c,1).
edge(b,d,5).
edge(c,e,4).
edge(c,f,1).
edge(d,h,3).
edge(e,h,7).
edge(f,g,2).
edge(g,h,1).

path(X,Y,M,[Y]) :- edge(X,Y,M).
path(X,Y,P,[Z|T]) :- edge(X,Z,M),path(Z,Y,N,T),
            P is M+N.

pravilo(X,Y,Z) :-  assert(min(100)),assert(minpath([])),!,
                path(X,Y,K,PATH1),
                (min(Z),K<Z,
                retract(min(Z));assert(min(K))),
                minpath(Q),retract(minpath(Q)),
                assert(minpath([X|PATH1])),
                fail.

?- pravilo(a,h,X);
    write("Minimal Path:"),
    minpath(PATH),
    write(PATH),
    nl,
    write("Path weight:"),
    min(Z),
    write(Z).

回答2:

Without knowing what the actual problem is, I can at least suggest that maybe shortest() and path() should take a maximum-length parameter that short-circuits the search.

Also, shortest() doesn't find the shortest path. It finds, for every possible pair of paths, the shortest of each pair.

来源：https://stackoverflow.com/questions/2564058/search-all-paths-and-the-shortest-path-for-a-graph-prolog