Question

In Prolog, how can I implement graph algorithm in order to find all path in order to implement travel salesman problem in directed graph ?

example :

                                                                         graph
                    expected input     expected output                 X -----> Y
    start X             X  Y               X Z                         Y -----> T
    end   Z             Y  T               X Y Z                       T -----> Z
                        T  Z               X Y T Z                     Y -----> Z
                        Y  Z                                           X -----> Z
                        X  Z

As you know, in directed graph, there could be a cycle. However, no need to pass same point two times.

             graph             expected output             
           X ----> Y            
           Y ----> X               X Y Z
           Y ----> Z 

Why I am elimineting this case is because ;

      output :

      X Y X Y ...   Z
              ^^^
              god knows this length ( when program terminates )
              termination is np problem

Answer 1

I placed some comment that explain what the code does…

% your directed graph
edge(x, y).
edge(y, t).
edge(t, z).
edge(y, z).
edge(x, z).

% get a path from start to end
path(Start, End, Path) :-
    path(Start, End, [Start], Path).

% when target reached, reverse the visited list
path(End, End, RPath, Path) :-
    reverse(RPath, Path).

% take non deterministically an edge, check if already visited before use
path(Start, End, Visited, Path) :-
    edge(Start, Next),
    \+ memberchk(Next, Visited),
    path(Next, End, [Next|Visited], Path).

test:

?- path(x,z,P).
P = [x, y, t, z] ;
P = [x, y, z] ;
P = [x, z] ;
false.