CODEWITHSUNDEEP
prologtransitive-closure
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Reputation: 49

Prolog, how to add a clause if the base case is accepted

I am new to Prolog. I am trying to write a predicate that is true when there is a path from X to Z, including X to Y and Y to Z.

For example

    foo(b,Y,[[a,b],[b,c],[b,d],[d,e]]).
    Y = c ;
    Y = d ;
    Y = e ;

I can write the code which is true when there is a path from X to Y but can't figure out how to add the code to get from Y to Z if the base case is true. 

    foo(Y, X, [Y, X]).
    foo(Y, X, [Head|_]) :- foo(Y, X, Head).
    foo(Y, X, [_|Tail]) :- foo(Y, X, Tail).

Upvotes: 2

Views: 190

Answers (1)

false
false

Reputation: 10102

(Assuming you want general transitive closure)

conn(XYss, X, Y) :-
   member([X,Y], XYss).

foo2(X, Y, XYss) :-
   closure(conn(XYss), X, Y).

using closure/3. Alternatively, using library(lambda):

foo2(X, Y, XYss) :-
   closure({XYss}+\Xi^Yi^member([Xi,Yi], XYss), X, Y).

And taking your question literally, you only want:

foo3(X, Y, XYss) :-
   member([X,Y], XYss).
foo3(X, Z, XYss) :-
   member([X,Y], XYss),
   member([Y,Z], XYss).

Upvotes: 2

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