Question

New to logic programming (Prolog). Come across a simple question but dont know how to code it in prolog.

Question is like you have several arguements: argument(a),argument(b)…, and several attack relations like attack(a,b) which means argument a attacks argument b. So given an argument I want to find out if it is a grounded one. “Grounded” for an argument a means if b attacks a, then there exists another argument, say c attacks b. If no argument attacks c then we say a and c are grounded.

Can you give an example how to implement this grounded/1 program to achieve this goal.

Not sure I make it clear….But welcome to give any advice (or code)!!

Answer 1

What I’ve understand from your explanation, an argument is grounded when there are no other grounded arguments attacking it.

You can define a procedure grounded/1 which obeys this rule straightforward in prolog:

grounded(A):-
  argument(A),    % A is an argument
  \+              % for which there does not exist
  (
    attack(B, A), % an attacker
    grounded(B)   % which is grounded
  ).

[Edit after comment by OP]:
If you have to deal with cycles, then you will probably need to keep a list of visited “attacks”, no forbid cycles:

grounded(A):-
  grounded(A, []).


grounded(A, L):-
  argument(A),
  \+
  (
    attack(B, A),
    \+ member(B, L),  % Here we forbid cycles
    grounded(B, [A|L])  % We add the current argument to the list of forbidden arguments
  ).