Question

:-dynamic listofQuestions/2.
myrule:-
    write('P = '), write(Percent), write('-'),write(X),
    ( listofQuestions(Percent,X) -> true ; assert(listofQuestions(Percent,X)) ),

The code snippet might not be required to answer my question.

I want to assert to a blank ‘listofQuestions’ everytime I call my rule. This only happens if I close my prolog window and restart it.

Any suggestions?

Answer 1

abolish/1 removes all clauses of a given predicate from the database. Hence, just add a call to abolish(PredName/Arity) whenever you need to remove the information about this predicate. Beware that after abolishing the call to the dynamic predicate does not fail but reports an error.

12 ?- f(X,Y).
false.

13 ?- assert(f(a,b)).
true.

14 ?- f(X,Y).
X = a,
Y = b.

15 ?- abolish(f/2).
true.

16 ?- f(X,Y).
ERROR: user://2:67:
        toplevel: Undefined procedure: f/2 (DWIM could not correct goal)

In SWI-Prolog, abolish works on static procedures, unless the prolog flag iso is set to true. If you intend to remove only dynamic predicates, you should better try retractall. Observe that in this case removal does not lead to an error being reported but to a failure.

17 ?- [user].
:- dynamic f/2.
|: 
% user://3 compiled 0.00 sec, 264 bytes
true.

18 ?- f(X,Y).
false.

19 ?- assert(f(a,b)).
true.

20 ?- f(X,Y).
X = a,
Y = b.

21 ?- retractall(f(X,Y)).
true.

22 ?- f(X,Y).
false.