CODEWITHSUNDEEP
listprolog
shurrok
shurrok

Reputation: 831

Assign variable in list

I've got a problem with Prolog lists. Let's say I've got this predicate: array(p, [A,B,C]). When I do: array(p,X). I got: X = [_,_,_]

Now, considering I've got this predicate: p1(1) :- array(p1, [1,B1,C1]). I expected to get: X = [1,_,_]

but instead, the result is the same as before. Is such a thing even possible in Prolog? Another question is if somehow we can set these values, could we overwrite these values in the same way? I understand that in the prolog variables are assigned only once but I would like to somehow get a dynamic list.

Upvotes: 0

Views: 87

Answers (1)

Daniel Lyons
Daniel Lyons

Reputation: 22803

I'm not sure what you mean by "paradigm," and I'm very unclear on what you're trying to do with this code. If you have this at the toplevel:

array(p, [A,B,C]).

you are defining a fact array/2, which associates p with a list of three uninstantiated variables. Your first query amounts to retrieving this fact.

Your second "paradigm" is really the definition of a rule or predicate p1/1, which takes a single argument, which must be 1 for the rule to fire. The body of this second predicate is a call to the predicate array/2 which is definitely going to fail. I don't see how you could possibly get the same result as before, because you defined array(p, ...) before and now you are looking for array(p1, ...). Furthermore, there is no X in your second query, so there is no reason for X to appear in the result, and it definitely would not, even if you had called array(p, ...) instead of array(p1, ...).

I think what you're trying to do here is probably set up some kind of set of three variables and then unify each of them in turn as you proceed along some calculation. To do something like that is possible and easy in Prolog, but the fact database is not going to participate in this process really. You're going to have to write predicates that pass your variables along to other predicates that will unify them and return them bound. None of this is very hard, but it looks like you're going to have to go back and understand the fundamentals here a little better. You're far enough off track here that I don't think anyone can really answer your question as stated, because there's too much confusion in it.

Upvotes: 2

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