Prolog. In a query, how to put a condition on a variable that I do not want in the results?

Question

Imagine that I have the following knowledge base which gives for each person his first name and his age.

person(mary, 39).
person(john, 24).
person(sandy, 17).

Now, I want to retrieve all the persons that are older than 20 years. Furthermore, I just want to collect their first names and not their age. Here, I want to retrieve mary and john.

How to do this generally in Prolog and more specifically in SWI-Prolog?

If we use a variable which is not anonymous, like:

?- person(X, Y), Y > 20.

Prolog will give me the values for both X and Y and I do not want Y.

I cannot use the anonymous variable _ because Prolog cannot link its two instantiations. The following gives an error:

?- person(X, _), _ > 20.

So, how to do this?

Scrubland

Answer 1

Why don't you define a predicate

ofintrest(X):- person(X,Y),Y>20.

an query

ofintrest(X).

If you don't want to define a predicate you could also use double negation

person(X,_) ,\+(\+ (person(X,Y), Y>20))

Antalya

Thanks for these 2 solutions, The first one has the drawback that I must modify my knowlege base. The second one is really the answer I was searching. – Scrubland

Answer 2

This answer directly follows up on this previous answer by @danielp.

With the prolog-toplevel of swi-prolog you can have either one:

• show answer substitutions of all variables (default)

• do not show answer substitutions for variables like _A

For detailed information, read the manual of the Prolog processor you are using!

For SWI: Environment Control (Prolog flags). current_prolog_flag/2. set_prolog_flag/2.

stefan@Lenovo ~ $ swipl
Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 7.3.15)
...

?- current_prolog_flag(toplevel_print_anon, Flag).   % get default
Flag = true.

?- _A = 1.
_A = 1.

?- _A = 1, X = _A.
_A = X, X = 1.

?- set_prolog_flag(toplevel_print_anon, false).      % toggle flag
true.

?- current_prolog_flag(toplevel_print_anon, Flag).
Flag = false.

?- _A = 1.   % watch out!
true.

?- _A = 1, X = _A.
X = 1.

?- set_prolog_flag(toplevel_print_anon, true).       % restore flag
true.

?- current_prolog_flag(toplevel_print_anon, Flag).
Flag = true.

?- _A = 1.
_A = 1.

?- _A = 1, X = _A.
_A = X, X = 1.

Boudicca

@MathieuVidal. The Prolog toplevel (REPL) is quite versatile; it helps me do a lot of I/O implicitly—simply by writing queries and reading answers—as opposed to using the corresponding read_term / write_term API. – Boudicca

@MathieuVidal. Want info on how other Prolog processors (not SWI or SICStus) behave in this regard? – Boudicca

Answer 3

You can define a predicate as already posted in CAFEBABE's answer. Alternatively, you can also give a name that starts with _ to variables whose values should not appear in the answer (as you already noted, occurrences of _ are always distinct variables):

person(X,_Age), _Age > 20.

Update: This is specific to the Prolog implementation. It works for SICStus, but not for SWI by default (see repeat's answer).

Chantell

This solution is perhaps good for some implementations of Prolog but it does not work with SWI-Prolog. There, variables starting with _ are just used to remove a warning given by the compiler and their values appear in the answer. Anyway, your answer could be effective for other implementations of Prolog. – Scrubland

@MathieuVidal: You are right, I'm using SICStus and there it works the way I described. I did not consider that other implementations handle it differently. – Chantell

s(X): also works with SWI, but needs to be explicitly enabled. see my post how it can be done. – Boudicca