CODEWITHSUNDEEP
prolog
ArtVandelay
ArtVandelay

Reputation: 21

Proper way to Declare and Search List in Prolog?

I'm new to Prolog. Is it possible to "declare" a list as a fact in Prolog, and then access it easily? I'm working on a family-tree type problem. To start, I would like to declare a bunch of males as belonging to a list of males. I then would like to be able to query if a particular person is a male. Here is the code I've written so far:

% ------------------------------------------------------------------------- %
% Facts:

males([john, joseph, aaron, peter, paul, mark, ben, adam, daniel]).


% ------------------------------------------------------------------------- %
% Rules:

% Member of a List
member(X, [X|_]).
member(X, [_|T]) :- member(X, T).

male(X) :- member(X, males).

However, my output is not as expected:

21 ?- male(john).

false.

I know my member() rules are working, since I can produce the following result:

23 ?- member(john, [peter, paul, john]).

true

Can anyone point me in the right direction? Thank you in advance.

Upvotes: 0

Views: 193

Answers (2)

user1812457
user1812457

Reputation:

The usual proper way to do this in Prolog would be to declare your list as a table of facts:

male(john).
male(joseph).
% etc

If you do it like this, you have already defined your male/1 and can query it either like this:

?- male(joseph).
true.

Or like this:

?- male(M).
M = john ;
M = joseph ;
% etc

You can start with Prolog by reading through the first chapter of "Learn Prolog Now!". Its first example is exactly the same as your problem.

Upvotes: 1

Nan Montaño
Nan Montaño

Reputation: 51

I'm not an expert in prolog but I know a solution to this (could be not the only one)

male(X) :- males(Y), member(X, Y)

The way I understand it is that predicates doesn't return variables, they assign values to the parameters given, so Y now have been assigned the array of males and now you can look for X in Y

Upvotes: 0

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