Prolog rules and query

Question

I need some help to find the rules and/or query for knowledgebase in Prolog with information about Costumers in a supermarket.

For example I have:

Customer(Name,Age,Sex,Wage).

customer(John,30,M,2000).
customer(Mary,35,F,2500).
customer(Mark,40,M,2500).

invoice(Number, CostumerName, Product, Price).

invoice(001, John, Potatoes, 20).
invoice(002, John, Tomatoes, 10).
invoice(003, Mary, Soap, 50).
invoice(004, Mark, Potatoes, 20).
invoice(005, Mary, Detergent, 15).

Food(Potatoes).
Food(Tomatoes).
CleanProd(Soap).
CleanProd(Detergent).

If I want to find a trend for example, to understand that Female bought more clean products than Male...which kind or what rule and query should I use ?

Answer 1

You can use findall followed by sort to collect unique results of a query (you don't want duplicate invoices in your list), and length to check the length of the collected results.

For example:

findall(X, (invoice(X, C, P, _), customer(C, _, f, _), cleanProd(P)), Xs),
sort(Xs, InvoicesOfWomenBuyingCleanProducts),
length(InvoicesOfMenBuyingCleanProducts, N).

Also, please note that variable start with uppercase, and atoms start with lower case (also applies for predicate names).

If you really want an atom starting with a upper case, you have to surround with single quotes.

Variable: M

Atom: 'M'

Atom: m

For example, you could rewrite your KB in this way:

%   this is not needed, however is useful as
%   a documentation of what your predicate means:
% customer(Name,Age,Sex,Wage).

customer('John',30,m,2000).
customer('Mary',35,f,2500).
customer('Mark',40,m,2500).

% invoice(Number, CostumerName, Product, Price).

invoice(001, 'John', potatoes, 20).
invoice(002, 'John', tomatoes, 10).
invoice(003, 'Mary', soap, 50).
invoice(004, 'Mark', potatoes, 20).
invoice(005, 'Mary', detergent, 15).

food(potatoes).
food(tomatoes).
cleanProd(soap).
cleanProd(detergent).