Rule writing in Prolog

Question

I would like to write a rule that tells me if a person passed a subject. The minimum grade to pass is 40%. I have successfully written a predicate that tells me if the person passed, however I am confused as to how I would get it to also give me person/subject pairs. i.e. julie/history. Thank you in advance.

% studies( Person, Subject)
% -------------------------
studies( julie, history).
studies( pete, chemistry).


% marks( Person, CourseWork, Exam)
% --------------------------------
marks( julie, 77, 63).
marks( pete, 55, 21).

passed(Person,_Subj):-
%get student work and exam grades
marks(Person, Work, Exam),
%calculate the final student grade
Perc is Work*0.25 + Exam*0.75,
%see if percentage is over 40%
Perc >= 40.

Answer 1

Your fact database is designed in such a way that one person could only participate in one subject (otherwise there is no way to know what marks are for what subject).

With this in mind, here is a simple modification of your code:

passed(Person, Subj):-
    studies(Person, Subj),
    marks(Person, Work, Exam),
    Perc is Work*0.25 + Exam*0.75,
    Perc >= 40.

Test run:

?- passed(X, Y).
X = julie,
Y = history ;
false.

Answer 2

Modify your passed predicate to provide a Person and Subj pair that passes:

passed(Person, Subj):-
    %get student work and exam grades
    marks(Person, Work, Exam),
    %calculate the final student grade
    Perc is Work*0.25 + Exam*0.75,
    %see if percentage is over 40%
    Perc >= 40,
    studies(Person, Subj).

Then collect all of them with findall/3:

passing(PassingStudents) :-
    findall(Person/Subj, passed(Person, Subj), PassingStudents).