Prolog help | how come 'tony' is true when it should be false? - list

Question

From the facts I have, 'tony' should not be true for pass(tony), but I am unsure what I am doing wrong.
I have an atom pass(Student) that passes if a Student has the following: their mark in programming > 29, they haven't failed an exam/none of their exam scores < 30, and if their average score for all their exams > 39
I also have an ERROR No permission to modify static procedure 'length/2'. I don't know what this is referring to but I would appreciate it if you could point me in the right direction.
I won't include all the facts, but here is an example of a student's:
coursework(tony, 57).
programming(tony, 65).
e1(tony, 20).
e2(tony, 80).
e3(tony, 90).
marks(tony, [20, 80, 90]).
length([], 0).
length([_|Tail], List + 1):-
length(Tail, List).
sum([], 0).
sum([X|Tail], Sum):-
sum(Tail, Temp),
Sum = Temp + X.
avg(List, Avg):-
sum(List, Sum),
length(List, Length),
Length > 0,
Avg is Sum / Length.
examFail(Student):-
e1(Student, Mark),
Mark < 30;
e2(Student, Mark),
Mark < 30;
e3(Student, Mark),
Mark < 30.
pass(Student):-
programming(Student, Mark),
Mark > 29,
\+ examFail(Mark),
marks(Student, List),
avg(List, Avg),
Avg > 39.
Is there something I am doing wrong?

Answer 1

As written in the title, Tony can neither be true nor false, Tony is a variable because it starts with a capital letter.
I have an atom pass(Student)
pass(Student) is a predicate, not an atom. An atom is a stringy thing like a or tony. A predicate can indeed yield true or false.
I also have an ERROR No permission to modify static procedure 'length/2'.
This is because you try to define
length([], 0).
length([_|Tail], List + 1):-
length(Tail, List).
But length/2 is a predicate that is built-in, to determine/set/check the length of a list No redefining allowed!
Evaluation is not done like this:
Sum = Temp + X.
This is unification ("The syntactic structures left and right shall be the same"). You want is:
Sum is Temp + X.
Which is correctly use here btw:
Avg is Sum / Length.
For example:
sum_list(L,Out) :-
sum_list(L,0,Out).
sum_list([],R,R).
sum_list([X|Xs],Rrun,Rfin) :-
Rnex is Rrun + X,
sum_list(Xs,Rnex,Rfin).
This ain't gonna work:
examFail(Student):-
e1(Student, Mark),
Mark < 30;
e2(Student, Mark),
Mark < 30;
e3(Student, Mark),
Mark < 30.
Mark is always the same "thing" in this whole clause. The above says that the Student must have the same Mark in all exams, and that Mark must be less than 30. This inequality is redundantly tested 2 more times. Use different variable names:
examFail(Student):-
(e1(Student, MarkE1), MarkE1 < 30);
(e2(Student, MarkE2), MarkE2 < 30);
(e3(Student, MarkE3), MarkE3 < 30).
Or even better, if data were stored in a more "normal form" (as in RDBMS):
marks(e1, tony, 20).
marks(e2, tony, 80).
marks(e3, tony, 90).
then one can use library(aggregate)
examFail(Student):- aggregate(min(M), marks(_,tony,M), Min), Min<20.
(I think, need to test)
This ain't gonna work either:
pass(Student):-
programming(Student, Mark),
Mark > 29,
\+ examFail(Mark),
marks(Student, List),
avg(List, Avg),
Avg > 39.
You call examFail(Mark). That should be examFail(Student).