Question

I want to calculate , in two lists (same lenght), the number of elements that are equal and in the same position.
For example:
Lets say we have the lists A = [3,6,7,9] and B = [2,6,4,9], i want to be printed in the screen the message, “2 bulls found”.

So far i have made this:

bulls([],[]).
bulls([Ha|Ta],[Hb|Tb]) :-
    Ha = Hb,
    writeln('bull found'),
    bulls(Ta,Tb);
    bulls(Ta,Tb).

Every time an element that exists in the same place in both lists, the message ‘bull found’ is printed.
And in my mind i want to make something like this:

bulls([],[],_).
bulls([Ha|Ta],[Hb|Tb],Counter) :-
    Ha = Hb,
    NewCounter is Counter + 1,
    bulls(Ta,Tb,NewCounter);
    bulls(Ta,Tb,NewCounter).

bulls(List1,List2):- bulls(List1,List2,0).

bulls is called from another rule that passes the lists two it.
How do i make it so it prints the value of ‘bulls’ to the screen. Any help?

---

Edit
So after Suki’s post, i made this test program testing 2 lists:

bulls([],[],X), write(X), write('bulls found'),fail.
bulls([Ha|Ta],[Hb|Tb],Counter) :-
    Ha = Hb,
    NewCounter is Counter + 1,
    bulls(Ta,Tb,NewCounter);
    bulls(Ta,Tb,NewCounter).

check(List1,List2):- 
    bulls(List1,List2,0).


start:-
    A=[1,1,1,1],
    B=[2,1,2,1],
    writeln(A),writeln(B),
    check(A,B).

and i get this result

1 ?- start.
[1,1,1,1]
[2,1,2,1]
ERROR: bulls/3: Arguments are not sufficiently instantiated

what am i doing wrong?

Answer 1

Regarding your edited program:

The first clause is not a clause, it’s a goal! It should look like this:

bulls([],[],X) :- write(X), write(' bulls found').

You should drop the fail, btw.

In the second clause, you need to use “if-then-else”, and use Counter instead of NewCounter in the “else”-branch:

bulls([Ha|Ta],[Hb|Tb],Counter) :-
  (
    Ha == Hb
  ->
    NewCounter is Counter + 1,
    bulls(Ta,Tb,NewCounter)
  ;
    bulls(Ta,Tb,Counter)
  ).