Predicate returns true and not some value - prolog

Question

I am trying to make a predicate that takes two numbers K, Num (0 when you use the predicate, it changes after each recursion), a list containing numbers from 1 to K and an associative tree with all keys from 1 to K having values 0. When all numbers from 1 to K are found on the list (Num has the numbers found until then) it returns the rest of the list NL and an associative tree where the value of each key is the times each number is found. It should be used like this:
first(3, 0, [1,3,1,3,1,3,3,2,2,1], T, NL, NT)
where T is the tree described above.
Here is my code:
first(K, K, L, T, L, T):- !.
first(_, _, [], _, [], NT) :-
empty_assoc(NT), !.
first(K, Num, [H|L], T, NL, NT) :-
get_assoc(H, T, V),
Newv is V+1,
put_assoc(H, T, Newv, TT),
V=:=0 -> Newnum is Num+1; Newnum is Num,
first(K, Newnum, L, TT, NL, NT).
My problem is than it returns true instead of the values of NL and NT.

Answer 1

The main problem here is operator precedence. If we ask the interpreter to generate a listing., we get:
first(A, A, B, C, B, C) :- !.
first(_, _, [], _, [], A) :-
empty_assoc(A), !.
first(G, E, [A|H], B, I, J) :-
( get_assoc(A, B, C),
D is C+1,
put_assoc(A, B, D, _),
C=:=0
-> F is E+1
; F is E,
first(G, F, H, _, I, J)
).
This learns us that only in case C =:= 0 does not hold, we will make a recursive call to first/6. This is probably not your intention. Since we only make a recursive call in case the condition does not hold, the TT
If we use brackets, like:
first(K, K, L, T, L, T):- !.
first(_, _, [], _, [], NT) :-
empty_assoc(NT), !.
first(K, Num, [H|L], T, NL, NT) :-
get_assoc(H, T, V),
Newv is V+1,
put_assoc(H, T, Newv, TT),
(V=:=0 -> Newnum is Num+1; Newnum is Num),
first(K, Newnum, L, TT, NL, NT).
With that fixed, we obtain an error:
?- first(3, 0, [1,3,1,3,1,3,3,2,2,1], T, NL, NT)
| .
ERROR: Arguments are not sufficiently instantiated
ERROR: In:
ERROR: [13] throw(error(instantiation_error,_7048))
ERROR: [9] assoc:get_assoc(1,_7080,_7082) at /usr/lib/swi-prolog/library/assoc.pl:178
ERROR: [8] first(3,0,[1,3|...],_7112,_7114,_7116) at /tmptest.pl:5
ERROR: [7] <user>
This means that we aim to call get_assoc/3, but with a non-constructed associative array. Note that the empty_assoc/1 is not constructed, it is only constructed at the end when the lists are exhausted.
I think the core problem here is that you are doing too much at once. We can make small predicates that each do limited work.
For example, we can generate an associative array that maps all values between 1 and K to 0 with:
gen_assoc(K, A) :-
empty_assoc(E),
gen_assoc(K, E, A).
gen_assoc(0, A, A).
gen_assoc(K, A, C) :-
K > 0,
put_assoc(K, A, 0, B),
K1 is K-1,
gen_assoc(K1, B, C).
So here gen_assoc(3, A) will unify A with an associative array that maps all numbers from 1 to 3 (both inclusive) to 0.
I leave the rest as an exercise.