Sum values of variables? - prolog

Question

I'm new with Prolog, and I'm trying to create a predicate that find a specific fact and sum the values.
This is my predicate:
position(X,T,P1,P2,P3,P4) :- object(X,C1,C2,C3,C4,T1),
T>T1,
move(X,S1,S2,S3,S4,T2),
T2>T1,
T2=<T,
P1 is C1+S1,P2 is C2+S2,P3 is C3+S3,P4 is C4+S4.
And what i get is this:
?- position(car0,31,P1,P2,P3,P4).
P1 = 930,
P2 = 278,
P3 = 1057,
P4 = 365 ;
P1 = 943,
P2 = 288,
P3 = 1058,
P4 = 370 ;
false.
I want the sum of all(S1,S2,S3,S4) values of move with the (C1,C2,C3,C4) of object but I don't know how to do this.

Answer 1

You try to do too much in a single predicate. It usually is better to split the logic in several predicates that each perform a certain task.
Cummulative coordinates
We can first create a list of list of values with findall/3 [swi-doc]:
all_moves(X, T, Diffs) :-
findall([T1, C1, C2, C3, C4], (move(X,C1,C2,C3,C4,T1), T1 <= T), Cs),
sort(Cs, Diffs).
We here thus sort the move/6s on timestamp, and create a list of 5-tuples with [T1, C1, C2, C3, C4] with T1 the time stamp, and C1, C2, C3, and C4 the timestamps.
Next we can define a cummulative sum function by making use of plus/3 [swi-doc] and maplist/3 [swi-doc]:
cumsum(P, _, P).
cumsum(P, [[_|D]|R], P2) :-
maplist(plus, P, D, P1),
cumsum(P1, R, P2).
then we can thus obtain the positions of an object with:
position(X,T,P1,P2,P3,P4) :-
object(X, C1, C2, C3, C4, T1),
T > T1,
all_moves(X, T, Mvs),
cumsum([C1, C2, C3, C4], Mvs, [P1, P2, P3, P4]).
Final coordinates
If we only need the final coordinates, we can improve the above. In that case, the order of the moves is irrelevant, as long as these are properly filtered. So we can define allmoves_unsorted/3, like:
all_moves_unsorted(X, T, Diffs) :-
findall([C1, C2, C3, C4], (move(X, C1, C2, C3, C4, T1), T1 <= T), Diffs).
Then we can sum these up with the object, with foldl/4 [swi-doc]:
final_position(X, T, S1, S2, S3, S4) :-
object(X, P1, P2, P3, P4, T1),
T1 < T,
all_moves_unsorted(X, T, Diffs),
foldl(maplist(plus), Diffs, [P1, P2, P3, P4], [S1, S2, S3, S4]).