CODEWITHSUNDEEP
prologclpfd
halcyon10
halcyon10

Reputation: 53

Constraint predicate not starting with the maximum value

I'm trying to get a match for the sum of the first Y elements of a certain list, for Y in the 1..10 range, and for the maximum of the sum. I would like the first match to be Res=10,Y=10, but it is just giving answers starting from Y=1, and increasing to Y=10. What am I missing here?

get_max(Res,Y):-
        Y in 1..10,
        add_list([1,1,1,1,1,1,1,1,1,1],Y,0,Res),
        labeling([max(Y)],[Y,Res]).

add_list(_,0,Res,Res).
add_list([H|Rest],C,Temp,Final):-
        NewTemp #= H+Temp,
        NewC #= C-1,
        add_list(Rest,NewC,NewTemp,Final).

?- get_max(Res,Y).
Res = Y, Y = 1 ;
Res = Y, Y = 2 ;
Res = Y, Y = 3 ;
Res = Y, Y = 4 ;
Res = Y, Y = 5 ;
Res = Y, Y = 6 ;
Res = Y, Y = 7 ;
Res = Y, Y = 8 ;
Res = Y, Y = 9 ;
Res = Y, Y = 10.

Upvotes: 1

Views: 63

Answers (2)

CapelliC
CapelliC

Reputation: 60004

You could force labeling/2 to take effect swapping the goals in this way:

get_max(Res,Y):-
        [Y,Res] ins 1..10,
        labeling([max(Y)],[Y,Res]),
        add_list([1,1,1,1,1,1,1,1,1,1],Y,0,Res).

?- get_max(Res,Y).
Res = Y, Y = 10 ;
Res = Y, Y = 9 .

but this requires the declaration on Res' domain as well.

To be true, seems more a bug than a feature. After all, the solution order should be as you declared, as advertised by library(clpfd). Otherwise, the clauses' order again come back to bite us...

Upvotes: 1

Isabelle Newbie
Isabelle Newbie

Reputation: 9378

?- add_list([1,1,1,1], Y, 0, Res).
Y = Res, Res = 0 ;
Y = Res, Res = 1 ;
Y = Res, Res = 2 ;
Y = Res, Res = 3 ;
Y = Res, Res = 4.

This goal enumerates values for Y and Res in this order. Labeling is not needed because the values are sufficiently determined. Later labeling also cannot force a different order once this goal enumerates the solutions like this.

(Also, if all you want to do is sum a list, you don't need this many arguments to the predicate. If you named them better, you might also see this. Your C had better be named UselessCounterFromZeroTowardsNegativeInfinity. Then you could eliminate it.)

Upvotes: 1

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