Prolog dynamic constraints

2
votes

I have a program in SICStus Prolog that runs the following constraints:

processList([Elem1,Elem2|_], X) :-
  (Elem1 #= 0 #/\ Elem2 #= X) #\/ 
   Elem1 #= X.

But I need to have it set up dynamically 

processList([Elem1,Elem2,Elem3|_], X) :-
  (Elem1 #= 0 #/\ Elem2 #= 0 #/\ Elem3 #= X) #\/ 
  (Elem1 #= 0 #/\ Elem2 #= X)                #\/
   Elem1 #= X.

And if I call it with 4 elements I will have a bigger restriction, but the pattern is always the same.

I have already looked into the table predicate (tuples_in/2, in SWI-Prolog), but that creates the need of me having to calculate all the possible valid combinations of values.

Is there any other way I can do this?

1
prologrestrictionclpfd

1 Answers

2
votes

Using built-in predicate append/3 and Prolog library predicate maplist/2 we write:

:- use_module(library(lists)), [maplist/2]).

processList(Zs, X) :-
   append(Prefix, [X|_], Zs),   % `Zs` comprises `Prefix`, followed by `X`
   maplist(=(0), Prefix).       % all items in `Prefix` are equal to 0

See it in action with SICStus Prolog 4.3.2!

| ?- processList(Zs, X).
Zs = [X|_A] ? ;
Zs = [0,X|_A] ? ;
Zs = [0,0,X|_A] ? ;
Zs = [0,0,0,X|_A] ? ;
Zs = [0,0,0,0,X|_A] ? ;
...