Question

I am trying to solve a specific problem using prologs constraint solvers, and I’m stuck 😀
A more general version of my problem requirement is the like this:

:- lib(ic).:- lib(ic).
solve( [A1*X+B1*Y=C1, A2*X+B2*Y=C2] ):-
X::[0..999],
Y::[0..999],
X #\= 0,
Y #\= 0,

A1*X+B1*Y#=C1, % line1
A2*X+B2*Y#=C2. % line2

And this is the query/goal I use:

solve( [2*X+3*Y=5, 3*X+2*Y=5] ).

And the program will compute the values of X and Y (in this case X=1, Y=1 is the solution).
What I am thinking is, what if the number of arguments in the goal/query can vary..in this case, my prolog program needs to have a dynamic suspended goals in place of lines commented with %line1 and %line2..

Question is, how do i make these expressions delayed..? I do not want to hard code these in the problem and think that only two expressions will be passed over through the goal..

Hope the question is clear.
Thanks.

Answer 1

I think you mean something like:

:- lib(ic).

solve([]).
solve([Eq | Eqs]) :-
        term_variables(Eq, Vars),
        Vars :: [0..999],
        ( foreach(Var, Vars) do Var #\= 0 ),
        Eq  = (Lhs  = Rhs),
        Eq0 = (Lhs #= Rhs),
        call(Eq0),
        solve(Eqs).

Note that this is ECLiPSe-CLP specific code (the foreach-loop which could be translated to a recursive helper predicate, of course, and the IC library).

Also note that call(Eq0) and writing just Lhs #= Rhs should have the same effect (afaik). But when the variables in Lhs #= Rhs are solver variables, that seems not to the case, at least I encountered such a problem some months ago using lib(cplex).