Re: [eclipse-clp-users] ECLiPSe - Real variables domain propagation problem

From: Kish Shen <kisshen_at_...5...>
Date: Mon, 27 Jul 2009 16:22:51 +0100
Wayne Mac Adams wrote:
> It might be a seperate issue but I am getting an unexpected result from the
> following query also (again assuming ic loaded)
> 
> ?- [A, B] :: 0.0 .. 0.5, C $= A + B, Cost $= -(C), minimize((locate([A, B,
> C], 1.0), get_min(Cost, Min)), Min).
> A = A{0.0 .. 0.5}
> B = B{0.0 .. 0.5}
> C = C{0.0 .. 0.5}
> Cost = Cost{-0.5 .. -0.0}
> Min = -0.5
> There are 2 delayed goals.
> Yes (0.00s cpu)
> 
> Surely the maximum for C is 1.0, and there should be a solution with Cost
> -1.0?
> 
> Am I simply using minimize wrong, or is there another problem?
> 
Hi Wayne,

There is no minimize/2 in lib(ic), and if you were using 
lib(branch_and_bound)'s minimize/2, you have not shown what it prints 
out with the query you gave:
Found a solution with cost -0.5
Found no solution with cost -1.0Inf .. -1.5

notice that the interval between -0.5 and -1.5 was not covered -- this 
should give you the clue to the problem.

If you read the documentation, you will see that miimize/2 is a 
short-hand for bb_min/3, with no options (i.e. run everything with the 
default). One of the option is delta, i.e. the minimum absolute 
improvement required for each step, and the default value is 1.0. Thus,
after finding a solution of -0.5, it will only try to find a solution 
that is at least -1.5, so there is no better solution is found.

Change your query to use bb_min, with a delta option, e.g.

[A,B]:: 0.0..0.5, C $= A + B, Cost $= -C, bb_min((locate([A,B,C], 1.0), 
get_min(Cost, Min)), Min, bb_options{delta:0.5}).

and this will give you Min = -1.0 `solution' [as Joachim pointed out, 
this is strictly not a solution, because you still have delayed goals 
and uninstantiated variables]

Cheers,

Kish

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Received on Mon Jul 27 2009 - 15:23:03 CEST

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