Originally posted by: VeraM

Hello!

I am trying to model a route planning problem in CP with MiniZinc and Gecode as a solver. If I run it without optimization function it is clear that solver returns absolutely identical solutions. All decision variables which I define have the same values. Why it can happen?
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Originally posted by: SystemAdmin

Hello.

Are you saying that two different solvers find the same solution? It can be no surprise if there are only few solutions. It could be also that the solvers are using similar search algorithm, but I would doubt that.

Anyway, I'm able to answer questions only about CP Optimizer, I do not know details about other solvers.

Best regards, Petr
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#DecisionOptimization

Originally posted by: VeraM

Thank you very much for so fast reply!
I am using only one solver. For one model I get several solutions but some of them are completely identical. As I understand all solutions should be different at least by one variable. For example, here are two solutions from one run:

aTime = array1d(1..16, 0, 12, 24, 47, 65, 0, 0, 0, 0, 11, 23, 40, 58, 65, 82, 0);
cost = 30904;
move = array1d(1..16, 1, 3, 1, 4, 1, 8, 8, 8, 1, 5, 7, 1, 6, 2, 1, 8);

---

aTime = array1d(1..16, 0, 12, 24, 47, 65, 0, 0, 0, 0, 11, 23, 40, 58, 65, 82, 0);
cost = 30904;
move = array1d(1..16, 1, 3, 1, 4, 1, 8, 8, 8, 1, 5, 7, 1, 6, 2, 1, 8);

---

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#DecisionOptimization

Originally posted by: SystemAdmin

I guess you have to ask Gecode guys about that to get reliable answer. It may be a bug, but it is not certain.
For example, if there is no objective function then CP Optimizer do not guarantee to report each solution only once. Typical reason for reporting the same solution multiple times is usage of multiple search strategies (typically on multiple CPUs or CPU cores).

Best regards, Petr
#ConstraintProgramming-General
#DecisionOptimization

Originally posted by: VeraM

I will ask them to be sure.However, probably, I get the same solution multiple times exactly because of the reason you mentioned. Thank you very much for quick and useful answer!

Respectfully,
Vera
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#DecisionOptimization