Why I get diffrents results??
Betty St-Pierre
Hello,
I get diffrents results for the same objective funcition when I use the CVRP model with 2 index decision variable x (i,j) and the CVRP model with 3 index decision variable x (k,i,j) and also when I change the Cplex version, whereas it suppose having the same results.
Example: optimal distance with CVRP model with 2 index= 33 km CPLEX 9.1.3
optimal distance with CVRP model with 3 index =30.98 km CPLEX 9.1.3
optimal distance with CVRP model with 2 index=31.7 km CPLEX 12.3
optimal distance with CVRP model with 3 index =31.7 km CPLEX 12.3
Can some body explain why these differences?
Thank you for your help.
Betty
Robert Fourer
More information would be necessary to explain the difference in optimal values for your models. Normally I repeat all of the runs with
option cplex_options 'mipdisplay=2';
(or with 'mipdisplay=2' added to the cplex_options string that is already being used). Then I look closely at the resulting listings to see what is different. For example some of the runs may have stopped before optimality was proved, or they may have stopped with different optimality gaps (the last column) -- by default CPLEX terminates as soon as it finds any solution with gap < 0.01%.
It is tempting however to disregard any results from CPLEX 9.1.3 because it is so old. With the recent version CPLEX 12.3 the two models do get the same optimal value.
If you are concerned that CPLEX might have a bug affecting your model (though that is unlikely), another possibility is to arrange to have the same models tested using another fast MIP solver such as Gurobi.
Bob Fourer
From: am...@googlegroups.com [mailto:am...@googlegroups.com]
On Behalf Of Betty St-Pierre
Sent: Monday, August 06, 2012 8:19 AM
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Subject: [AMPL 6045] Why I get diffrents results??
Betty St-Pierre
Thank you Bob for responding.
I run the same model for 4 and 7 vertecies with option cplex_options ' mipdisplay=2' as you suggest but I still get the difference between gurob and cplex. In attached the results.
the difference in optimal values for the model with 2 index variable decision aned 3 index variable decision is not regular as you can see in attached result-3and 2 index file
If more information would be necessary tell me what you need exactely.
Thanks again.
Betty
