Hi, everyone.
I am using Cplex 12.4 to solve my MIP model with minimization objective. The optimal solution is 203.
I then add several constraints to the model (they acutally indicate values for some decision variables), and the optimal solution is 200.
It seems impossible theoritically, but Cplex solved both models to optimality without reporting error. I am confused...
I have uploaded log files of these two runs. Thanks for your help!
Topic
This topic has been locked.
4 replies
Latest Post
 20130130T03:14:52Z by SystemAdmin
ACCEPTED ANSWER
Pinned topic Adding constraints improves the optimal solution?
20130129T11:42:12Z

Answered question
This question has been answered.
Unanswered question
This question has not been answered yet.
Attachments
Updated on 20130130T03:14:52Z at 20130130T03:14:52Z by SystemAdmin

ACCEPTED ANSWER
Re: Adding constraints improves the optimal solution?
20130129T22:50:39Z in response to xhanIf you add the number of columns eliminated by presolve and the number of columns in the reduced MIP, you get 12,222 in the first model and 12,230 in the second model. In the process of adding constraints, did you also add variables? That could explain the improved objective value.
Paul
Mathematicians are like Frenchmen: whenever you say something to them, they translate it into their own language, and at once it is something entirely different. (Goethe)
ACCEPTED ANSWER
Re: Adding constraints improves the optimal solution?
20130130T01:12:13Z in response to SystemAdminThank you, Paul!
In the added constraints, I just specify the values for some existing variables in the first model. I didn't introduce new variables
Could this difference between 12,222 and 12,230 caused by the "substitutions" did by "Aggregator"?
Besides, I am not sure, but shouldn't adding variables also degrade the objective value?
ACCEPTED ANSWER
Re: Adding constraints improves the optimal solution?
20130130T03:14:52Z in response to xhanI'm not sue about the substitutions. Adding a variable never hurts the objective value (set it to zero and the previous solution remains feasible) and may help.
Paul
Mathematicians are like Frenchmen: whenever you say something to them, they translate it into their own language, and at once it is something entirely different. (Goethe)


ACCEPTED ANSWER
Re: Adding constraints improves the optimal solution?
20130130T01:59:17Z in response to SystemAdminProblem solved. I tried the latest version of 12.5, and the first model is optimized to 200. Please see the attached log file.
Thanks again for your help!Attachments
