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  • 4 replies
  • Latest Post - ‏2013-01-30T03:14:52Z by SystemAdmin
xhan
xhan
7 Posts

Pinned topic Adding constraints improves the optimal solution?

‏2013-01-29T11:42:12Z |
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!

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Updated on 2013-01-30T03:14:52Z at 2013-01-30T03:14:52Z by SystemAdmin
  • SystemAdmin
    SystemAdmin
    7929 Posts

    Re: Adding constraints improves the optimal solution?

    ‏2013-01-29T22:50:39Z  
    If 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)
  • xhan
    xhan
    7 Posts

    Re: Adding constraints improves the optimal solution?

    ‏2013-01-30T01:12:13Z  
    If 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)
    Thank 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?
  • xhan
    xhan
    7 Posts

    Re: Adding constraints improves the optimal solution?

    ‏2013-01-30T01:59:17Z  
    If 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)
    Problem 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!
  • SystemAdmin
    SystemAdmin
    7929 Posts

    Re: Adding constraints improves the optimal solution?

    ‏2013-01-30T03:14:52Z  
    • xhan
    • ‏2013-01-30T01:12:13Z
    Thank 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?
    I'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)