GUROBI results / solutions interpretation

[Asif Ali Khan Lecturer DCSE]

Dear all,

I have some difficulties in understanding the gurobi results. As obvious, the time it takes to find optimal solution(s) directly depends on my input size. For smaller input size it works fine but for larger, it stucks.

Now, I am interrupting the model after 3 hours. In some case, I get solutions and obj; in other I don't (sol cont is 0). Does this mean no solution has been reached in this duration?

What I am not sure about is, when solution count is 0, I am expecting obj to be 0 as well. However, I am getting solution count: 0 and obj: Obj: 1e+100 (in some cases). Can somebody clarify this please.

Asif

[Daniel Espinoza]

Hi Khan,

Could you post the logs to give you a detailed answer?

Best,

Daniel

[Robert Luce]

Asif,

if the solution count is 0, then Gurobi has not been able to find a solution to your model.  In that case, the attribute ObjVal doesn't have a meaning, and the value 1e100 is simply a placeholder for the value "infinity".  The value of the attribute objVal only has a meaning in terms of the objective function if the value of the attribute SolCount is >0.

Robert

[Asif Ali Khan Lecturer DCSE]

Thank you Robert for the explanation. Another quick thing, I want to export the intermediate results of my model while it is running. I have added the following.

def mycllback(model, where):
  if where == GRB.Callback.MIPSOL:
    for i in range(n):
      print(str(m.getVarByName("v"+str(i))))

Again, for smaller input size it works fine. It gives me intermediate results. However, when the input is large, the simulation stops and gives me 0 for both solution count and obj. Though I have set a time limit of 3 hours, it returns immediately (in few seconds). Any thought on that?

Thanks again.

Asif

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