How to get the optimal value of objective function

[auz107]

I am solving a simple LP in pyomo (using cplex) and need to write the optimal value of the objective function into a file. While I can get the optimal values of variables, I couldn't find anyway of doing the same thing for objective function value. Any help is greatly appreciated.

[Watson, Jean-paul]

Assuming you are scripting (it sounds like you are), and have loaded the results into your Pyomo model…

You just need to say something like: obj_value = my_model.my_objective()

The () operator computes the value of the objective on-the-fly, using current values for the variables.

jpw

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[auz107]

Thanks. This is something that is missing in the manual and is not easy to find. I was able to find another way of getting the value of objective function by looking at details of results.Solution:

results = opt.solve(instance)

obj_value = results.Solution.Objective.__default_objective__['value']

but obviously the way you did is much easier.

[JolleG]

Hi,

I agree that this is still not well documented. I tried to print the value of the objective function as

print instance.OBJ,

when my objective was declared as model.OBJ. But this just printed: "OBJ". Furthermore,

print instance.OBJ.value

gave me: 'None'. However, with the help of auz107 I tried

print instance.OBJ()

which gave me the desired result.