Getting the value of the objective from AMPL console

[Fulya Trösser]

Hi,

I'm working with the AMPL console in the terminal window. I have a MIP for which I defined all the variables and constraints, and finally an objective function as follows:

minimize objective1_UnsatDemand: sum{d in Demands} Unsat[d];
objective objective1_UnsatDemand;

Then I run the "solve;" command, and get the optimal solution. What I want at this point is to fetch the value of objective1_UnsatDemand, define a new constraint:

subject to cstr_ObjUnsat:

sum{d in Demands} Unsat[d] <= objective1_UnsatDemand.val;

and then delete objective1_UnsatDemand and define a new objective function, to run the model with this new constraint above and the new objective function.

However, when I try to do so, I get the error below:

MIP.mod, line 214 (offset 8323):

   cstr_ObjUnsat must be redeclared without objective1_UnsatDemand.val because objective1_UnsatDemand is a defined variable.

context:  sum{d in Demands} Unsat[d] <=  >>> objective1_UnsatDemand.val; <<< 

Any help or workaround is much appreciated!

Thanks in advance,
Fulya

[AMPL Google Group]

Your constraint "subject to cstr_ObjUnsat: sum{d in Demands} Unsat[d] <= objective1_UnsatDemand.val;" depends on objective1_UnsatDemand, so deleting objective1_UnsatDemand will cause an error.

However, a model can have more than one objective function. So in addition to defining cstr_ObjUnsat, you can define

minimize objective2_UnsatDemand: . . . ;
objective objective2_UnsatDemand;

where . . . is replaced by the expression for your second objective function. Then you can run "solve;" again, and the second objective will be optimized.

--
Robert Fourer
am...@googlegroups.com

{#HS:1944645602-110921#}

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[Fulya Trösser]

Dear Robert,

Many thanks for your answer.

I can indeed do that, however, my problem of not being able to use the value of the first objective function as the right hand side of a new constraint persists. How can I do this?

Best regards,

Fulya

[AMPL Google Group]

Try something like this, after solving the first time:

param obj1_unsatdemand;
let obj1_unsatdemand := objective1_UnsatDemand.val;

subject to cstr_ObjUnsat:
   sum {d in Demands} Unsat[d] <= obj1_unsatdemand;

minimize objective2_UnsatDemand: ... ;
objective objective2_UnsatDemand;
solve;

--
Robert Fourer
am...@googlegroups.com

{#HS:1944645602-110921#}

On Wed, Jul 13, 2022 at 6:43 AM UTC, AMPL Modeling Language <am...@googlegroups.com> wrote:

Dear Robert,

Many thanks for your answer.

I can indeed do that, however, my problem of not being able to use the value of the first objective function as the right hand side of a new constraint persists. How can I do this?

Best regards,
Fulya

On Sun, Jul 10, 2022 at 4:28 AM UTC, AMPL Google Group <am...@googlegroups.com> wrote:

Your constraint "subject to cstr_ObjUnsat: sum{d in Demands} Unsat[d] <= objective1_UnsatDemand.val;" depends on objective1_UnsatDemand, so deleting objective1_UnsatDemand will cause an error.

However, a model can have more than one objective function. So in addition to defining cstr_ObjUnsat, you can define

minimize objective2_UnsatDemand: . . . ;
objective objective2_UnsatDemand;

where . . . is replaced by the expression for your second objective function. Then you can run "solve;" again, and the second objective will be optimized.

--
Robert Fourer
am...@googlegroups.com