Computing Kappa statistics

[Lukas Barth]

Hi,

I'd like to collect statistics about the Kappa values of the solutions encountered during solving. I know I can query Kappa via 

http://www.gurobi.com/documentation/8.0/refman/kappa.html#attr:Kappa

However, there are some things I don't understand from the documentation:

* When I query Kappa, I assume I always get the value for the last LP that Gurobi solved at a Branch-and-Bound node?

* Kappa can only be computed for basic LP solutions. So, if the last solution seen was not a basic solution, what do I get? The value from the last basic solution seen, or some meaningless value?

* The documentation states that "a basis is available when a continuous model has been solved using primal simplex, dual simplex, or barrier with crossover" - so, as long as I use any of simplex or barrier with crossover, should every solution be a basic solution (my intuition about the Simplex algorithm tells me this should be true)? 

I would like to sample Kappa values by periodically querying that attribute in a callback. However, I would need to make sure that I'm not counting the same value for the same solution multiple times. Is there a way of seeing (in a callback) whether the last seen (basic) solution has changed?

Thanks for any help,

Lukas

[Daniel Espinoza]

Hi Lukas,

Unfortunately, you can not query kappa during callbacks (or at least is not supported).... what is available, and when, is described here:

http://www.gurobi.com/documentation/8.0/refman/callback_codes.html

Sorry about that.

Best,

Daniel

[Lukas Barth]

Hi Daniel,

On Wednesday, July 18, 2018 at 3:57:25 PM UTC+2, Daniel Espinoza wrote:

Unfortunately, you can not query kappa during callbacks (or at least is not supported).... what is available, and when, is described here:

http://www.gurobi.com/documentation/8.0/refman/callback_codes.html

Thanks! That's unfortunate… So when can I query Kappa? Only after the MIP model is completely solved? What is the meaning of Kappa then? Or is Kappa only available if I use Gurobi to solve an LP?

Having access to statistics of the Kappas of the relaxed LPs during MIP solve would be a very nice feature, I think.

Regards,

Lukas 

[Daniel Espinoza]

Hi Lukas,

Kappa and KappaExact are only available for LP's, not for MIPs.

I'll make a note of your request thought.

Best,

Daniel