Obtain slack variables value of constraints?

[Fernando Banez-Chicharro]

Hi!

Is it possible to obtain the value of the slack variables of the constraints? 

I have obtained the value of all the variables of the model with:

value = MathProgBase.getsolution(model)

But I don't know how to obtain the value of the slack variables.

Thanks!

[Miles Lubin]

I don't believe there's a method to obtain slacks. If you're talking about linear constraints, just query the constraint matrix and bounds and compute the slacks yourself.

[Fernando Banez-Chicharro]

Ok.

What do you mean by "query the constraint matrix"? Can I retrieve the value of the constraint to compute the slack by comparing to the bounds?

Thanks!

[Fernando Banez-Chicharro]

I think I got it!

I am using slack = get_dblattrarray( m.inner, "Slack", 1, num_constrs(m.inner)) to obtain the slack variable.

[Miles Lubin]

That works if you're happy using Gurobi. More generally, you can use MathProgBase.getconstrmatrix, MathProgBase.getconstrLB, and MathProgBase.getconstrUB to recompute the slacks.

[Robert Schwarz]

I want to compute the constraint slacks for specific constraints only, but I don't know what row they are in.
Is there a way to extract the row coefficients and lhs/rhs for JuMP constraint references?
I built my linear constraints using @constraint, and would a solver independent solution.

[Robert Schwarz]

OK, I got something working with using JuMP.getinternalmodel and JuMP.linearindex (for both rows and colums). Sorry for the noise, I couldn't find linearindex(ConstraintRef) in the documentation at first.