[SCIP] Driving the constraint slack variables out of the basis?

[Nelson Uhan]

Hi all,

I have a question about basis information in SCIP and SoPlex.

I’m solving an LP of this form:

min c’x
s.t. Ax = b
x >= 0

In particular, I’m interested in getting an optimal basis to this LP. After solving the LP in SCIP with SoPlex, I get a list of basic indices using SCIPgetLPBasisInd. Some of these indices are negative. I understand that these negative indices correspond constraint slack variables. These slack variables all have a value of 0 at optimality (the LP is feasible).

I would like to obtain a list of basic indices corresponding only to columns of A (i.e. nonnegative indices). Presumably, one could do this by pivoting on variables with zero reduced cost to drive the constraint slack variables out of the basis. Is there a (relatively) easy way to do this with SCIP/SoPlex?

Any help would be much appreciated. Thanks,
Nelson