custom cutter has invalid Lhs or Rhs

Question

I'm trying to write a custom separator (aka, MIP cutter) to my SCIP model (in Python). To do this I need to detect which constraints are tight/binding at the given MIP node. I thought that I could do this by comparing LHS to RHS on the rows (aka, the constraints). However, one of the two always reports +/- 1e20. Why is that? How do I iterate on the binding constraints?

    class Cutter(scip.Sepa):
        def sepaexeclp(self):
            m: scip.Model = self.model
            if not m.getLPSolstat() or not m.isLPSolBasic():
                return {"result": scip.PY_SCIP_RESULT.DIDNOTRUN}
        
            cols = m.getLPColsData()
            rows = m.getLPRowsData()
        
            values = [col.getPrimsol() for col in cols]
            constraints = [row for row in rows if row.getLhs() == row.getRhs()]  # always empty
            ...

Answer 1

I think there is a misunderstanding. All rows in SCIP are ranged rows, i.e., they are of the form lhs <= a^Tx (+constant) <= rhs. When a model contains inequalities with only one side (as seems to be the case for you) then the non-existant side is set to either 1e20 for the rhs (SCIPs infinity value) or -1e20 for the lhs.

I'm assuming what you really want to do is check which constraints are tight w.r.t. the current LP at that node? In that case you should use getRowActivity