Re: [Help-glpk] Double bounded vs. fixed

[Neill Clift]

The problem case is the branch of the if with the commented out code.
It sets the row bounds to lb = 0.0 and ub = 0.0. The return from 
glp_get_status is GLB_UNDEF. I didn't notice this till after I sent the 
mail.
I would have expected this code to work either way.

On 10/25/2015 9:30 AM, Heinrich Schuchardt wrote:
> A double bounded row with upper bound -1 and lower bound 0 created a 
> infeasability.
>
> A fixed row with value 0 is not by itself infeasable.
>
> So where do you see an issue?
>
> Best regards
>
> Heinrich Schuchardt
>
> http://www.xypron.de
>
> Am 25.10.15 um 14:34 schrieb Neill Clift
>
> > Hi,
> >
> >
> >
> > I have the following fragment of code in my program:
> >
> >
> >
> >           ia[iai] = irow;
> >
> >           ja[iai] = EdgeStartIndex(d) + (i - 1) * 2 + 1;
> >
> >           ar[iai] = 1.0;
> >
> >           iai++;
> >
> >           ia[iai] = irow;
> >
> >           ja[iai] = VertexStartIndex() + Val[i].k;
> >
> >           ar[iai] = -1.0;
> >
> >           if (Val[i].j == Val[i].k /* && Val[i].k == i - 1 */) {
> >
> >               int Lower = 1 - Val[Val[i].k].maxv;
> >
> > #if defined(PRINT)
> >
> >               std::cout << irow << ")" << Lower << " <= e[" << ja[iai-1]
> >
> > << "] - v[" << ja[iai] << "] <= -1" << std::endl;;
> >
> > #endif
> >
> >               glp_set_row_bnds(lp, irow, GLP_DB, Lower, -1.0);
> >
> >           } else {
> >
> >               int Lower = 0 - Val[Val[i].k].maxv;
> >
> > #if defined(PRINT)
> >
> >               std::cout << irow << ")" << Lower << " <= e[" << ja[iai-1]
> >
> > << "] - v[" << ja[iai] << "] <= 0" << std::endl;;
> >
> > #endif
> >
> > //            if (Lower == 0) {
> >
> > //                glp_set_row_bnds(lp, irow, GLP_FX, Lower, 0.0);
> >
> > //            } else {
> >
> >                      glp_set_row_bnds(lp, irow, GLP_DB, Lower, 0.0);
> >
> > //            }
> >
> >           }
> >
> >           iai++;
> >
> >           irow++;
> >
> >
> >
> > Note the code that's commented out at the bottom. The code as it is as
> >
> > shown says the overall linear system is infeasible (in the reals).
> >
> > The row causing the infeasibility generated by the code above when Lower
> >
> > == 0.
> >
> > So the code is generating a double bounded row with ub = lb = 0.
> >
> > Uncommenting those 4 lines generates a system that glpk finds feasible.
> >
> > Is this expected behavior?
> >
> > Thanks.
> >
> > Neill.
> >
> >
> >
> > _______________________________________________
> >
> > Help-glpk mailing list
> >
> > address@hidden
> >
> > https://lists.gnu.org/mailman/listinfo/help-glpk