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

 From: Heinrich Schuchardt Subject: Re: [Help-glpk] Double bounded vs. fixed Date: Sun, 25 Oct 2015 17:30:56 +0100

```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.
>
>
>
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