Re: [Fwd: identifying infeasible problem without using the LP presolver]
On Mon, 21 Mar 2022, Andrew Makhorin wrote: Forwarded Message From: Will Tipton To: help-glpk@gnu.org Cc: John Rice Subject: identifying infeasible problem without using the LP presolver Date: Mon, 21 Mar 2022 11:15:44 -0400 Running without the presolver usually works great. However, we can't rely on the results in this case, because the simplex algorithm returns an OK error code even when it fails to find a feasible solution. If the presolver is not used, glp_simplex will return 0 iff it was able to do its job. That includes discovering that the problem has no solution. To get the desired information, use glp_get_status. If the presolver is used, glp_simplex will return 0 iff it was able to do its job and the presolver did not determine that the problem was primal or dual infeasible. -- Michael henne...@web.cs.ndsu.nodak.edu Electic cars run on coal.
[Fwd: identifying infeasible problem without using the LP presolver]
Forwarded Message From: Will Tipton To: help-glpk@gnu.org Cc: John Rice Subject: identifying infeasible problem without using the LP presolver Date: Mon, 21 Mar 2022 11:15:44 -0400 > Hi help-glpk, > > We're using GLPK's simplex algorithm to solve some medium-sized lp > problems (hundreds of variables and constraints). We've found that the > presolver produces bad solutions, and we suspect that it's due to > numerical issues, since the constraints may be near-degenerate in many > cases. > > Running without the presolver usually works great. However, we can't > rely on the results in this case, because the simplex algorithm > returns an OK error code even when it fails to find a feasible > solution. > > So: > How hard would it be to get errors in case there is no feasible > solution when not using the presolver? > Any tips on debugging presolver stability problems? > Thanks in advance, > Will
