[Help-glpk] Re: Bridge And Torch Problem
Apologies for replying to myself twice, but a bit of experimenting has shown me a partial explanation for the mystery I raised in the quoted text below. As well as the fact that the MIP software used by the authors of the paper linked below might have been specifically written for transport problems, I have also found, to my surprise (though it makes sense when you think about it), that, in a "bridge and torch" problem, the larger the number of people who are allowed to "cross the river" in one go, the more easily GLPK is able to resolve it. Since the large scale problems they claim to have resolved in the paper are about optimising transport in a city, they will have been discussing buses and trains - which, of course, carry large numbers of people simultaneously. I wasn't expecting this, because according to the academic paper at http://www.zib.de/Publications/Reports/SC-95-27.ps.Z (from Konrad-Zuse-Zentrum für Informationstechnik Berlin), river crossing problems can be resolved using integer programming with planar cuts. Would I be correct in thinking that to achieve the level of results described in that paper, one would have to program the planar cuts explicitly for this problem, rather than use the generalised cuts provided with GLPK, or is there something I can do to make the model resolve more quickly? Add other email accounts to Hotmail in 3 easy steps. Find out how. _ Learn how to add other email accounts to Hotmail in 3 easy steps. http://clk.atdmt.com/UKM/go/167688463/direct/01/___ Help-glpk mailing list Help-glpk@gnu.org http://lists.gnu.org/mailman/listinfo/help-glpk
[Help-glpk] initial basis
Hello, I've got a question about LP initial basis. I create a problem, then optimize it, and save status for variables and constraints, and delete the problem. I create a new problem, early the same as the first one : I only remove some variables (and so modify/remove some constraints). Is there a way, in this configuration, to use the statuses of the first optimization ? The number of basis variable in the first optimization is not the same as the number of rows in the second, but the basis of the first optimization could be usefull for the second. Thank you very much, Best regards, -- Anne-Laurence Putz Optimization Engineer EURODECISION Operational Research 9 rue de la Porte de Buc 78000 VERSAILLES - FRANCE Tel : + 33 (0)1 39 07 12 40 Fax: + 33 (0)1 39 07 12 41 www.eurodecision.com ___ Help-glpk mailing list Help-glpk@gnu.org http://lists.gnu.org/mailman/listinfo/help-glpk
Re: [Help-glpk] initial basis
> I've got a question about LP initial basis. > I create a problem, then optimize it, and save status for variables and > constraints, and delete the problem. > I create a new problem, early the same as the first one : I only remove > some variables (and so modify/remove some constraints). > Is there a way, in this configuration, to use the statuses of the first > optimization ? The number of basis variable in the first optimization is > not the same as the number of rows in the second, but the basis of the > first optimization could be usefull for the second. If the number of basic variables is not equal to the number of rows, the basis is invalid and therefore cannot be used as initial basis (at least glpk does not provide such a feature). Rather than removing basic variables, that invalidates the basis, you could fix them at zero; this gives the same effect, however, keeps the basis valid. ___ Help-glpk mailing list Help-glpk@gnu.org http://lists.gnu.org/mailman/listinfo/help-glpk
