[Cbc] Suboptimal solution as optimal?

[Haroldo Santos]

Hi,

Tested in trunk now, still gives 825 as optimal,  without cuts gives  70.
Could not get zero as optimal yet.



On Fri, Dec 29, 2017 at 8:39 AM, John Forrest <john.forrest at fastercoin.com>
wrote:

> Fixed hopefully - stable and trunk.
>
> The original model is not well scaled - one element was 2.1e14 and even
> after preprocessing the problem had to be solved applying scaling factors.
> The small fast branch and bound is deliberately a bit forgiving on this.
> When the code thinks it has got a solution it double checks.  In this case
> it did that and threw that solution out - but it had left some variables
> fixed when they should not have been.
>
> John Forrest
>
>
> On 28/12/17 18:30, John Forrest wrote:
>
> Ryan,
>
> Bug in Cbc.  My first try gave correct result, but I managed to get
> error.  For small problems, Cbc will do a simpler faster branch and bound
> on a portion of the tree.  This is returning infeasible - I will look into
> it.
>
> If you add -depth -100 (which means only go into this simpler version at
> depth 100) then all looked fine.
>
> John Forrest
> On 27/12/17 18:37, Rhavar wrote:
>
> I have a problem:
> https://gist.github.com/RHavar/dfb9de631363ecb9e1c326fff5ebd09b
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__gist.github.com_RHavar_dfb9de631363ecb9e1c326fff5ebd09b&d=DwMDaQ&c=Ngd-ta5yRYsqeUsEDgxhcqsYYY1Xs5ogLxWPA_2Wlc4&r=js2M0T-3OIMIVDvokcKjokJbk0F8QOCd0mT4FsVFE88&m=S4VZrfTJ9vrNsPyEyqDoTZUqb0eG9rdZ8l7-jr8EA9Y&s=dWErV9QjwaNhpQZZkV1ZmdJ_Ofpqgb80lh0-5ORAiSU&e=>
>
> (lp file and solution file attached)
>
> In the solution it says: "Optimal - objective value"  but I think I have
> found a superior solution by hand:
>
>  o6 = 1
> i22 = 1
>
> Which gives a better objective (70 instead of 825)
>
> --
>
> So I guess my question is, does "Optimal - objective value"  mean that
> it's guaranteed to be the optimal solution? Or am I doing something wrong?
> Or is there a bug in cbc?
>
>
> -Ryan
>
>
>
>
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-- 
=============================================================
Haroldo Gambini Santos
Computing Department
Universidade Federal de Ouro Preto - UFOP
email: haroldo [at ] iceb.ufop.br
home/research page: www.decom.ufop.br/haroldo


It has long been an axiom of mine that the little things are infinitely
the most important.
-- Sir Arthur Conan Doyle, "A Case of Identity"
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