Re: [Help-glpk] shiftcover.mod - generate different solutions

[Xypron]

Hello Andreas,

the MIP solver rejects branches of the tree of all solutions in the 
course of the solution process. Hence rerunning the solver is not avoidable.

If you use the GLPK library you could add the last solution as 
additional constraints to the problem and resolve the problem.

If you use the glpsol executable, you will have to restart it and will 
need some external storage. Using an ODBC database is an alternative to 
files. I think it is preferable because you can use it to accumulate the 
solutions, while in the case of files, you have to write all old 
solutions to the file again.

In the appendix the shiftcover model is changed to only use binary 
variables. This makes excluding possible solutions much easier.

The idea for the conversion is replacing integer variables (crew[s]) by 
sums of powers of two times binary variables (sum{i in I} 2**i * crew[s, 
i]).

Best regards

Xypron

Andreas F wrote:
>    
> > From: address@hidden
> > On Wed, 2011-02-16 at 14:48 +0000, Andreas F wrote:
> >      
> > > However, is it possible to generate multiple, different solutions [..] ?
> > >
> > >        
> > Currently glpk does not have such a feature. However, you may obtain an
> > alternate mip solution by introducing a packing inequality to disable
> > the current optimal solution. See
> > http://lists.gnu.org/archive/html/help-glpk/2010-02/msg00033.html
> > for example.
> >      
> The solution is indeed elegant, but it requires use of files and one has to 
> re-run the solver.  Is it possible to generate more than one solution without 
> using files?
>
> Best regards,
> Andreas
>

shiftcover2.gz
Description: GNU Zip compressed data