[Scip] Fwd: getting column coefficients and realloc error

[Stefan Heinz]

Hi Mattia,
> In my branching plugin, I try doing this:
> 
>  for (int v = 0; v < nlpcands; v++) {
> 
>         cVar = lpcands[v];
> 
>         *//include var*
>         SCIPstartDive(scip);
> 
>         SCIPchgVarLbDive(scip, cVar, 1.0);
>         SCIPchgVarUbDive(scip, cVar, 1.0);
>         SCIPsolveDiveLP(scip, -1, error);
> 
>         LPval1 = SCIPgetLowerbound(scip);
> 
>         SCIPendDive(scip);
> 
>        * //exclude var*
>         SCIPstartDive(scip);
> 
>         SCIPchgVarLbDive(scip, cVar, 0.0);
>         SCIPchgVarUbDive(scip, cVar, 0.0);
>         SCIPsolveDiveLP(scip, -1, error);
> 
>         LPval1 = SCIPgetLowerbound(scip);
> 
>         SCIPendDive(scip);
> 
>         LPval = MIN(LPval1, LPval2);
> 
>         *//take max of LPval of all fractional variables*
>         if (LPval > bestLPval) {
>             bestLPval = LPval;
>             bestVarIndex = v;
>         }
>     }
> 
> But what I get is an incorrect primal bound (integer) solution. I suppose
> this happens because a change in Lower/Upper bound in this way is done
> permanently, while what I hope to get is a change only during the time I
> solve the LP to select the best variable. Then I want the variable's
> Upper/Lower bound come back to its original values, letting me going on to
> the usual branching process without any changes in the variables (I will
> include or exclude "arcs" from paths, not entire variables).
I do not understand the Problem yet. First, if the above code is just a copy 
of the real code, then there is bug. Since one of these lines
>         LPval1 = SCIPgetLowerbound(scip);
should be 
>         LPval2 = SCIPgetLowerbound(scip);
Second, with this call you should get a dual value and not a primal value.
Besides that you should try SCIPgetNodeLowerbound() instead of 
SCIPgetLowerbound().
http://scip.zib.de/doc/html/scip_8c.html#f365112efafb8219d42ee2ab205be2e1

> > -  After few nodes the actives variables are a lot (1000 to 20k
> > variables,
> >
> > > considering 70-nodes problems). These are all the variables that my
> >
> > pricer
> >
> > > plugin find, but in practice only a half (or less) are active columns
> > > of the current LP at one node (I suppose that SCIP has a own mechanism
> > > to define a pool of active columns).
> >
> > Yes that is the case.
> >
> > > Thousands of columns are anyway too
> > >  much, because the LP solving process is very slow. Is there anyway to
> > >  better manage this problem?
> >
> > I am not sure if I got your question right. I try to answer it in the
> > follwing
> > way:
> > There is one problem in the current version of SCIP. It is not possible
> > to remove variables from the problem. In your case I would suggest to
> > select only
> > a certain number of the variables your pricer creates for adding to your
> > problem in each round. That  might help to reduce the amount of useless
> > variables in the end.
> 
> Instead it may help to remove from the current LP variables whit a very
>  high positive (min object sense) reduced cost. How can I deal whit this
>  approach?
SCIP does that for you already. The only thing you have to do, is to allow 
that a variable can be removed from the LP. To do this you have to set the 
parameter "removable" in the method call SCIPcreateVar() to TRUE. 
http://scip.zib.de/doc/html/scip_8c.html#9025d19ffd6d4fd3d10714485709fefc

Hope that helps.

-- Stefan