[Scip] update constraint

[Mattia Barbieri]

On 16 July 2010 11:57, Stefan Vigerske <stefan at math.hu-berlin.de> wrote:

> Hi,
>
> >> A dirty way could be to construct your penalty term as an SCIP_ROW, add
> >> this one to the LP with the flag that it should not be removed, and then
> >> modify this row as you like. Changes in the row should be passed to the
> >> LP.
> >
> > What does this mean? I understand this:
> >  create an initial row:
> >    *SCIPcreateEmptyRow* (...flag it removable..);
>
> Better flag it *not* removable, so that it will stay always in the LP.
>
> >  add variables and their coefficients (penalty terms):
> >    *SCIPaddVarsToRow*(...);
> > then tell the LP this changes:
> >    *???*
>
> If the row is in the LP and then changes, then SCIP passes these changes
> into the LP, so there is no need to tell the LP.
>
> I do this:

//trickRow
    SCIP_CALL(SCIPcreateEmptyRow(scip, &sp->*trickRow*, "trickRow", 0.0,
0.0, FALSE, TRUE, FALSE));

*//sp is a struct to mantain some data like penalties, ecc.*

    SCIP_VAR* transVar;
    for (int p = 0; p < Qdim; p++) {

*//z_m[ ], y_m[ ], y_p[ ], z_p[ ] are the variables associated with the
penalties*
*//g_m[ ], d.., are the coefficients associated with the penalties*

        SCIP_CALL(*SCIPtransformVar*(scip, sp->z_m[p], &transVar));
*//I suppose SCIP want trasformed variables, because I obtain an error
passing the original variables *
*[src/scip/var.c:9329] ERROR: cannot add untransformed original variable
<_var3_> to LP row <trickRow>*
        SCIP_CALL(SCIPaddVarToRow(scip, sp->trickRow, transVar,
-sp->g_m[p]));
        SCIP_CALL(SCIPtransformVar(scip, sp->y_m[p], &transVar));
        SCIP_CALL(SCIPaddVarToRow(scip, sp->trickRow, transVar,
-sp->d_m[p]));
        SCIP_CALL(SCIPtransformVar(scip, sp->y_p[p], &transVar));
        SCIP_CALL(SCIPaddVarToRow(scip, sp->trickRow, transVar,
sp->d_p[p]));
        SCIP_CALL(SCIPtransformVar(scip, sp->z_p[p], &transVar));
        SCIP_CALL(SCIPaddVarToRow(scip, sp->trickRow, transVar,
sp->g_p[p]));
    }
    SCIP_CALL(SCIPtransformVar(scip, sp->*trickVa*r, &transVar));

*//trickVar is a variables add to the objective function of the LP, equal to
the sum of the penalty variables*

    SCIP_CALL(SCIPaddVarToRow(scip, sp->trickRow, transVar, -1.0));

SCIP told me this:
 *solution violates original bounds of variable <_var43_> [-1e+20,1e+20]
solution value <-2e+20>
best solution is not feasible in original problem*

while if I add a linear constraint with the same variables/values the
solution of the problem is correct (but as you told me in this way I can't
change the coefficient's values during the LP iterations).

Perhaps I am missing something...what? (Is that the right way to transform a
variable?)

> after each pricer iteration (at each branch), modify the row whit new
> > coefficients (new penalty values):
> >    *???*
>
> OK, I see that there are no methods in scip.h or pub_lp.h to change row
> coefficients. :-(
> You can try with those in lp.h, e.g.
> SCIP_RETCODE    SCIProwChgCoef (SCIP_ROW *row, BMS_BLKMEM *blkmem,
> SCIP_SET *set, SCIP_LP *lp, SCIP_COL *col, SCIP_Real val).
> blkmem you can get via SCIPblkmem(scip).
> For set and lp, you would need to include struct_scip.h, so you can do
> scip->set and scip->lp.
>
> > Can you give me more details? Forgive me but really I am stuck in this
> > problem.
>
> It may help to send mails to the scip mailing list... :-)
>
> Stefan
>
> > (I will be in Darmstadt for the COLGEN in August 23-27, but now I
> > have time to work, so I am trying to solve my problem anyway)
> >
> >> (You could even ask your row for its position in the LP by using
> >> SCIProwGetLPPos, get access to the LP via SCIPgetLPI, and then modify
> >> the LP there, but that is something one should really not do...)
> >>
> >> Stefan
> >>
> >> --
> >> Stefan Vigerske
> >> Humboldt University Berlin, Numerical Mathematics
> >> http://www.math.hu-berlin.de/~stefan<http://www.math.hu-berlin.de/%7Estefan>
> <http://www.math.hu-berlin.de/%7Estefan>
> >>
> >
> >
> >
>
>
> --
> Stefan Vigerske
> Humboldt University Berlin, Numerical Mathematics
> http://www.math.hu-berlin.de/~stefan<http://www.math.hu-berlin.de/%7Estefan>
>



-- 
Mattia Barbieri
barbieri.mattia at gmail.com
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