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XPRSpresolverow

XPRSpresolverow

Purpose
Presolves a row formulated in terms of the original variables such that it can be added to a presolved matrix.
Synopsis
int XPRS_CC XPRSpresolverow(XPRSprob prob, char qrtype, int nzo, const int mcolso[], const double dvalo[], double drhso, int maxcoeffs, int * nzp, int mcolsp[], double dvalp[], double * drhsp, int * status);
Arguments
prob 
The current problem.
qrtype 
The type of the row:
L 
indicates a ≤ row;
G 
indicates a ≥ row.
nzo 
Number of elements in the mcolso and dvalo arrays.
mcolso 
Integer array of length nzo containing the column indices of the row to presolve.
dvalo 
Double array of length nzo containing the non-zero coefficients of the row to presolve.
drhso 
The right-hand side constant of the row to presolve.
maxcoeffs 
Maximum number of elements to return in the mcolsp and dvalp arrays.
nzp 
Pointer to the integer where the number of elements in the mcolsp and dvalp arrays will be returned.
mcolsp 
Integer array which will be filled with the column indices of the presolved row. It must be allocated to hold at least COLS elements.
dvalp 
Double array which will be filled with the coefficients of the presolved row. It must be allocated to hold at least COLS elements.
drhsp 
Pointer to the double where the presolved right-hand side will be returned.
status 
Status of the presolved row:
-3 
Failed to presolve the row due to presolve dual reductions;
-2 
Failed to presolve the row due to presolve duplicate column reductions;
-1 
Failed to presolve the row due to an error. Check the Optimizer error code for the cause;
0 
The row was successfully presolved;
1 
The row was presolved, but may be relaxed.
Related controls
Integer
PRESOLVE 
Turns presolve on or off.
PRESOLVEOPS 
Selects the presolve operations.

Example
Suppose we want to add the row 2x 1 + x 2 ≤ 1 to our presolved matrix. This could be done in the following way: 
int mindo[] = { 1, 2 };
int dvalo[] = { 2.0, 1.0 };
char qrtype = 'L';
double drhso = 1.0;
int nzp, status, mtype, mstart[2], *mindp;
double drhsp, *dvalp;
...
XPRSgetintattrib(prob, XPRS_COLS, &ncols);
mindp = (int*) malloc(ncols*sizeof(int));
dvalp = (double*) malloc(ncols*sizeof(double));
XPRSpresolverow(prob, qrtype, 2, mindo, dvalo, drhso, ncols,
                &nzp, mindp, dvalp, &drhsp, &status);
if (status >= 0) {
  mtype = 0;
  mstart[0] = 0; mstart[1] = nzp;
  XPRSaddcuts(prob, 1, &mtype, &qrtype, &drhsp, mstart, mindp,
              dvalp);
}
Further information

There are certain presolve operations that can prevent a row from being presolved exactly. If the row contains a coefficient for a column that was eliminated due to duplicate column reductions or singleton column reductions, the row might have to be relaxed to remain valid for the presolved problem. The relaxation will be done automatically by the XPRSpresolverow function, but a return status of +1 will be returned. If it is not possible to relax the row, a status of -2 will be returned instead. Likewise, it is possible that certain dual reductions prevents the row from being presolved. In such a case a status of -3 will be returned instead.

If XPRSpresolverow will be used for presolving e.g. branching bounds or constraints, then dual reductions and duplicate column reductions should be disabled, by clearing the corresponding bits of PRESOLVEOPS. By clearing these bits, the default value for PRESOLVEOPS changes to 471.

If the user knows in advance which columns will have non-zero coefficients in rows that will be presolved, it is possible to protect these individual columns through the XPRSloadsecurevecs function. This way the Optimizer is left free to apply all possible reductions to the remaining columns.


Related topics
XPRSaddcuts, XPRSloadsecurevecs, XPRSsetbranchcuts, XPRSstorecuts.

Parent Topic Console and Library Functions

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