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 rowtype, int norigcoefs, const int origcolind[], const double origrowcoef[], double origrhs, int maxcoefs, int * p_ncoefs, int colind[], double rowcoef[], double * p_rhs, int * p_status);

Arguments

prob

The current problem.

rowtype

The type of the row:

L	indicates a ≤ row;
G	indicates a ≥ row.

norigcoefs

Number of elements in the origcolind and origrowcoef arrays.

origcolind

Integer array of length norigcoefs containing the column indices of the row to presolve.

origrowcoef

Double array of length norigcoefs containing the non-zero coefficients of the row to presolve.

origrhs

The right-hand side constant of the row to presolve.

maxcoefs

Maximum number of elements to return in the colind and rowcoef arrays.

p_ncoefs

Pointer to the integer where the number of elements in the colind and rowcoef arrays will be returned.

colind

Integer array which will be filled with the column indices of the presolved row. It must be allocated to hold at least COLS elements.

rowcoef

Double array which will be filled with the coefficients of the presolved row. It must be allocated to hold at least COLS elements.

p_rhs

Pointer to the double where the presolved right-hand side will be returned.

p_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 origrowcoef[] = { 2.0, 1 };
char rowtype = 'L';
double origrhs = 1;
int ncols, ncoefs, status, mtype, mstart[2], *mindp;
double rhs, *rowcoef;
...
XPRSgetintattrib(prob, XPRS_COLS, &ncols);
mindp = (int*) malloc(ncols*sizeof(int));
rowcoef = (double*) malloc(ncols*sizeof(double));
XPRSpresolverow(prob, rowtype, 2, mindo, origrowcoef, origrhs, ncols,
                &ncoefs, mindp, rowcoef, &rhs, &status);
if (status >= 0) {
  mtype = 0;
  mstart[0] = 0; mstart[1] = ncoefs;
  XPRSaddcuts(prob, 1, &mtype, &rowtype, &rhs, mstart, mindp,
              rowcoef);
}

See also example tsp.c.

Further information

1. This function applies to linear and convex quadratic (including second order conic) problems only.

2. 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.

3. 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.

4. 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.

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XPRSpresolverow