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XPRSpresolvecut

Purpose

Presolves a cut formulated in terms of the original variables such that it can be added to a presolved matrix.

Synopsis

               int XPRS_CC XPRSpresolvecut(XPRSprob prob, char qrtype, double drhso, int nzo, const int mcolso[], const double dvalo[], 
               double *drhsp, int *nzp, int mcolsp[], double dvalp[], int *status);
               

Arguments

prob  

The current problem.

qrtype  

The row type of the cut:

L	indicates a cut;
E	indicates a = cut;
G	indicates a cut.

drhso  

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

nzo

Number of elements in the mcolso and dvalo arrays.

mcolso  

Integer array of length nzo containing the column indices of the cut to presolve.

dvalo  

Double array of length nzo containing the non-zero coefficients of the cut to presolve.

drhsp  

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

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 cut. It must be allocated to hold at least COLS elements.

dvalp  

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

status  

Status of the presolved cut:

0	The cut was successfully presolved;
1	Presolving the cut failed.

Related Controls

Integer

PRESOLVE	Turns presolve on or off.
PRESOLVEOPS	Selects the presolve operations.

Example

Suppose we want to add the cut 2x₁ + x₂ Maths/leq.png

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;
...
XPRSpresolvecut(prob, qrtype, drhso, 2, mindo, dvalo, &drhsp, 
                &nzp, mindp, dvalp, &status);
if (!status) {
  mtype = 0;
  mstart[0] = 0; mstart[1] = nzp;
  XPRSaddcuts(prob, 1, &mtype, &qrtype, &drhsp, mstart, mindp, 
              dvalp);
}

Further information

The are certain presolve operations that can prevent presolving a cut successfully. These are singleton column removal, duplicate column removal and variable eliminations. Thus, bits 0, 5 and 8 (total value 289) of PRESOLVEOPS should be cleared before calling XPRSminim or XPRSmaxim on a matrix. Furthermore, bit 11 (value 2048) should be set to prevent some further reductions that conflict with presolving cuts. A safe settting of PRESOLVEOPS is therefore 2270.