set_presolve

Specifies if a presolve must be done before solving.

void set_presolve(lprec *lp, int do_presolve);

Return Value

set_presolve has no return value.

Parameters

Pointer to previously created lp model. See return value of make_lp, read_lp, read_LP, read_mps, read_freemps, read_MPS, read_freeMPS, read_XLI

do_presolve

Specifies presolve level. Can be the (OR) combination of any of the following values:

PRESOLVE_NONE (0)	No presolve at all
PRESOLVE_ROWS (1)	Presolve rows
PRESOLVE_COLS (2)	Presolve columns
PRESOLVE_LINDEP (4)	Eliminate linearly dependent rows
PRESOLVE_SOS (32)	Convert constraints to SOSes (only SOS1 handled)
PRESOLVE_REDUCEMIP (64)	If the phase 1 solution process finds that a constraint is redundant then this constraint is deleted
PRESOLVE_DUALS (128)	Calculate duals
PRESOLVE_SENSDUALS (256)	Calculate sensitivity if there are integer variables

Remarks

The set_presolve function specifies if a presolve must be done before solving. Presolve looks at the model and tries to simplify it so that solving times are shorter. For example a constraint on only one variable is converted to a bound on this variable (and the constraint is deleted). Note that the model dimensions can change because of this, so be careful with this. Both rows and columns can be deleted by the presolve.
Note that PRESOLVE_LINDEP can result in deletion of rows (the linear dependent ones). get_constraints, get_ptr_constraints will then return only the values of the rows that are kept and the values of the deleted rows are not known anymore. The default is not (PRESOLVE_NONE) doing a presolve.

Example

#include <stdio.h>
#include <stdlib.h>
#include "lp_lib.h"

int main(void)
{
  lprec *lp;

  /* Create a new LP model */
  lp = make_lp(0, 0);
  if(lp == NULL) {
    fprintf(stderr, "Unable to create new LP model\n");
    return(1);
  }

  set_presolve(lp, PRESOLVE_ROWS | PRESOLVE_COLS | PRESOLVE_LINDEP);

  delete_lp(lp);
  return(0);
}

lp_solve API reference