Try our new documentation site.


lpmod_c.c


/* Copyright 2019, Gurobi Optimization, LLC */

/* This example reads an LP model from a file and solves it.
   If the model can be solved, then it finds the smallest positive variable,
   sets its upper bound to zero, and resolves the model two ways:
   first with an advanced start, then without an advanced start
   (i.e. 'from scratch'). */

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <string.h>
#include "gurobi_c.h"

int
main(int   argc,
     char *argv[])
{
  GRBenv   *env   = NULL;
  GRBmodel *model = NULL;
  int       error = 0;
  int       j;
  int       numvars, isMIP, status, minVar = 0;
  double    minVal = GRB_INFINITY, sol, lb;
  char     *varname;
  double    warmCount, warmTime, coldCount, coldTime;

  if (argc < 2)
  {
    fprintf(stderr, "Usage: lpmod_c filename\n");
    exit(1);
  }

  error = GRBloadenv(&env, "lpmod.log");
  if (error) goto QUIT;

  /* Read model and determine whether it is an LP */
  error = GRBreadmodel(env, argv[1], &model);
  if (error) goto QUIT;
  error = GRBgetintattr(model, "IsMIP", &isMIP);
  if (error) goto QUIT;
  if (isMIP)
  {
    printf("The model is not a linear program\n");
    goto QUIT;
  }

  error = GRBoptimize(model);
  if (error) goto QUIT;

  error = GRBgetintattr(model, "Status", &status);
  if (error) goto QUIT;

  if ((status == GRB_INF_OR_UNBD) || (status == GRB_INFEASIBLE) ||
      (status == GRB_UNBOUNDED))
  {
    printf("The model cannot be solved because it is ");
    printf("infeasible or unbounded\n");
    goto QUIT;
  }

  if (status != GRB_OPTIMAL)
  {
    printf("Optimization was stopped with status %i\n", status);
    goto QUIT;
  }

  /* Find the smallest variable value */
  error = GRBgetintattr(model, "NumVars", &numvars);
  if (error) goto QUIT;
  for (j = 0; j < numvars; ++j)
  {
    error = GRBgetdblattrelement(model, "X", j, &sol);
    if (error) goto QUIT;
    error = GRBgetdblattrelement(model, "LB", j, &lb);
    if (error) goto QUIT;
    if ((sol > 0.0001) && (sol < minVal) &&
        (lb == 0.0))
    {
      minVal = sol;
      minVar = j;
    }
  }

  error = GRBgetstrattrelement(model, "VarName", minVar, &varname);
  if (error) goto QUIT;
  printf("\n*** Setting %s from %f to zero ***\n\n", varname, minVal);
  error = GRBsetdblattrelement(model, "UB", minVar, 0.0);
  if (error) goto QUIT;

  /* Solve from this starting point */
  error = GRBoptimize(model);
  if (error) goto QUIT;

  /* Save iteration & time info */
  error = GRBgetdblattr(model, "IterCount", &warmCount);
  if (error) goto QUIT;
  error = GRBgetdblattr(model, "Runtime", &warmTime);
  if (error) goto QUIT;

  /* Reset the model and resolve */
  printf("\n*** Resetting and solving ");
  printf("without an advanced start ***\n\n");
  error = GRBreset(model, 0);
  if (error) goto QUIT;
  error = GRBoptimize(model);
  if (error) goto QUIT;

  /* Save iteration & time info */
  error = GRBgetdblattr(model, "IterCount", &coldCount);
  if (error) goto QUIT;
  error = GRBgetdblattr(model, "Runtime", &coldTime);
  if (error) goto QUIT;

  printf("\n*** Warm start: %f iterations, %f seconds\n",
         warmCount, warmTime);
  printf("*** Cold start: %f iterations, %f seconds\n",
         coldCount, coldTime);


QUIT:

  /* Error reporting */

  if (error)
  {
    printf("ERROR: %s\n", GRBgeterrormsg(env));
    exit(1);
  }

  /* Free model */

  GRBfreemodel(model);

  /* Free environment */

  GRBfreeenv(env);

  return 0;
}

Try Gurobi for Free

Choose the evaluation license that fits you best, and start working with our Expert Team for technical guidance and support.

Evaluation License
Get a free, full-featured license of the Gurobi Optimizer to experience the performance, support, benchmarking and tuning services we provide as part of our product offering.
Academic License
Gurobi supports the teaching and use of optimization within academic institutions. We offer free, full-featured copies of Gurobi for use in class, and for research.
Cloud Trial

Request free trial hours, so you can see how quickly and easily a model can be solved on the cloud.

Search