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### lpmod_c.c

/* Copyright 2016, Gurobi Optimization, Inc. */

/* 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:
(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);
}

if (error) goto QUIT;

/* Read model and determine whether it is an LP */
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, "LB", 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 ");
error = GRBresetmodel(model);
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;
}


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