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### lpmod_c++.cpp

/* Copyright 2023, 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 "gurobi_c++.h"
using namespace std;

int
main(int argc,
char *argv[])
{
if (argc < 2)
{
cout << "Usage: lpmod_c++ filename" << endl;
return 1;
}

GRBEnv* env = 0;
GRBVar* v = 0;
try
{
// Read model and determine whether it is an LP
env = new GRBEnv();
GRBModel model = GRBModel(*env, argv[1]);
if (model.get(GRB_IntAttr_IsMIP) != 0)
{
cout << "The model is not a linear program" << endl;
return 1;
}

model.optimize();

int status = model.get(GRB_IntAttr_Status);

if ((status == GRB_INF_OR_UNBD) || (status == GRB_INFEASIBLE) ||
(status == GRB_UNBOUNDED))
{
cout << "The model cannot be solved because it is "
<< "infeasible or unbounded" << endl;
return 1;
}

if (status != GRB_OPTIMAL)
{
cout << "Optimization was stopped with status " << status << endl;
return 0;
}

// Find the smallest variable value
double minVal = GRB_INFINITY;
int minVar = 0;
v = model.getVars();
for (int j = 0; j < model.get(GRB_IntAttr_NumVars); ++j)
{
double sol = v[j].get(GRB_DoubleAttr_X);
if ((sol > 0.0001) && (sol < minVal) &&
(v[j].get(GRB_DoubleAttr_LB) == 0.0))
{
minVal = sol;
minVar = j;
}
}

cout << "\n*** Setting " << v[minVar].get(GRB_StringAttr_VarName)
<< " from " << minVal << " to zero ***" << endl << endl;
v[minVar].set(GRB_DoubleAttr_UB, 0.0);

// Solve from this starting point
model.optimize();

// Save iteration & time info
double warmCount = model.get(GRB_DoubleAttr_IterCount);
double warmTime = model.get(GRB_DoubleAttr_Runtime);

// Reset the model and resolve
cout << "\n*** Resetting and solving "
<< "without an advanced start ***\n" << endl;
model.reset();
model.optimize();

// Save iteration & time info
double coldCount = model.get(GRB_DoubleAttr_IterCount);
double coldTime = model.get(GRB_DoubleAttr_Runtime);

cout << "\n*** Warm start: " << warmCount << " iterations, " <<
warmTime << " seconds" << endl;
cout << "*** Cold start: " << coldCount << " iterations, " <<
coldTime << " seconds" << endl;

}
catch (GRBException e)
{
cout << "Error code = " << e.getErrorCode() << endl;
cout << e.getMessage() << endl;
}
catch (...)
{
cout << "Error during optimization" << endl;
}

delete[] v;
delete env;
return 0;
}


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