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### workforce3_c.c

/* Copyright 2023, Gurobi Optimization, LLC */

/* Assign workers to shifts; each worker may or may not be available on a
particular day. If the problem cannot be solved, relax the model
to determine which constraints cannot be satisfied, and how much
they need to be relaxed. */

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

#define xcol(w,s)  nShifts*w+s
#define MAXSTR     128

int
main(int   argc,
char *argv[])
{
GRBenv   *env = NULL;
GRBmodel *model = NULL;
int       error = 0, status;
int       s, w, col;
int      *cbeg = NULL;
int      *cind = NULL;
int       idx;
double   *cval = NULL;
char     *sense = NULL;
char      vname[MAXSTR];
double    obj;
int       i, j, orignumvars, numvars, numconstrs;
double   *rhspen = NULL;
double    sol;
char     *sname;

/* Sample data */
const int nShifts = 14;
const int nWorkers = 7;

/* Sets of days and workers */
char* Shifts[] =
{ "Mon1", "Tue2", "Wed3", "Thu4", "Fri5", "Sat6",
"Sun7", "Mon8", "Tue9", "Wed10", "Thu11", "Fri12", "Sat13",
"Sun14" };
char* Workers[] =
{ "Amy", "Bob", "Cathy", "Dan", "Ed", "Fred", "Gu" };

/* Number of workers required for each shift */
double shiftRequirements[] =
{ 3, 2, 4, 4, 5, 6, 5, 2, 2, 3, 4, 6, 7, 5 };

/* Amount each worker is paid to work one shift */
double pay[] = { 10, 12, 10, 8, 8, 9, 11 };

/* Worker availability: 0 if the worker is unavailable for a shift */
double availability[][14] =
{ { 0, 1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 1 },
{ 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0 },
{ 0, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1 },
{ 0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1 },
{ 1, 1, 1, 1, 1, 0, 1, 1, 1, 0, 1, 0, 1, 1 },
{ 1, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 1 },
{ 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 } };

/* Create environment */
if (error) goto QUIT;

/* Create initial model */
error = GRBnewmodel(env, &model, "workforce3", nWorkers * nShifts,
NULL, NULL, NULL, NULL, NULL);
if (error) goto QUIT;

/* Initialize assignment decision variables:
x[w][s] == 1 if worker w is assigned
to shift s. Since an assignment model always produces integer
solutions, we use continuous variables and solve as an LP. */
for (w = 0; w < nWorkers; ++w)
{
for (s = 0; s < nShifts; ++s)
{
col = xcol(w, s);
sprintf(vname, "%s.%s", Workers[w], Shifts[s]);
error = GRBsetdblattrelement(model, "UB", col, availability[w][s]);
if (error) goto QUIT;
error = GRBsetdblattrelement(model, "Obj", col, pay[w]);
if (error) goto QUIT;
error = GRBsetstrattrelement(model, "VarName", col, vname);
if (error) goto QUIT;
}
}

/* The objective is to minimize the total pay costs */
error = GRBsetintattr(model, "ModelSense", GRB_MINIMIZE);
if (error) goto QUIT;

/* Make space for constraint data */
cbeg = malloc(sizeof(int) * nShifts);
if (!cbeg) goto QUIT;
cind = malloc(sizeof(int) * nShifts * nWorkers);
if (!cind) goto QUIT;
cval = malloc(sizeof(double) * nShifts * nWorkers);
if (!cval) goto QUIT;
sense = malloc(sizeof(char) * nShifts);
if (!sense) goto QUIT;

/* Constraint: assign exactly shiftRequirements[s] workers
to each shift s */
idx = 0;
for (s = 0; s < nShifts; ++s)
{
cbeg[s] = idx;
sense[s] = GRB_EQUAL;
for (w = 0; w < nWorkers; ++w)
{
cind[idx] = xcol(w, s);
cval[idx++] = 1.0;
}
}
error = GRBaddconstrs(model, nShifts, idx, cbeg, cind, cval, sense,
shiftRequirements, Shifts);
if (error) goto QUIT;

/* Optimize */
error = GRBoptimize(model);
if (error) goto QUIT;
error = GRBgetintattr(model, "Status", &status);
if (error) goto QUIT;
if (status == GRB_UNBOUNDED)
{
printf("The model cannot be solved because it is unbounded\n");
goto QUIT;
}
if (status == GRB_OPTIMAL)
{
error = GRBgetdblattr(model, "ObjVal", &obj);
if (error) goto QUIT;
printf("The optimal objective is %f\n", obj);
goto QUIT;
}
if ((status != GRB_INF_OR_UNBD) && (status != GRB_INFEASIBLE))
{
printf("Optimization was stopped with status %i\n", status);
goto QUIT;
}

/* Relax the constraints to make the model feasible */
printf("The model is infeasible; relaxing the constraints\n");

/* Determine the matrix size before relaxing the constraints */
error = GRBgetintattr(model, "NumVars", &orignumvars);
if (error) goto QUIT;
error = GRBgetintattr(model, "NumConstrs", &numconstrs);
if (error) goto QUIT;

/* Use FeasRelax feature with penalties for constraint violations */
rhspen = malloc(sizeof(double) * numconstrs);
if (!rhspen) goto QUIT;
for (i = 0; i < numconstrs; i++) rhspen[i] = 1;
error = GRBfeasrelax(model, GRB_FEASRELAX_LINEAR, 0,
NULL, NULL, rhspen, NULL);
if (error) 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 relaxed model cannot be solved "
"because it is infeasible or unbounded\n");
goto QUIT;
}
if (status != GRB_OPTIMAL)
{
printf("Optimization was stopped with status %i\n", status);
goto QUIT;
}

printf("\nSlack values:\n");
error = GRBgetintattr(model, "NumVars", &numvars);
if (error) goto QUIT;
for (j = orignumvars; j < numvars; ++j)
{
error = GRBgetdblattrelement(model, "X", j, &sol);
if (error) goto QUIT;
if (sol > 1e-6)
{
error = GRBgetstrattrelement(model, "VarName", j, &sname);
if (error) goto QUIT;
printf("%s = %f\n", sname, sol);
}
}

QUIT:

/* Error reporting */

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

/* Free data */

free(cbeg);
free(cind);
free(cval);
free(sense);
free(rhspen);

/* Free model */

GRBfreemodel(model);

/* Free environment */

GRBfreeenv(env);

return 0;
}


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