```/* nag_zero_nonlin_eqns_expert (c05qcc) Example Program.
*
* Copyright 2017 Numerical Algorithms Group.
*
* Mark 26.1, 2017.
*/

#include <nag.h>
#include <nagx04.h>
#include <stdio.h>
#include <nag_stdlib.h>
#include <math.h>
#include <nagc05.h>
#include <nagx02.h>

#ifdef __cplusplus
extern "C"
{
#endif
static void NAG_CALL fcn(Integer n, const double x[], double fvec[],
Nag_Comm *comm, Integer *iflag);
#ifdef __cplusplus
}
#endif

static Integer nprint = 0;

int main(void)
{

static double ruser[1] = { -1.0 };
Integer exit_status = 0, i, n = 9, maxfev, ml, mu, nfev;
double *diag = 0, *fjac = 0, *fvec = 0, *qtf = 0, *r = 0, *x = 0;
double epsfcn, factor, xtol;
/* Nag Types */
NagError fail;
Nag_Comm comm;
Nag_ScaleType scale_mode;

INIT_FAIL(fail);

printf("nag_zero_nonlin_eqns_expert (c05qcc) Example Program Results\n");

/* For communication with user-supplied functions: */
comm.user = ruser;

if (n > 0) {
if (!(diag = NAG_ALLOC(n, double)) ||
!(fjac = NAG_ALLOC(n * n, double)) ||
!(fvec = NAG_ALLOC(n, double)) ||
!(qtf = NAG_ALLOC(n, double)) ||
!(r = NAG_ALLOC(n * (n + 1) / 2, double)) ||
!(x = NAG_ALLOC(n, double)))
{
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
}
else {
printf("Invalid n.\n");
exit_status = 1;
goto END;
}

/* The following starting values provide a rough solution. */
for (i = 0; i < n; i++)
x[i] = -1.0;

/* nag_machine_precision (x02ajc).
* The machine precision
*/
xtol = sqrt(nag_machine_precision);

for (i = 0; i < n; i++)
diag[i] = 1.0;

maxfev = 2000;
ml = 1;
mu = 1;
epsfcn = 0.0;
scale_mode = Nag_ScaleProvided;
factor = 100.0;

/* nag_zero_nonlin_eqns_expert (c05qcc).
* Solution of a system of nonlinear equations (function
* values only)
*/
nag_zero_nonlin_eqns_expert(fcn, n, x, fvec, xtol, maxfev, ml, mu,
epsfcn, scale_mode, diag, factor, nprint, &nfev,
fjac, r, qtf, &comm, &fail);

if (fail.code != NE_NOERROR) {
printf("Error from nag_zero_nonlin_eqns_expert (c05qcc).\n%s\n",
fail.message);
exit_status = 1;
if (fail.code != NE_TOO_MANY_FEVALS &&
fail.code != NE_TOO_SMALL && fail.code != NE_NO_IMPROVEMENT)
goto END;
}

printf(fail.code == NE_NOERROR ? "Final approximate" : "Approximate");
printf(" solution\n\n");
for (i = 0; i < n; i++)
printf("%12.4f%s", x[i], (i % 3 == 2 || i == n - 1) ? "\n" : " ");

if (fail.code != NE_NOERROR)
exit_status = 2;

END:
NAG_FREE(diag);
NAG_FREE(fjac);
NAG_FREE(fvec);
NAG_FREE(qtf);
NAG_FREE(r);
NAG_FREE(x);
return exit_status;
}

static void NAG_CALL fcn(Integer n, const double x[], double fvec[],
Nag_Comm *comm, Integer *iflag)
{
Integer k;

if (comm->user[0] == -1.0) {
printf("(User-supplied callback fcn, first invocation.)\n");
comm->user[0] = 0.0;
}
if (*iflag == 0) {
if (nprint > 0) {
/* Insert print statements here if desired. */
}
}
else {
for (k = 0; k < n; ++k) {
fvec[k] = (3.0 - x[k] * 2.0) * x[k] + 1.0;
if (k > 0)
fvec[k] -= x[k - 1];
if (k < n - 1)
fvec[k] -= x[k + 1] * 2.0;
}
}
/* Set iflag negative to terminate execution for any reason. */
*iflag = 0;
}
```