NAG Library Manual, Mark 28.4
```/* nag_rand_field_2d_predef_setup (g05zrc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.4, 2022.
*/
#include <math.h>
#include <nag.h>

static void display_results(Integer approx, Integer *m, double rho, double *eig,
Integer icount, double *lam);
static void read_input_data(Nag_Variogram *cov, Integer *np, double *params,
Nag_NormType *norm, double *var, double *xmin,
double *xmax, double *ymin, double *ymax,
Integer *ns, Integer *maxm, Nag_EmbedScale *corr,

int main(void) {
/*  Scalars */
Integer exit_status = 0;
double rho, var, xmax, xmin, ymax, ymin;
Integer approx, icount, np;
/*  Arrays */
double eig[3], params[5];
double *lam = 0, *xx = 0, *yy = 0;
Integer m[2], maxm[2], ns[2];
/* Nag types */
Nag_Variogram cov;
Nag_NormType norm;
Nag_EmbedScale corr;
NagError fail;

INIT_FAIL(fail);

printf("nag_rand_field_2d_predef_setup (g05zrc) Example Program Results\n\n");
/* Get problem specifications from data file */
read_input_data(&cov, &np, params, &norm, &var, &xmin, &xmax, &ymin, &ymax,
if (!(lam = NAG_ALLOC(maxm[0] * maxm[1], double)) ||
!(xx = NAG_ALLOC(ns[0], double)) || !(yy = NAG_ALLOC(ns[1], double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
/* Get square roots of the eigenvalues of the embedding matrix. These are
* obtained from the setup for simulating two-dimensional random fields,
* with a predefined variogram, by the circulant embedding method using
* nag_rand_field_2d_predef_setup (g05zrc).
*/
nag_rand_field_2d_predef_setup(ns, xmin, xmax, ymin, ymax, maxm, var, cov,
norm, np, params, pad, corr, lam, xx, yy, m,
&approx, &rho, &icount, eig, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_rand_field_2d_predef_setup (g05zrc).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}
/* Output results */
display_results(approx, m, rho, eig, icount, lam);
END:
NAG_FREE(lam);
NAG_FREE(xx);
NAG_FREE(yy);
return exit_status;
}

void read_input_data(Nag_Variogram *cov, Integer *np, double *params,
Nag_NormType *norm, double *var, double *xmin,
double *xmax, double *ymin, double *ymax, Integer *ns,
Integer j;
char nag_enum_arg[40];

/* Read in covariance function name and convert to value using
* nag_enum_name_to_value (x04nac).
*/
scanf("%*[^\n] %39s%*[^\n]", nag_enum_arg);
*cov = (Nag_Variogram)nag_enum_name_to_value(nag_enum_arg);
scanf("%" NAG_IFMT "%*[^\n]", np);
for (j = 0; j < *np; j++)
scanf("%lf", &params[j]);
scanf("%*[^\n]");
/* Read choice of norm to use, and convert name to value. */
scanf(" %39s%*[^\n]", nag_enum_arg);
*norm = (Nag_NormType)nag_enum_name_to_value(nag_enum_arg);
/* Read in variance of random field */
scanf("%lf%*[^\n]", var);
/* Read in domain endpoints */
scanf("%lf %lf%*[^\n]", xmin, xmax);
scanf("%lf %lf%*[^\n]", ymin, ymax);
/* Read in number of sample points in each direction */
scanf("%" NAG_IFMT " %" NAG_IFMT "%*[^\n]", &ns[0], &ns[1]);
/* Read in maximum size of embedding matrix */
scanf("%" NAG_IFMT " %" NAG_IFMT "%*[^\n]", &maxm[0], &maxm[1]);
/* Read name of scaling in case of approximation and convert to value. */
scanf(" %39s%*[^\n]", nag_enum_arg);
*corr = (Nag_EmbedScale)nag_enum_name_to_value(nag_enum_arg);
/* Read in choice of padding and convert name to value. */
scanf(" %39s%*[^\n]", nag_enum_arg);
}

void display_results(Integer approx, Integer *m, double rho, double *eig,
Integer icount, double *lam) {
/*  Scalars */
Integer i, j;

/* Display size of embedding matrix */
printf("\nSize of embedding matrix = %" NAG_IFMT "\n\n", m[0] * m[1]);
/* Display approximation information if approximation used. */
if (approx == 1) {
printf("Approximation required\n\n");
printf("rho = %10.5f\n", rho);
printf("eig = ");
for (j = 0; j < 3; j++)
printf("%10.5f", eig[j]);
printf("\nicount = %" NAG_IFMT "\n", icount);
} else {
printf("Approximation not required\n");
}
/* Display square roots of the eigenvalues of the embedding matrix. */
printf("\nSquare roots of eigenvalues of embedding matrix:\n\n");
for (i = 0; i < m[0]; i++) {
for (j = 0; j < m[1]; j++) {
printf("%8.4f", lam[i + j * m[0]]);
}
printf("\n");
}
}
```