NAG Library Manual, Mark 28.4
/* nag_lapackeig_zhpev (f08gnc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.4, 2022.
*/

#include <math.h>
#include <nag.h>
#include <stdio.h>

int main(void) {
/* Scalars */
double eerrbd, eps;
Integer exit_status = 0, i, j, n;
/* Arrays */
char nag_enum_arg[40];
Complex *ap = 0, *dummy = 0;
double *w = 0;
/* Nag Types */
Nag_OrderType order;
Nag_UploType uplo;
NagError fail;

#ifdef NAG_COLUMN_MAJOR
#define AP_UPPER(I, J) ap[J * (J - 1) / 2 + I - 1]
#define AP_LOWER(I, J) ap[(2 * n - J) * (J - 1) / 2 + I - 1]
order = Nag_ColMajor;
#else
#define AP_LOWER(I, J) ap[I * (I - 1) / 2 + J - 1]
#define AP_UPPER(I, J) ap[(2 * n - I) * (I - 1) / 2 + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapackeig_zhpev (f08gnc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n]");
scanf("%" NAG_IFMT "%*[^\n]", &n);

scanf("%39s%*[^\n]", nag_enum_arg);
/* nag_enum_name_to_value (x04nac).
* Converts NAG enum member name to value
*/
uplo = (Nag_UploType)nag_enum_name_to_value(nag_enum_arg);

/* Allocate memory */
if (!(ap = NAG_ALLOC(n * (n + 1) / 2, Complex)) ||
!(dummy = NAG_ALLOC(1, Complex)) || !(w = NAG_ALLOC(n, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read the upper or lower triangular part of the matrix A from data file */
if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i)
for (j = i; j <= n; ++j)
scanf(" ( %lf , %lf )", &AP_UPPER(i, j).re, &AP_UPPER(i, j).im);
scanf("%*[^\n]");
} else if (uplo == Nag_Lower) {
for (i = 1; i <= n; ++i)
for (j = 1; j <= i; ++j)
scanf(" ( %lf , %lf )", &AP_LOWER(i, j).re, &AP_LOWER(i, j).im);
scanf("%*[^\n]");
}

/* nag_lapackeig_zhpev (f08gnc).
* Solve the Hermitian eigenvalue problem.
*/
nag_lapackeig_zhpev(order, Nag_EigVals, uplo, n, ap, w, dummy, 1, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_zhpev (f08gnc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print solution */
printf("Eigenvalues\n");
for (j = 0; j < n; ++j)
printf("%8.4f%s", w[j], (j + 1) % 8 == 0 ? "\n" : " ");
printf("\n");

/* Get the machine precision, eps, using nag_machine_precision (X02AJC)
* and compute the approximate error bound for the computed eigenvalues.
* Note that for the 2-norm, ||A|| = max{|w[i]|,i=0..n-1}), and since
* the eigenvalues are in ascending order: ||A|| = max(|w[0]|,|w[n-1]|).
*/
eps = nag_machine_precision;
eerrbd = eps * MAX(fabs(w[0]), fabs(w[n - 1]));

/* Print the approximate error bound for the eigenvalues */
printf("\nError estimate for the eigenvalues\n");
printf("%11.1e\n", eerrbd);

END:
NAG_FREE(ap);
NAG_FREE(dummy);
NAG_FREE(w);

return exit_status;
}

#undef AP_UPPER
#undef AP_LOWER