NAG Library Manual, Mark 30.1
```/* nag_lapackeig_dspevd (f08gcc) Example Program.
*
* Copyright 2024 Numerical Algorithms Group.
*
* Mark 30.1, 2024.
*/

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

int main(void) {
/* Scalars */
Integer i, j, n, ap_len, pdz, w_len;
Integer exit_status = 0;
NagError fail;
Nag_JobType job;
Nag_UploType uplo;
Nag_OrderType order;
/* Arrays */
char nag_enum_arg[40];
double *ap = 0, *w = 0, *z = 0;

#ifdef NAG_COLUMN_MAJOR
#define A_UPPER(I, J) ap[J * (J - 1) / 2 + I - 1]
#define A_LOWER(I, J) ap[(2 * n - J) * (J - 1) / 2 + I - 1]
#define Z(I, J) z[(J - 1) * pdz + I - 1]
order = Nag_ColMajor;
#else
#define A_LOWER(I, J) ap[I * (I - 1) / 2 + J - 1]
#define A_UPPER(I, J) ap[(2 * n - I) * (I - 1) / 2 + J - 1]
#define Z(I, J) z[(I - 1) * pdz + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapackeig_dspevd (f08gcc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%*[^\n] ", &n);
ap_len = n * (n + 1) / 2;
w_len = n;
pdz = n;

/* Allocate memory */
if (!(ap = NAG_ALLOC(ap_len, double)) || !(z = NAG_ALLOC(n * n, double)) ||
!(w = NAG_ALLOC(w_len, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
/* Read whether Upper or Lower part of A is stored */
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);
/* Read A from data file */
if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i) {
for (j = i; j <= n; ++j)
scanf("%lf", &A_UPPER(i, j));
}
scanf("%*[^\n] ");
} else {
for (i = 1; i <= n; ++i) {
for (j = 1; j <= i; ++j)
scanf("%lf", &A_LOWER(i, j));
}
scanf("%*[^\n] ");
}
/* Read type of job to be performed */
scanf("%39s%*[^\n] ", nag_enum_arg);
job = (Nag_JobType)nag_enum_name_to_value(nag_enum_arg);
/* Calculate all the eigenvalues and eigenvectors of A */
/* nag_lapackeig_dspevd (f08gcc).
* All eigenvalues and optionally all eigenvectors of real
* symmetric matrix, packed storage (divide-and-conquer)
*/
nag_lapackeig_dspevd(order, job, uplo, n, ap, w, z, pdz, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_dspevd (f08gcc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Normalize the eigenvectors */
for (j = 1; j <= n; j++) {
for (i = n; i >= 1; i--) {
Z(i, j) = Z(i, j) / Z(1, j);
}
}
/* Print eigenvalues and eigenvectors */
printf("Eigenvalues \n");
for (i = 0; i < n; ++i)
printf(" %8.4f", w[i]);
printf("\n");
/* nag_file_print_matrix_real_gen (x04cac).
* Print real general matrix (easy-to-use)
*/
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
n, z, pdz, "Eigenvectors", 0, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_real_gen (x04cac).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}
END:
NAG_FREE(ap);
NAG_FREE(w);
NAG_FREE(z);
return exit_status;
}
```