NAG Library Manual, Mark 30.1
```/* nag_lapackeig_dstedc (f08jhc) Example Program.
*
* Copyright 2024 Numerical Algorithms Group.
*
* Mark 30.1, 2024.
*/

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

int main(void) {
/* Scalars */
Integer i, j, k, kd, n, pdab, pdq;
Integer exit_status = 0;
/* Arrays */
char nag_enum_arg[40];
double *ab = 0, *d = 0, *e = 0, *q = 0;
/* Nag Types */
Nag_OrderType order;
Nag_UploType uplo;
NagError fail;

#ifdef NAG_COLUMN_MAJOR
#define AB_UPPER(I, J) ab[(J - 1) * pdab + k + I - J - 1]
#define AB_LOWER(I, J) ab[(J - 1) * pdab + I - J]
#define Q(I, J) q[(J - 1) * pdq + I - 1]
order = Nag_ColMajor;
#else
#define AB_UPPER(I, J) ab[(I - 1) * pdab + J - I]
#define AB_LOWER(I, J) ab[(I - 1) * pdab + k + J - I - 1]
#define Q(I, J) q[(I - 1) * pdq + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapackeig_dstedc (f08jhc) Example Program Results\n\n");

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

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);

pdab = kd + 1;

#ifdef NAG_COLUMN_MAJOR
pdq = n;
#else
pdq = n;
#endif

/* Allocate memory */
if (!(ab = NAG_ALLOC(pdab * n, double)) || !(d = NAG_ALLOC(n, double)) ||
!(e = NAG_ALLOC(n - 1, double)) || !(q = NAG_ALLOC(n * n, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read the upper or lower triangular part of the band matrix A
* from data file.
*/
k = kd + 1;
if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i)
for (j = i; j <= MIN(n, i + kd); ++j)
scanf("%lf", &AB_UPPER(i, j));
scanf("%*[^\n]");
} else if (uplo == Nag_Lower) {
for (i = 1; i <= n; ++i)
for (j = MAX(1, i - kd); j <= i; ++j)
scanf("%lf", &AB_LOWER(i, j));
scanf("%*[^\n]");
}

/* nag_lapackeig_dsbtrd (f08hec).
* Reduce A to tridiagonal form T = (Q^T)*A*Q, and form Q.
*/
nag_lapackeig_dsbtrd(order, Nag_FormQ, uplo, n, kd, ab, pdab, d, e, q, pdq,
&fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_dsbtrd (f08hec).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* nag_lapackeig_dstedc (f08jhc)
* Calculate all the eigenvalues and eigenvectors of A,
* from T and Q.
*/
nag_lapackeig_dstedc(order, Nag_OrigEigVecs, n, d, e, q, pdq, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_dstedc (f08jhc).\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--)
Q(i, j) = Q(i, j) / Q(1, j);

/* Print eigenvalues and eigenvectors */
printf("%s\n", "Eigenvalues");
for (i = 0; i < n; ++i)
printf("%8.4f%s", d[i], (i + 1) % 8 == 0 ? "\n" : " ");
printf("\n\n");

/* nag_file_print_matrix_real_gen (x04cac).
* Print eigenvectors.
*/
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
n, q, pdq, "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(ab);
NAG_FREE(d);
NAG_FREE(e);
NAG_FREE(q);

return exit_status;
}

#undef AB_UPPER
#undef AB_LOWER
#undef Q
```