NAG Library Manual, Mark 30.1
```/* nag_lapackeig_zhpevx (f08gpc) Example Program.
*
* Copyright 2024 Numerical Algorithms Group.
*
* Mark 30.1, 2024.
*/

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

int main(void) {
/* Scalars */
double abstol, vl, vu;
Integer exit_status = 0, i, il = 0, iu = 0, j, m, n, pdz;
/* Arrays */
char nag_enum_arg[40];
Complex *ap = 0, *z = 0;
double *w = 0;
Integer *index = 0;
/* Nag Types */
Nag_OrderType order;
Nag_UploType uplo;
NagError fail, fail_print;

#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]
#define Z(I, J) z[(J - 1) * pdz + 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]
#define Z(I, J) z[(I - 1) * pdz + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapackeig_zhpevx (f08gpc) 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)) ||
!(z = NAG_ALLOC(n * n, Complex)) || !(w = NAG_ALLOC(n, double)) ||
!(index = NAG_ALLOC(n, Integer))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
pdz = n;

/* Read the lower and upper bounds of the interval to be searched, and
* read the upper or lower triangular part of the matrix A from data file.
*/
scanf("%lf%lf%*[^\n]", &vl, &vu);
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]");
}

/* Set the absolute error tolerance for eigenvalues.
* With abstol set to zero, the default value is used instead.
*/
abstol = 0.0;

/* nag_lapackeig_zhpevx (f08gpc).
* Solve the Hermitian eigenvalue problem.
*/
nag_lapackeig_zhpevx(order, Nag_DoBoth, Nag_Interval, uplo, n, ap, vl, vu, il,
iu, abstol, &m, w, z, pdz, index, &fail);
if (fail.code != NE_NOERROR && fail.code != NE_CONVERGENCE) {
printf("Error from nag_lapackeig_zhpevx (f08gpc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* nag_complex_divide (a02cdc).
* Normalize the eigenvectors.
*/
for (j = 1; j <= m; j++)
for (i = n; i >= 1; i--)
Z(i, j) = nag_complex_divide(Z(i, j), Z(1, j));

/* Print solution */
printf("Number of eigenvalues found =%5" NAG_IFMT "\n", m);

printf("\nEigenvalues\n");
for (j = 0; j < m; ++j)
printf("%8.4f%s", w[j], (j + 1) % 8 == 0 ? "\n" : " ");
printf("\n\n");

/* nag_file_print_matrix_complex_gen (x04dac).
* Print selected eigenvectors.
*/
INIT_FAIL(fail_print);
fflush(stdout);
nag_file_print_matrix_complex_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag,
n, m, z, pdz, "Selected eigenvectors", 0,
&fail_print);
if (fail_print.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_complex_gen (x04dac).\n%s\n",
fail_print.message);
exit_status = 1;
goto END;
}
if (fail.code == NE_CONVERGENCE) {
printf("eigenvectors failed to converge\n");
printf("Indices of eigenvectors that did not converge\n");
for (j = 0; j < m; ++j)
printf("%8" NAG_IFMT "%s", index[j], (j + 1) % 8 == 0 ? "\n" : " ");
}

END:
NAG_FREE(ap);
NAG_FREE(z);
NAG_FREE(w);
NAG_FREE(index);

return exit_status;
}

#undef AP_UPPER
#undef AP_LOWER
#undef Z
```