```/* nag_lapackeig_zhpgvx (f08tpc) Example Program.
*
* Copyright 2019 Numerical Algorithms Group.
*
* Mark 27.0, 2019.
*/

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

int main(void)
{
/* Scalars */
double abstol, vl, vu;
Integer i, il = 0, iu = 0, j, m, n, pdz;
Integer exit_status = 0;

/* Arrays */
Complex *ap = 0, *bp = 0, *z = 0;
double *w = 0;
Integer *index = 0;
char nag_enum_arg[40];

/* Nag Types */
NagError fail;
Nag_OrderType order;
Nag_UploType uplo;

#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 B_UPPER(I, J) bp[J*(J-1)/2 + I - 1]
#define B_LOWER(I, J) bp[(2*n-J)*(J-1)/2 + I - 1]
#define Z(I, J) z[(J-1)*pdz + I - 1]
order = Nag_ColMajor;
#else
#define A_UPPER(I, J) ap[(2*n-I)*(I-1)/2 + J - 1]
#define A_LOWER(I, J) ap[I*(I-1)/2 + J - 1]
#define B_UPPER(I, J) bp[(2*n-I)*(I-1)/2 + J - 1]
#define B_LOWER(I, J) bp[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_zhpgvx (f08tpc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n]");
scanf("%" NAG_IFMT "%*[^\n]", &n);
if (n < 0) {
printf("Invalid n\n");
exit_status = 1;
goto END;;
}
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);

m = n;
#ifdef NAG_COLUMN_MAJOR
pdz = n;
#else
pdz = m;
#endif
/* Allocate memory */
if (!(ap = NAG_ALLOC(n * (n + 1) / 2, Complex)) ||
!(bp = NAG_ALLOC(n * (n + 1) / 2, Complex)) ||
!(z = NAG_ALLOC(n * m, Complex)) ||
!(w = NAG_ALLOC(n, double)) || !(index = NAG_ALLOC(n, Integer)))
{
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read the lower and upper bounds of the interval to be searched. */
scanf("%lf%lf%*[^\n]", &vl, &vu);
/* Read the triangular parts of the matrices A and B from data file. */
if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i)
for (j = i; j <= n; ++j)
scanf(" ( %lf , %lf )", &A_UPPER(i, j).re, &A_UPPER(i, j).im);
scanf("%*[^\n]");
for (i = 1; i <= n; ++i)
for (j = i; j <= n; ++j)
scanf(" ( %lf , %lf )", &B_UPPER(i, j).re, &B_UPPER(i, j).im);
}
else if (uplo == Nag_Lower) {
for (i = 1; i <= n; ++i)
for (j = 1; j <= i; ++j)
scanf(" ( %lf , %lf )", &A_LOWER(i, j).re, &A_LOWER(i, j).im);
scanf("%*[^\n]");
for (i = 1; i <= n; ++i)
for (j = 1; j <= i; ++j)
scanf(" ( %lf , %lf )", &B_LOWER(i, j).re, &B_LOWER(i, j).im);
}
scanf("%*[^\n]");

/* Use the default absolute error tolerance for eigenvalues. */
abstol = 0.0;

/* Solve the generalized Hermitian eigenvalue problem
* A*x = lambda*B*x (itype = 1). using nag_lapackeig_zhpgvx (f08tpc).
*/
nag_lapackeig_zhpgvx(order, 1, Nag_DoBoth, Nag_Interval, uplo, n, ap, bp, vl, vu,
il, iu, abstol, &m, w, z, pdz, index, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_zhpgvx (f08tpc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* 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 eigensolution */
printf("Number of eigenvalues found =%5" NAG_IFMT "\n\n", m);
printf(" Eigenvalues\n  ");
for (j = 0; j < m; ++j)
printf(" %11.4f%s", w[j], j % 8 == 7 ? "\n" : "");
printf("\n");

/* Print normalized vectors using nag_file_print_matrix_complex_gen (x04dac). */
fflush(stdout);
nag_file_print_matrix_complex_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n, m,
z, pdz, "Selected eigenvectors", 0, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_complex_gen (x04dac).\n%s\n",
fail.message);
exit_status = 1;
}

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

return exit_status;
}
```