NAG Library Manual, Mark 27.2
```/* nag_lapackeig_dgehrd (f08nec) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.2, 2021.
*/

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

int main(void) {
/* Scalars */
Integer i, j, n, pda, tau_len;
Integer exit_status = 0;
NagError fail;
Nag_OrderType order;
/* Arrays */
double *a = 0, *tau = 0;

#ifdef NAG_COLUMN_MAJOR
#define A(I, J) a[(J - 1) * pda + I - 1]
order = Nag_ColMajor;
#else
#define A(I, J) a[(I - 1) * pda + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapackeig_dgehrd (f08nec) Example Program Results\n\n");

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

pda = n;
tau_len = n - 1;

/* Allocate memory */
if (!(a = NAG_ALLOC(n * n, double)) || !(tau = NAG_ALLOC(tau_len, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read A from data file */
for (i = 1; i <= n; ++i) {
for (j = 1; j <= n; ++j)
scanf("%lf", &A(i, j));
}
scanf("%*[^\n] ");

/* nag_lapackeig_dgehrd (f08nec): Reduce A to upper Hessenberg form */
nag_lapackeig_dgehrd(order, n, 1, n, a, pda, tau, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_dgehrd (f08nec).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Set the elements below the first subdiagonal to zero */
for (i = 1; i <= n - 2; ++i) {
for (j = i + 2; j <= n; ++j)
A(j, i) = 0.0;
}

/* nag_file_print_matrix_real_gen (x04cac): Print upper Hessenberg form. */
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
n, a, pda, "Upper Hessenberg form", 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(a);
NAG_FREE(tau);
return exit_status;
}

#undef A
```