NAG Library Manual, Mark 27.2
```/* nag_lapackeig_ztrexc (f08qtc) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.2, 2021.
*/

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

int main(void) {
/* Scalars */
Integer i, ifst, ilst, j, n, pdq, pdt;
Integer exit_status = 0;
NagError fail;
Nag_OrderType order;
/* Arrays */
Complex *q = 0, *t = 0;

#ifdef NAG_COLUMN_MAJOR
#define T(I, J) t[(J - 1) * pdt + I - 1]
order = Nag_ColMajor;
#else
#define T(I, J) t[(I - 1) * pdt + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapackeig_ztrexc (f08qtc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%*[^\n] ", &n);
#ifdef NAG_COLUMN_MAJOR
pdq = 1;
pdt = n;
#else
pdq = 1;
pdt = n;
#endif

/* Allocate memory */
if (!(q = NAG_ALLOC(1 * 1, Complex)) || !(t = NAG_ALLOC(n * n, Complex))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read T from data file */
for (i = 1; i <= n; ++i) {
for (j = 1; j <= n; ++j)
scanf(" ( %lf , %lf )", &T(i, j).re, &T(i, j).im);
}
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%" NAG_IFMT "%*[^\n] ", &ifst, &ilst);

/* Reorder the Schur factorization T */
/* nag_lapackeig_ztrexc (f08qtc).
* Reorder Schur factorization of complex matrix using
* unitary similarity transformation
*/
nag_lapackeig_ztrexc(order, Nag_NotQ, n, t, pdt, q, pdq, ifst, ilst, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_ztrexc (f08qtc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Print reordered Schur form */
/* nag_file_print_matrix_complex_gen_comp (x04dbc).
* Print complex general matrix (comprehensive)
*/
fflush(stdout);
nag_file_print_matrix_complex_gen_comp(
order, Nag_GeneralMatrix, Nag_NonUnitDiag, n, n, t, pdt, Nag_BracketForm,
"%7.4f", "Reordered Schur form", Nag_IntegerLabels, 0, Nag_IntegerLabels,
0, 80, 0, 0, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_complex_gen_comp (x04dbc).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}
END:
NAG_FREE(q);
NAG_FREE(t);

return exit_status;
}
```