NAG Library Manual, Mark 28.4
Interfaces:  FL   CL   CPP   AD
```/* nag_lapackeig_dtgexc (f08yfc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.4, 2022.
*/

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

int main(void) {
/* Scalars */
double alpha, beta;
Integer i, ifst, ilst, j, n, pdq, pds;
Integer pdt, pdz, exit_status = 0;
/* Arrays */
double *q = 0, *s = 0, *t = 0, *z = 0;
char nag_enum_arg[40];

/* Nag Types */
NagError fail;
Nag_OrderType order;
Nag_Boolean wantq, wantz;

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

INIT_FAIL(fail);

printf("nag_lapackeig_dtgexc (f08yfc) 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
*/
wantq = (Nag_Boolean)nag_enum_name_to_value(nag_enum_arg);
scanf(" %39s%*[^\n]", nag_enum_arg);
wantz = (Nag_Boolean)nag_enum_name_to_value(nag_enum_arg);

pds = n;
pdt = n;

pdq = (wantq ? n : 1);
pdz = (wantz ? n : 1);

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

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

/* Initialize Q an Z to identity matrices using nag_blast_dge_load (f16qhc).
*/
alpha = 0.0;
beta = 1.0;
if (wantq)
nag_blast_dge_load(order, n, n, alpha, beta, q, pdq, &fail);
if (wantz)
nag_blast_dge_load(order, n, n, alpha, beta, z, pdz, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_blast_dge_load (f16qhc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Read the row indices of diagonal elements or blocks to be swapped. */
scanf("%" NAG_IFMT "%" NAG_IFMT "%*[^\n]", &ifst, &ilst);

/* Reorder S and T */
nag_lapackeig_dtgexc(order, wantq, wantz, n, s, pds, t, pdt, q, pdq, z, pdz,
&ifst, &ilst, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_dtgexc (f08yfc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* normalize signs so that last column and diagonal of T is positive. */
for (i = 1; i <= n; i++) {
if (T(i, n) < 0.0) {
for (j = i; j <= n; j++) {
S(i, j) = -S(i, j);
T(i, j) = -T(i, j);
}
if (i > 1 && fabs(S(i, i - 1)) > 0.0) {
S(i, i - 1) = -S(i, i - 1);
}
}
}
for (j = 1; j <= n - 1; j++) {
if (T(j, j) < 0.0) {
for (i = 1; i <= j; i++) {
S(i, j) = -S(i, j);
T(i, j) = -T(i, j);
}
if (fabs(S(j + 1, j)) > 0.0) {
S(j + 1, j) = -S(j + 1, j);
}
}
}

/* nag_file_print_matrix_real_gen (x04cac): Print reordered S and T. */
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
n, s, pds, "Reordered matrix S", 0, &fail);
printf("\n");
if (fail.code != NE_NOERROR)
goto PRERR;
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
n, t, pdt, "Reordered matrix T", 0, &fail);
printf("\n");
PRERR:
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(q);
NAG_FREE(s);
NAG_FREE(t);
NAG_FREE(z);

return exit_status;
}
```