```/* nag_linsys_real_tridiag_solve (f04bcc) Example Program.
*
* Copyright 2019 Numerical Algorithms Group.
*
* Mark 27.0, 2019.
*/

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

int main(void)
{
/* Scalars */
double errbnd, rcond;
Integer exit_status, i, j, n, nrhs, pdb;

/* Arrays */
double *b = 0, *d = 0, *dl = 0, *du = 0, *du2 = 0;
Integer *ipiv = 0;

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

#ifdef NAG_COLUMN_MAJOR
#define B(I, J) b[(J-1)*pdb + I - 1]
order = Nag_ColMajor;
#else
#define B(I, J) b[(I-1)*pdb + J - 1]
order = Nag_RowMajor;
#endif

exit_status = 0;
INIT_FAIL(fail);

printf("nag_linsys_real_tridiag_solve (f04bcc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%" NAG_IFMT "%*[^\n] ", &n, &nrhs);
if (n >= 0 && nrhs >= 0) {
/* Allocate memory */
if (!(b = NAG_ALLOC(n * nrhs, double)) ||
!(d = NAG_ALLOC(n, double)) ||
!(dl = NAG_ALLOC(n - 1, double)) ||
!(du = NAG_ALLOC(n - 1, double)) ||
!(du2 = NAG_ALLOC(n - 2, double)) || !(ipiv = NAG_ALLOC(n, Integer)))
{
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
#ifdef NAG_COLUMN_MAJOR
pdb = n;
#else
pdb = nrhs;
#endif
}
else {
printf("%s\n", "n and/or nrhs too small");
exit_status = 1;
return exit_status;
}
/* Read A and B from data file */
for (i = 1; i <= n - 1; ++i) {
scanf("%lf", &du[i - 1]);
}
scanf("%*[^\n] ");

for (i = 1; i <= n; ++i) {
scanf("%lf", &d[i - 1]);
}
scanf("%*[^\n] ");

for (i = 1; i <= n - 1; ++i) {
scanf("%lf", &dl[i - 1]);
}
scanf("%*[^\n] ");

for (i = 1; i <= n; ++i) {
for (j = 1; j <= nrhs; ++j) {
scanf("%lf", &B(i, j));
}
}
scanf("%*[^\n] ");

/* Solve the equations AX = B for X */
/* nag_linsys_real_tridiag_solve (f04bcc).
* Computes the solution and error-bound to a real
* tridiagonal system of linear equations
*/
nag_linsys_real_tridiag_solve(order, n, nrhs, dl, d, du, du2, ipiv, b, pdb,
&rcond, &errbnd, &fail);
if (fail.code == NE_NOERROR) {
/* Print solution, estimate of condition number and approximate */
/* error bound */
/* nag_file_print_matrix_real_gen (x04cac).
* Print real general matrix (easy-to-use)
*/
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
nrhs, b, pdb, "Solution", 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;
}
printf("\n");
printf("%s\n      %10.1e\n", "Estimate of condition number", 1.0 / rcond);
printf("\n\n");

printf("%s\n      %10.1e\n\n",
"Estimate of error bound for computed solutions", errbnd);
}
else if (fail.code == NE_RCOND) {
/* Matrix A is numerically singular.  Print estimate of */
/* reciprocal of condition number and solution */

printf("\n%s\n%6s%10.1e\n\n\n",
"Estimate of reciprocal of condition number", "", rcond);
/* nag_file_print_matrix_real_gen (x04cac), see above. */
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
nrhs, b, pdb, "Solution", 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;
}
}
else if (fail.code == NE_SINGULAR) {
/* The upper triangular matrix U is exactly singular.  Print */
/* details of factorization */

printf("%s\n\n", "Details of factorization");
printf("%s\n", " Second superdiagonal of U");

for (i = 1; i <= n - 2; ++i) {
printf("%9.4f%s", du2[i - 1], i % 8 == 0 || i == n - 2 ? "\n" : " ");
}
printf("\n");

printf("\n%s\n", " First superdiagonal of U");
for (i = 1; i <= n - 1; ++i) {
printf("%9.4f%s", du[i - 1], i % 8 == 0 || i == n - 1 ? "\n" : " ");
}
printf("\n\n");

printf("%s\n", " Main diagonal of U");
for (i = 1; i <= n; ++i) {
printf("%9.4f%s", d[i - 1], i % 8 == 0 || i == n ? "\n" : " ");
}
printf("\n\n");

printf("%s\n", " Multipliers");
for (i = 1; i <= n - 1; ++i) {
printf("%9.4f%s", dl[i - 1], i % 8 == 0 || i == n - 1 ? "\n" : " ");
}
printf("\n\n");

printf("%s\n", " Vector of interchanges");
for (i = 1; i <= n; ++i) {
printf("%9" NAG_IFMT "%s", ipiv[i - 1], i % 8 == 0
|| i == n ? "\n" : " ");
}
printf("\n");
}
else {
printf("Error from nag_linsys_real_tridiag_solve (f04bcc).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}
END:
NAG_FREE(b);
NAG_FREE(d);
NAG_FREE(dl);
NAG_FREE(du);
NAG_FREE(du2);
NAG_FREE(ipiv);

return exit_status;
}
```