NAG Library Manual, Mark 28.5
Interfaces:  FL   CL   CPP   AD
```/* nag_lapacklin_dptrfs (f07jhc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.5, 2022.
*/

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

int main(void) {

/* Scalars */
Integer exit_status = 0, i, j, n, nrhs, pdb, pdx;
Nag_OrderType order;

/* Arrays */
double *b = 0, *berr = 0, *d = 0, *df = 0, *e = 0, *ef = 0, *ferr = 0;
double *x = 0;

/* Nag Types */
NagError fail;

#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

INIT_FAIL(fail);

printf("nag_lapacklin_dptrfs (f07jhc) 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) {
printf("Invalid n or nrhs\n");
exit_status = 1;
goto END;
}
/* Allocate memory */
if (!(b = NAG_ALLOC(n * nrhs, double)) || !(berr = NAG_ALLOC(nrhs, double)) ||
!(d = NAG_ALLOC(n, double)) || !(df = NAG_ALLOC(n, double)) ||
!(e = NAG_ALLOC(n - 1, double)) || !(ef = NAG_ALLOC(n - 1, double)) ||
!(ferr = NAG_ALLOC(nrhs, double)) || !(x = NAG_ALLOC(n * nrhs, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
#ifdef NAG_COLUMN_MAJOR
pdb = n;
pdx = n;
#else
pdb = nrhs;
pdx = nrhs;
#endif

/* Read the lower bidiagonal part of the tridiagonal matrix A from */
/* data file */
for (i = 0; i < n; ++i)
scanf("%lf", &d[i]);
scanf("%*[^\n]");
for (i = 0; i < n - 1; ++i)
scanf("%lf", &e[i]);
scanf("%*[^\n]");

/* Read the right hand matrix B */
for (i = 1; i <= n; ++i)
for (j = 1; j <= nrhs; ++j)
scanf("%lf", &B(i, j));
scanf("%*[^\n]");

/* Copy A into DF and EF */
for (i = 0; i < n; ++i)
df[i] = d[i];
for (i = 0; i < n - 1; ++i)
ef[i] = e[i];

/* Copy B into X using nag_blast_dge_copy (f16qfc). */
nag_blast_dge_copy(order, Nag_NoTrans, n, nrhs, b, pdb, x, pdx, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_blast_dge_copy (f16qfc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Factorize the copy of the tridiagonal matrix A using
* nag_lapacklin_dpttrf (f07jdc).
*/
nag_lapacklin_dpttrf(n, df, ef, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpttrf (f07jdc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Solve the equations AX = B using nag_lapacklin_dpttrs (f07jec). */
nag_lapacklin_dpttrs(order, n, nrhs, df, ef, x, pdx, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpttrs (f07jec).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Improve the solution and compute error estimates
* using nag_lapacklin_dptrfs (f07jhc).
*/
nag_lapacklin_dptrfs(order, n, nrhs, d, e, df, ef, b, pdb, x, pdx, ferr, berr,
&fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dptrfs (f07jhc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print the solution and the forward and backward error estimates
* using nag_file_print_matrix_real_gen (x04cac).
*/
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
nrhs, x, pdx, "Solution(s)", 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("\nBackward errors (machine-dependent)\n");
for (j = 0; j < nrhs; ++j)
printf("%11.1e%s", berr[j], j % 7 == 6 ? "\n" : " ");

printf("\n\nEstimated forward error bounds (machine-dependent)\n");
for (j = 0; j < nrhs; ++j)
printf("%11.1e%s", ferr[j], j % 7 == 6 ? "\n" : " ");

printf("\n");

END:
NAG_FREE(b);
NAG_FREE(berr);
NAG_FREE(d);
NAG_FREE(df);
NAG_FREE(e);
NAG_FREE(ef);
NAG_FREE(ferr);
NAG_FREE(x);

return exit_status;
}

#undef B
```