```/* nag_dtbmv (f16pgc) Example Program.
*
* NAGPRODCODE Version.
*
* Copyright 2016 Numerical Algorithms Group.
*
* Mark 26, 2016.
*/

#include <stdio.h>
#include <nag.h>
#include <nag_stdlib.h>
#include <nagf16.h>

int main(void)
{

/* Scalars */
double alpha;
Integer exit_status, i, incx, j, k, kd, n, pdab, xlen;

/* Arrays */
double *ab = 0, *x = 0;
char nag_enum_arg[40];

/* Nag Types */
NagError fail;
Nag_DiagType diag;
Nag_OrderType order;
Nag_TransType trans;
Nag_UploType uplo;

#ifdef NAG_COLUMN_MAJOR
#define AB_UPPER(I, J) ab[(J-1)*pdab + k + I - J - 1]
#define AB_LOWER(I, J) ab[(J-1)*pdab + I - J]
order = Nag_ColMajor;
#else
#define AB_UPPER(I, J) ab[(I-1)*pdab + J - I]
#define AB_LOWER(I, J) ab[(I-1)*pdab + k + J - I - 1]
order = Nag_RowMajor;
#endif

exit_status = 0;
INIT_FAIL(fail);

printf("nag_dtbmv (f16pgc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
/* Read the problem dimension */
scanf("%" NAG_IFMT "%" NAG_IFMT "%*[^\n] ", &n, &kd);
scanf("%39s%*[^\n] ", nag_enum_arg);
/* nag_enum_name_to_value (x04nac).
* Converts NAG enum member name to value
*/
uplo = (Nag_UploType) nag_enum_name_to_value(nag_enum_arg);
scanf("%39s%*[^\n] ", nag_enum_arg);
/* nag_enum_name_to_value (x04nac).
* Converts NAG enum member name to value
*/
trans = (Nag_TransType) nag_enum_name_to_value(nag_enum_arg);
scanf("%39s%*[^\n] ", nag_enum_arg);
/* nag_enum_name_to_value (x04nac).
* Converts NAG enum member name to value
*/
diag = (Nag_DiagType) nag_enum_name_to_value(nag_enum_arg);
scanf("%lf%*[^\n] ", &alpha);
scanf("%" NAG_IFMT "%*[^\n] ", &incx);

pdab = kd + 1;
xlen = MAX(1, 1 + (n - 1) * ABS(incx));

if (n > 0) {
/* Allocate memory */
if (!(ab = NAG_ALLOC(pdab * n, double)) || !(x = NAG_ALLOC(xlen, double)))
{
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
}
else {
printf("Invalid n\n");
exit_status = 1;
return exit_status;
}

/* Read A from data file */
k = kd + 1;
if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i) {
for (j = i; j <= MIN(i + kd, n); ++j)
scanf("%lf", &AB_UPPER(i, j));
}
scanf("%*[^\n] ");
}
else {
for (i = 1; i <= n; ++i) {
for (j = MAX(1, i - kd); j <= i; ++j)
scanf("%lf", &AB_LOWER(i, j));
}
scanf("%*[^\n] ");
}

/* Input vector x */
for (i = 1; i <= xlen; ++i)
scanf("%lf%*[^\n] ", &x[i - 1]);

/* nag_dtbmv (f16pgc).
* Triangular banded matrix-vector multiply.
*
*/
nag_dtbmv(order, uplo, trans, diag, n, kd, alpha, ab, pdab, x, incx, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_dtbmv (f16pgc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print output vector x */
printf("%s\n", "  x");
for (i = 1; i <= xlen; ++i) {
printf("%11f\n", x[i - 1]);
}

END:
NAG_FREE(ab);
NAG_FREE(x);

return exit_status;
}
```