NAG Library Manual, Mark 29.3
```/* nag_lapacklin_dpbtrf (f07hdc) Example Program.
*
* Copyright 2023 Numerical Algorithms Group.
*
* Mark 29.3, 2023.
*/

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

int main(void) {
/* Scalars */
Integer i, j, k, kd, n, pdab;
Integer exit_status = 0;
Nag_UploType uplo;
NagError fail;
Nag_OrderType order;
/* Arrays */
char nag_enum_arg[40];
double *ab = 0;

#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

INIT_FAIL(fail);

printf("nag_lapacklin_dpbtrf (f07hdc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%" NAG_IFMT "%*[^\n] ", &n, &kd);
pdab = kd + 1;

/* Allocate memory */
if (!(ab = NAG_ALLOC((kd + 1) * n, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read A from data file */
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);

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] ");
}
/* Factorize A */
/* nag_lapacklin_dpbtrf (f07hdc).
* Cholesky factorization of real symmetric
* positive-definite band matrix
*/
fflush(stdout);
nag_lapacklin_dpbtrf(order, uplo, n, kd, ab, pdab, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpbtrf (f07hdc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Print details of factorization */
if (uplo == Nag_Upper)
/* nag_file_print_matrix_real_band (x04cec).
* Print real packed banded matrix (easy-to-use)
*/
nag_file_print_matrix_real_band(order, n, n, 0, kd, ab, pdab, "Factor", 0,
&fail);
else
/* nag_file_print_matrix_real_band (x04cec), see above. */
fflush(stdout);
nag_file_print_matrix_real_band(order, n, n, kd, 0, ab, pdab, "Factor", 0,
&fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_real_band (x04cec).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}
END:
NAG_FREE(ab);
return exit_status;
}
```