NAG Library Manual, Mark 27.3
```/* nag_lapacklin_dpptrf (f07gdc) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.3, 2021.
*/

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

int main(void) {
/* Scalars */
Integer ap_len, i, j, n;
Integer exit_status = 0;
Nag_UploType uplo;
NagError fail;
Nag_OrderType order;

/* Arrays */
char nag_enum_arg[40];
double *ap = 0;

#ifdef NAG_COLUMN_MAJOR
#define A_UPPER(I, J) ap[J * (J - 1) / 2 + I - 1]
#define A_LOWER(I, J) ap[(2 * n - J) * (J - 1) / 2 + I - 1]
order = Nag_ColMajor;
#else
#define A_LOWER(I, J) ap[I * (I - 1) / 2 + J - 1]
#define A_UPPER(I, J) ap[(2 * n - I) * (I - 1) / 2 + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapacklin_dpptrf (f07gdc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%*[^\n] ", &n);
ap_len = n * (n + 1) / 2;

/* Allocate memory */
if (!(ap = NAG_ALLOC(ap_len, 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);

if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i) {
for (j = i; j <= n; ++j)
scanf("%lf", &A_UPPER(i, j));
}
scanf("%*[^\n] ");
} else {
for (i = 1; i <= n; ++i) {
for (j = 1; j <= i; ++j)
scanf("%lf", &A_LOWER(i, j));
}
scanf("%*[^\n] ");
}
/* Factorize A */
/* nag_lapacklin_dpptrf (f07gdc).
* Cholesky factorization of real symmetric
* positive-definite matrix, packed storage
*/
nag_lapacklin_dpptrf(order, uplo, n, ap, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpptrf (f07gdc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Print factor */
/* nag_file_print_matrix_real_packed (x04ccc).
* Print real packed triangular matrix (easy-to-use)
*/
fflush(stdout);
nag_file_print_matrix_real_packed(order, uplo, Nag_NonUnitDiag, n, ap,
"Factor", 0, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_real_packed (x04ccc).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}

END:
NAG_FREE(ap);
return exit_status;
}
```