NAG Library Manual, Mark 29.3
```/* nag_lapacklin_dpstrf (f07kdc) Example Program.
*
* Copyright 2023 Numerical Algorithms Group.
*
* Mark 29.3, 2023.
*/

#include <nag.h>

int main(void) {
/* Scalars */
Integer exit_status = 0;
Integer i, j, n, pda, rank;
double tol;
/* Arrays */
double *a = 0;
Integer *piv = 0;
char nag_enum_arg[40];
/* Nag Types */
Nag_UploType uplo;
Nag_OrderType order;
Nag_MatrixType matrix;
NagError fail;

INIT_FAIL(fail);

printf("nag_lapacklin_dpstrf (f07kdc) Example Program Results\n");
/* Skip heading in data file and retrieve data */
scanf("%*[^\n]%" NAG_IFMT "%39s%*[^\n]", &n, nag_enum_arg);
uplo = (Nag_UploType)nag_enum_name_to_value(nag_enum_arg);
if (!(a = NAG_ALLOC(n * n, double)) || !(piv = NAG_ALLOC(n, Integer))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

pda = n;
#ifdef NAG_COLUMN_MAJOR
order = Nag_ColMajor;
#define A(I, J) a[(J - 1) * pda + I - 1]
#else
order = Nag_RowMajor;
#define A(I, J) a[(I - 1) * pda + J - 1]
#endif

/* Read triangular part of A from data file */
if (uplo == Nag_Upper) {
matrix = Nag_UpperMatrix;
for (i = 1; i <= n; i++)
for (j = i; j <= n; j++)
scanf("%lf", &A(i, j));
} else if (uplo == Nag_Lower) {
matrix = Nag_LowerMatrix;
for (i = 1; i <= n; i++)
for (j = 1; j <= i; j++)
scanf("%lf", &A(i, j));
} else {
printf("Invalid uplo.\n");
exit_status = 1;
goto END;
}
scanf("%*[^\n]");

tol = -1.0;

/* Factorize A using nag_lapacklin_dpstrf (f07kdc) which performs a Cholesky
* factorization of real symmetric positive semidefinite matrix.
*/
nag_lapacklin_dpstrf(order, uplo, n, a, pda, piv, &rank, tol, &fail);

if (fail.code == NW_NOT_POS_DEF) {
/* A is not of full rank.
* Zero out columns rank+1 to n.
*/
if (uplo == Nag_Upper)
for (j = rank + 1; j <= n; j++)
for (i = rank + 1; i <= j; i++)
A(i, j) = 0.0;
else if (uplo == Nag_Lower)
for (j = rank + 1; j <= n; j++)
for (i = j; i <= n; i++)
A(i, j) = 0.0;
} else if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpstrf (f07kdc)\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print rank of A. */
printf("\nComputed rank: %" NAG_IFMT "\n\n", rank);

/* Print factorization using
* nag_file_print_matrix_real_gen (x04cac).
* Print real general matrix (easy-to-use)
*/
fflush(stdout);
nag_file_print_matrix_real_gen(order, matrix, Nag_NonUnitDiag, n, n, a, pda,
"Factor", 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;
}

/* Print pivot indices. */
printf("\nPivots:\n");
for (i = 0; i < n; i++)
printf("%11" NAG_IFMT "", piv[i]);
printf("\n");

END:
NAG_FREE(a);
NAG_FREE(piv);
return exit_status;
}
```