NAG Library Manual, Mark 28.5
/* nag_lapacklin_dpocon (f07fgc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.5, 2022.
*/

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

int main(void) {
/* Scalars */
double anorm, rcond;
Integer i, j, n, pda;
Integer exit_status = 0;
NagError fail;
Nag_OrderType order;
/* Arrays */
char nag_enum_arg[40];
double *a = 0;
Nag_UploType uplo;

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

INIT_FAIL(fail);

printf("nag_lapacklin_dpocon (f07fgc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%*[^\n] ", &n);
#ifdef NAG_COLUMN_MAJOR
pda = n;
#else
pda = n;
#endif

/* Allocate memory */
if (!(a = NAG_ALLOC(n * 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);

if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i) {
for (j = i; j <= n; ++j)
scanf("%lf", &A(i, j));
}
scanf("%*[^\n] ");
} else {
for (i = 1; i <= n; ++i) {
for (j = 1; j <= i; ++j)
scanf("%lf", &A(i, j));
}
scanf("%*[^\n] ");
}
/* Compute norm of A */
/* nag_blast_dsy_norm (f16rcc).
* 1-norm, infinity-norm, Frobenius norm, largest absolute
* element, real symmetric matrix
*/
nag_blast_dsy_norm(order, Nag_OneNorm, uplo, n, a, pda, &anorm, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_blast_dsy_norm (f16rcc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Factorize A */
/* nag_lapacklin_dpotrf (f07fdc).
* Cholesky factorization of real symmetric
* positive-definite matrix
*/
nag_lapacklin_dpotrf(order, uplo, n, a, pda, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpotrf (f07fdc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Estimate condition number */
/* nag_lapacklin_dpocon (f07fgc).
* Estimate condition number of real symmetric
* positive-definite matrix, matrix already factorized by
* nag_lapacklin_dpotrf (f07fdc)
*/
nag_lapacklin_dpocon(order, uplo, n, a, pda, anorm, &rcond, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpocon (f07fgc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* nag_machine_precision (x02ajc).
* The machine precision
*/
if (rcond >= nag_machine_precision)
printf("Estimate of condition number = %11.2e\n\n", 1.0 / rcond);
else
printf("A is singular to working precision\n");
END:
NAG_FREE(a);
return exit_status;
}