NAG Library Manual, Mark 28.5
/* nag_lapacklin_dpftri (f07wjc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.5, 2022.
*/

#include <nag.h>

int main(void) {
/* Scalars */
Integer exit_status = 0;
Integer i, j, k, lar1, lar2, lenar, n, pdar, pda, q;
/* Arrays */
double *ar = 0, *a = 0;
char nag_enum_arg[40];
/* NAG types */
Nag_RFP_Store transr;
Nag_UploType uplo;
Nag_OrderType order;
NagError fail;

#ifdef NAG_COLUMN_MAJOR
order = Nag_ColMajor;
#define AR(I, J) ar[J * pdar + I]
#else
order = Nag_RowMajor;
#define AR(I, J) ar[I * pdar + J]
#endif

INIT_FAIL(fail);

printf("nag_lapacklin_dpftri (f07wjc) Example Program Results\n\n");
/* Skip heading in data file */
scanf("%*[^\n]");
scanf("%" NAG_IFMT "", &n);
scanf("%39s", nag_enum_arg);
uplo = (Nag_UploType)nag_enum_name_to_value(nag_enum_arg);
scanf("%39s%*[^\n]", nag_enum_arg);
transr = (Nag_RFP_Store)nag_enum_name_to_value(nag_enum_arg);

lenar = (n * (n + 1)) / 2;
pda = n;
if (!(ar = NAG_ALLOC(lenar, double)) || !(a = NAG_ALLOC(pda * n, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Setup dimensions for RFP array ar. */
k = n / 2;
q = n - k;
if (transr == Nag_RFP_Normal) {
lar1 = 2 * k + 1;
lar2 = q;
} else {
lar1 = q;
lar2 = 2 * k + 1;
}
if (order == Nag_RowMajor) {
pdar = lar2;
} else {
pdar = lar1;
}
/* Read matrix into RFP array ar. */
for (i = 0; i < lar1; i++) {
for (j = 0; j < lar2; j++) {
scanf("%lf ", &AR(i, j));
}
}

/* Factorize A using nag_lapacklin_dpftrf (f07wdc) which performs a Cholesky
* factorization of a real symmetric positive definite matrix in
* Rectangular Full Packed format
*/
nag_lapacklin_dpftrf(order, transr, uplo, n, ar, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpftrf (f07wdc)\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Compute inverse of A using nag_lapacklin_dpftri (f07wjc) */
nag_lapacklin_dpftri(order, transr, uplo, n, ar, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dpftri (f07wjc)\n%s\n", fail.message);
exit_status = 2;
goto END;
}

/* Convert inverse to full array form using nag_matop_dtfttr (f01vgc). */
nag_matop_dtfttr(order, transr, uplo, n, ar, a, pda, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_matop_dtfttr (f01vgc)\n%s\n", fail.message);
exit_status = 3;
goto END;
}

/* nag_file_print_matrix_real_gen (x04cac).
* Print real general matrix (easy-to-use)
*/
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_LowerMatrix, Nag_NonUnitDiag, n, n,
a, pda, "Inverse", 0, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_real_gen (x04cac)\n%s\n",
fail.message);
exit_status = 4;
}

END:
NAG_FREE(ar);
NAG_FREE(a);
return exit_status;
}