NAG Library Manual, Mark 28.4
/* nag_blast_dtpsv (f16plc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.4, 2022.
*/

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

int main(void) {

/* Scalars */
double alpha;
Integer ap_len, exit_status, i, incx, j, n, xlen;

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

/* Nag Types */
NagError fail;
Nag_OrderType order;
Nag_TransType trans;
Nag_UploType uplo;
Nag_DiagType diag;

#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

exit_status = 0;
INIT_FAIL(fail);

printf("nag_blast_dtpsv (f16plc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");

/* Read the problem dimensions */
scanf("%" NAG_IFMT "%*[^\n] ", &n);

/* Read the uplo storage parameter */
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);
/* Read the transpose parameter */
scanf("%39s%*[^\n] ", nag_enum_arg);
/* nag_enum_name_to_value (x04nac), see above. */
trans = (Nag_TransType)nag_enum_name_to_value(nag_enum_arg);
/* Read the unit-diagonal parameter */
scanf("%39s%*[^\n] ", nag_enum_arg);
/* nag_enum_name_to_value (x04nac), see above. */
diag = (Nag_DiagType)nag_enum_name_to_value(nag_enum_arg);

scanf("%lf%*[^\n] ", &alpha);
scanf("%" NAG_IFMT "%*[^\n] ", &incx);

ap_len = n * (n + 1) / 2;
xlen = MAX(1, 1 + (n - 1) * ABS(incx));

if (n > 0) {
/* Allocate memory */
if (!(ap = NAG_ALLOC(ap_len, double)) || !(x = NAG_ALLOC(xlen, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
} else {
printf("Invalid n\n");
exit_status = 1;
return exit_status;
}

/* Input matrix A and vector x */

if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i) {
if (diag == Nag_NonUnitDiag)
scanf("%lf", &A_UPPER(i, i));
for (j = i + 1; 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));
if (diag == Nag_NonUnitDiag)
scanf("%lf", &A_LOWER(i, i));
}
scanf("%*[^\n] ");
}
for (i = 0; i < xlen; ++i)
scanf("%lf%*[^\n] ", &x[i]);

/* nag_blast_dtpsv (f16plc).
* Solution of real triangular system of linear equations,
* using packed storage.
*/
nag_blast_dtpsv(order, uplo, trans, diag, n, alpha, ap, x, incx, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_blast_dtpsv.\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print output vector x */
printf("%s\n", " Solution x:");
for (i = 0; i < xlen; ++i) {
printf("%11f\n", x[i]);
}

END:
NAG_FREE(ap);
NAG_FREE(x);

return exit_status;
}