NAG Library Manual, Mark 29.3
```/* nag_blast_dspr2 (f16psc) Example Program.
*
* Copyright 2023 Numerical Algorithms Group.
*
* Mark 29.3, 2023.
*/

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

int main(void) {

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

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

/* Nag Types */
NagError fail;
Nag_OrderType order;
Nag_UploType uplo;

#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_dspr2 (f16psc) Example Program Results\n\n");

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

/* Read the problem dimension */
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);

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

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

if (n > 0) {
/* Allocate memory */
if (!(ap = NAG_ALLOC(ap_len, double)) || !(x = NAG_ALLOC(xlen, double)) ||
!(y = NAG_ALLOC(ylen, 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) {
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] ");
}
}
for (i = 0; i < xlen; ++i)
scanf("%lf%*[^\n] ", &x[i]);
for (i = 0; i < ylen; ++i)
scanf("%lf%*[^\n] ", &y[i]);

/* nag_blast_dspr2 (f16psc).
* Rank two update of real symmetric matrix,
* packed storage.
*/
nag_blast_dspr2(order, uplo, n, alpha, x, incx, y, incy, beta, ap, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_blast_dspr2.\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print updated matrix A */
/* 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,
"Updated Matrix A", 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);
NAG_FREE(x);
NAG_FREE(y);

return exit_status;
}
```