NAG Library Manual, Mark 27.2
Interfaces:  FL   CL   CPP   AD
```/* nag_sparse_real_gen_matvec (f11xac) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.2, 2021.
*/

#include <nag.h>

int main(void) {
/* Scalars */
Integer exit_status = 0;
Integer i, j, n, nnz;
/* Arrays */
char nag_enum_arg[40];
Integer *irow = 0, *icol = 0;
double *a = 0, *x = 0, *y = 0;
/* NAG types */
NagError fail;
Nag_TransType trans;
Nag_SparseNsym_CheckData check;

INIT_FAIL(fail);

printf("nag_sparse_real_gen_matvec (f11xac) Example Program Results\n");

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

/* Read order of matrix and number of nonzero entries */
scanf("%" NAG_IFMT "%*[^\n]", &n);
scanf("%" NAG_IFMT "%*[^\n]", &nnz);

/* Allocate memory */
if (!(a = NAG_ALLOC(nnz, double)) || !(x = NAG_ALLOC(n, double)) ||
!(y = NAG_ALLOC(n, double)) || !(icol = NAG_ALLOC(nnz, Integer)) ||
!(irow = NAG_ALLOC(nnz, Integer))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read the matrix A */
for (i = 0; i < nnz; i++)
scanf("%lf"
"%" NAG_IFMT "%" NAG_IFMT "%*[^\n]",
&a[i], &irow[i], &icol[i]);

/* Read the vector x */
for (j = 0; j < n; j++)
scanf("%lf"
"%*[^\n]",
&x[j]);
/* Nag_NoTrans */
scanf("%39s%*[^\n]", nag_enum_arg);
trans = (Nag_TransType)nag_enum_name_to_value(nag_enum_arg);

/* Nag_SparseNsym_Check */
scanf("%39s%*[^\n]", nag_enum_arg);
check = (Nag_SparseNsym_CheckData)nag_enum_name_to_value(nag_enum_arg);

/* nag_sparse_real_gen_matvec (f11xac)
* Calculate matrix-vector product without transposed matrix.
*/
nag_sparse_real_gen_matvec(trans, n, nnz, a, irow, icol, check, x, y, &fail);

if (fail.code != NE_NOERROR) {
printf("Error from nag_sparse_real_gen_matvec (f11xac)\n%s\n",
fail.message);
exit_status = 1;
goto END;
}

/* Output results */
printf("\n Matrix-vector product\n");
for (j = 0; j < n; j++)
printf("%16.4e\n", y[j]);

/* Calculate transposed matrix-vector product */
/* Nag_Trans */
scanf("%39s%*[^\n]", nag_enum_arg);
trans = (Nag_TransType)nag_enum_name_to_value(nag_enum_arg);

/* Nag_SparseNsym_NoCheck */
scanf("%39s%*[^\n]", nag_enum_arg);
check = (Nag_SparseNsym_CheckData)nag_enum_name_to_value(nag_enum_arg);

/* nag_sparse_real_gen_matvec (f11xac)
* Calculate matrix-vector product with transposed matrix.
*/
nag_sparse_real_gen_matvec(trans, n, nnz, a, irow, icol, check, x, y, &fail);

if (fail.code != NE_NOERROR) {
printf("Error from nag_sparse_real_gen_matvec (f11xac)\n%s\n",
fail.message);
printf("%s\n", fail.message);
exit_status = 2;
goto END;
}

/* Output results */
printf("\nTransposed matrix-vector product\n");
for (j = 0; j < n; j++)
printf("%16.4e\n", y[j]);

END:
NAG_FREE(a);
NAG_FREE(x);
NAG_FREE(y);
NAG_FREE(icol);
NAG_FREE(irow);

return exit_status;
}
```