```/* nag_sparse_nsym_precon_ssor_solve (f11ddc) Example Program.
*
* Copyright 2014 Numerical Algorithms Group.
*
* Mark 23, 2011.
*/
#include <nag.h>
#include <nag_stdlib.h>
#include <nagf11.h>
int main(void)
{
/* Scalars */
Integer                     exit_status = 0;
double                      anorm, omega, sigmax, stplhs, stprhs, tol;
Integer                     i, irevcm, iterm, itn, la, liwork,
lwneed, lwork, m, maxitn, monit, n, nnz;
/* Arrays */
char                        nag_enum_arg[100];
double                      *a = 0, *b = 0, *rdiag = 0, *wgt = 0,
*work = 0, *x = 0;
Integer                     *icol = 0, *irow = 0, *iwork = 0;
/* NAG types */
Nag_SparseNsym_CheckData    ckdd,ckxa;
Nag_NormType                norm;
Nag_SparseNsym_PrecType     precon;
Nag_TransType               trans;
Nag_SparseNsym_Weight       weight;
Nag_SparseNsym_Method       method;
NagError                    fail, fail1;

INIT_FAIL(fail);
INIT_FAIL(fail1);

printf("nag_sparse_nsym_precon_ssor_solve (f11ddc) Example Program Results");
printf("\n\n");
/* Skip heading in data file*/
scanf("%*[^\n]");
scanf("%ld%ld%*[^\n]", &n, &m);
scanf("%ld%*[^\n]", &nnz);
la = 3 * nnz;
lwork = MAX(n * (m + 3) + m * (m + 5) + 101, 7 * n + 100);
liwork = 2 * n + 1;
if (
!(a = NAG_ALLOC((la), double)) ||
!(b = NAG_ALLOC((n), double)) ||
!(rdiag = NAG_ALLOC((n), double)) ||
!(wgt = NAG_ALLOC((n), double)) ||
!(work = NAG_ALLOC((lwork), double)) ||
!(x = NAG_ALLOC((n), double)) ||
!(icol = NAG_ALLOC((la), Integer)) ||
!(irow = NAG_ALLOC((la), Integer)) ||
!(iwork = NAG_ALLOC((liwork), Integer))
)
{
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
/* Read or initialize the parameters for the iterative solver*/
scanf("%99s%*[^\n] ", nag_enum_arg);
/* nag_enum_name_to_value (x04nac).
* Converts NAG enum member name to value
*/
method = (Nag_SparseNsym_Method) nag_enum_name_to_value(nag_enum_arg);
scanf("%99s%*[^\n]", nag_enum_arg);
precon = (Nag_SparseNsym_PrecType) nag_enum_name_to_value(nag_enum_arg);
scanf("%99s%*[^\n]", nag_enum_arg);
norm = (Nag_NormType) nag_enum_name_to_value(nag_enum_arg);
scanf("%ld%*[^\n]", &iterm);
scanf("%lf%ld%*[^\n]", &tol, &maxitn);
scanf("%lf%lf%*[^\n]", &anorm, &sigmax);
scanf("%lf%*[^\n]", &omega);

/* Read the non-zero elements of the matrix a*/
for (i = 0; i < nnz; i++)
scanf("%lf%ld%ld%*[^\n] ", &a[i], &irow[i], &icol[i]);

/* Read right-hand side vector b and initial approximate solution x*/
for (i = 0; i < n; i++)  scanf("%lf", &b[i]);
scanf("%*[^\n]");
for (i = 0; i < n; i++)  scanf("%lf", &x[i]);

weight = Nag_SparseNsym_UnWeighted;
monit = 0;
/* Call to initialize the solver
* nag_sparse_nsym_basic_setup (f11bdc)
* Real sparse nonsymmetric linear systems, setup routine
*/
nag_sparse_nsym_basic_setup(method, precon, norm, weight, iterm, n, m, tol,
maxitn, anorm, sigmax, monit, &lwneed, work,
lwork, &fail);
/* Calculate reciprocal diagonal matrix elements if necessary*/
if (precon == Nag_SparseNsym_Prec) {
for (i = 0; i < n; i++)  iwork[i] = 0;
for (i = 0; i < nnz; i++) {
if (irow[i] == icol[i]) {
iwork[irow[i]-1]++;
if (a[i] == 0.0) {
printf("Matrix has a zero diagonal element \n");
goto END;
}
rdiag[(irow[i]-1)] = 1.0/a[i];
}
}
for (i = 0; i < n; i++) {
if (iwork[i] == 0) {
printf("Matrix has a missing diagonal element \n");
goto END;
}
if (iwork[i] >= 2) {
printf("Matrix has a multiple diagonal element \n");
goto END;
}
}
}
/* call solver repeatedly to solve the equations*/
irevcm = 0;
ckxa = Nag_SparseNsym_Check;
ckdd = Nag_SparseNsym_Check;
while (irevcm!=4)
{
/* nag_sparse_nsym_basic_solver (f11bec)
* Real sparse nonsymmetric linear systems, solver routine
* preconditioned RGMRES, CGS, Bi-CGSTAB or TFQMR method
*/
nag_sparse_nsym_basic_solver(&irevcm, x, b, wgt, work, lwork, &fail);
switch (irevcm) {
case 1:
/* Compute matrix-vector product using
* nag_sparse_nsym_matvec (f11xac)
* Real sparse nonsymmetric matrix vector multiply
*/
trans = Nag_NoTrans;
nag_sparse_nsym_matvec(trans, n, nnz, a, irow, icol, ckxa, x, b,
&fail1);
ckxa = Nag_SparseNsym_NoCheck;
break;
case -1:
/* Compute transposed matrix-vector product */
trans = Nag_Trans;
nag_sparse_nsym_matvec(trans, n, nnz, a, irow, icol, ckxa, x, b,
&fail1);
ckxa = Nag_SparseNsym_NoCheck;
break;
case 2:
/* SSOR preconditioning using
* nag_sparse_nsym_precon_ssor_solve (f11ddc)
* Solution of linear system involving preconditioning matrix generated
* by applying SSOR to real sparse nonsymmetric matrix
*/
trans = Nag_NoTrans;
nag_sparse_nsym_precon_ssor_solve(trans, n, nnz, a, irow, icol, rdiag,
omega, ckdd, x, b,&fail1);
ckdd = Nag_SparseNsym_NoCheck;
break;
}
if (fail1.code != NE_NOERROR) irevcm = 6;
}
if (fail.code != NE_NOERROR)
{
printf("Error from nag_sparse_nsym_basic_solver (f11bec)\n%s\n",
fail.message);
exit_status = 1;
goto END;
}

/* nag_sparse_nsym_basic_diagnostic (f11bfc)
* Real sparse nonsymmetric linear systems, diagnostic
*/
nag_sparse_nsym_basic_diagnostic(&itn, &stplhs, &stprhs, &anorm, &sigmax,
work, lwork, &fail);
printf(" Converged in %11ld iterations\n", itn);
printf(" Matrix norm         = %9.3e\n", anorm);
printf(" Final residual norm = %9.3e\n\n", stplhs);
/* Output x*/
printf(" Solution of linear system\n");
for (i = 0; i < n; i++)  printf("%16.4e\n", x[i]);

END:
NAG_FREE(a);
NAG_FREE(b);
NAG_FREE(rdiag);
NAG_FREE(wgt);
NAG_FREE(work);
NAG_FREE(x);
NAG_FREE(icol);
NAG_FREE(irow);
NAG_FREE(iwork);
return exit_status;
}
```