NAG Library Manual, Mark 30.2
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NAG CL Interface Introduction
Example description
/* nag_sparse_real_gen_precon_ssor_solve (f11ddc) Example Program.
 *
 * Copyright 2024 Numerical Algorithms Group.
 *
 * Mark 30.2, 2024.
 */
#include <nag.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_real_gen_precon_ssor_solve (f11ddc) Example Program Results");
  printf("\n\n");
  /* Skip heading in data file */
  scanf("%*[^\n]");
  /* Read algorithmic parameters */
  scanf("%" NAG_IFMT "%" NAG_IFMT "%*[^\n]", &n, &m);
  scanf("%" NAG_IFMT "%*[^\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("%" NAG_IFMT "%*[^\n]", &iterm);
  scanf("%lf%" NAG_IFMT "%*[^\n]", &tol, &maxitn);
  scanf("%lf%lf%*[^\n]", &anorm, &sigmax);
  scanf("%lf%*[^\n]", &omega);

  /* Read the nonzero elements of the matrix a */
  for (i = 0; i < nnz; i++)
    scanf("%lf%" NAG_IFMT "%" NAG_IFMT "%*[^\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_real_gen_basic_setup (f11bdc)
   * Real sparse nonsymmetric linear systems, setup routine
   */
  nag_sparse_real_gen_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_real_gen_basic_solver (f11bec)
     * Real sparse nonsymmetric linear systems, solver routine
     * preconditioned RGMRES, CGS, Bi-CGSTAB or TFQMR method
     */
    nag_sparse_real_gen_basic_solver(&irevcm, x, b, wgt, work, lwork, &fail);
    switch (irevcm) {
    case 1:
      /* Compute matrix-vector product using
       * nag_sparse_real_gen_matvec (f11xac)
       * Real sparse nonsymmetric matrix vector multiply
       */
      trans = Nag_NoTrans;
      nag_sparse_real_gen_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_real_gen_matvec(trans, n, nnz, a, irow, icol, ckxa, x, b,
                                 &fail1);
      ckxa = Nag_SparseNsym_NoCheck;
      break;
    case 2:
      /* SSOR preconditioning using
       * nag_sparse_real_gen_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_real_gen_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_real_gen_basic_solver (f11bec)\n%s\n",
           fail.message);
    exit_status = 1;
    goto END;
  }

  /* nag_sparse_real_gen_basic_diag (f11bfc)
   * Real sparse nonsymmetric linear systems, diagnostic
   */
  nag_sparse_real_gen_basic_diag(&itn, &stplhs, &stprhs, &anorm, &sigmax, work,
                                 lwork, &fail);
  printf(" Converged in %11" NAG_IFMT " 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;
}