NAG Library Manual, Mark 28.3
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NAG CL Interface Introduction
Example description
/* nag_sparse_real_symm_precon_ssor_solve (f11jdc) Example Program.
 *
 * Copyright 2022 Numerical Algorithms Group.
 *
 * Mark 28.3, 2022.
 */
#include <nag.h>
int main(void) {
  /* Scalars */
  Integer exit_status = 0;
  double anorm, omega, sigerr, sigmax, sigtol, stplhs, stprhs, tol;
  Integer i, irevcm, iterm, itn, its, j, listr, lcneed, lcomm, maxitn, maxits,
      monit, n, nnz, nnz1;
  /* Arrays */
  char nag_enum_arg[100];
  double *a = 0, *b = 0, *rdiag = 0, *wgt = 0, *commarray = 0, *x = 0;
  Integer *icol = 0, *irow = 0, *istr = 0;
  /* NAG types */
  Nag_NormType norm;
  Nag_SparseSym_Method method;
  Nag_SparseSym_PrecType precon;
  Nag_SparseSym_Bisection sigcmp;
  Nag_SparseSym_CheckData ckjd, ckxe;
  Nag_SparseSym_Dups dup;
  Nag_SparseSym_Weight weight;
  Nag_SparseSym_Zeros zero;
  NagError fail, fail1;

  INIT_FAIL(fail);

  printf(
      "nag_sparse_real_symm_precon_ssor_solve (f11jdc) Example Program Results");
  printf("\n\n");
  /* Skip heading in data file */
  scanf("%*[^\n]");
  /* Read algorithmic parameters */
  scanf("%" NAG_IFMT "%*[^\n]", &n);
  scanf("%" NAG_IFMT "%*[^\n]", &nnz);

  /* Allocate memory */
  listr = n + 1;
  lcomm = 6 * n + 120;
  if (!(a = NAG_ALLOC(nnz, double)) || !(b = NAG_ALLOC(n, double)) ||
      !(rdiag = NAG_ALLOC(n, double)) || !(wgt = NAG_ALLOC(n, double)) ||
      !(commarray = NAG_ALLOC(lcomm, double)) || !(x = NAG_ALLOC(n, double)) ||
      !(icol = NAG_ALLOC(nnz, Integer)) || !(irow = NAG_ALLOC(nnz, Integer)) ||
      !(istr = NAG_ALLOC(listr, Integer))) {
    printf("Allocation failure\n");
    exit_status = -1;
    goto END;
  }
  scanf("%99s%*[^\n] ", nag_enum_arg);
  /* nag_enum_name_to_value (x04nac).
   * Converts NAG enum member name to value
   */
  method = (Nag_SparseSym_Method)nag_enum_name_to_value(nag_enum_arg);
  scanf("%99s%*[^\n] ", nag_enum_arg);
  precon = (Nag_SparseSym_PrecType)nag_enum_name_to_value(nag_enum_arg);
  scanf("%99s%*[^\n] ", nag_enum_arg);
  sigcmp = (Nag_SparseSym_Bisection)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%" NAG_IFMT "%*[^\n]", &sigtol, &maxits);
  scanf("%lf%*[^\n]", &omega);

  /* Read the matrix a */
  for (i = 0; i <= nnz - 1; i++)
    scanf("%lf%" NAG_IFMT "%" NAG_IFMT "%*[^\n] ", &a[i], &irow[i], &icol[i]);

  /* Sort matrix a removing zero or duplicate elements using
   * nag_sparse_real_symm_sort (f11zbc).
   */
  nnz1 = nnz;
  dup = Nag_SparseSym_RemoveDups;
  zero = Nag_SparseSym_RemoveZeros;
  nag_sparse_real_symm_sort(n, &nnz1, a, irow, icol, dup, zero, istr, &fail);
  if (fail.code != NE_NOERROR) {
    printf("Error from nag_sparse_real_symm_sort (f11zbc).\n%s\n",
           fail.message);
    exit_status = 1;
    goto END;
  }
  if (nnz != nnz1) {
    printf("Warning, input Matrix has zero or duplicate elements\n");
    printf("         nnz has been reduced from %" NAG_IFMT " to %" NAG_IFMT
           "\n",
           nnz, nnz1);
    nnz = nnz1;
  }

  /* Check for zero diagonal matrix elements and calculate reciprocals. */
  for (i = 0; i < n; i++) {
    /* j points to last element in row i */
    j = istr[i + 1] - 2;
    if (irow[j] == icol[j])
      rdiag[irow[j] - 1] = 1.0 / a[j];
    else {
      printf("Matrix has a missing element for diagonal %" NAG_IFMT "\n", i);
      goto END;
    }
  }

  /* Read right-hand side vector b and initial approximate solution x */
  for (i = 0; i <= n - 1; i++)
    scanf("%lf", &b[i]);
  scanf("%*[^\n]");
  for (i = 0; i <= n - 1; i++)
    scanf("%lf", &x[i]);

  /* Initialize the basic symmteric solver (f11gec) using
   * nag_sparse_real_symm_basic_setup (f11gdc)
   */
  weight = Nag_SparseSym_UnWeighted;
  monit = 0;
  nag_sparse_real_symm_basic_setup(
      method, precon, sigcmp, norm, weight, iterm, n, tol, maxitn, anorm,
      sigmax, sigtol, maxits, monit, &lcneed, commarray, lcomm, &fail);
  if (fail.code != NE_NOERROR) {
    printf("Error from nag_sparse_sym_setup (f11gdc).\n%s\n", fail.message);
    exit_status = 2;
    goto END;
  }

  /* call solver repeatedly to solve the equations */
  irevcm = 0;
  ckxe = Nag_SparseSym_Check;
  ckjd = Nag_SparseSym_Check;
  while (1) {
    /* nag_sparse_real_symm_basic_solver (f11gec).
     * Real sparse symmetric linear systems, preconditioned conjugate gradient
     * or Lanczos method.
     */
    nag_sparse_real_symm_basic_solver(&irevcm, x, b, wgt, commarray, lcomm,
                                      &fail);
    if (irevcm != 4) {
      INIT_FAIL(fail1);
      switch (irevcm) {
      case 1:
        /* Compute sparse symmetric matrix vector product using
         * nag_sparse_real_symm_matvec (f11xec).
         */
        nag_sparse_real_symm_matvec(n, nnz, a, irow, icol, ckxe, x, b, &fail1);
        ckxe = Nag_SparseSym_NoCheck;
        break;
      case 2:
        /* SSOR preconditioning
         * nag_sparse_real_symm_precon_ssor_solve (f11jdc).
         * Solution of linear system involving preconditioning matrix
         * generated by applying SSOR to real sparse symmetric matrix
         */
        nag_sparse_real_symm_precon_ssor_solve(n, nnz, a, irow, icol, rdiag,
                                               omega, ckjd, x, b, &fail1);
        ckjd = Nag_SparseSym_NoCheck;
      }
      if (fail1.code != NE_NOERROR)
        irevcm = 6;
    } else if (fail.code != NE_NOERROR) {
      printf("Error from nag_sparse_real_symm_basic_solver (f11gec).\n%s\n",
             fail.message);
      exit_status = 3;
      goto END;
    } else
      goto END_LOOP;
  }
END_LOOP:
  /* Obtain and print diagnostic statistics using
   * nag_sparse_real_symm_basic_diag (f11gfc).
   */
  nag_sparse_real_symm_basic_diag(&itn, &stplhs, &stprhs, &anorm, &sigmax, &its,
                                  &sigerr, commarray, lcomm, &fail);
  if (fail.code != NE_NOERROR) {
    printf("Error from nag_sparse_real_symm_basic_diag (f11gfc).\n%s\n",
           fail.message);
    exit_status = 4;
    goto END;
  }
  printf("Converged in   %10" NAG_IFMT " iterations \n", itn);
  printf("Final residual norm = %11.3e\n\n", stplhs);
  /* Output solution */
  printf("%16s\n", "Solution");
  for (i = 0; i <= n - 1; i++)
    printf("%16.4e\n", x[i]);

END:
  NAG_FREE(a);
  NAG_FREE(b);
  NAG_FREE(rdiag);
  NAG_FREE(wgt);
  NAG_FREE(commarray);
  NAG_FREE(x);
  NAG_FREE(icol);
  NAG_FREE(irow);
  NAG_FREE(istr);
  return exit_status;
}