static char help[] = "Tests MatCreateSubMatrices().\n\n";

#include <petscmat.h>

int main(int argc, char **args)
{
  Mat         A, B, *Bsub;
  PetscInt    i, j, m = 6, n = 6, N = 36, Ii, J;
  PetscScalar v;
  IS          isrow;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &args, NULL, help));
  PetscCall(MatCreateSeqAIJ(PETSC_COMM_WORLD, N, N, 5, NULL, &A));
  for (i = 0; i < m; i++) {
    for (j = 0; j < n; j++) {
      v  = -1.0;
      Ii = j + n * i;
      if (i > 0) {
        J = Ii - n;
        PetscCall(MatSetValues(A, 1, &Ii, 1, &J, &v, INSERT_VALUES));
      }
      if (i < m - 1) {
        J = Ii + n;
        PetscCall(MatSetValues(A, 1, &Ii, 1, &J, &v, INSERT_VALUES));
      }
      if (j > 0) {
        J = Ii - 1;
        PetscCall(MatSetValues(A, 1, &Ii, 1, &J, &v, INSERT_VALUES));
      }
      if (j < n - 1) {
        J = Ii + 1;
        PetscCall(MatSetValues(A, 1, &Ii, 1, &J, &v, INSERT_VALUES));
      }
      v = 4.0;
      PetscCall(MatSetValues(A, 1, &Ii, 1, &Ii, &v, INSERT_VALUES));
    }
  }
  PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
  PetscCall(MatView(A, PETSC_VIEWER_STDOUT_SELF));

  /* take the first diagonal block */
  PetscCall(ISCreateStride(PETSC_COMM_WORLD, m, 0, 1, &isrow));
  PetscCall(MatCreateSubMatrices(A, 1, &isrow, &isrow, MAT_INITIAL_MATRIX, &Bsub));
  B = *Bsub;
  PetscCall(ISDestroy(&isrow));
  PetscCall(MatView(B, PETSC_VIEWER_STDOUT_SELF));
  PetscCall(MatDestroySubMatrices(1, &Bsub));

  /* take a strided block */
  PetscCall(ISCreateStride(PETSC_COMM_WORLD, m, 0, 2, &isrow));
  PetscCall(MatCreateSubMatrices(A, 1, &isrow, &isrow, MAT_INITIAL_MATRIX, &Bsub));
  B = *Bsub;
  PetscCall(ISDestroy(&isrow));
  PetscCall(MatView(B, PETSC_VIEWER_STDOUT_SELF));
  PetscCall(MatDestroySubMatrices(1, &Bsub));

  /* take the last block */
  PetscCall(ISCreateStride(PETSC_COMM_WORLD, m, N - m - 1, 1, &isrow));
  PetscCall(MatCreateSubMatrices(A, 1, &isrow, &isrow, MAT_INITIAL_MATRIX, &Bsub));
  B = *Bsub;
  PetscCall(ISDestroy(&isrow));
  PetscCall(MatView(B, PETSC_VIEWER_STDOUT_SELF));

  PetscCall(MatDestroySubMatrices(1, &Bsub));
  PetscCall(MatDestroy(&A));

  PetscCall(PetscFinalize());
  return 0;
}

/*TEST

   test:

TEST*/