program main
#include <petsc/finclude/petscvec.h>
#include <petsc/finclude/petscmat.h>

  use petscvec
  use petscmat

  implicit none

  Mat A
  PetscInt, parameter ::  n = 5, m = 5
  PetscScalar, parameter ::  two = 2.0, one = 1.0
  PetscInt, pointer, dimension(:) ::  dnnz, onnz
  PetscInt    ::  i, rstart, rend, M1, N1
  PetscErrorCode ierr

  PetscCallA(PetscInitialize(ierr))

  allocate (dnnz(0:m - 1))
  allocate (onnz(0:m - 1))

  do i = 0, m - 1
    dnnz(i) = 1
    onnz(i) = 1
  end do

  PetscCallA(MatCreateAIJ(PETSC_COMM_WORLD, m, n, PETSC_DETERMINE, PETSC_DETERMINE, PETSC_DECIDE, dnnz, PETSC_DECIDE, onnz, A, ierr))
  PetscCallA(MatSetFromOptions(A, ierr))
  PetscCallA(MatSetUp(A, ierr))
  deallocate (dnnz)
  deallocate (onnz)

  !/* This assembly shrinks memory because we do not insert enough number of values */
  PetscCallA(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY, ierr))
  PetscCallA(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY, ierr))

  !/* MatResetPreallocation restores the memory required by users */
  PetscCallA(MatResetPreallocation(A, ierr))
  PetscCallA(MatSetOption(A, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_TRUE, ierr))
  PetscCallA(MatGetOwnershipRange(A, rstart, rend, ierr))
  PetscCallA(MatGetSize(A, M1, N1, ierr))
  do i = rstart, rend - 1
    PetscCallA(MatSetValue(A, i, i, two, INSERT_VALUES, ierr))
    if (rend < N1) PetscCallA(MatSetValue(A, i, rend, one, INSERT_VALUES, ierr))
  end do
  PetscCallA(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY, ierr))
  PetscCallA(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY, ierr))
  PetscCallA(MatView(A, PETSC_VIEWER_STDOUT_WORLD, ierr))
  PetscCallA(MatDestroy(A, ierr))
  PetscCallA(PetscFinalize(ierr))

end program

!/*TEST
!
!   test:
!      suffix: 1
!      output_file: output/ex4_1.out
!
!   test:
!      suffix: 2
!      nsize: 2
!      output_file: output/ex4_2.out
!
!TEST*/