1: #include <petsc/finclude/petscmat.h>
  2: program main
  3:   use petscmat

  5:   implicit none

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

 14:   PetscCallA(PetscInitialize(ierr))

 16:   allocate (dnnz(0:m - 1), source=1_PETSC_INT_KIND)
 17:   allocate (onnz(0:m - 1), source=1_PETSC_INT_KIND)

 19:   PetscCallA(MatCreateAIJ(PETSC_COMM_WORLD, m, n, PETSC_DETERMINE, PETSC_DETERMINE, PETSC_DECIDE, dnnz, PETSC_DECIDE, onnz, A, ierr))
 20:   PetscCallA(MatSetFromOptions(A, ierr))
 21:   PetscCallA(MatSetUp(A, ierr))
 22:   deallocate (dnnz)
 23:   deallocate (onnz)

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

 29:   !/* MatResetPreallocation restores the memory required by users */
 30:   PetscCallA(MatResetPreallocation(A, ierr))
 31:   PetscCallA(MatSetOption(A, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_TRUE, ierr))
 32:   PetscCallA(MatGetOwnershipRange(A, rstart, rend, ierr))
 33:   PetscCallA(MatGetSize(A, M1, N1, ierr))
 34:   do i = rstart, rend - 1
 35:     PetscCallA(MatSetValue(A, i, i, two, INSERT_VALUES, ierr))
 36:     if (rend < N1) PetscCallA(MatSetValue(A, i, rend, one, INSERT_VALUES, ierr))
 37:   end do
 38:   PetscCallA(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY, ierr))
 39:   PetscCallA(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY, ierr))
 40:   PetscCallA(MatView(A, PETSC_VIEWER_STDOUT_WORLD, ierr))
 41:   PetscCallA(MatDestroy(A, ierr))
 42:   PetscCallA(PetscFinalize(ierr))

 44: end program

 46: !/*TEST
 47: !
 48: !   test:
 49: !      suffix: 1
 50: !      output_file: output/ex4_1.out
 51: !
 52: !   test:
 53: !      suffix: 2
 54: !      nsize: 2
 55: !      output_file: output/ex4_2.out
 56: !
 57: !TEST*/