program foo
  implicit none

  character(len=80) :: filename
  character(len=5) :: nchar, nchar2

  integer :: ilun=1

  call title(1, nchar2)
  nchar = '00001'

  filename = trim(nchar)//'.txt'

  write(*, '(a, "<", a, ">")') 'filename ', trim(filename)
  open(newunit=ilun, file=trim(filename), form='formatted', status='replace')
  write(ilun, '(a1,a12,a10)') '#', 'Family', 'Count'
  close(ilun)
end program foo

subroutine title(n, nchar)
  implicit none
  integer, intent(in) :: n
  character(len=5), intent(out) :: nchar
  write(nchar, '(i0.5)') n

end subroutine title

!   use iso_fortran_env
  ! implicit none
  ! include 'mpif.h'
  ! character(len=80) :: filename

  ! integer :: unit_out
  ! character(len=80) :: fileloc
  ! character(len=5) :: nchar
  ! logical :: ivar


  ! filename='bar'
  ! fileloc = trim(filename) // trim(nchar)
  ! ! unit_out = 11
  ! open(newunit=unit_out, file=trim(fileloc), form='formatted', status='unknown')
  ! write(unit_out, '(a1,a12,a10)') '#', 'Family', 'Count'
  ! return

  ! implicit none

  ! integer :: ilun, ifam, npart_family(3) = [1, 2, 3], npart_all = 6

  ! character(len=128) :: filename = 'bar', particle_family_keys(3) = ['x', 'y', 'z']

  ! open(newunit=ilun,file=trim(filename),form='formatted')

  ! write(ilun, '(a1,a12,a10)') '#', 'Family', 'Count'
  ! do ifam = 1, 3
  !    write(ilun, '(a13, i10)') &
  !         trim(particle_family_keys(ifam)), npart_family(ifam)
  ! end do
  ! write(ilun, '(a13, i10)') &
  !      'undefined', npart_all - sum(npart_family)

  ! ! Keep track of what particle fields are present
  ! write(ilun,*)'Particle fields'
  ! write(ilun,'(a)',advance='no')'pos vel mass iord level family tag '
  ! close(ilun)
! end program foo