I have a subroutine that’s essentially a logger. It takes in a required string, as well as two optional parameters that it formats.

Here’s a simplified version. Currently this fails when writing to action_local

subroutine log(required_param, action, idx)
    character(len=*), intent(in) :: required_param
    character(len=*), intent(in), optional :: action
    integer, intent(in), optional :: idx
    character(:), allocatable :: action_local

10  format(A, A, I)

    if (present(action) .and. present(idx)) then
        write(action_local, 10) ' ', action, idx ! fails: action_local is not allocated
    else
        action_local = ''
    end if

    print *, required_param // action_local
end subroutine

Ideally I wouldn’t guess at the size required – I’d just let the output of the write statement allocate what it needs. Is there a way to write to an allocatable character without allocating it ahead of time?