Fortran, passing array using starting index only

Question

In the program below, there are two methods presented for passing an array:

program main
    integer, dimension(4) :: x = [9, 8, 7, 6]
    call print_x(x(2:3))   ! Method 1
    call print_x(x(2))     ! Method 2
end program

subroutine print_x(x)
    integer, dimension(2), intent(in) :: x
    print *, x
end subroutine

Both methods produce the same result: the numbers 8 and 7 are printed. Personally, I would never code this using Method 2 because it looks like a single value is being passed rather than an array.

Can you give an example of when Method 2 MUST be used instead of Method 1?

I would use neither. Stick all your subprograms in a module or as contained subprograms and use assumed shape for your dummy arguments. Both the above methods are holdovers from archaic standards. — Ian Bush
– Ian Bush, Commented Jul 16, 2019 at 19:34
@IanBush, when you want to have elements 2 and 3 of the array associated with the dummy then certainly case 1 seems quite natural to me. What have you against that? — francescalus
– francescalus, Commented Jul 16, 2019 at 19:35
@francescalus Lack of an interface in scope is what I have against it — Ian Bush
– Ian Bush, Commented Jul 16, 2019 at 19:42
@IanBush, ok. Assuming an explicit interface in scope would your statement still hold? — francescalus
– francescalus, Commented Jul 16, 2019 at 19:46
@francescalus If there's an interface in scope I would still much prefer assumed shape for the dummy argument - we've recently seen compilers unable to diagnose a mismatch between the declared shape of the dummy argument and the actual argument. But if you applied the thumb screws I would eventually, reluctantly, come down in favour of method 1. — Ian Bush
– Ian Bush, Commented Jul 16, 2019 at 19:50

francescalus · Accepted Answer · 2019-07-16 20:23:09Z

2

Consider the program

  implicit none
  integer :: x(2,2)=0
  call set(x(2,1))
  print*, x

contains

  subroutine set(y)
    integer y(2)
    y = [1,2]
  end subroutine set

end program

The dummy argument y in this subroutine call is argument associated with the elements x(2,1) and x(1,2). There is no array section of x which consists of exactly these two elements.

answered Jul 16, 2019 at 20:23

francescalus

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3 Comments

David Hansen Over a year ago

I see! This does indeed seem to be an application where Method 1 doesn't work but Method 2 does. This answers the question. That being said, I think we would probably agree that this isn't an ideal methodology, as I mentioned in the original post and as mentioned by @IanBush. Thanks!

francescalus Over a year ago

Indeed. This is a very specific scenario and the casual reader shouldn't jump to conclusions about its wider utility or desirability.

David Hansen Over a year ago

Right. I am a little surprised that this feature exists in Fortran, but perhaps that is a discussion for another day.

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