non-contiguous data and temporary array creation

Question

A similar question was answered in Fortran runtime warning: temporary array. However, the solutions do not quite help me in my case. Inside a subroutine, I have a subroutine call as:

subroutine initialize_prim(prim)

 real(kind=wp), dimension(2, -4:204), intent(out) :: prim

call double_gaussian(prim(1, :))

end subroutine initialize_prim

subroutine double_gaussian(y)

    real(kind=wp), dimension(-4:204), intent(out) :: y

    integer :: i

    do i = -4, 204
      y(i) = 0.5 * ( &
         exp(-((r(i) - r0))**2) + exp(-((r(i) + r0)/std_dev)**2))
    end do

  end subroutine double_gaussian

This gives an error message saying that fortran creates a temporary array for "y" in "double_gaussian". Having read a bit about continguous arrays, I understand why this error appears. Now, looking at my whole program, it would be very tedious to invert the order of the arrays for "prim", so that solution is not really possible. For creating assumed-shapes in "double_gaussian", I tried doing,


    real(kind=wp), dimension(:), intent(out) :: y

    integer :: i

    do i = -4, 204
      y(i) = 0.5 * ( &
         exp(-((r(i) - r0))**2) + exp(-((r(i) + r0)/std_dev)**2))
    end do

  end subroutine double_gaussian

This, however, causes fortran to crash with the error message "Index '-4' of dimension 1 of array 'y' below lower bound of 1". It seems that for the assumed-shape format, the indexing is nonetheless assumed to start with 1, whereas it starts at -4 as in my case. Is there a way to resolve this issue?

Can up show the actual 'error messages' about a non-contiguous array? Can you provide a self-contained compilable code? — evets, Jun 08 '19 at 16:38
Assumed shape starts wherever you declare it to start. `y(-4:)` will start at -4. — Vladimir F Героям слава, Jun 08 '19 at 17:20

High Performance Mark · Accepted Answer · 2019-06-08T21:18:27.103

I think that you have perhaps misinterpreted a compiler warning as an error. Usually compilers issue a warning when they create temporary arrays - it's a useful aid to high-performance programming. But I'm not sure a compiler ever regards that as an error. And yes, I understand why you might not want to re-order your array just to avoid that

As for the crash - you have discovered that Fortran routines don't automagically know about the lower bounds of arrays which you have carefully set to be other than 1 (nor their upper bounds either). If it is necessary you have to pass the bounds (usually only the lower bound, the routine can figure out the upper bound itself) in the argument list.

However, it rarely is necessary, and it doesn't seem to be in your code - the loop to set each value of the y array could (if I understand correctly) be replaced by

y = 0.5 * (exp(-((r - r0))**2) + exp(-((r + r0)/std_dev)**2))

PS I think that this part of your question, about routines not respecting other-than-1 array lower bounds, is almost certainly a duplicate of several others asked hereabouts but which I couldn't immediately find.

non-contiguous data and temporary array creation

1 Answers1