Fortran pointer to parameter array target

Question

I have a few parameter arrays with different names in a module:

real*8, parameter :: para1(*) = [43.234, 34.0498, ...
real*8, parameter :: para2...

In a routine in this module

subroutine sub(n,...
...
end

I want to use para1 when n=1, para2 when n=2, etc. There are some solutions to that, one is to make an array paras=[para1,para2...] and index properly which works fine. But I want to try using a pointer

real*8, pointer :: ptr(:)

and assign it to different parameter arrays depending on n, but the problem is that "PARAMETER attribute conflicts with TARGET attribute at (1)". If I remove the parameter attribute, the routine is less safe and the SAVE attribute is assumed.

Am I missing something or why can we not combine parameter and target? And is there a good way around it for this purpose?

Yes, you are missing something. Fortran 2018, Constraint C860: An entity with the TARGET attribute shall be a variable. A entity with the PARAMETER attribute is a named constant. A named constant is not a variable. — evets
– evets, Commented Jun 5, 2019 at 15:22
@ja72 They have different lengths so a structured 2D parameter array is trickier to construct. Then you still need to store the lengths. In 1D you stack them and store the positions so it's much easier. — Jonatan Öström
– Jonatan Öström, Commented Jun 7, 2019 at 20:24
@ja72 No I don't need that :) Also it is not a parameter. See the answer of 'roygvib' for a 2D solution. — Jonatan Öström
– Jonatan Öström, Commented Jun 8, 2019 at 14:05

francescalus · Accepted Answer · 2019-06-05 21:54:49Z

2

The parameter and target attributes indeed conflict. An object with the target attribute must be a variable (Fortran 2018 8.5.17, C861); a named constant (object with the parameter attribute) is not a variable (F2018, 8.5.13, C850).

To use target arrays you must, then, use variables. It is tricky to have a variable which is "safe" from having its value modified by a programming mistake or such. There are several considerations which prohibit a variable from appearing in a variable definition context. If you can arrange such a state, then the compiler may have a chance of detecting your mistake. Can that happen easily?

Outside a pure procedure and an intent(in) dummy argument, the most tempting prohibition is with a protected module variable:

module pars
  real, save, target, protected :: para1(74) = [...]
  real, save, target, protected :: para2(1) = [6]
end module

subroutine sub (...)
  use pars
  real, pointer :: p
  p => para1
end subroutine sub

Being protected, the values are safe from modification outside the module pars? Alas, even if this were true it isn't helpful: being protected, we can't even point a pointer to module variables.

In summary, your compiler isn't going to find it easy to detect a programming mistake which modifies the variable target array, so if you want to use an array as a target, you'll have to be careful.

edited Jun 5, 2019 at 21:54

answered Jun 5, 2019 at 15:33

francescalus

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4 Comments

IanH Over a year ago

The last paragraph is a bit of a stretch. It is trivial to modify the values of those module variables in a procedure outside the module.

francescalus Over a year ago

It's worse than that, isn't it? The whole target, protected part is nonsense.

Jonatan Öström Over a year ago

Do you know if there is a specific reason that that targets must be variables?

francescalus Over a year ago

@Jonatan, I can't speak for the writers of the standard, but a couple of points: if a pointer can be a variable then whatever it points to must for simplicity be a variable; a named constant needn't exist at "run time".

roygvib · Accepted Answer · 2019-06-06 13:21:28Z

0

Following the suggestion by @ja72 in the comment, this is an attempt to use a single 2D array for the parameters. This works nicely with gfortran-8.2 (on MacOS10.11).

program main
    implicit none
    integer i
    integer, parameter :: para1(*) = [1, 2, 3, 4, 5]
    integer, parameter :: para2(*) = [6, 7]
    integer, parameter :: N1 = size(para1), N2 = size(para2), N = max(N1, N2)
    integer, parameter :: params(N, 2) = &
            reshape( [ para1, (0, i = 1, N - N1), &
                       para2, (0, i = 1, N - N2) ], [N, 2] )

    print *, "para1 = ", params( :, 1 )
    print *, "para2 = ", params( :, 2 )

    print *, "Input i"
    read *, i
    print *, params( :, i )
end

$ gfortran-8 test.f90 && ./a.out

 para1 =            1           2           3           4           5
 para2 =            6           7           0           0           0
 Input i
1
           1           2           3           4           5

However, because the code becomes a bit complicated (because of reshape) and may not work with old compilers, things may be more straightforward to just use non-parameter arrays...

answered Jun 6, 2019 at 13:21

roygvib

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1 Comment

Jonatan Öström Over a year ago

Nice, but the 1D is indeed simpler:)

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