errors when transforming program into module with subroutine

Question

Given a simple program, that runs just fine, e.g.:

program test
implicit none
real, allocatable :: x(:)
real :: su, si
allocate(x(3))
x(:) = (/1,2,3/)
su=sum(x(:))
si=size(x(:))
end program

I tried to transform it into an external module with a subroutine:

module test
implicit none
real, allocatable, intent(in) :: x(:)
real, intent(out) :: su, si
contains
subroutine sumsize(x(:),su,si)
    su=sum(x(:))
    si=size(x(:))
end subroutine
end module

However I cannot get it compiled with several error messages of which I believe most are based on x(:), su ,and si all not being DUMMY variables.

Where and how are dummies defined? It seems like this is a simple formal error.

Another error states junk in the subroutine head, and fades when defining it with subroutine sumsize(x,su,si). Why can the input for the subroutine not be a vector?

Compiling with gfortran - gcc v. 8.3.0-6 (Debian) via gfortran -ffree-form -c module_test.f -o test.o.

Vladimir F Героям слава · Accepted Answer · 2019-10-03 18:57:18Z

2

Dummy arguments, also called function parameters in other programming languages, must be declared inside the function or subroutine.

Also, the argument list (the argument names in the parentheses after the subroutine name) should not contain any additional parentheses.

module test
  implicit none

contains

  subroutine sumsize(x,su,si)    
    real,  intent(in) :: x(:)
    real, intent(out) :: su, si

    su=sum(x(:))
    si=size(x(:))
  end subroutine
end module

Also, you typically do not want the dummy argument to be allocatable. That is normally used when the array is allocated or deallocated inside the subroutine or if you want to allow working with an unallocated array.

program test
  implicit none

  real, allocatable :: main_x(:)
  real :: main_su, main_si

  allocate(main_x(3))
  main_x(:) = (/1,2,3/)

  call  sumsize(main_x, main_su, main_si)
end program

I renamed the variables with the main_prefix. It is not needed, it is just to make clear that they are distinct from the names inside the subroutine.

edited Oct 3, 2019 at 18:57

answered Oct 3, 2019 at 18:47

Vladimir F Героям слава

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3 Comments

FelixJN Over a year ago

Is the array then directly allocated when called from the main program? And dummies are "anything in the parentheses of the subroutine"?

Vladimir F Героям слава Over a year ago

The array should be allocated in the main program before calling the subroutine. Dummy arguments are basically placeholders for the actual variables from the main program or other code that calls the subroutine.

FelixJN Over a year ago

Simple as that. TY.

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