将公共块数组大小传递给Fortran中的子例程

Question

I would like to pass the array dimension as a dummy variable to a subroutine. The array itself is on a common block. Here is the code:

PROGRAM test
integer i, nn
integer PARAMETER(Nt=10)
real x(Nt), y(nt), z(Nt)
Common /Bdat/ z
nn=Nt
do i=1,Nt
x(i)=i+1
z(i)=i-1
enddo
call estimate(x,y,nn)
print*, y
return
end

subroutine estimate(x,y,jj)
integer i,jj
real x(jj), y(jj), zq(jj)
COMMON /Bdat/ zq
do i=1, jj
y(i)=x(i)+zq(i)
enddo
return
end

this is the error I get from the subroutine:

real x(jj), y(jj), zq(jj)
                      1

Error: Variable 'jj' at (1) in this context must be constant

I would really appreciate it if anybody could solve the issue.

Answer 1

You have a scope problem. Read: Scope in Fortran . That is, your subroutine estimate needs access to the variable Nt which you need to pass as an additional argument, or you can move the entire subroutine inside your program using the contains statement. This will allow your program to run successfully, but I highly encourage you to abstain from using common blocks. If you cannot avoid them due to legacy codes see: Improve your FORTRAN 77 programs using some Fortran 90 features

Try using modules instead:

    module bdat

      implicit none

      private
      public :: NT, z

      integer, parameter :: NT = 10
      real               :: z(NT) 

    end module bdat

    module my_sub

      use bdat, only: &
           zq => z ! You're free to rename the variable

      implicit none
      private
      public :: estimate

    contains

      subroutine estimate(x,y)
        ! calling arguments
        real, intent (in) :: x(:)
        real, intent (out) :: y(:)

        ! local variables
        integer :: i, jj

        jj = size(x)

        do i=1, jj
           y(i)=x(i)+zq(i)
        end do

      end subroutine estimate

    end module my_sub

    program test

      use bdat, only: &
           NT, z

      use my_sub, only: &
           estimate

      implicit none

      integer :: i
      real :: x(NT), y(NT)

      do i=1,NT
         x(i)=i+1
         z(i)=i-1
      end do

      call estimate(x,y)

      print *, y

    end program test