Simple incompatible rank problem

[dsal]

Hello, I have programmed in Fortran in the past, but it is safe to say that I am very much a novice. My current goal is to write a function, or subroutine for that matter, that accepts an array of rank one as its argument, and determines it's size from within the function/subroutine. I figure that this way I wont have to pass the dimensions as arguments, making the function/subroutine more general.

It seems simple enough, however, I am getting rank mismatch issues when compiling with gfortran. More specifically, it complains at n = lbound(vec), saying that n has rank 0 while lbound(vec) has rank 1. I don't understand why these two would have different ranks? Do they have different ranks, or is this a gfortran-specific problem? Any and all help is much appreciated. Below is the code that I'm working with.

PROGRAM arrayarg

REAL, DIMENSION(10):: a

DO i = 1, 10
a(i) = i
END DO

CALL showsize(a)

END PROGRAM arrayarg


SUBROUTINE showsize(vec)
REAL, DIMENSION)::vec
INTEGER n
n = lbound(vec)
END SUBROUTINE showsize


Thanks,
Daniel

 

[FJacq]

First of all, put your subroutine into a module and USE the module in the main program.

Indeed, you need an explicit interface to call a subroutine which has an argument vector declared with ")". The use of a module is the easiest way to define this interface.

From a more general point of view, define always all your subroutines of functions within modules. They are very nice containers.

      MODULE my_module
      IMPLICIT NONE
      CONTAINS
      SUBROUTINE showsize(vec)
      REAL, DIMENSION(:)::vec
      WRITE(*,*) SIZE(vec)
      END SUBROUTINE showsize
      END MODULE

      PROGRAM arrayarg

      USE my_module
      IMPLICIT NONE

      REAL, DIMENSION(10):: a
      INTEGER :: i

      DO i = 1, 10
         a(i) = i
      END DO

      CALL showsize(a)
      
      END PROGRAM arrayarg

Notice also that I have replaced the function LBOUND by SIZE. LBLOUND will return 1 in general (the lowest bound of a dummy array declared ")"). SIZE, on the contrary, will return 10 in your case.

You have apparently chosen a fixed format (each instruction starts at the column 7). I would prefer a free format (file suffix .f90), the fixed format being usually associated to the suffixes ".f" and ".for".

François Jacq

 

[dsal]

Thank you for your reply, FJacq. I figured it was only a matter of time before I needed to start using modules, but I'm not sure I understand their role within the programming language. Could you or anyone out there direct me to a good place to learn the language along with good programming practices? I plan to use fortran to solve equations that arise from computational fluid dynamics, if it makes any difference.

-Daniel

 

[FJacq]

Few advices :
- put everything in modules except the main program which is normally in its own source file
- a module is a container well suited to group together related things like derived type definitions, data and procedures (subroutines or functions)
- try to limit the size of a module... A module of 10000 instruction is too large (2000 instructions is OK). Inside a module, try also to limit the size of a procedure (500 instructions is already quite large).
- put at the top of the module the needed USE statements (a module may use modules). But the circular use is forbidden (A uses B and B uses C => C cannot use A). It is very convenient to precise, in each USE statement, the list of symbols you want to import (keyword ONLY)
- just after, add the instruction IMPLICIT NONE. USE and IMPLICIT statements are automatically valid for all the procedures within the module (no need to repeat inside the procedures).
- it may be useful to declare symbols you want to make public from the current module, but only when the software becomes large enough. For that, I start by the instruction PRIVATE and I continue by the declaration of public symbols (procedure names, data, types) (PUBLIC instructions).

They are books available like "Fortran 95/2003 explained" by Michael Metcalf, John Ker Reid, Malcolm Cohen.

Have also a look in the website

http://fortranwiki.org/fortran/show/HomePage

which presents many things about Fortran.

François Jacq