Assumed-shape Arrays

 

Declaring dummy arrays as assumed-shape arrays is the recommended method in Fortran 90. Consider,

    PROGRAM TV
     INTERFACE
      SUBROUTINE gimlet(a,b)
       REAL, INTENT(IN)   :: a(:), b(:,:)
      END SUBROUTINE gimlet
     END INTERFACE
      ...
     REAL, DIMENSION(40)    :: X
     REAL, DIMENSION(40,40) :: Y
      ...
     CALL gimlet(X,Y)
     CALL gimlet(X(1:39:2),Y(2:4,4:4))
     CALL gimlet(X(1:39:2),Y(2:4,4)) ! invalid
      ...
    END PROGRAM TV

    SUBROUTINE gimlet(a,b)
     REAL, INTENT(IN)   :: a(:), b(:,:)
      ...
    END SUBROUTINE gimlet

An assumed-shape array declaration must have the same type, rank and kind as the actual argument. An INTERFACE block is needed to transfer the type, kind and bounds of the actual argument into the procedure. These can then be checked alongside the declaration of the dummy at compile time.

Note:

• array sections can be passed so long as they are regular, that is, not defined by vector subscripts. The reason for this is concerned with efficiency. A vector subscripted section will be non-trivial to find in the memory, it is likely to be widely scattered and would probably need to be copied on entry to the procedure and then copied back on exit, this will create all sorts of runtime penalties.
• the actual argument cannot be an assumed-size array. If an actual argument were an assumed-size array then the bound / extent information of the last dimension would not be known meaning that the relevant information could not be passed on to a further procedure.

The third call is invalid because the second section reference has one dimensions whereas the declaration of the dummy has two.