FORTRAN - allocatable array in subroutine

Question

I'm trying to use an allocatable array in a subroutine but the compiler complains that

Error: Dummy argument 'locs' with INTENT(IN) in variable definition context (ALLOCATE object) at (1)

The only thing I could find was that I am supposed to use an explicit interface, which I am doing. Here the relevant code for the subroutine:

    RECURSIVE SUBROUTINE together(locs, LL, RL)

    INTEGER, DIMENSION(:,:), ALLOCATABLE, INTENT(IN)            :: locs
    INTEGER, INTENT(IN)                                         :: LL, RL


    ALLOCATE(locs(LL,RL))


END SUBROUTINE together

Answer 1

The compiler's error message is one descriptive of the problem. With INTENT(IN) you are saying that the object will not change, but you then go on to attempt to ALLOCATE it.

Yes, an explicit interface will be required for the calling, but that isn't the problem.

The Fortran 2008 standard says in section 5.3.10 that

A nonpointer object with the INTENT (IN) attribute shall not appear in a variable denition context

Allocation is one such context: section 16.6.7, point (11).

Answer 2

The locs dummy argument is allocatable, and has the INTENT(IN) attribute - the intent attribute here indicating that the calling procedure is providing information to the subroutine.

A consequence of the INTENT(IN) attribute is that you cannot change the allocation status (or value) of locs. Your ALLOCATE statement is attempting to do just that.

Answer 3

Try allocating your array in your main program, then when locs is pushed to your subroutine, use INTENT(INOUT) to tell the compiler you also want to change the contents of your array.