Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!ukma!psuvm.bitnet!cunyvm!byuvax!hallidayd
From: hallidayd@yvax.byu.edu
Newsgroups: comp.lang.fortran
Subject: Re: Pointer examples and 8x
Message-ID: <638hallidayd@yvax.byu.edu>
Date: 20 Jun 89 01:38:44 GMT
Lines: 92

By the way Walt, even your Fortran 8x example needs only one pointer (for the
walk) since (according to your own code, message <158@unmvax.unm.edu>) Fortran
8x allows references of the form

      TRAIL_PTR % NEXT % NEXT => TMP_PTR

thus your code for "walking" through the list may be implemented as

   SUBROUTINE INSERT (PTR, NUMBER)
      ! Single pointer version

      TYPE (NODE), POINTER, INTENT (IN) :: PTR
      TYPE (NODE), POINTER :: TRAIL_PTR, TMP_PTR
      INTEGER, INTENT (IN) :: NUMBER

      ! "Walk" through the list to
      ! determine where NUMBER goes
      TRAIL_PTR => PTR
      DO
         IF (.NOT. ASSOCIATED (TRAIL_PTR % NEXT)) EXIT
         IF (NUMBER <= TRAIL_PTR % NEXT % VALUE) EXIT
         TRAIL_PTR => TRAIL_PTR % NEXT      ! Please excuse me if this should
                                            ! actually use the = assignment
                                            ! operator.  (I have not read the
                                            ! latest proposed standard yet.)
      END DO

      ! Insert into sublist
      ALLOCATE (TMP_PTR)
      TMP_PTR % VALUE = NUMBER
      TMP_PTR % NEXT => TRAIL_PTR % NEXT
      TRAIL_PTR % NEXT => TMP_PTR

   END SUBROUTINE INSERT

Actually, if I understand the ``Aliasing'' mode for Fortran 8x pointers,
Fortran 8x pointers are a lot closer to the concept of true recursive data
structures than they are to C style ``addresses''.  Thus the construct

         TRAIL_PTR = TMP_PTR

used in Walt's code will accomplish the wrong thing, it will reassign the value
of the list node ``pointed to''(aliased) by TRAIL_PTR the value of the next
node (the node aliased by TMP_PTR).  This will, in effect, delete the original
node aliased by TRAIL_PTR and leave a duplicate node, aliased by TMP_PTR,
``dangling'' after the execution of the next statement
(  TMP_PTR => TRAIL_PTR % NEXT  ).
This is an example of how most any form of pointer implementation can allow the
programmer to produce erroneous code.  (Unless the standard declares that there
is NO garbage collection, that code which leaves allocated memory unreferenced
is in error, and, further, that constructs which cause such unreferenced
allocated memory will be flagged as erroneous by the compiler.  However, as has
been pointed out by people in the comp.lang.ada news group, there are cases
where data structures may be referenced though several different paths and where
the program (and thus, all the more so, the compiler) may have no idea whether a
given abject has become unreferenced, therefore some form of garbage collection
becomes needed---perhaps being turned on or off by compiler directives.)

Furthermore, Walt, were you not trying to insert _before_ the node that had a
VALUE greater than or equal to the given NUMBER?  This is not what your
recursive routine _appears_ to do.  Instead, it creates a ``dangling'' node,
containing the inserted number, pointing to the node _after_ the one with
the VALUE greater than or less than the NUMBER.  What I expect you wanted
was:

   RECURSIVE SUBROUTINE INSERT (L, NUMBER)

      TYPE (NODE), POINTER, INTENT(IN OUT) :: L
      TYPE (NODE), POINTER :: TMP_L
      INTEGER, INTENT (IN) :: NUMBER

      ! If NUMBER goes here, insert it in new node
      IF (NUMBER <= L % VALUE) THEN
         ALLOCATE (TMP_L)
         TMP_L % VALUE = NUMBER
         TMP_L % NEXT => L   ! Modified
         L => TMP_L          ! lines
      ! Otherwise, insert into sublist
      ELSE
         CALL INSERT (L % NEXT, NUMBER)
      END IF

   END SUBROUTINE INSERT

(Just in the interest of accuracy---I hope! ;-)

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