Re: Sets in Pascal, etc.

hbaker@netcom.com (Henry G. Baker) writes:


> When I taught Pascal, I found sets to be extremely useful, but I also
> found their arbitrary size limitations very irritating.
>
> For twenty years, I have been one of the champions of extending boolean
> operations to Lisp's bignums, which then have the capability of emulating
> any finite (positive) or co-finite (negative numbers) set.


I agree that the limitation on set size is most annoying. However, with
the advent of Object Pascal and Oberon-2, this limitation can be neatly
hidden through the use of a Set object. This means that the compiler can
implement sets using whatever width is most efficient, and the programmer
doesn't have to be constrained by this.


Though the implementation of a Set class (object) is simple, I would like
to point out that the book: "Object Oriented Programming in Oberon-2" by
Hanspeter Mossenboeck contains the code for such a thing. It looks like
the following: (I have modified this slightly)




MODULE Sets;




TYPE
    Set* = RECORD
        max-: INTEGER; (*largest element*)
        val: POINTER TO ARRAY OF SET; (*open-ended array*)
    END;


VAR
    setSize: INTEGER; (* size of type SET *)




PROCEDURE (VAR s: Set) Init* (max: INTEGER);
BEGIN
    s.max := max;
    NEW(s.val, (max+setSize) DIV setSize);
END Init;


(* you get the picture *)
(*PROCEDURE (VAR s: Set) CopyTo* (var s1: Set);
[code deleted]
PROCEDURE (VAR s: Set) Clear*;
[code deleted]


PROCEDURE (VAR s: Set) Incl* (x: INTEGER);
[code deleted]
PROCEDURE (VAR s: Set) Excl* (x: INTEGER);
[code deleted]
PROCEDURE (VAR s: Set) Contains* (x: INTEGER);
[code deleted]
PROCEDURE (VAR s: Set) Incl* (x: INTEGER);
[code deleted]
PROCEDURE (VAR s: Set) Add* (s1: Set);
[code deleted]
PROCEDURE (VAR s: Set) Subtract* (s1: Set);
[code deleted]
PROCEDURE (VAR s: Set) Intersect* (s1: Set);
[code deleted]*)




BEGIN (* initialisation for module *)
    setSize := MAX(SET) + 1;
END Sets.




Cheers,
Andrew McVeigh
--

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