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From: micha@ecrc.de (Micha Meier)
Newsgroups: comp.lang.prolog
Subject: Re: Arrays in Prolog
Message-ID: <990@ecrc.de>
Date: 29 Aug 90 11:02:22 GMT
References: <90239.175243SCHMIED@DB0TUI11.BITNET> <9653@bunny.GTE.COM>
Sender: news@ecrc.de
Reply-To: micha@ecrc.de (Micha Meier)
Organization: ECRC
Lines: 56

In article <9653@bunny.GTE.COM> tomf@GTE.COM (Tom Fawcett) writes:
>
>I too have needed to use arrays in Prolog, and have been generally
>dissatisfied with what I've found.
...
>When asking about this deficiency, the reaction I've gotten is that you
>shouldn't need to use arrays - there is no logical interpretation of
>destructive modifications to a data structure.

    In every Prolog system, it is possible to use the arg/3
    predicate to have a constant-time access to a structure
    argument, and so for example instead of having
	char_in_sequence4(1, c).
	char_in_sequence4(2, u).
	char_in_sequence4(3, c).
	...
    one can write

	char_in_sequence4(I, Element) :-
		arg(I, chars(c, u, c, ...), Element).

    however in many Prolog systems the arity of functors is limited, and
    some of the structure-copying compilers will try to construct the
    whole chars/N structure on every char_in_sequence4/2 call.

    The ECRC SEPIA 3.0 system provides both these logical arrays
    (implemented efficiently when the array structure is ground),
    and real arrays. The latter are mainly a means to pass
    compound terms between Prolog and C, so that e.g. C structures
    can be mapped on Prolog arrays and handled in Prolog,
    however the arrays can also be used only in Prolog.

    For example, the sequence

    :- make_array(rotate(3, 3), integer),
       setval(rotate(0, 0), 1),
       setval(rotate(1, 2), -1),
       setval(rotate(2, 1), 1).

    creates a matrix for a rotation about the x-axis, the predicate
    getval/2 can be used to retrieve elements of the array.
    The SEPIA system is available for a nominal fee to academic sites
    and it has a number of other very useful features.

-- Micha Meier

USA     micha%ecrc.de@pyramid.com	MAIL	Micha Meier
						ECRC, Arabellastr. 17
EUROPE  micha@ecrc.de				8000 Munich 81
						West Germany

--
USA     micha%ecrc.de@pyramid.com	MAIL	Micha Meier
						ECRC, Arabellastr. 17
EUROPE  micha@ecrc.de				8000 Munich 81
						West Germany