Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!willett!ForthNet
From: ForthNet@willett.UUCP (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: Data Structures
Message-ID: <572.UUL1.3#5129@willett.UUCP>
Date: 27 Feb 90 23:55:15 GMT
Organization: Latest link in the ForthNet chain.  (Pgh, PA)
Lines: 54


 Date: 02-26-90 (08:06)              Number: 2973 (Echo)
   To: IAN GREEN                     Refer#: NONE
 From: JACK WOEHR                      Read: NO
 Subj: FORTH DATA STRUCTURES         Status: PUBLIC MESSAGE

 >
 >   How about declaring data structures like an array, or a queue. A 
 >stack is automatic but I am unsure how to implement these other 
 >elementary data structures.
 >


        Ian, the CREATE ... DOES> syntax is Forth's way of declaring
 a custom object. Anything created thusly pushes its data address to
 the stack when executed, then performs the associated DOES> code.

        : ARRAY ( #entries ---) \ this "defining" word will create arrays
           CREATE ALLOT \ allot { #entries } bytes
           DOES> ( index --- addr) \ which at runtime self-indexes
           + ;

        20 ARRAY FOO ok
        0 FOO . 1234 ok
        1 FOO . 1235 ok

        ... etc.

        Note there are no limit checks in this construct! However, limit
 checks can be implemented.

        : CHECKARRAY ( #entries ---)
           CREATE DUP , ALLOT
           DOES> ( index --- addr | abort)
           OVER 0< ABORT" Negative array index!"
           2DUP @
           >= ABORT" Index out of range!"
           CELL + + ;

        4 CHECKARRAY FOO ok
        0 FOO . 1234 ok
        1 FOO . 1235 ok
        -1 FOO . Negative array index!
        4 FOO . Index out of range!

        For a more elaborate example, a self-indexing n-dimensional
 array, see DIMARRAY.ARC on most of these fine ForthNet stations.

                =jax=

 NET/Mail : RCFB Golden, CO (303) 278-0364 VESTA & Denver FIG for Forth!
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