Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.lisp
Subject: Re: Partially displaced arrays?
Message-ID: <25364@news.Think.COM>
Date: 25 Jul 89 01:49:53 GMT
References: <9562@phoenix.Princeton.EDU>
Sender: news@Think.COM
Reply-To: barmar@kulla.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 27

In article <9562@phoenix.Princeton.EDU> eliot@phoenix.Princeton.EDU (Eliot Handelman) writes:
>Supposing this is the array:
>	   -----------------
>           |   | X | X | X |
>	   -----------------
>           |   |   |   |   |
>	   -----------------
>I need to create a second array that shares only those locations marked
>X, so that changes in those slots are reflected in the "displaced" array.

Common Lisp requires that arrays be stored in row-major format.
Therefore, assuming your diagram is consistent with that
representation, the following should work:

(setq array-2 (make-array 3 :displaced-to array-1
			    :displaced-index-offset 1))

This specifies that the displaced array should only be three elements
long and should start at the second element of the original array.
Elements 0, 1, and 2 of the displaced array will correspond to
elements (0, 1), (0, 2), and (0, 3) of the target array.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
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