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From: bothner@sevenlayer.cs.wisc.edu (Per Bothner)
Newsgroups: comp.lang.lisp
Subject: Clarification of array semantics needed
Message-ID: <11765@spool.cs.wisc.edu>
Date: 20 Nov 90 06:58:18 GMT
Sender: news@spool.cs.wisc.edu
Organization: U of Wisconsin CS Dept
Lines: 36

I'm trying to implement (in C++) CL arrays, but I
find CLtL:2 rather confusing on some of the details.
Perhaps someone here can help a poor non-Lisp-hacker (and
perhaps ANSI could be more specific about the details).

* If an array A is displaced to a vector B with a
fill-pointer, is B's fill-pointer ignored when
accessing A? That seems a reasonable interpretation
(confirmed by a simple test with kcl). It does make
the remark "(Note, however, that one can create a multidimensional
array that a *displaced* to a vector that has a fill pointer.)"
[p. 454, 6th line from bottom] seem rather irrelevant.
Or are *all* fill-pointers checked when following a chain
of displacements?

* Is the following legal?
	(setq A (make-array ... :adjustable t :displaced-to B))
A later (adjust-array A ...) should just change A's diminsions?
What if :initial-contents is given to the adjust-array?

* I find section 17.6 on adjust-array totally confusing.
The various "clarifications" don't help. For example:

* Page 456, line 12: "but this may be achieved ... by creating
a new array and modifying the array argument to be displaced..."
seems to be contradicted by p 457, line 19.

* The whole "clarification" on p 458 is unclear to me.
If I have:
	(setq B (adjust-array A ...))
does the phrase "the result" refer to B or A?


--
	--Per Bothner
bothner@cs.wisc.edu Computer Sciences Dept, U. of Wisconsin-Madison