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Adjustable and displaced arrays
- To: common-lisp%SU-AI@THINK.ARPA
- Subject: Adjustable and displaced arrays
- From: Guy Steele <gls%AQUINAS@THINK.ARPA>
- Date: Thu, 23 May 1985 14:51:00 -0000
- Cc: gls%AQUINAS@THINK.ARPA
- In-reply-to: <RAM.12113193061.BABYL@CMU-CS-C.ARPA>
Date: Wed, 22 May 1985 23:56 EDT
From: Rob MacLachlan <RAM@CMU-CS-C.ARPA@think>
I am somewhat confused about the interaction between array adjustment
In CLTL p298:
Note that if the array a is created displaced to array b and
subsequently array b is given to adjust-array, array a will still
be displaced to array b; the effects of this displacement and the
rule of row-major storage order must be taken into account.
What is this trying to say? Is this a warning to the user or a threat
to the implementor? The obvious interpretation of this statement
seems to require that all non-simple arrays contain a list of all the
other array headers that share the same data so that they can be fixed
up when the array is adjusted. The answer that you stick in a
forwarding pointer is not acceptable.
This should be elucidated in the next edition. If you put this paragraph
beside the descriptions of :DISPLACED-TO and :DISPLACED-INDEX-OFFSET on pages
288-289, and assume that array a is essentially unaffected, one concludes
that array a is still displaced to array b, the mapping between elements
still being determined by taking the elements in row-major order and then
mapping element k of array a to element k+n of array b, where n is the
:displaced-index-offset for array a. The only difference is that array b,
having been adjusted, may have a different row-major ordering. (A nasty
problem is that the roles of arrays A and B are reversed on pages 288 and
298! Sorry about that.)
The statement in the manual also doesn't answer the question of what
happens to array b if array a is adjusted. Is it legal or meaningful
to have arrays which are both displaced to another array and
adjustable? What are the semantics of adjusting such an array?
Well, page 298 says that if A is displaced to X, then after adjustment of A,
A has no relationship to X, nor does the returned result, unless X is
respecified explicitly in the :displaced-to option to ADJUST-ARRAY.
I admit that this fails to state explicitly that X is unaffected.