<sandia.gov at svspire> wrote:

<well.com at [mlb> wrote about adjust-array being guaranteed to copy the
array in one particular case.]

> It's not clear to me from reading the hyperspec that this is always
> guaranteed, and I trust the hyperspec more than CLtL. However, the
> hyperspec _does_ offer an alternative guarantee involving
> adjustable-array-p.

The adjust-array guarantee looks pretty solid to me:

  A is displaced to B before the call, but not displaced afterward.

   (adjust-array A ... :displaced-to B)
   (adjust-array A ... :displaced-to nil)

  A gets a new ``data region,'' and contents of B are copied into it
  as appropriate to maintain the existing old contents; additional
  elements of A are taken from initial-element if supplied. However,
  the use of initial-contents causes all old contents to be discarded.

(From
<http://www-2.cs.cmu.edu/Groups/AI/html/hyperspec/HyperSpec/Body/fun_adjust-array.html>.
Also see
<http://groups.google.com/groups?selm=sfwwwmgwhuh.fsf%40world.std.com>,
where at least someone else has come to the same conclusion.)

> http://www.xanalys.com/software_tools/reference/HyperSpec/Issues/iss005_w.htm
> 
> The telling line is:
> 
>   5. The predicate ADJUSTABLE-ARRAY-P is true if and only if
>   ADJUST-ARRAY will return a value EQ to this array when given this
>   array as its first argument.

Ah.  Unfortunately, issues are not part of the spec, and are only
there to give background and help you to understand the original
intent.

So I think that this code of Marc's is probably the easiest, and is
correct:

  (defun copy-array (array)
     (let ((dims (array-dimensions array)))
       (adjust-array
         (make-array dims :displaced-to array)
         dims)))


John Wiseman


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