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From: Tamas K Papp <tkp...@gmail.com>
Newsgroups: comp.lang.lisp
Subject: Re: adjustable arrays
Date: 29 Apr 2010 15:45:19 GMT
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On Thu, 29 Apr 2010 15:13:30 +0000, Peter Keller wrote:

> RG <rNOSPA...@flownet.com> wrote:
>> You want a displaced (and adjustable) array.
>> 
>> ? (setf a1 (make-array 30))
>> #(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0) ?
>> (dotimes (i 30) (setf (aref a1 i) i)) ...
>> ? a1
>> #(0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
>> 26 27 28 29)
>> ? (setf a2 (make-array 10 :displaced-to a1 :displaced-index-offset 15
>> :adjustable t))
>> #(15 16 17 18 19 20 21 22 23 24)
>> ? (adjust-array a1 '(15) :displaced-to a1 :displaced-index-offset 10)
>> #(10 11 12 13 14 15 16 17 18 19 20 21 22 23 24)
> 
> I see.
> 
> If I knew how big the final sizes of the arrays were I could use this
> method, but sadly I don't. I'll file away the method for another day. It
> looks like under my circumstances, a new array into which I copy all of
> my disparate pieces looks to be one of the only solutions.

Sometimes I face a similar problem (collecting an number of items that
are either unknown a prior, or I am lazy to calculate).  I usually
solve it the following way: create array with extra elements, keep
using them until I run out, then create a new array with a buffer of
extra elements, copy and continue.  This way, you will copy
occasionally, but not all the time.  Example:

(use-package '(:bind))

(defun prepend-elements (vector n &optional (extra 256))
  (bind (((:values displaced-to index-offset) (array-displacement vector))
         (new-length (+ n (length vector))))
    (if (and displaced-to (<= n index-offset))
        (make-array new-length 
                    :displaced-index-offset (- index-offset n)
                    :displaced-to displaced-to)
        (let ((new-vector (make-array (+ extra new-length))))
          (replace new-vector vector :start1 (+ extra n))
          (make-array new-length
                      :displaced-index-offset extra
                      :displaced-to new-vector)))))

(defparameter *a* #(1 2 3))
(setf *a* (prepend-elements *a* 7))
(array-displacement *a*)                ; #(0 0 0 ...), 256

You can add bells and whistles, eg prepended element initialization,
etc.

Tamas