[Openmcl-devel] Strange LISP Clozure problems

Jon S. Anthony j-anthony at comcast.net
Fri Oct 1 15:49:11 UTC 2010


Hi,

I would also suggest looking at copy-tree.  However, I would suggest
even more strongly to reflect on why the general case of "make copy" is
as vague and difficult as the general case of "is equal" and would refer
you to:

http://www.nhplace.com/kent/PS/EQUAL.html

for some great elucidation.


/Jon


On Fri, 2010-10-01 at 09:57 -0400, Lonnie Cumberland wrote:
> Greetings All,
> 
> I am working on some code in lisp and basically have 2 lists now after
> originally trying with just one where I had the same problem. (I also
> had a plist version but still get the same problem.)
> 
> I make a list for a format of
> 
> 
> ((1  nil nil 1)
> .
> .
> .
> (n nil nil 1))
> 
> call it *db*. I then add some values such that the first "nil" is made
> into a list. [ie.. (1 (2) nil 1) ] and then do this for the whole list
> in *db* with the function below.
> 
> (defun add-mod-pres (b)
> 	(mapcar
>          #'(lambda (row) (setf  (second row) (nconc (second row) (list
> (mod (first row) b))))) *db*)
> )
> 
> The problem comes when I try to make a copy of the *db* and then
> change some value in the "copied" db.
> 
> (setf tempdb (copy-list (remove-if-not #'(lambda (row) (equal (first
> (last row)) (list j))) *db*)))
> 
> The result I am getting is that things as also being changed in the
> original *db* as well and it look as though there is something going
> on with LISP using pointers instead of actually making a copy.
> 
> The basic code problem is to have my list like:
> 
> ((1  nil nil 1)
> .
> .
> .
> (n nil nil 1))
> 
> and while iterating over some other lists that I have, I grow each
> list like the below example of a single sublist within *db*:
> 
> Iteration 1
> (1 (2) (1) 1)
> 
> Iteration 2
> (1 (2 1) (1 5) 1)
> 
> .
> .
> 
> Iteration 10
> (1 (2 1 3 4 5 4 3 2 1 0) (1 5 3 2 4 5 4 4 5 3) 1)
> 
> etc....
> 
> What I am currently getting is that when I modify the (1 5 .... ) list
> then it also seems to modify the (2 1 .....) list in the *db* and this
> also happens if I modify the (1 5 ....) list in the tempdb which also
> modifies the *db* list.
> 
> There seems to be some shared memory in the lists going on and I don't
> know how to stop it.
> 
> Could the "lambda" or "mapcar" functions be the problem?
> 
> Kind Regards and have a great day,
> Lonnie T. Cumberland
> 
> "BTE, bringing people together today to meet the world's energy needs
> of tomorrow......"
> 
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