[Openmcl-devel] Strange LISP Clozure problems

[Doug Currie]

Hi Lonnie,

The function copy-list is a shallow copy (as is remove-if-not). 

From http://www.lispworks.com/documentation/HyperSpec/Body/f_cp_lis.htm
> Only the list structure of list is copied; the elements of the resulting list are the same as the corresponding elements of the given list.

http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_l.htm#list_structure

You should take a look at copy-tree
http://www.lispworks.com/documentation/HyperSpec/Body/f_cp_tre.htm

e

On Oct 1, 2010, at 9:57 AM, 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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Doug Currie, CTO
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