From: rmagere
Subject: Problem with global variables [i.e. showing my ignorance]
Date: Mon, 02 Feb 2004 10:21:55 +0000
Message-ID: <bvl8c7$g4o$1@news.ox.ac.uk>

Hi,
the problem I have is that I want to have a global variable holding some
information about actions that have to be taken at certain times, that
information needs to get changed with time until when it gets re-initialized
to what its original values where.
Here is an example:

(defvar *trials* nil)

(defclass trial ()
   ((name :accessor name :initarg :name :initform nil)
     (data :accessor data :initarg :data :initform nil)))

(defun trials ()
   (list (list (make-instance 'trial :name 'trial-0 :data '(0 1 2))
               (make-instance 'trial :name 'trial-1 :data '(2 3 4)))
         (list (make-instance 'trial :name 'trial-0 :data '(a b c))
               (make-instance 'trial :name 'trial-1 :data '(d e f)))))

(setf *trials* (trials))

(defun initialize-trials ()
   (setf *trials* (trials)))

So when I call (first (data (first (first *trials*)))) I get 0, then if I
setf it to 111 calling it again gives me 111 as expected. However if I then
run (initialize-trials) and then call (first ....) I still get 111 rather
than 0 as I (obviously erroneously) expected.

Can anyone highlight what I have done wrong and how to around the problem?

Is it that (defun initialize-trials) creates another level and the *trials*
in it isn't the same as the *trials* visible on the outside?

Thanks for any help,
 rmagere

• Re: Problem with global variables [i.e. showing my ignorance] Christophe Rhodes
  • Re: Problem with global variables [i.e. showing my ignorance] rmagere

From: Christophe Rhodes
Subject: Re: Problem with global variables [i.e. showing my ignorance]
Date: Mon, 02 Feb 2004 10:26:57 +0000
Message-ID: <squ129hj4e.fsf@lambda.dyndns.org>

"rmagere" <·······@*the-mail-that-burns*.com> writes:

> (defun trials ()
>    (list (list (make-instance 'trial :name 'trial-0 :data '(0 1 2))
                                                            ^
                   literal data ----------------------------'

> So when I call (first (data (first (first *trials*)))) I get 0, then if I
> setf it to 111 calling it again gives me 111 as expected. However if I then
> run (initialize-trials) and then call (first ....) I still get 111 rather
> than 0 as I (obviously erroneously) expected.
>
> Can anyone highlight what I have done wrong and how to around the problem?

You're modifying literal data when you setf it to 111.  This is
undefined.  If you change the call to
 ... :data (list 0 1 2)
everything should start working.

Christophe
-- 
http://www-jcsu.jesus.cam.ac.uk/~csr21/       +44 1223 510 299/+44 7729 383 757
(set-pprint-dispatch 'number (lambda (s o) (declare (special b)) (format s b)))
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From: rmagere
Subject: Re: Problem with global variables [i.e. showing my ignorance]
Date: Mon, 02 Feb 2004 10:40:57 +0000
Message-ID: <bvl9fu$gm7$1@news.ox.ac.uk>

Christophe Rhodes wrote:
> You're modifying literal data when you setf it to 111.  This is
> undefined.  If you change the call to
>  ... :data (list 0 1 2)
> everything should start working.

Thanks everything works now - I have actually realized that I have had the
same problem sometime ago in another context and then it had dissappeard, I
guess that I must have changed to list for some reason or another without
actually understading the issue involved.

Thanks again.