From: Jean-Christophe Helary <brandelune@gmail.com>
To: help-gnu-emacs <help-gnu-emacs@gnu.org>
Subject: Re: using setq to create lists based on other lists...
Date: Sun, 2 Dec 2018 21:03:04 +0900 [thread overview]
Message-ID: <C78FF94F-DB39-49BB-BE0F-41433CBA3394@gmail.com> (raw)
In-Reply-To: <barmar-BF2125.06211202122018@reader.eternal-september.org>
Barry, thank you for the reply.
> On Dec 2, 2018, at 20:21, Barry Margolin <barmar@alum.mit.edu> wrote:
>
>> I spend most of the day investigating why creating a list with setq was not
>> "working".
>>
>> For ex:
>> (setq list0 '(1 2))
>> (setq list1 list0)
>>
>> If you do
>>
>> (setcar list0 0)
>>
>> then for some reason (for which I could not find an explanation in the elisp
>> reference) the car of list1 also changes, and vice-versa.
>>
>> Which is totally unexpected since when you do:
>>
>> (setq list0 0)
>>
>> list1 does not become 0
>>
>> I don't suppose that's a bug, but really it ought the be very clearly
>> documented in the reference. Also, I'd like to know why that's happening.
>
> list0 and list1 both contain references to the same cons.
The reference says:
> Note that the first form is evaluated, then the first symbol is set...
So in my case I expected to have list0 evaluated to (1 2) and list1 set to (1 2).
You mean that list0 evaluates to a pointer to a cons that holds the values (1 2) and that list1 is set to that pointer.
I'm ok if that is true but I wish it were plainly written in the reference. Because I can't find a place where it is clearly written that lists evaluate to a pointer to their cons.
> If you're familiar with C, you can think of Lisp variables as being like
> pointers, and conses are like structures. Your code is equivalent to:
I am not familiar with C but think you for the explanations.
> cons *list0 = malloc(sizeof(cons));
> list0->car = make_number(1);
> list0->cdr = malloc(sizeof(cons));
> list0->cdr->car = make_number(2);
> list0->cdr->cdr = null;
> cons *list1 = list0;
> list0->car = make_number(0);
Jean-Christophe Helary
-----------------------------------------------
http://mac4translators.blogspot.com @brandelune
next prev parent reply other threads:[~2018-12-02 12:03 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.5010.1543748027.1284.help-gnu-emacs@gnu.org>
2018-12-02 11:21 ` using setq to create lists based on other lists Barry Margolin
2018-12-02 11:51 ` Stephen Berman
2018-12-02 12:22 ` Jean-Christophe Helary
2018-12-02 13:08 ` Stephen Berman
2018-12-02 13:28 ` Jean-Christophe Helary
2018-12-02 14:40 ` Michael Heerdegen
2018-12-02 15:34 ` Jean-Christophe Helary
2018-12-02 15:44 ` Michael Heerdegen
2018-12-02 15:57 ` Jean-Christophe Helary
2018-12-02 15:00 ` Stephen Berman
2018-12-02 15:30 ` Jean-Christophe Helary
[not found] ` <mailman.5026.1543764670.1284.help-gnu-emacs@gnu.org>
2018-12-04 9:00 ` Barry Margolin
2018-12-02 12:03 ` Jean-Christophe Helary [this message]
[not found] <mailman.5042.1543777897.1284.help-gnu-emacs@gnu.org>
2018-12-04 9:04 ` Barry Margolin
[not found] ` <(message>
[not found] ` <from>
[not found] ` <Barry>
[not found] ` <Margolin>
[not found] ` <on>
[not found] ` <Tue>
[not found] ` <04>
[not found] ` <Dec>
[not found] ` <2018>
[not found] ` <04:04:52>
2018-12-04 13:56 ` Stefan Monnier
2018-12-05 1:07 ` Robert Thorpe
2018-12-05 2:32 ` Drew Adams
2018-12-05 6:45 ` Jean-Christophe Helary
2018-12-05 8:00 ` Marcin Borkowski
2018-12-05 8:11 ` Jean-Christophe Helary
2018-12-05 14:57 ` Drew Adams
[not found] ` <mailman.5218.1544021892.1284.help-gnu-emacs@gnu.org>
2018-12-05 16:59 ` Barry Margolin
[not found] ` <mailman.5186.1543978155.1284.help-gnu-emacs@gnu.org>
2018-12-05 16:50 ` Barry Margolin
[not found] ` <mailman.5145.1543931778.1284.help-gnu-emacs@gnu.org>
2018-12-05 16:47 ` Barry Margolin
2018-12-02 10:53 Jean-Christophe Helary
2018-12-02 15:07 ` Stefan Monnier
2018-12-02 15:41 ` Jean-Christophe Helary
2018-12-02 16:05 ` Stefan Monnier
2018-12-02 16:23 ` Jean-Christophe Helary
2018-12-02 17:02 ` Stefan Monnier
2018-12-02 17:21 ` Jean-Christophe Helary
2018-12-02 19:11 ` Robert Thorpe
2018-12-02 23:44 ` Jean-Christophe Helary
[not found] ` <mailman.5028.1543765273.1284.help-gnu-emacs@gnu.org>
2018-12-03 13:43 ` Rusi
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