Re: [External] : Re: Appending lists

[Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

Jean Louis wrote:

> But then I can use the previously defined variables to
> define a new one:
>
> (setq appended (append list-a list-b)) ⇒ (1 2 3 a b c)
>
> and nothing changed in those:
>
> list-a ⇒ (1 2 3)
> list-b ⇒ (a b c)

Actually `append' can also be trouble because the last list
isn't copied, the new list just has a cdr somewhere to it -
yes, to the actual list.

Try this:

  (setq list-head '(1))                          ; (1)

  (setq list-tail '(2 3 4))                      ; (2 3 4)

  (setq whole-list (append list-head list-tail)) ; (1 2 3 4)

  (setcdr list-tail '(3.5 4))                    ; (3.5 4)

  whole-list                                     ; (1 2 3.5 4)

See that "whole-list" changed even tho we did it with `setq',
append, and didn't mention it...

I don't know why append does this, either to speed things up
or it has something to do with the car/cdr dynamic, maybe it
is considered safe enough to move the car out of action with
an actual copy ...

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