Chopping the last element of a list

Chopping the last element of a list

[emacsq via Users list for the GNU Emacs text editor]

If I want to do this then I can do, for example:

(prog1
(car (last mylist)) (setq mylist (nbutlast mylist)))

But here last and nbutlast walks the list twice unnecessarily.

Shouldn't emacs provide a function which does it in one step, so the list isn't walked twice?

E.g. (choplast mylist)

which returns a cons cell of (LAST . CHOPPEDLIST)

Of course, returning two values is not very lispy, but at least it could be more efficient.

Is there an existing function which does this in one step? If not, shouldn't there be one built-in emacs, for efficient manipulation of the list's end?

RE: Chopping the last element of a list

[Drew Adams]

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> If I want to do this then I can do, for example:
> 
> (prog1 (car (last mylist)) (setq mylist (nbutlast mylist)))
> 
> But here last and nbutlast walks the list twice unnecessarily.
> 
> Shouldn't emacs provide a function which does it in one step, so the
> list isn't walked twice?
> 
> E.g. (choplast mylist)
> 
> which returns a cons cell of (LAST . CHOPPEDLIST)
> 
> Of course, returning two values is not very lispy, but at least it
> could be more efficient.
> 
> Is there an existing function which does this in one step? If not,
> shouldn't there be one built-in emacs, for efficient manipulation of
> the list's end?

Here.  It returns the last element, and it
chops that last element off the list.

(You don't need to also return the updated
list, since you already have it as the arg.
But if you want to return it then return a
cons, as you did.)

(defun choplast (xs)
  (let ((m  (length xs)))
    (and (< 1 m)
         (let ((cons  (nthcdr (- m 2) xs)))
           (prog1 (cadr cons) (setcdr cons nil))))))

(setq foo  '(1 2 3 4 5 6 7 8 9))
(choplast foo) ; -> 9
 ; foo = (1 2 3 4 5 6 7 8)

(setq foo  '(1 2))
(choplast foo) ; -> 2
 ; foo = (1)

(setq foo  '(1))
(choplast foo) ; -> nil
 ; foo = (1)

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Re: Chopping the last element of a list

[Michael Heerdegen]

emacsq via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:

> If I want to do this then I can do, for example:
>
> (prog1
> (car (last mylist)) (setq mylist (nbutlast mylist)))
>
> But here last and nbutlast walks the list twice unnecessarily.

I would do that like

#+begin_src emacs-lisp
(let ((last2 (last my-list 2)))
  (prog1 (cons (cadr last2) my-list) (setcdr last2 nil)))
#+end_src

> Is there an existing function which does this in one step? If not,
> shouldn't there be one built-in emacs, for efficient manipulation of
> the list's end?

Dunno.  Does one need that so often?

I remember that I once implemented `rotate-left' and `rotate-right'
functions for lists, but I never used them.  Most of the time you need a
pointer to the last cdr more than once, and then it is more efficient to
remember that pointer in a variable.

What's your use case?

Michael.

Re: Chopping the last element of a list

[Stefan Monnier via Users list for the GNU Emacs text editor]

emacsq via Users list for the GNU Emacs text editor [2022-04-28 09:21:23] wrote:
> If I want to do this then I can do, for example:
>
> (prog1
> (car (last mylist)) (setq mylist (nbutlast mylist)))
>
> But here last and nbutlast walks the list twice unnecessarily.

Right, so you see the problem with functions like `last` and `butlast`
which work on the end of a list: they are only useful if that's the
only thing you do with that list.  As soon as you try to do more, then
you end up traversing the list multiple times.

> Shouldn't emacs provide a function which does it in one step, so the
> list isn't walked twice?
>
> E.g. (choplast mylist)

That would just suffer from the same problem you just experienced
(i.e. everytime you need to do `choplast` plus something else, you'll
end up traversing the list twice).

Instead, every time you need to work on the end of a list, you should
start by getting a reference to that end (presumably with `last` with an
appropriate N) and then you work locally on that end.


        Stefan

Re: Chopping the last element of a list

[Drew Adams]

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> Right, so you see the problem with functions like `last` and `butlast`
> which work on the end of a list: they are only useful if that's the
> only thing you do with that list.  As soon as you try to do more, then
> you end up traversing the list multiple times.
> 
> > Shouldn't emacs provide a function which does it in one step, so the
> > list isn't walked twice? E.g. (choplast mylist)
> 
> That would just suffer from the same problem you just experienced
> (i.e. everytime you need to do `choplast` plus something else, you'll
> end up traversing the list twice).
> 
> Instead, every time you need to work on the end of a list, you should
> start by getting a reference to that end (presumably with `last` with
> an appropriate N) and then you work locally on that end.

+1.  Well put.

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RE: [External] : Re: Chopping the last element of a list

[Drew Adams]

> I would do that like
> 
> (let ((last2 (last my-list 2)))
>   (prog1 (cons (cadr last2) my-list) (setcdr last2 nil)))

Yes.  I was just showing essentially the code
that `last' uses (in the example I gave).

RE: Chopping the last element of a list

[emacsq]

>
> Here. It returns the last element, and it
> chops that last element off the list.

Thanks. It's quite simple and useful  to have along with last/butlast/etc.

Will you put it into subr.el, so emacs has it built-in ?

Re: Chopping the last element of a list

[tomas]

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On Fri, Apr 29, 2022 at 04:26:51AM +0000, emacsq wrote:
> >
> > Here. It returns the last element, and it
> > chops that last element off the list.
> 
> Thanks. It's quite simple and useful  to have along with last/butlast/etc.
> 
> Will you put it into subr.el, so emacs has it built-in ?

Read Drew's code again. Read the examples he provided.

Now, answer this one question: for a list with exactly
one element, say

  (setq foo '(1))

what is its last element? What would you expect choplast
to do? What does Drew's implementation do? Surprise?

Cheers
-- 
t

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