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How to specify a list, which's last element is referring to the first element?

By specify I mean, for example, how to write a setq statement with this list.
I have seen this in the documentation, but I can't find it anymore. Here some pointers:

  • You could loop over the elements of the list forever, but it uses only about as much memory as it has real elements.
  • Its cdr points to its car.
  • You could call it a ring list, but I do not mean stuff from the ring package.
  • It is something basic.

I do not have a use in mind, it's just for learning and experimenting.

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  • So you want an infinite list or you want a circular list? (-1 since this appears in search results for the title, but the contents are different)
    – mihai
    Commented Jun 6, 2019 at 11:32
  • @mihai I don't know what you are talking about. Maybe writing an answer would be more helpful than downvoting.
    – jue
    Commented Jun 19, 2019 at 19:56
  • see this answer. It has a nice picture, too. Cyclic and infinite are two different things. You ask for one in the title, but for the other one in your actual question.
    – mihai
    Commented Jun 20, 2019 at 6:11
  • @mihai thats the way learning goes, It didn't occured to me there is difference. Also I'm no native english speaker, so I rely on (maybe inaccurate) dictionarys and approximations ... Anyway, thanks for the hint.
    – jue
    Commented Sep 17, 2019 at 10:28

1 Answer 1

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See the Elisp manual, node Circular Objects.

A circular list is one way to implement an infinite list.

For example (from the doc), this creates a list in which the first element recurs as the third element:

(setq x  '(#1=(a) b #1#))

In your case:

(setq ys  '(1 2 3 4))
(setq ll  (last ys))
(setcdr ll ys)

C-h v ys:

#1=(1 2 3 4 . #1#)
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  • 1
    It would be better to write (setq ys (list 1 2 3 4)) since you're going to modify the list object
    – npostavs
    Commented Jun 29, 2017 at 14:25
  • This is exactly the documentation I was searching for. This #1=(a #1#) was what I meant. Thank you for the nice setq example.
    – jue
    Commented Jun 29, 2017 at 14:25
  • @npostavs Thanks for the hint, I thought ' and list are just different notations. I better have an extra look into the manual. :)
    – jue
    Commented Jun 29, 2017 at 14:29
  • list vs ' doesn't matter for this simple illustration. What matters is that you must not expect ' to always create new list structure (it doesn't). list always creates a new cons.
    – Drew
    Commented Jun 29, 2017 at 14:51
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    @npostavs: Agreed. For its purpose here it is OK. Plugged into a different context, the behavior might surprise someone unaware of how ' is handled.
    – Drew
    Commented Jun 29, 2017 at 17:53

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