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* bug#55395: What does (1 2 3 . #2) mean?
@ 2022-05-13 11:32 Mattias Engdegård
  2022-05-13 15:22 ` Lars Ingebrigtsen
  2022-05-13 16:08 ` Andreas Schwab
  0 siblings, 2 replies; 10+ messages in thread
From: Mattias Engdegård @ 2022-05-13 11:32 UTC (permalink / raw)
  To: 55395

What, exactly, does the #N print notation mean (with print-circle=nil)?

Let's define (rho LEAD LOOP) as the iota list that has a loop LOOP long after LEAD initial elements:

(defun rho (lead loop)
  (let ((l (number-sequence 1 (+ lead loop))))
    (setcdr (nthcdr (+ lead loop -1) l) (nthcdr lead l))
    l))

Then we have:

(rho 0 1) => (1 . #0)
(rho 0 2) => (1 2 1 2 . #2)
(rho 0 3) => (1 2 3 1 2 . #2)
(rho 0 4) => (1 2 3 4 1 2 3 4 1 2 . #5)
(rho 0 5) => (1 2 3 4 5 1 2 3 4 5 1 . #5)
(rho 1 4) => (1 2 3 4 5 2 3 4 5 2 . #5)
(rho 4 1) => (1 2 3 4 5 5 5 . #3)

and so on. The pattern is not obvious to me.

It may have made more sense before the switch of cycle-detection algorithm from Floyd to Brent. This can be fixed by hand-coding the list iteration and explicitly remembering the index of the tortoise, but would that be correct? What's the spec?

If #N means 'Nth object from the top along the path to the current object, starting at 0' then we should have

(rho 2 3) => (1 2 3 4 5 . #2)
(list (rho 2 3)) => ((1 2 3 4 5 . #3))

ie, adding the print depth to the index in the list. Do you agree?






^ permalink raw reply	[flat|nested] 10+ messages in thread

end of thread, other threads:[~2022-05-23 14:59 UTC | newest]

Thread overview: 10+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2022-05-13 11:32 bug#55395: What does (1 2 3 . #2) mean? Mattias Engdegård
2022-05-13 15:22 ` Lars Ingebrigtsen
2022-05-13 17:20   ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2022-05-13 19:54     ` Lars Ingebrigtsen
2022-05-13 20:01     ` Mattias Engdegård
2022-05-14 13:45       ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2022-05-18 14:29         ` Mattias Engdegård
2022-05-18 21:16           ` Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors
2022-05-23 14:59             ` Mattias Engdegård
2022-05-13 16:08 ` Andreas Schwab

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