From: "Mattias Engdegård" <mattiase@acm.org>
To: 55395@debbugs.gnu.org
Subject: bug#55395: What does (1 2 3 . #2) mean?
Date: Fri, 13 May 2022 13:32:34 +0200 [thread overview]
Message-ID: <4B70C9BE-B784-4B74-A081-7A8B3F8D5136@acm.org> (raw)
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?
next reply other threads:[~2022-05-13 11:32 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-13 11:32 Mattias Engdegård [this message]
2022-05-13 15:22 ` bug#55395: What does (1 2 3 . #2) mean? 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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