Re: using setq to create lists based on other lists...

[Jean-Christophe Helary <brandelune@gmail.com>]

> On Dec 3, 2018, at 1:05, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
>> When I check "Modifying Existing List Structure" the only things I find of
>> any use in my case are setcar/setcdr.
> 
> That's because "modifying" is the wrong way to think about it.
> 
> That section should start by telling the user that most likely what they
> really want to do is something else (i.e. get a similar list structure
> with some differences).

I see. Eventually I used copy-tree instead to initialize the new lists and then modified them separately with setcar.

>> I would certainly have considered that if I had known that setq was
>> linking the lists :)
> 
> Clearly, "linking the lists" means something to you, but it means
> nothing to me, so the manual can't really say something like that.

I think somebody used that term in the thread. I didn't even have that concept before the discussion started :)

Just out of curiosity, let me post what I did. There are probably better ways to do it, but that's the best I could come up with today. I'd love to be able to think in terms closer to what elisp allows though.

Jean-Christophe Helary
-----------------------------------------------
http://mac4translators.blogspot.com @brandelune


;; The function takes a date d and returns a (d m y) list where (d m y) is the closest day to today
;; For ex, today is 2018/12/3
;; d=1 -> (1 12 2018)
;; d=30 -> (30 11 2018)
;; d=15 -> (15 12 2018)
;; I use the function in a template for a blog that I write one page a day.
;; When I launch the template for a given day, it creates links for the previous day and the next day
;; I used the "closest" assumption because I'm either writing the blog late, or outputing future days templates for preparation.


(defun getMyDate (myDay)
  (interactive)
  (if (or (> 1 myDay) (< 32 myDay))
      (setq myDay (nth 3 (decode-time))))
  (setq now (decode-time (float-time))
	myDateLastMonth (copy-tree now)
	myDateThisMonth (copy-tree now)
	myDateNextMonth (copy-tree now)
	now (encode-time now 'integer))

  ;; need to create "last month", "next month", "last year", "next year"
  ;; 1 day is 84600 seconds
  ;; 1 month is approximately 2,592,000 seconds
  ;; 1 year is approximately 31,536,000 seconds
  (setq yesterDay (nth 3 (decode-time (- (float-time) 84600))) ;; not used in the function
	toMorrow (nth 3 (decode-time (+ (float-time) 84600))) ;; not used in the function
	lastMonth (nth 4 (decode-time (- (float-time) 2592000)))
	thisMonth (nth 4 (decode-time)) ;; not used in the function
	nextMonth (nth 4 (decode-time (+ (float-time) 2592000)))
	lastYear (nth 5 (decode-time (- (float-time) 31536000)))
	thisYear (nth 5 (decode-time)) ;; not used in the function
	nextYear (nth 5 (decode-time (+ (float-time) 31536000))))

  ;; create the data for last month
  (setcar (cdr (cdr (cdr myDateLastMonth))) myDay)
  (setcar (cdr (cdr (cdr (cdr myDateLastMonth)))) lastMonth)
  (if (= lastMonth 12)
      (setcar (cdr (cdr (cdr (cdr (cdr myDateLastMonth))))) lastYear))

  ;; create the data for this month
  (setcar (cdr (cdr (cdr myDateThisMonth))) myDay)

  ;; create the data for next month
  (setcar (cdr (cdr (cdr myDateNextMonth))) myDay)
  (setcar (cdr (cdr (cdr (cdr myDateNextMonth)))) nextMonth)
  (if (= nextMonth 1)
      (setcar (cdr (cdr (cdr (cdr (cdr myDateNextMonth))))) nextYear))

  ;; compare the dates to find the closest
  (setq a (abs (- now (encode-time myDateLastMonth 'integer)))
	b (abs (- now (encode-time myDateThisMonth 'integer)))
	c (abs (- (encode-time myDateNextMonth 'integer))))

  (if (and (< a b) (< b c))
      (setq myMonth (nth 4 myDateLastMonth)
	    myYear (nth 5 myDateLastMonth))
    (if (and (< b a) (< b c))
	(setq myMonth (nth 4 myDateThisMonth)
	      myYear (nth 5 myDateThisMonth))
      (setq myMonth (nth 4 myDateNextMonth)
	    myYear (nth 5 myDateNextMonth))))
  ;; et voilà
  (list myDay myMonth myYear))

(getMyDate 30)