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I have a list, and want to clear it with (setq slist '()). But I only want to do this when the number of elements exceeds the value N.

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    (when (> N (length slist)) (setq slist nil)). Check out the on-board introduction to emacs lisp: C-h R eintr which will help you answer several of your recent questions. Aug 19, 2022 at 8:46
  • Have seen (setq slist '()). Is (setq slist nil) preferable for reading?
    – Dilna
    Aug 19, 2022 at 8:56
  • Since (eq nil '()) is true, it doesn't matter which you use. I think the latter is less noisy. Aug 19, 2022 at 14:31

1 Answer 1

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You can do it using this macro

(defmacro normalize-list(l n)
  "empties the list L when its size reaches N"
  `(if(>=(length ,l),n)(setq ,l '())))

Examples

(setq slist'(foo bar baz)) ;->(foo bar baz)

(normalize-list slist 2);->nil

slist ;->(foo bar baz)

(normalize-list slist 5);->nil

slist;->nil
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    Why do you need a macro for this? Aug 19, 2022 at 14:30
  • The argument l (for the list) must not be evaluated. In case l is bound to '(foo bar baz) What would elisp (setq l'()) render if the symbol l is evaluated? And what about (length l) if the argument l is quoted ?
    – gigiair
    Aug 19, 2022 at 15:04
  • yup, good point. I was not thinking clearly earlier. Aug 19, 2022 at 20:23

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