Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: counting nils in a list
Date: Wed, 17 Jul 2024 04:12:35 -0000 (UTC)
Organization: A noiseless patient Spider
Lines: 74
Message-ID: <v77gbh$1m8bf$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Injection-Date: Wed, 17 Jul 2024 06:12:35 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="c61a3943c524c0aaf773b15bc0bf86a0";
	logging-data="1778031"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+e2WuDmIU1svIW1W81MJGM"
User-Agent: XanaNews/1.18.1.6
Cancel-Lock: sha1:V0sRGpRbAy3Mau4CbaezbiJfZHk=
Bytes: 2819

Pascal J. Bourguignon wrote:

> > For example, if we have a function such as cut-list-if, which can cut
> > a list between elements according to a predicate taking two successive
> > elements in the list, such as:
> >
> > (cut-list-if (lambda (a b) (or a b))
> >             '(nil nil nil a nil nil nil a b nil nil nil
> >               a b c nil nil nil a b c d e nil))
> > --> ((NIL NIL NIL) (A) (NIL NIL NIL) (A) (B) (NIL NIL NIL) (A) (B) (C)
> >      (NIL NIL NIL) (A) (B) (C) (D) (E) (NIL))
> 
> Oops, I forgot to present a CUT-LIST-IF:
> 
> (defun cut-list-if (predicate list)
>   (loop
>      :with results = '()
>      :with sublist = '()
>      :for (a b) :on list
>      :do (if (funcall predicate a b)
>            (progn
>              (push a sublist)
>              (push sublist results)
>              (setf sublist '()))
>            (push a sublist))
>      :finally (when sublist (push sublist results)) (return (reverse results))))


Buggy:

* (cut-list-if (lambda (a b) (or (numberp a) (numberp b)))
            '(a b c 8 d e f 7 6 g 5 h i))

((C B A) (8) (F E D) (7) (6) (G) (5) (I H))



Gauche Scheme

(use gauche.collection) ;; fold2

;; Correcting his bug.
(define (cut-list-if predicate the-list)
  (receive (tmp res)
    (fold2
      (lambda (a tmp res)
        (if (or (null? tmp) (not (predicate (car tmp) a)))
          (values (cons a tmp) res)
          (values (list a) (cons tmp res))))
      '() '()
      the-list)
    (reverse (map reverse (cons tmp res)))))


(cut-list-if (lambda (a b) (or (number? a) (number? b)))
  '(a b c 8 d e f 7 6 g 5 h i 5))

  ===>
((a b c) (8) (d e f) (7) (6) (g) (5) (h i) (5))


(cut-list-if (lambda (a b) (or a b))
  '(#f #f #f a #f #f #f a b #f #f #f
    a b c #f #f #f a b c d e #f))

  ===>
((#f #f #f) (a) (#f #f #f) (a) (b) (#f #f #f) (a) (b) (c)
 (#f #f #f) (a) (b) (c) (d) (e) (#f))


(cut-list-if > '(0 2 3 5 4 5 6))

  ===>
((0 2 3 5) (4 5 6))