• #### Re: counting nils in a list

From B. Pym@21:1/5 to Pascal J. Bourguignon on Wed Jul 17 04:12:35 2024
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)
(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))

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