Re: counting nils in a list

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))