Re: Better way to write this function?

B. Pym wrote:

Kenny Tilton wrote:
 

Peter Seibel wrote:

As part of a larger program I needed a function which given a number
(n) and a vector (row), returns the index into row of the n'th nil in
row. Here's the first (working) version I came up with:
 
  (defun nth-slot (n v)
    "Find the nth nil cell in a vector v."
    (loop with slots-seen = 0
          for item across v
          for idx from 0
          counting (not item) into slots-seen
          until (> slots-seen n)
          finally (if (> slots-seen n) (return idx) (error "No slot"))))

 
Please take that inscrutal glop to comp.lang.loop. <g>
 
(defun nth-null-slot-index (n v &optional must-find-p &aux (nil-ct 0))
    "Return the index of the nth nil cell in vector v"
    (dotimes (x (length v) (when must-find-p (error "bzzzt")))
       (when (null (elt v x))
          (when (= n (incf nil-ct)) ;; [1]
             (return-from nth-null-slot-index x)))))
 
(nth-null-slot-index 3 #(nil 1 2 nil 3 4 nil 5 nil nil))
=>6
 

 
The first nil found is considered to be number 1,
not number 0.
 
Gauche Scheme
 
(use gauche.generator)
 
;; "We don't need no stinkin' loops!"
(define (nth-null-slot-index n vec)
  (list-ref
    (generator->list
      ;; Gauche coerces the vector to a generator.
      (gselect (grange 0) (gmap not vec)))
    (- n 1)))
 
 
(nth-null-slot-index 3 #(#f a b #f c d #f e #f #f))
  ===>
6

Without using generators:

(define (nth-null-slot-index n vec)
  (list-ref
    (filter-map
      (^(a b) (and b a))
      (lrange 0)
      (map not (vector->list vec)))
    (- n 1)))