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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Better way to write this function?
Date: Tue, 10 Sep 2024 06:37:35 -0000 (UTC)
Organization: A noiseless patient Spider
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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