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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 20:17:24 -0000 (UTC)
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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)))