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comp / comp.lang.scheme / Re: Better way to write this function?

Subject: Re: Better way to write this function?
From: B. Pym
Newsgroups: comp.lang.lisp, comp.lang.scheme
Organization: A noiseless patient Spider
Date: Tue, 10 Sep 2024 06:37 UTC

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From: Nobody447095@here-nor-there.org (B. Pym)
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)
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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

Subject: Re: Better way to write this function?
From: B. Pym
Newsgroups: comp.lang.lisp, comp.lang.scheme
Organization: A noiseless patient Spider
Date: Tue, 10 Sep 2024 20:17 UTC
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From: Nobody447095@here-nor-there.org (B. Pym)
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Better way to write this function?
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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)))

Subject	Author
Re: Better way to write this function?	B. Pym
Re: Better way to write this function?	B. Pym