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comp / comp.lang.lisp / Re: every-other revisited (with series)

SubjectAuthor
o Re: every-other revisited (with series)B. Pym

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Subject: Re: every-other revisited (with series)
From: B. Pym
Newsgroups: comp.lang.lisp, comp.lang.scheme
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Date: Wed, 18 Sep 2024 21:15 UTC
Path: eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Nobody447095@here-nor-there.org (B. Pym)
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: every-other revisited (with series)
Date: Wed, 18 Sep 2024 21:15:55 -0000 (UTC)
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Fred Gilham wrote:

> For those who are interested, here are two versions of the
> `every-other' function we were discussing a couple months ago, done
> for amusement value with the SERIES package that was mentioned
> recently:
>
>
> (defun every-other (list &key (start 0))
> (collect
> (choose (series t nil)
> (scan (nthcdr start list)))))
>
>
> This one uses SERIES to create a series of (t nil t nil ...); CHOOSE
> uses this series to determine whether or not to take the current item
> from the series created from the original list by SCAN. This was the

Gauche Scheme

(use srfi-1) ; circular-list

(define (every-other lst)
(filter-map
(cut and <> <>)
(circular-list #t #f)
lst))

(every-other (iota 22))
===>
(0 2 4 6 8 10 12 14 16 18 20)

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