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Re: srfi-1 take and drop
From: |
Rob Browning |
Subject: |
Re: srfi-1 take and drop |
Date: |
Thu, 08 May 2003 11:35:10 -0500 |
User-agent: |
Gnus/5.1001 (Gnus v5.10.1) Emacs/21.3 (gnu/linux) |
address@hidden (Paul Jarc) writes:
> Rob Browning <address@hidden> wrote:
>> Yep, I have a "take" I haven't committed yet that avoids both stack
>> growth and reverse! by tracking the end pair...
>
> Are you using (ice-9 q)? It seems to be the right tool for that job.
Nope. Didn't even know that was there. I had just done this as a
first pass:
(define (take lst k)
;; avoid a reverse! or stack growth -- easy on the cache,
;; hard on the eyes...
(cond
((zero? k) '())
((negative? k) (error "negative count in call to take."))
(else
(let ((result (cons (car lst) '())))
(let lp ((n (- k 1)) (rest (cdr lst)) (end-pair result))
(if (zero? n)
result
(let ((new-end (cons (car rest) '())))
(set-cdr! end-pair new-end)
(lp (- n 1) (cdr rest) new-end))))))))
though thanks for pointing out (ice-9 q). That might be a better idea...
--
Rob Browning
rlb @defaultvalue.org, @linuxdevel.com, and @debian.org
Previously @cs.utexas.edu
GPG starting 2002-11-03 = 14DD 432F AE39 534D B592 F9A0 25C8 D377 8C7E 73A4
- srfi-1 take and drop, Kevin Ryde, 2003/05/05
- Re: srfi-1 take and drop, Rob Browning, 2003/05/05
- Re: srfi-1 take and drop, Kevin Ryde, 2003/05/05
- Re: srfi-1 take and drop, Rob Browning, 2003/05/05
- Re: srfi-1 take and drop, Rob Browning, 2003/05/05
- Re: srfi-1 take and drop, Kevin Ryde, 2003/05/07
- Re: srfi-1 take and drop, Rob Browning, 2003/05/08
- Re: srfi-1 take and drop, Kevin Ryde, 2003/05/09
- Re: srfi-1 take and drop, Paul Jarc, 2003/05/08
- Re: srfi-1 take and drop,
Rob Browning <=