From: Kevin Rodgers <kevin.d.rodgers@gmail.com>
To: help-gnu-emacs@gnu.org
Subject: Re: Incredible shrinking list....
Date: Fri, 17 Oct 2008 22:15:56 -0600 [thread overview]
Message-ID: <gdbnth$ebm$1@ger.gmane.org> (raw)
In-Reply-To: <20081017204742.GB1614@drmemory.local>
Scott wrote:
> Hi,
>
> I am very much a newbie, so forgive if this is a stupid question.
>
> I am trying to write a function which returns a randomized permutation
> of a certain number of items. Here is my stab at it, and the problem
> which baffles me:
>
>
> (defun pick-ten ()
> "Pick a series of ten unique random numbers from 0-9.
> You should ensure that random is properly 'seeded', as eg:
> (random t)
> prior to using this function."
> (let ((all-ten '(0 1 2 3 4 5 6 7 8 9)) ;initial list
> (rnd-list '()) ;return value
> (nxt 0)) ;placeholder for current number
> (while all-ten ;step through the list
> (setq all-ten ;trim by
> (delq ;deleting selected number
> (setq nxt ;but remember number for later
> (car
> (nthcdr (mod (random)(length all-ten)) all-ten)
> )
> ) all-ten))
> (setq rnd-list (cons nxt rnd-list)) ;and put into the return value
> )
> rnd-list ;now return it.
> ))
>
> ;; works fine on the first evaluation,
> ;; but the series somehow shrinks with subsequent calls!
> ;; what am I doing wrong???!
> ;;
> (pick-ten)(3 2 9 7 1 6 0 5 8 4)
> (pick-ten)(1 0 2 3)
> (pick-ten)(0 1)
> (pick-ten)(0)
Probably destructively modifying a constant -- try this:
(let ((all-ten (list 0 1 2 3 4 5 6 7 8 9)) ;initial list
or this:
(setq all-ten (remq ... all-ten))
--
Kevin Rodgers
Denver, Colorado, USA
next prev parent reply other threads:[~2008-10-18 4:15 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-10-17 20:47 Incredible shrinking list Scott
2008-10-18 4:15 ` Kevin Rodgers [this message]
2008-10-21 8:09 ` Alex Schroeder
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