* Incredible shrinking list....
@ 2008-10-17 20:47 Scott
2008-10-18 4:15 ` Kevin Rodgers
0 siblings, 1 reply; 3+ messages in thread
From: Scott @ 2008-10-17 20:47 UTC (permalink / raw
To: help-gnu-emacs
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)
(version)"GNU Emacs 21.3.2 (i586-pc-linux-gnu, X toolkit)
of 2004-07-02 on DrMemory"
(time-to-upgradep (version))t
I'm sure there is a far better way to write this, but I can't for the
life of me figure out why my ham-handed attempt is acting so
bizarrely, and would appreciate any help!
thanks,
Scott swanson
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Incredible shrinking list....
2008-10-17 20:47 Incredible shrinking list Scott
@ 2008-10-18 4:15 ` Kevin Rodgers
2008-10-21 8:09 ` Alex Schroeder
0 siblings, 1 reply; 3+ messages in thread
From: Kevin Rodgers @ 2008-10-18 4:15 UTC (permalink / raw
To: help-gnu-emacs
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
^ permalink raw reply [flat|nested] 3+ messages in thread
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2008-10-17 20:47 Incredible shrinking list Scott
2008-10-18 4:15 ` Kevin Rodgers
2008-10-21 8:09 ` Alex Schroeder
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