Re: Incredible shrinking list....

[Kevin Rodgers <kevin.d.rodgers@gmail.com>]

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