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From: "WJ" <w_a_x_...@yahoo.com>
Newsgroups: comp.lang.lisp
Subject: Re: Problem with iteration
Date: 2 May 2012 09:09:25 GMT
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Erik Naggum wrote:

> * Raffael Cavallaro 
> | (defun my-reverse (a-list) 
> |      (do ((n (- 1) (incf n)) (result '() (cons (nth n a-list) result))) 
> |          ((= n (- (length a-list) 1))  result ))) 
> 
>   yikes. 
> 
> (defun my-reverse (list) 
>   (do ((reversed (list (pop list)) 
>                  (cons (pop list) reversed))) 
>       ((endp list) reversed)))

Buggy:

* (my-reverse '())

(NIL)


And POP won't work on immutable lists.

Let's do it the right way using Racket:

(define (my-reverse alist)
  (do ((accum '()  (cons (car alist) accum))
       (alist  alist  (cdr alist)))
      ((null? alist)   accum)))