Re: lisp newbie here

From: Pascal Costanza (costanza_at_web.de)
Date: 04/10/04 

Date: Sat, 10 Apr 2004 23:18:32 +0200

Kenny Tilton wrote:

> (loop for (a b) on '(a b c d e) by 'cddr
> collecting (if b
> (list a b)
> (list a)))
>
> ...or if you don't mind padding the last pair:
>
> (loop for (a b) on '(a b c d e) by 'cddr
> collecting (list a b))

Hmm, this looks like a good way to iterate over property lists. I have
implemented a relatively complex doplist some time ago, and I was
surprised that CL doesn't offer anything predefined for that purpose.
Now this is already considerably simpler.

However, you probably want to make sure that the length of a property
list is even, so that all properties get one value, and that the last
property doesn't accidentally get a nil value. So I came up with this:

(loop for (a b) on '(:a b :c d :e nil) by (lambda (list)
                                             (assert (cdr list))
                                             (cddr list))
       collect (list a b))

Is there a simpler way to do this?

Pascal 

-- 
1st European Lisp and Scheme Workshop
June 13 - Oslo, Norway - co-located with ECOOP 2004
http://www.cs.uni-bonn.de/~costanza/lisp-ecoop/

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