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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: doubling list elements at every level using recursion
Date: Wed, 17 Jul 2024 15:32:54 -0000 (UTC)
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Pascal J. Bourguignon wrote:

> You could slightly generalize double, and accept any atoms. This would
> allow you to simplify it.
> 
> You could also use emacs and let it do the indentation for you!
> 
> (defun double (object)
>   (typecase object
>     (cons    (cons (double (car object)) (double (cdr object))))
>     (number  (* 2 object))
>     (t       object)))
> 
> (mapcar (function double)
>         '( ()  abc  123  (1 2.0 #C(3 4) a b c (5/2 6 d e f) 7 8 9) ))
> --> (NIL ABC 246 (2 4.0 #C(6 8) A B C (5 12 D E F) 14 16 18))


Why not simply

(double '(() abc 123 (1 2.0 #C(3 4) a b c (5/2 6 d e f) 7 8 9)))
  ===>
(NIL ABC 246 (2 4.0 #C(6 8) A B C (5 12 D E F) 14 16 18))


Scheme

(define (double obj)
  (cond ((pair? obj) `(,(double (car obj)) ,@(double (cdr obj))))
        ((number? obj) (* 2 obj))
        (#t obj)))

(double 3)
  ===>
6

(double '(() abc 123 (1 2.0 (3 4) a b c (5/2 6 d e f) 7 8 9)))
  ===>
(() abc 246 (2 4.0 (6 8) a b c (5 12 d e f) 14 16 18))