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Re: Help with recursive destructive function

From: Michael Heerdegen
Subject: Re: Help with recursive destructive function
Date: Mon, 07 May 2018 04:01:03 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Eric Abrahamsen <address@hidden> writes:

> Oh hang on, this doesn't work for all cases: the "by #'cdr" prevents
> it from hitting the cdr of cons cells:
> (let ((tree '("c" (2 ("d" . 3)) (4 . "e") "f")))
>   (iter-do (_ (iter-tree-example tree)))
>   tree)
> --> ("C" (2 ("D" . 3)) (4 . "e") "F")

Not only that (it's easy to fix), but I also realized that this approach
doesn't circumvent the recursion issue.  But if your data is not
extremely deeply nested, recursion should not be such a problem.

> The best I've been able to come up with is:
> (defun useless (val)
>   (while (consp val)
>     (let ((head (car val)))
>       (cond ((stringp head)
>            (setcar val (upcase head)))
>           ((listp head)
>            (useless head)))
>       (when (stringp (cdr val))
>       (setcdr val (upcase (cdr val))))
>       (setq val (cdr val)))))
> Recurses on car, and only repeats the test twice. I'd love to know if
> that could be simplified...

Looks ok to me in principle.


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