On Wed, 16 Mar 2022 07:47:18 -0700 (PDT)
Bigos <
ruby.object@googlemail.com> wrote:
This is my lame attempt. Only if I knew how to replace your cars and
cdrs following the setf I would have my solution.
(defparameter *bintree* '(((1) . (2)) (3 . 4)))
(defun bintrav (tr)
(format t "trying ~S~%" tr)
(when (equal tr '(2))
(progn
(setf tr (cons 2222 nil))
(format t "trying changed ~S~%" tr)))
(if (atom tr)
nil
(progn
(format t "trying with ~S~%" tr)
(bintrav (car tr))
(bintrav (cdr tr)))))
A few general remarks first :
1. It is much more useful if you specify what is the initial and final tree
you want. Instead you are leaving us to guess that the initial tree is the value of *bintree* and for the final tree we must guess based on the code
for bintrav which does not actually achieve what you want !
2. If you are doing (bintrav *bintree*) then it's undefined behaviour
because *bintree* is a quoted object and you are not supposed to modify those. So what you really want is
(defparameter *bintree* (list (cons (list 1) (list 2)) (cons 3 4))))
3. Note that
(equalp '(((1) . (2)) (3 . 4)) '(((1) 2) (3 . 4)))
returns T so your quoted object is more complicated than it needs to be.
4. WHEN is an implicit PROGN so instead of
(when (equal tr '(2))
(progn
(setf tr (cons 2222 nil))
(format t "trying changed ~S~%" tr)))
you can simply do
(when (equal tr '(2))
(setf tr (cons 2222 nil))
(format t "trying changed ~S~%" tr))
I didn't examine carefully your subsequent posts using EVAL because it is almost certain that you don't want EVAL for this. Instead I think you want something like
(defun bintrav2 (arg &aux (tr arg))
(format t "trying ~S~%" tr)
(when (equalp tr '(2))
(setf (car tr) (cons 2222 nil))
(format t "trying changed ~S~%" tr))
(if (atom tr)
nil
(progn
(format t "trying with ~S~%" tr)
(bintrav2 (car tr))
(bintrav2 (cdr tr)))))
--
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