modify/replace in LISP

Question

I have an implementation of a function called modify list shown below but it only works for top level lists.

(defun modify-list (old new a-list)
  (cond
   ((null a-list) nil)
   ((eql (car a-list) old) (cons new (modify-list old new (cdr a-list))))
   (T (cons (car a-list)(modify-list old new (cdr a-list))))))

CL-USER 16 : 6 > (modify-list 'a 'x '(p a d g c a)) (P X D G C X) <-- GOOD!

CL-USER 17 : 6 > (modify-list 'a 'x '(p a d (g a) c a)) (P X D (G A) C X) <----NOT GOOD!

can anyone help me make this function work on nested lists?

6502 · Accepted Answer · 2011-10-26 13:34:49Z

3

Why not working at an higher level? It would make the code simpler...

(defun modify (old new x)
  (cond
    ((eq x old) new)
    ((listp x)
     (mapcar (lambda (y) (modify old new y)) x))
    (t x)))

Basically instead of assuming x must be a list (actually a tree) you just return new if x is old, recursively map if it's a list or otherwise return x unchanged...

With this approach also (modify 'a 'x 'a) --> X (and that IMO seems right).

answered Oct 26, 2011 at 13:34

6502

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1 Comment

user1013905 Over a year ago

this is why I love stack overflow...always helpful THANKS

Óscar López · Accepted Answer · 2011-10-26 04:22:15Z

0

Here's an idea:

(defun modify-list (old new a-list)
  (cond ((null a-list) nil)
        ((not (listp (car a-list)))
         (if (eql (car a-list) old)
             (cons new (modify-list old new (cdr a-list)))
             (cons (car a-list) (modify-list old new (cdr a-list)))))
        (T (cons (modify-list old new (car a-list))
                 (modify-list old new (cdr a-list))))))

I don't have access to a LISP interpreter (anyone can verify the above procedure, please?), so you'll have to test it first!

edited Oct 26, 2011 at 4:22

answered Oct 26, 2011 at 4:10

Óscar López

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