novice question about CONS

Hello,

I just started lisp, and i have a question I cant resolve

how can I flatten a list like this ((1 2 3) (4 5 6) (7 8 9)) into a list ( 1 2 3 4 5 6 7 8 9) ?

cons '((1 2 3) (4 5 6) ( 7 8 9)) does not work, i guess since it takes only one argument in this case.

thanks

Use recursion. Check out IF or COND and ATOM.

Go over all the elements in the list.
If an element is a list, do here as described to it and append to result.(Here you recurse; you call the function itself.)
If an element is not a list just append that to the result too. (But in this case you append a single element.)

If you happen to append a tree to a list, you won't flatten all the lists together, you'll get this:
(append '(1 2 (2) ) '(1 2))
(1 2 (2) 1 2)

So you need to use 'double recursion', something like this:

(defun oct (tr)
  (if (atom tr)
      (cons tr nil)
    (append (oct (car tr))
          (oct (cdr tr)))))

This may be of help if you haven't seen it:

CL-USER> (concatenate 'list (list 1 2 3)(list 4 5 6)(list 7 8 9))
(1 2 3 4 5 6 7 8 9)

This was also discussed in another thread.

viewtopic.php?f=2&t=268&start=0