Raffael Cavallaro <raffaelcavallaro@pas-d'espam-s'il-vous-plait-mac.com> wrote:
+---------------
| vippstar@gmail.com said:
| > Unfortunately, I don't know how to do step 1.
| 
| There is a very common lisp utility called flatten.
| Paul Graham gives a version in On Lisp thus:
| (defun flatten (x)
|   (labels ((rec (x acc)
| 		(cond ((null x) acc)
| 		      ((atom x) (cons x acc))
| 		      (t (rec (car x) (rec (cdr x) acc))))))
|     (rec x nil)))
+---------------

Note that this function should be more properly called FLATTEN-TREE,
as contrasted with FLATTEN-LIST, e.g.:

    (defun flatten-list (list)
      (loop for item in list
	when (consp item)
	  nconc (flatten-list item)
	else
	  collect item))

The difference appears when the data really is a "list of lists"
and some of the lists contain NIL as an member:

    > (flatten-tree '(1 2 nil (3 4 nil (5 6 nil 7) nil) nil))

    (1 2 3 4 5 6 7)
    > (flatten-lists '(1 2 nil (3 4 nil (5 6 nil 7) nil) nil))

    (1 2 NIL 3 4 NIL 5 6 NIL 7 NIL NIL)
    > 

Sometimes you want one, and sometimes you want the other.
It all depends on your particular application.


-Rob

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Rob Warnock			<rpw3@rpw3.org>
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