So I've been looking at flattening a list in lisp.
However, what I wanted to do is flatten a list level by level.
So instead of having
(flatten '(a (b (d (g f))) e)) = (a b d g f e)
i want
(flatten '(a (b (d (g f))) e)) = (a b (d (g f )) e )
Any idea on how to do this guys?
Much appreciated =)
You could do something like this, for example:
(defun fletten-level (tree)
(loop for e in tree
nconc
(if (consp e)
(copy-list e)
(list e))))
(fletten-level '(a (b (d (g f))) e))
;; (A B (D (G F)) E)
This loops over the original tree top-level branches and creates a new list containing if the branch was a leaf, that leaf, and if the branch had two other branches, then both the first leaf and the rest of the branches.
EDIT: sorry, it wasn't good the first time actually, now it should be fixed.
EDIT2: just because I almost got confused myself. (cons (car e) (cdr e)) looks a little weird because it is basically the same as saying just e. However, I realized that nconc will destructively modify the conses, so it has to be this way (i.e. create a new cons cell to be concatenated rather than reuse the old one).
EDIT3: Wow... actually, it has to be copy-list, because it will modify the original list this way.
Heres my quick and dirty take on a non-destructive version:
(defun flatten-level (tree)
(cond ((null tree) ())
((consp (car tree)) (concatenate 'list (car tree)
(flatten-level (cdr tree))))
(t (cons (car tree) (flatten-level (cdr tree))))))
This is a recursive function that walks the tree and for each element, if it is a cons'ed element it concatenates it onto the rest of the tree, flattened.
