<div>On 7/30/07, Zeng Fucen <<a href="mailto:zengfucen@gmail.com">zengfucen@gmail.com</a>> wrote:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> I'm a scheme novice, I want to travel a binary tree, how to ? (depth first) Thanks for your tips , first! I define a tree like this:  (1 (2 4 5) (3 6 7)) _________________________________________________   For list-related administrative tasks:   <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </blockquote></div> The syntax for defining a list of any sort is either: (list ...) '(...) <- What's inside will NOT be evaluated. Hans gave a nice example: "(1 (2 (4 () ()) (5 () ())) (3 (6 () ()) (7 () ()))" You don't *need* it, but it makes more sense that way. (define (travel-tree l)    (cond ((null? l) ())             ((list? l) (travel-tree (car l)))             (t (travel-tree (cdr l)))) The above method _should_ work, but I can't guarantee it because I have my own problems. I assume that it'll be tail-recursive, considering that it will not evaluate the rest of the condition if it succeeds. All in all, I hope this helps.