On Mon, Jun 8, 2009 at 3:59 AM, Mark Polesky
<address@hidden> wrote:
my version:
(define-public (split-at-predicate predicate lst)
"Split LST (into 2 lists) at the first element that returns #f for
(PREDICATE previous_element element), and return the 2 new lists as a
pair. Example: (split-at-predicate < '(1 2 3 2 1)) ==> ((1 2 3) 2 1)"
(if (< (length lst) 2)
(cons lst '())
(let loop ((L0 (list (car lst))) (L1 (cdr lst)))
(cond ((null? L1) (cons L0 L1))
((predicate (car (last-pair L0))
last-pair is an O(n) operation, since it has to traverse the whole list, making split-at-predicate O(n^2) instead of O(n), as it should be. You'd be better off building L0 backwards and reversing it at the end, so that the element you need is always at the beginning of a list.
Joe