Gregory Rickeard <s9905...@sms.ed.ac.uk> writes:
> (defun crossover (tree &optional (subst nil) (prob-no-recursion .75))
>   "args: tree &optional (subst nil) (prob-no-recursion .75)

> finds a random subtree (not including the entire tree) of tree and returns
> it, possibly
> after replacing it with subst if subst is non nil.."
>   (if (< (random 1.0) (expt prob-no-recursion (1- (max-depth tree))))
>       ;; swap with a sub tree
>       (let ((args (cdr tree))
>        (rnd (random 2))
>        (tmp nil))
>    (if (null subst)
>        (cond ((= rnd 0) (car args))
>              (t (second args)))
>      (cond ((= rnd 0) (setf tmp (car args))
>                       (rplaca args subst)
>                       tmp)
>            (t (setf tmp (second args))
>               (rplacd args (list subst))
>               tmp))))
>     ;; swap with another sub-tree
>     (let* ((dl (max-depth (second tree)))
>       (dr (max-depth (third tree)))
>       (rnd (random (+ dl dr))))
>       (if (< rnd dl)
>      (crossover (second tree) subst)
>    (crossover (third tree) subst)))))