Setf(？)导致树中的循环

Question

我在Common Lisp(CLISP)中实现了一个进化算法,我遇到了一个问题.

我有一个树状的课:

(defclass node ()
  ((item :initarg :item :initform nil :accessor item)
   (children :initarg :children :initform nil :accessor children)
   (number-of-descendants :initarg :descs :initform nil :accessor descs)))

还有一些方法:

(defmethod copy-node ((n node))
  (make-instance
   'node
   :item (item n)
   :descs (descs n)
   :children (mapcar #'copy-node (children n))))

(defmethod get-subtree ((n node) nr)
 (gsth (children n) nr))
(defmethod (setf get-subtree) ((val node) (n node) nr)
  (setf (gsth (children n) nr) val))
(defmethod get-random-subtree ((n node))
  (gsth (children n) (random (descs n))))
(defmethod (setf get-random-subtree) ((val node) (n node))
  (setf (get-subtree n (random (descs n))) val))

(defun gsth (lst nr)    
  (let ((candidate (car lst)))
    (cond
      ((zerop nr) candidate)
      ((<= nr (descs candidate)) (gsth (children candidate) (1- nr)))
      (t (gsth (cdr lst) (- nr (descs candidate) 1))))))

(defun (setf gsth) (val lst nr)    
  (let ((candidate (car lst)))
    (cond
      ((zerop nr) (setf (car lst) val))
      ((<= nr (descs candidate))
       (setf (gsth (children candidate) (1- nr)) val))
      (t (setf (gsth (cdr lst) (- nr (descs candidate) 1)) val)))
    val))

我要做的是从群体中交换两个随机树的两个随机子树.但是,当我做这样的事情时:

(defun stdx (population)
  (let ((n (length population))
        (npop))
    (do ((done 0 (+ done 2)))
        ((>= done n) npop)
      (push (stdx2 (copy-node (random-el population))
                   (copy-node (random-el population)))
            npop))))

(defun stdx2 (father mother)
  ;; swap subtrees
  (rotatef (get-random-subtree father)
           (get-random-subtree mother))
  (check-for-cycles father)
  (check-for-cycles mother))

有时检测到一个循环,显然不应该发生.

检查循环是否正常,我也检测到循环(跟踪).我一直在更新后代数.

我想(setf get-subtree)有问题.我是LISP的新手,我对setf扩展不太满意.请帮我.

Answer 1

想想如何实现这个:

;; swap subtrees
(rotatef (get-random-subtree father)
         (get-random-subtree mother))

该rotatef表格将按照以下方式进行宏观扩展:

(let ((a (get-subtree father (random (descs father))))
      (b (get-subtree mother (random (descs mother)))))
  (setf (get-subtree father (random (descs father))) b)
  (setf (get-subtree mother (random (descs mother))) a))

(您可以使用macroexpand以确切了解您的扩展情况.)

换句话说,随机子树将被选择两次(一次在读取时和一次在更新时),因此不是相互交换子树,而是将子树的引用复制到另一棵树中的随机位置.

例如,在下图中,算法可能会选择要交换的蓝色和红色子树.但是当它附加它们时,它会将它们放在标有点的点上.

图的下半部分显示了将子树附加到新点之后的结果数据结构:您可以看到已创建一个循环.

因此,您需要修改代码,以便您只需选择一次随机子树.也许是这样的事情:

(let ((a (random (descs father)))
      (b (random (descs mother))))
  (rotatef (get-subtree father a)
           (get-subtree mother b)))