1

我无法打印出所有可能的路径。目前我只能打印出一个路径，如果 (path-demo "J" "I")，程序会显示这个错误 mcdr: expects argument of type <mutable-pair>; 给定#f

 (define net
  '(("A" "B")
    ("B" "A" "C")
    ("C" "B" "D")
    ("D" "C" "E" "F")
    ("F" "I" "D")
    ("I" "F")
    ("E" "D" "J")
    ("J" "E" "G")
    ("G" "J" "H"))) 

 (define (path-demo start finish)
         (for-each (lambda (x) (display x) (display " "))
                   (cons "Route:" (shortest-path start finish net))))

 (define (shortest-path start end net)
    (bfs end (list (list start)) net))

 ;; Breadth-first search
  (define (bfs end queue net)
    (display queue) (newline) (newline) ; entertainment
    (if (null? queue)
      '()
      (let ((path (car queue)))
       (let ((node (car path)))
         (if (equal? node end) ;; Graham used CL eql
             (reverse path)
             (bfs end 
                  (append (cdr queue)
                          (new-paths path node net))
                  net))))))

 (define (new-paths path node net)
   (map (lambda (n) (cons n path)) (cdr (assoc node net))))

;;
(path-demo "J" "I")
4

2 回答 2

1

在您对 net 的定义中，您忘记列出 H 所连接的节点。

发生错误时 node 和 net 具有以下值：

node: H 
net: ((A B) (B A C) (C B D) (D C E F) (F I D) (I F) (E D J) 
      (J E G) (G J H)))

因此 

(assoc node net))

将返回 #f 因为 H 在 net.h 中没有关联。这导致来自 cdr 的错误：

cdr: expects argument of type <pair>; given #f
于 2011-10-22T12:12:20.000 回答
0

很可能会返回以下内容#f：

(cdr (assoc node net))

关于评论（用于格式化）：

(define (new-paths path node net)
   (write node)
   (newline)
   (map (lambda (n) (cons n path)) (cdr (assoc node net))))
于 2011-10-18T09:31:13.800 回答