问题描述
我要在树形图t
中插入一个键key
和一个值val
,以返回一个新树,该树的节点在适当的位置包含键和值.
I am inserting a key key
and a value val
into the tree-map t
returning a new tree with a node containing the key and the value in the appropriate location.
我有这个定义:
(define (tree-node key value left right)(list key value left right))
(define (get-key tn) (node_key tn))
(define (get-val tn) (node_val tn))
(define (get-left tn) (node_left tn))
(define (get-right tn) (node_right tn))
(define (node_key tn) (list-ref tn 0))
(define (node_val tn) (list-ref tn 1))
(define (node_left tn) (list-ref tn 2))
(define (node_right tn) (list-ref tn 3))
我正在尝试这种insert
方法
(define (insert t key val)
(cond (empty? t))
(tree-node (key val '() '()))
((equal key (get-key t))
(tree-node (key val (get-left t) (get-right t)))
((<key (get-key t))
(tree-node ((get-key t) (get-val t) (insert (get-left t key val)) (get-right t)))
(tree-node ((get-key t) (get-val t) (get-left t) (insert (get-right t key val)))))))
我正在使用它来定义
(define right (insert (insert empty 3 10) 5 20))
我在DrRacket中遇到此错误
and I'm getting this error in DrRacket
application: not a procedure;
expected a procedure that can be applied to arguments
given: (list 0 1 2)
arguments...: [none]
推荐答案
正在发生的事情是,在期望功能的位置返回了一个树节点(仅是一个列表). Scheme无法执行该列表-它不是一个函数,因此很抱怨.
What is happening is that a tree-node (which is just a list) is being returned in a position where a function is expected. Scheme cannot execute the list - it's not a function, so it's complaining.
我怀疑这是因为您弄乱了括号.
I suspect this is because you have messed up your parentheses.
(cond (empty? t))
是一个自包含的复合表达.将对其求值,并返回t的值.之后的表达式不作为cond的一部分进行评估.
Is a self-contained compound expression. It is evaluated, returning the value of t. The expressions after that are not evaluated as part of the cond.
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