解析列表并生成该列表的结构

Question

;; structure representing homework points
;; nr: number - the number of the homework
;; points: number - the number of points reached
(define-struct homework (nr points))



;; parse-homework: (list of number pairs) -> (list of homework)
;; The procedure takes a list of number pairs and produces a list of homework     structures
;; Example: (parse-homework (list (list 1 6) (list 2 7) (list 3 0))) should produce  (list (make-homework 1 6) (make-homework 2 7) (make-homework 3 0)) 

(define (parse-homework homework-entries) 
  (if (and (= (length (first homework-entries) 2))(= (length (parse-homework (rest homework-entries)) 2)))
    (make-homework (first homework-entries) (parse-homework (rest homework-entries)))
    (error 'Non-valid-input "entered list is not of length two"))

  )        


(parse-homework (list (list 1 6) (list 2 7) (list 3 0))) 

这段代码产生错误 length: Expects 1 argument, 鉴于 2: (list 1 6) 2

我真的很感激你能给我的每一个解释，让我参与这个方案......

谢谢非常

Answer 1

您的括号是错误的（见下文）

(define (parse-homework homework-entries) 
   (if (and (= (length (first homework-entries) 2))  ;; <---- Parens wrong here
            (= (length (parse-homework (rest homework-entries)) 2)))  ;; <---- ... and here 
    (make-homework (first homework-entries) (parse-homework (rest homework-entries)))
    (error 'Non-valid-input "entered list is not of length two"))

)      

您需要使用一个参数调用 length 函数，使用两个参数调用 = 函数：

(= (length (first homework-entries)) 2)

对于另一条标记行也是如此。

编辑 当您解析家庭作业列表时，请考虑：

• 您何时解析了 homework-entries 的所有元素？即，什么时候必须停止递归？ (null?) 错误说明了一切：输入列表已用尽。
• 根据您的示例，将 parse-homework 应用于项目列表的预期结果是什么？您实际上并没有产生有意义的结果。

尝试将问题分解为更小的部分：

(define (parse-homework-item item)
    ;; Parse a single homework item, returning an instance of the 
    ;; Homework type for it. 
    ...)

(define (parse-homework list-of-items)
    ;; Loop over the elements of list-of-items, processing each in turn
    ;; using parse-homework-item. Collect each result object into a list
    ;; which is returned as final result.
    ...)