解析的列表和產生的該

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))) 

此代碼將產生錯誤長度的結構：預計1個參數，給定2：（列表1 6）2

我真正體會每解釋，即可以給我得到在該方案中，東西...

非常感謝您

Answer 1

你的括號是錯誤的（見下文）

(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")) 

)  

你需要調用length功能與一體，=函數有兩個參數：

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

同樣對於其他標記線

。

編輯當你解析家庭作業的列表，可以考慮：

• 當你解析的homework-entries所有元素？也就是說，您何時必須停止遞歸？ （null?）錯誤說明了一切：輸入列表已用盡。
• 根據您的示例將parse-homework應用於項目列表的預期結果是什麼？你實際上並沒有產生有意義的結果。

嘗試將問題分解成更小的部分：

(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. 
    ...) 

Answer 2

;; 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)))  

現在我得到錯誤：第一：預計類型非空列表的說法;鑑於空

代碼HAST是完全錯誤的，我認爲，但在這一刻，我沒有其他的解決方案...

希望你能給我一些提示！

謝謝