魔术八球？在方案中

Question

就像 Magic-8-ball 游戏一样，我试图建立一个模型，可以按照以下顺序回答不同的问题：

• “这是肯定的”
• “前景良好”
• “稍后再问”
• “前景不太好”。

如果问题是空的，则生成“你没有问问题，请重试”，用（无答案）表示

我写下了这个问题的代码，但空的情况不起作用。

也就是说，(magic-8-ball "") 应该生成“您没有问问题，请重试”。

;; 8-ball-answers: (listof string)

(define 8-ball-answers (list "It is certain" "Outlook good" "Ask again later" "Outlook not so good"))

;; no-answer: string
;; Purpose: correct form of string to produce when magic-8-ball consumes empty string

(define no-answer "you did not ask a question, try again")

;;magic-8-ball: string -> string
;;Purpose: consumes a string and produces a string
;;Effects: modifies (8-ball-answers). If the string is empty, produces
;;         "you did not ask a question, try again". Otherwise, changes 
;;         "It is certain" to "Outlook good",
;;         "Outlook is good" to "Ask again later",
;;         "Ask again later" to "Outlook not so good",
;;         and "Outlook not so good" to "It is certain".

(define (magic-8-ball s)
  (local 
    [
     ;; new-list represents the new value of the list 8-ball-answers
     ;;          after every time the function is called.
     (define next-answer (first 8-ball-answers))]
    (begin
      (cond [(equal? s "") no-answer]
            [else (set! 8-ball-answers (append (rest 8-ball-answers)
                                               (list next-answer)))])
      next-answer)))

这是我的测试用例：

(check-expect (and (equal? (magic-8-ball "Do you love me?")
                       "It is certain")
               (equal? (magic-8-ball "How is your life?")
                       "Outlook good")
               (equal? (magic-8-ball "") 
                       "you did not ask a question, try again")
               (equal? (magic-8-ball "2nd0A-wmQ232.asdA?") 
                       "Ask again later")
               (equal? (magic-8-ball "No questions here")
                       "Outlook not so good")
               (equal? (magic-8-ball "Now do you hate me?")
                       "It is certain"))
               true))

测试应该通过，但我的没有。当字符串为空时就会出现问题。

谁能告诉我问题是什么？ 谢谢！

Answer 1

很好，你修好了空支票。现在我可以运行代码了:)

空支票并不是您唯一的问题。查看以下代码块并告诉我您的期望：

(begin
  (cond [(equal? s "") no-answer]
        [else (set! 8-ball-answers (append (rest 8-ball-answers)
                                           (list next-answer)))])
  next-answer)

我期望 next-answer，与条件结果无关。我想您可能希望您的 else 条件在循环答案后返回 next-answer。问题是你的开始。您测试一个条件，然后返回next-answer。也许您想要类似的东西：

(cond [(equal? s "") no-answer]
      [else (begin (set! 8-ball-answers (append (rest 8-ball-answers)
                                                (list next-answer)))
                   next-answer)])

如果字符串为空，则返回no-answer，否则返回循环的next-answer。虽然可能是正确的，但我认为这段代码相当丑陋。考虑重构循环并返回下一个答案位。我的结构很糟糕。