提问人：BTDN 提问时间：2/18/2023 最后编辑：BTDN 更新时间：2/18/2023 访问量：59
函数体的函数体，该函数使用大学的复合数据列表并生成学费最低的学校名称。BSL球拍
Function body for a function that consumes a compound data list of universities and produces the name of school with the lowest tuition. BSL RACKET
问：
大家好，我在为函数编码正文时遇到了问题 使用大学的复合数据列表，并生成学费最低的大学名称。
语言是BSL Racket。
复合数据定义：
(define-struct uni (name tuition))
interp. a university name as a string and its tuition as a
natural
(define UNI-1 (make-uni "UBC" 28500))
; example of compound data definition
大学的列表数据定义：
ListOfUni is one of:
- empty
- (cons uni ListOfUni)
interp. a list of unis
(cons (make-uni "SFU" 27797) (cons (make-uni "UVIC" 26000)
empty))
功能介绍：
(lowest? lou)
This is a function that consumes list of unis and produces the
name of school with the lowest tuition
检查预期：
(check-expect (lowest? empty) "none")
(check-expect (lowest? (cons (make-uni "UBC" 28500) empty))
"UBC")
(check-expect (lowest? (cons (make-uni "SFU" 27797) (cons (make-
uni "UVIC" 26000) empty))) "UVIC")
函数体的尝试：
(define (lowest? lou)
(cond [(empty? lou) "none"]
[(empty? (rest lou)) (uni-name (first lou))]
[else (if (<(uni-tuition (first lou)) (uni-tuition (lowest? (rest lou))))
(uni-name (first lou))
(lowest? (rest lou)))]))
给出的错误消息：
1 of the 3 tests failed.
Check failures:
check-expect encountered the following error instead of the
expected value, "UVIC".
:: uni-tuition: expects an uni, given "UVIC"
我不明白如何解决这个错误，并且仍然让代码是递归的。
另外，请原谅任何格式错误，这是我在 StackOverFlow 上的第一篇文章
答：
0赞
Martin Půda
2/18/2023
#1
当你遍历大学列表时，你必须从每个步骤返回结构，因为你调用了结果：uniuni-tuition(uni-tuition (lowest? (rest lou)))
但是当只有一个元素时，你的函数会返回一个字符串。这就是错误消息的含义： ：： uni-tuition： expect an uni， given “UVIC”lou(uni-name (first lou))
可能还有更多方法可以解决这个练习，但考虑到 BSL 的限制（no 和 work only for 和 values），我不得不想出 helper 函数和“none”大学创建：letorand#true#false(make-uni "none" 0)
(define-struct uni (name tuition))
(define (lowest-helper lou)
(cond [(empty? lou) (make-uni "none" 0)]
[(empty? (rest lou)) (first lou)]
[(< (uni-tuition (first lou))
(uni-tuition (lowest-helper (rest lou))))
(first lou)]
[else
(lowest-helper (rest lou))]))
(define (lowest? lou)
(uni-name (lowest-helper lou)))
(check-expect (lowest? empty) "none")
(check-expect (lowest? (cons (make-uni "UBC" 28500) empty))
"UBC")
(check-expect (lowest? (cons (make-uni "SFU" 27797)
(cons (make-uni "UVIC" 26000) empty))) "UVIC")
或者，您可以返回 either 或 struct 并添加一个新函数来区分它们："none"uni
(define (get-name result)
(if (string? result)
result
(uni-name result)))
(define (lowest? lou)
(get-name (lowest-helper lou)))
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