Question

作为参考，我使用Scheme编译DrRacket。

我正在尝试对使用城市名称和航班列表的定义daily-to进行编码，并返回航空公司每天从指定城市起飞的城市（字符串）列表。

根据我的理解，此定义需要同时使用filter和map以及lambda，因为这是higher order functions和lambda的问题。我似乎没有正确地做到这一点。

希望我可以获得daily-to定义的帮助，因为它运行不正确。

这是我的计划：

(define-struct flight (from to frequency))
;; Example: (make-flight "Boston" "Chicago" “daily”))
;; The frequency is one of “daily”, “weekday” or “weekend”.

(define myflights 
  (list
   (make-flight "Boston" "Chicago" "daily")
   (make-flight "New York" "Providence" "weekend")
   (make-flight "Paris" "Moscow" "weekday")))

;; Signature: service-between?: String String ListOfFlights -> Boolean
;; Purpose: consumes two city names and a list of flights and determines 
;;          whether (true or false) there is a flight from the first city to the second.
;; Tests:
(check-expect (service-between? "Boston" "Chicago" myflights) true)
(check-expect (service-between? "New York" "Providence" myflights) true)
(check-expect (service-between? "Paris" "Moscow" myflights) true)
(check-expect (service-between? "Boston" "Providence" myflights) false)
;; Definition: service-between?
(define (service-between? city1 city2 alof)
  (ormap
   (lambda (str)
   (and (string=? city1 (flight-from str))
        (string=? city2 (flight-to str)))) alof))

;; Signature: daily-to: String ListOfFlights  -> ListOfString
;; Purpose: consumes a city name and a list of flights and returns a list 
;;          of cities (strings) to which the airline has a daily flight from the given city.
;; Tests:
(check-expect (daily-to "Boston" myflights) (list "Chicago"))
(check-expect (daily-to "New York" myflights) empty)
(check-expect (daily-to "Paris" myflights) empty)
;; Definition: daily-to
(define (daily-to city alof)
  (filter (map
           (lambda (str)
             (cond
               [(and (string=? city (flight-from str)) (string=? "daily" (flight-frequency str))) (flight-to str)]
               [else empty])) alof)))

提前谢谢！

Answer 1

这里有一个daily-to的定义，似乎有点理解。它没有经过测试。

(define (daily-to city alof)
  (map flight-to
       (filter (lambda (flight)
                 (and (string=? city (flight-from flight))
                      (string=? "daily" (flight-frequency flight))))
               alof)))

Scheme（DrRacket） - 在定义中将两个高阶函数与Lambda相结合

1 个答案: