方案（DrRacket） - 将两个高阶函数与Lambda结合定义

仅供参考我正在使用Scheme使用DrRacket进行编程。方案（DrRacket） - 将两个高阶函数与Lambda结合定义

我试图代码，消耗了城市名称和航班的列表，并返回城市（串）的列表定义daily-to到航空公司有从给定的城市每日航班。

从我的理解这个定义需要同时使用filter和map，与lambda一起，因为这是一个问题，关于higher order functions和lambda。虽然我似乎没有正确地做到这一点。

希望我能得到我的daily-to定义的帮助，因为它运行不正确。

这里是我的程序：

(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)))

预先感谢您！

来源

2014-12-07 BBladem83

你的问题是什么？ – 2014-12-07 08:36:22

希望我可以在我的'daily-to'定义获得帮助，因为它运行不正确。 @WillNess – BBladem83 2014-12-07 08:46:51

波士顿没有每日航班*，波士顿每日航班*。 – uselpa 2014-12-07 09:27:30

这里有一个daily-to的定义，这似乎是有道理的我的头顶。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)))

来源

2014-12-07 09:27:21

方案（DrRacket） - 将两个高阶函数与Lambda结合定义

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