Racket - 使用两个列表并输出字符串

Question

作为参考，我在DrRacket 中编程Racket。

我使用的“语言”是Intermediate Student with lambda。

也就是说，我应该注意这些没有使用高阶函数或 lambda。

我正在编写一个名为winner-by-all 的函数，它使用候选人列表和投票列表，并返回获胜者的姓名（得票最多的人）。

我可以让它输出最大的数字，但不是获胜者本身的名字（字符串）。

这是我的winner-by-all 函数，不正确：

;; Signature: winner-by-all: list-of-candidates list-of-votes -> string
;; Purpose: Consumes a list of candidates and a list of votes, and returns 
;;          the name of the winner by the winner-takes-all strategy.
;; Tests:
(check-expect (winner-by-all empty empty) empty)
(check-expect (winner-by-all listofCandidates listofVotes) "Blake")
;; Define:
(define (winner-by-all aloc alov)
  (cond
    [(empty? aloc) empty]
    [else (voting-tally-candidate (max (voting-tally-numVotes (first (tally-by-all listofCandidates listofVotes)))))]
    )
  )

运行时，我收到一条错误消息：

voting-tally-candidate: expects a voting-tally, given 3

所以我的函数更有意义，下面是我的代码的其他一些部分来填补空白：

tally-by:

;; Signature: tally-by: (helper function) list-of-candidates list-of-votes -> list-of-Voting-Tallies
;; Purpose: Consumes a helper function, a list of candidate names, and a list of
;;          votes and produces a list of voting-tallies.
;; Define:
(define (tally-by helper aloc alov)
  (cond
    [(empty? aloc) empty]
    [else (cons (make-voting-tally (first aloc) 
                                   (helper (first aloc) alov))
                (tally-by helper (rest aloc) alov))]
    )
  )

tally-by-all:

;; Signature: tally-by-all: list-of-candidates list-of-votes -> list-of-Voting-Tallies
;; Purpose: Consumes a list of candidate names and a list of votes and produces a
;;          list of voting-tallies.
;;          (Winner-Takes-All strategy).
;; Tests:
(check-expect (tally-by-all empty empty) empty)
(check-expect (tally-by-all listofCandidates listofVotes) (cons (make-voting-tally "Blake" 3)
                                                          (cons (make-voting-tally "Ash" 0)
                                                          (cons (make-voting-tally "Bob" 0)
                                                          (cons (make-voting-tally "Will" 0)
                                                          (cons (make-voting-tally "Joey" 0) empty))))))
;; Define:
(define (tally-by-all aloc alov)
  (tally-by top-votes-for aloc alov))

listofVotes:

;; Data Definition
(define-struct vote (choice1 choice2 choice3))
;; A vote is a structure: (make-vote String String String). 
;; interp. 3 candidates that one person has voted for (String).

(define vote1
  (make-vote "Blake" "Joey" "Will"))

(define vote2
  (make-vote "Blake" "Bob" "Ash"))

(define vote3
  (make-vote "Blake" "Ash" "Blake"))

(define listofVotes
  (list vote1 vote2 vote3))

top-votes-for:

;; Signature: top-votes-for: string list-of-strings -> number
;; Purpose: Consumes a name and a list of votes and produces the number of
;;          times that the given name was the first choice vote in the list of votes.
;;          (This tallies points under winner-takes-all strategy.)
;; Tests:
(check-expect (top-votes-for "Blake" empty) 0)
(check-expect (top-votes-for "Blake" listofVotes) 3)
(check-expect (top-votes-for "Bob" listofVotes) 0)
(check-expect (top-votes-for "Ash" listofVotes) 0)
(check-expect (top-votes-for "Joey" listofVotes) 0)
(check-expect (top-votes-for "Will" listofVotes) 0)
;; Define:
(define (top-votes-for cand alov)
  (cond
    [(empty? alov) 0]
    [(string=? (vote-choice1 (first alov)) cand) (+ 1 (top-votes-for cand (rest alov)))]
    [else (top-votes-for cand (rest alov))]
    )
  )

listofCandidates:

(define listofCandidates
  (list "Blake" "Ash" "Bob" "Will" "Joey"))

voting-tally结构：

;; Data Definition
(define-struct voting-tally (candidate numVotes))
;; A voting-tally is a structure: (make-voting-tally String Number). 
;; interp. a candidate (String) and how many votes said
;;         candidate has gotten (Number).

我希望有人能够在 winner-by-all 函数定义方面为我提供帮助，因为我不确定如何使其按预期方式工作。

Answer 1

在Intermediate student with lambda中可以使用argmax等高阶函数：

(define (winner-by-all aloc alov)
  (cond
    [(empty? aloc) empty]
    [else (voting-tally-candidate (argmax voting-tally-numVotes
                                          (tally-by-all listofCandidates listofVotes)))]
    )
  )

---

如果您确实需要在没有高阶函数和 lambda 表达式的情况下实现它，您可以使用自己的函数模拟 argmax，该函数递归迭代 tally-by-all 的所有结果并选择具有最高数量的投票计数投票。

(define (best-candidate left-candidates best)
  (if (null? left-candidates)
      best
      (let ([fst (car left-candidates)])
        (if (or (null? best)
                (> (voting-tally-numVotes fst) (voting-tally-numVotes best)))
            (best-candidate (cdr left-candidates) fst)
            (best-candidate (cdr left-candidates) best)))))

然后使用best-candidate 代替argmax 来完成这项工作：

(define (winner-by-all aloc alov)
  (cond
    [(empty? aloc) empty]
    [else (voting-tally-candidate (best-candidate (tally-by-all listofCandidates listofVotes) null))]
    )
  )