两个清单作为生产清单的输入-方案

Question

For reference, I am programming with Scheme using DrRacket .

I am trying to make the function tally-by-all which consumes a list-of-candidates and list-of-votes to output a list-of-voting-tallies with the help of the helper function that is already defined, top-votes-for .

When I run it as is, an error occurs because the helper function top-votes-for consumes a string and list-of-votes so it can't exactly consume a list-of-strings in place of a string , at least I think so anyways.

The output I am supposed to get is:

(cons (make-voting-tally "Blake" 2)
(cons (make-voting-tally "Ash" 0)
(cons (make-voting-tally "Bob" 1)
(cons (make-voting-tally "Will" 0)
(cons (make-voting-tally "Joey" 0) empty))))))

Keep in mind that everything in the program is correct except for the definition of tally-by-all .

Any assistance would be greatly appreciated.

Thank you in advance!

Here is my program:

;; 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 "Bob" "Ash" "Blake"))

(define listofVotes
  (list vote1 vote2 vote3))

;; 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) 2)
(check-expect (top-votes-for "Bob" listofVotes) 1)
(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))]
    )
  )

;; 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).

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

;; Signature: tally-by-all: list-of-canidates 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.
;; Tests:
(check-expect (tally-by-all empty empty) empty)
(check-expect (tally-by-all listofCandidates listofVotes) (cons (make-voting-tally "Blake" 2)
                                                          (cons (make-voting-tally "Ash" 0)
                                                          (cons (make-voting-tally "Bob" 1)
                                                          (cons (make-voting-tally "Will" 0)
                                                          (cons (make-voting-tally "Joey" 0) 
                                                                 empty))))))
;; Define:
(define (tally-by-all aloc alov)
  (cond
    [(empty? alov) empty]
    [else (cons (make-voting-tally (first aloc) (tally-by-all alov (rest (top-votes-for aloc alov)))))]
    )
  )

Answer 1

I think what you want is to recurse over the list of candidates ( aloc ), and for each one, compute his/her tally from all of the votes:

(define (tally-by-all aloc alov)
  (cond
    [(empty? aloc) empty]
    [else (cons (make-voting-tally (first aloc) (top-votes-for (first aloc) alov))
                (tally-by-all (rest aloc) alov)))]
    )
  )