How to correctly return value from function?

debugcn 投稿 Dev

Bob Smith

I am trying to calculate the seats left in each class by subtracting "Enrollment" from "Capacity" and then printing the result in a separate function. However my open-seats function returns no value unless I use the display function. How can I fix this?

    #lang racket
       ( define course-list (cons '("Dept" "Number" "Section" "Class Nbr" "Capacity" "Enrollment")
                                         '(("CMSC" "201" "1" "1052" 100 30)
                                         ("CMSC" "341" "6" "7447" 40 27)
                                         ("CMSC" "341" "3" "7443" "40" 29)
                                         ("CMSC" "331" "5" "7746" 40 36)
                                         ("CMSC" "331" "6" "7747" 40 "40")
                                         ("CMSC" "471" "3" "8196" 40 31))

                                  )
            )

   (define (open-seats section)
      (for ([e (in-list  course-list)])
        (if (equal? section (string->number (list-ref e 2))) (- (list-ref e 4) (list-ref e 5)) 'something)
       ;(if (equal? section (string->number (list-ref e 2))) (display(- (list-ref e 4) (list-ref e 5))) 'something)
            )
        )
  ;test open-seats          
     (open-seats 1)

  (define (report-open-seats list-of-courses)
    (for ([e (in-list course-list)])
      (if (and (number? (list-ref e 4)) (number? (list-ref e 5))) (displayln(string-append (list-ref e 0) (list-ref e 1) " (Section " (list-ref e 2) ")=> " (open-seats 1))) newline)
            )
   )

  ; leave the following function call intact
  ;(report-open-seats course-list)

rain1

To make up a list of the items you can use for/list like this:

(define (open-seats section)
  (for/list ([e (in-list  course-list)]
             #:when (equal? section (string->number (list-ref e 2))))
    (- (list-ref e 4) (list-ref e 5))))

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