Racket博士中的语法错误

Question

The window keeps saying there is a error on the last part where it says (...(count-even-strings (rest alist)))
saying that it found on extra part but I just don't know how to fix it.

;; consumes a list of strings, list, and produces the number of strings in list that have an even length

(define (count-even-strings alist)
  (cond
    [(empty? alist) 0]
    [else (+ (cond [(even? (string-length (first alist))) true]) 1)]
[else 0]) (count-even-strings (rest alist)))

Answer 1

Using an editor that aligns properly the code, like DrRacket, you could see that your function has the following structure:

(define (count-even-string1s alist)
  (cond
    [(empty? alist) 0]
    [else (+ (cond [(even? (string-length (first alist))) true])
             1)]
    [else 0])
  (count-even-strings1 (rest alist)))

and has two else inside the same cond , which is not correct sintactically, while the last part is outside of the cond .

A correct solution with the use of if is the following:

 (define (count-even-strings alist)
  (if (empty? alist)
      0
      (+ (if (even? (string-length (first alist)))
             1
             0)
         (count-even-strings (rest alist)))))

(count-even-strings '("a" "ab" "c""))   ;; => 2

or, using a single conditional:

(define (count-even-strings alist)
  (cond ((empty? alist) 0)
        ((even? (string-length (first alist))) (+ 1 (count-even-strings (rest alist))))
        (else (count-even-strings (rest alist)))))