(define letter-to-number
  (lambda (letter)
    (if (eq? letter 'A)
        4.0
        (if (eq? letter 'B+)
            3.5
            (if (eq? letter 'B)
                3.0
                0)))))

(define letter-to-number
  (lambda (letter)
    (cond [(eq? letter 'A) 4.0]
          [(eq? letter 'B+) 3.5]
          [(eq? letter 'B) 3.0]
          [else 0])))

(define letter-to-number
  (lambda (letter)
    (case letter
      [(A) 4.0]
      [(B+) 3.5]
      [(B C) 3.0]
      [else 0])))

(define vector-sum
  (lambda (u v)
    (list (+ (car u) (car v))
	  (+ (cadr u) (cadr v))
	  (+ (caddr u) (caddr v)))))

(define vector-sum
  (lambda (u v)
    (map + u v)))


(vector-sum '(1 2 3) '(4 5 6)) ; (5 7 9)

(define largest-in-list
  (lambda (lon)
    (if (null? lon)
	(errorf 'largest-in-list "empty list")
	(largest-in-non-empty (car lon) (cdr lon)))))

(define largest-in-non-empty
  (lambda (largest-so-far unseen)
    (if (null? unseen)
	largest-so-far
	(largest-in-non-empty (max largest-so-far
				   (car unseen))
			      (cdr unseen)))))