More Examples

Sum of a list of numbers:

(define (sum lst) (sumb lst 0))

(define (sumb lst sum)
  (if (pair? lst)
      (sumb (cdr lst)
            (+ (car lst) sum))
      sum))

Average of a list of numbers:

(define (average lst) (averageb lst 0 0))

(define (averageb lst sum n)
  (if (pair? lst)
      (averageb (cdr lst)
                (+ (car lst) sum)
                (+ 1 n))
      (/ sum n) ))

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