src/sicp/ex3_74.rkt

   1 #lang racket
   2
   3 (define (make-zero-crossings input-stream last-value)
   4   (cons-stream
   5    (sign-change-detector (stream-car input-stream) last-value)
   6    (make-zero-crossings (stream-cdr input-stream)
   7                         (stream-car input-stream))))
   8
   9 (define (sign-change-detector current-value previous-value)
  10   (cond [(and (positive? previous-value)
  11               (positive? current-value)) 0]
  12         [(and (positive? previous-value)
  13               (negative? current-value)) -1]
  14         [(and (negative? previous-value?)
  15               (positive? current-value)) +1]
  16         [else 0]))
  17
  18 (define zero-crossings (make-zero-crossings sense-data 0))
  19
  20 ;; eva's implementation
  21 (define zc (stream-map sign-change-detector sense-data (cons-stream 0 sense-data)))