sicp-ex-3.74

  
  
  
 (define zero-crossings 
         (stream-map sign-change-detector sense-data (cons-stream 0 sense-data))) 

I don't think that is right. If the first element of the sense-data stream were negative above solution will show a sign-change crossing immediately when there is none.

  
 (define (zero-crossings sense-data) 
   (stream-map sign-change-detector sense-data (stream-cdr sense-data))) 
  

I agree with hoonji that meteorgan's solution correctly answers the problem. However I would like to point out that the last-value parameter is completely superfluous when the next value in the stream is always easily accessible. In this manner, we can rewrite make-zero-crossings elegantly to require only a single argument:

 (define (make-zero-crossings input-stream) 
   (stream-map sign-change-detector (stream-cdr sense-data) sense-data)) 

karthikk's solution addresses this issue too, but as pointed out already, that is not what the book is asking for literally in this exercise. In my opinion this seems like a rare example of lapse of judgment on the part of the authors.

 (define zero-crossings 
     (stream-map 
         sign-change-detector 
         sense-data 
         (cons-stream 
             (stream-car sense-data) 
             sense-data))) 
  

 ;; helpers 
 (define (sign-change-detector old new) 
       (cond ((or (positive? old) (zero? old)) 
              (if (negative? new) -1 0)) 
             ((negative? old) 
              (if (negative? new) 0 1)))) 
  
 (define (random-in-range low high)  
    (let ((range (- high low)))  
      (+ low (* (random) range))))  
  
 (define (random-stream low high) 
   (cons-stream (random-in-range low high) 
                (random-stream low high))) 
  
 (define sense-data (random-stream -10 10)) 
  
 (define (make-zero-crossings input-stream last-value) 
   (cons-stream 
     (sign-change-detector 
       (stream-car input-stream) 
       last-value) 
     (make-zero-crossings 
       (stream-cdr input-stream) 
       (stream-car input-stream)))) 
  
 (define zero-crossings-alysa 
   (make-zero-crossings sense-data 0)) 
  
  
 (define zero-crossings-eva 
   (stream-map sign-change-detector 
               sense-data 
               (stream-cdr sense-data))) 
  
 (display "eva:") 
 (newline) 
 (stream-ref zero-crossings-eva 0) 
 (stream-ref zero-crossings-eva 1) 
 (stream-ref zero-crossings-eva 2) 
 (stream-ref zero-crossings-eva 3) 
 (stream-ref zero-crossings-eva 4) 
 (stream-ref zero-crossings-eva 5) 
 (stream-ref zero-crossings-eva 6) 
 (stream-ref zero-crossings-eva 7) 
 (stream-ref zero-crossings-eva 8) 
 (stream-ref zero-crossings-eva 9) 
 (stream-ref zero-crossings-eva 10) 
  
 (display "alyssa:") 
 (newline) 
 (stream-ref zero-crossings-eva 0) 
 (stream-ref zero-crossings-eva 1) 
 (stream-ref zero-crossings-eva 2) 
 (stream-ref zero-crossings-eva 3) 
 (stream-ref zero-crossings-eva 4) 
 (stream-ref zero-crossings-eva 5) 
 (stream-ref zero-crossings-eva 6) 
 (stream-ref zero-crossings-eva 7) 
 (stream-ref zero-crossings-eva 8) 
 (stream-ref zero-crossings-eva 9) 
 (stream-ref zero-crossings-eva 10)