sicp-ex-3.74



<< Previous exercise (3.73) | Index | Next exercise (3.75) >>


meteorgan

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

karthikk

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))) 
  

meteorgan's solution complies with the original zero-crossings in the book. The first two arguments to sign-change-detector should be: (stream-car sense-data) and 0



xdavidliu

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.