Exercise 3.70

Read Exercise 3.70 ~ Solution 70a

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First the merge procedure with weighting

(define (merge-weighted s1 s2 weight)
  (cond ((stream-null? s1) s2)
        ((stream-null? s2) s1)
        (else
         (let ((s1car (stream-car s1))
               (s2car (stream-car s2)))
           (if (<= (weight s1car) (weight s2car))
               (cons-stream s1car 
                            (merge-weighted (stream-cdr s1) 
                                            s2 
                                            weight))
               (cons-stream s2car 
                            (merge-weighted s1 
                                            (stream-cdr s2) 
                                            weight)))))))

(define (weighted-pairs s t weight) 
  (cons-stream
   (list (stream-car s) (stream-car t))
   (merge-weighted
    (stream-map (lambda (x) (list (stream-car s) x))
                (stream-cdr t))
    (weighted-pairs (stream-cdr s) (stream-cdr t) weight)
    weight)))

a

(weighted-pairs integers
                  integers 
                  (lambda (x) 
                    (apply + x))))

b

Solution 70b

(define (factored-weight x)
  (let ((i (car x))
        (j (cadr x)))
    (+ (* 2 i) 
       (* 3 j) 
       (* 5 i j))))

(define (unfactored? x)
  (not (or (even? x)
           (zero? (remainder x 3))
           (zero? (remainder x 5)))))

(define unfactored
  (stream-filter unfactored? integers))

(weighted-pairs unfactored
                unfactored
                factored-weight))

I’m sure there’s a fizz-buzz interview question in here somewhere.