►zthomae’s SICP solutions
▼3 Chapter 3
3.1 Exercise 3.1
3.2 Exercise 3.2
3.3 Exercise 3.3
3.4 Exercise 3.4
3.5 Exercise 3.5
3.6 Exercise 3.6
3.7 Exercise 3.7
3.8 Exercise 3.8
3.9 Exercise 3.9
3.10 Exercise 3.10
3.11 Exercise 3.11
3.12 Exercise 3.12
3.13 Exercise 3.13
3.14 Exercise 3.14
3.15 Exercise 3.15
3.16 Exercise 3.16
3.17 Exercise 3.17
3.18 Exercise 3.18
3.19 Exercise 3.19
3.20 Exercise 3.20
3.21 Exercise 3.21
3.22 Exercise 3.22
3.23 Exercise 3.23
3.24 Exercise 3.24
3.25 Exercise 3.25
3.26 Exercise 3.26
3.27 Exercise 3.27
3.28 Exercise 3.28
3.29 Exercise 3.29
3.30 Exercise 3.30
3.31 Exercise 3.31
3.32 Exercise 3.32
3.33 Exercise 3.33
3.34 Exercise 3.34
3.35 Exercise 3.35
3.36 Exercise 3.36
3.37 Exercise 3.37
3.38 Exercise 3.38
3.39 Exercise 3.39
3.40 Exercise 3.40
3.41 Exercise 3.41
3.42 Exercise 3.42
3.43 Exercise 3.43
3.44 Exercise 3.44
3.45 Exercise 3.45
3.46 Exercise 3.46
3.47 Exercise 3.47
3.48 Exercise 3.48
3.49 Exercise 3.49
3.50 Exercise 3.50
3.51 Exercise 3.51
3.52 Exercise 3.52
3.53 Exercise 3.53
3.54 Exercise 3.54
3.55 Exercise 3.55
3.56 Exercise 3.56
3.57 Exercise 3.57
3.58 Exercise 3.58
3.59 Exercise 3.59
3.60 Exercise 3.60
3.61 Exercise 3.61
3.62 Exercise 3.62
3.63 Exercise 3.63
3.64 Exercise 3.64
3.65 Exercise 3.65
3.66 Exercise 3.66
3.67 Exercise 3.67
3.68 Exercise 3.68
3.69 Exercise 3.69
3.70 Exercise 3.70
3.71 Exercise 3.71
3.72 Exercise 3.72
3.73 Exercise 3.73
3.74 Exercise 3.74
3.75 Exercise 3.75
3.76 Exercise 3.76
3.77 Exercise 3.77
3.78 Exercise 3.78
3.79 Exercise 3.79
3.80 Exercise 3.80
3.81 Exercise 3.81
3.82 Exercise 3.82
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3.72 Exercise 3.72

(define (find-three-consecutive-by f g s)
  (if (stream-null? s)
      the-empty-stream
      (let ((first (stream-car s))
            (rest (stream-cdr s)))
        (if (stream-null? rest)
            the-empty-stream
            (let ((second (stream-car rest))
                  (rrest (stream-cdr rest)))
              (if (stream-null? rrest)
                  the-empty-stream
                  (let ((third (stream-car rrest)))
                    (if (= (f first) (f second) (f third))
                        (cons-stream (cons (f first) (map g (list first second third)))
                                     (find-three-consecutive-by
                                      f
                                      g
                                      (drop-while (lambda (x) (= (f x) (f first)))
                                                  (stream-cdr rrest))))
                        (find-three-consecutive-by f g rest)))))))))
(define (weight-squared p)
  (+ (square (car p)) (square (cadr p))))
(define squares-three-ways
  (find-three-consecutive-by
   car
   cadr
   (stream-map (lambda (p) (list (weight-squared p) p))
               (weighted-pairs weight-squared integers integers))))
> (display-stream (take-stream squares-three-ways 10))

(325 (10 15) (6 17) (1 18))

(425 (13 16) (8 19) (5 20))

(650 (17 19) (11 23) (5 25))

(725 (14 23) (10 25) (7 26))

(845 (19 22) (13 26) (2 29))

(850 (15 25) (11 27) (3 29))

(925 (21 22) (14 27) (5 30))

(1025 (20 25) (8 31) (1 32))

(1105 (23 24) (12 31) (9 32))

(1250 (25 25) (17 31) (5 35))

'done

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