►zthomae’s SICP solutions
▼2 Chapter 2
2.1 Exercise 2.1
2.2 Exercise 2.2
2.3 Exercise 2.3
2.4 Exercise 2.4
2.5 Exercise 2.5
2.6 Exercise 2.6
2.7 Exercise 2.7
2.8 Exercise 2.8
2.9 Exercise 2.9
2.10 Exercise 2.10
2.11 Exercise 2.11
2.12 Exercise 2.12
2.13 Exercise 2.13
2.14 Exercise 2.14
2.15 Exercise 2.15
2.16 Exercise 2.16
2.17 Exercise 2.17
2.18 Exercise 2.18
2.19 Exercise 2.19
2.20 Exercise 2.20
2.21 Exercise 2.21
2.22 Exercise 2.22
2.23 Exercise 2.23
2.24 Exercise 2.24
2.25 Exercise 2.25
2.26 Exercise 2.26
2.27 Exercise 2.27
2.28 Exercise 2.28
2.29 Exercise 2.29
2.30 Exercise 2.30
2.31 Exercise 2.31
2.32 Exercise 2.32
2.33 Exercise 2.33
2.34 Exercise 2.34
2.35 Exercise 2.35
2.36 Exercise 2.36
2.37 Exercise 2.37
2.38 Exercise 2.38
2.39 Exercise 2.39
2.40 Exercise 2.40
2.41 Exercise 2.41
2.42 Exercise 2.42
2.43 Exercise 2.43
2.44 Exercise 2.44
2.45 Exercise 2.45
2.46 Exercise 2.46
2.47 Exercise 2.47
2.48 Exercise 2.48
2.49 Exercise 2.49
2.50 Exercise 2.50
2.51 Exercise 2.51
2.52 Exercise 2.52
2.53 Exercise 2.53
2.54 Exercise 2.54
2.55 Exercise 2.55
2.56 Exercise 2.56
2.57 Exercise 2.57
2.58 Exercise 2.58
2.59 Exercise 2.59
2.60 Exercise 2.60
2.61 Exercise 2.61
2.62 Exercise 2.62
2.63 Exercise 2.63
2.64 Exercise 2.64
2.65 Exercise 2.65
2.66 Exercise 2.66
2.67 Exercise 2.67
2.68 Exercise 2.68
2.69 Exercise 2.69
2.70 Exercise 2.70
2.71 Exercise 2.71
2.72 Exercise 2.72
2.73 Exercise 2.73
2.74 Exercise 2.74
2.75 Exercise 2.75
2.76 Exercise 2.76
2.77 Exercise 2.77
2.78 Exercise 2.78
2.79 Exercise 2.79
2.80 Exercise 2.80
2.81 Exercise 2.81
2.82 Exercise 2.82
2.83 Exercise 2.83
2.84 Exercise 2.84
2.85 Exercise 2.85
2.86 Exercise 2.86
2.87 Exercise 2.87
2.88 Exercise 2.88
2.89 Exercise 2.89
2.90 Exercise 2.90
2.91 Exercise 2.91
2.92 Exercise 2.92
2.93 Exercise 2.93
2.94 Exercise 2.94
2.95 Exercise 2.95
2.96 Exercise 2.96
2.97 Exercise 2.97
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2.49 Exercise 2.49

These procedures are victims to creeping indentation due to the nested lets, but we don’t have the tools to do better at the moment.

As a note: The painter considers the upper bound of 1.0 to not be inside the image – that is, points at an X or Y coordinate of 1.0 will not be drawn. To get the semantics we want, we define an upper-bound that can be drawn:

(define upper-bound 0.995)

Since we’re reusing the four corner points, we’ll define them separately:
(define bottom-left (make-vect 0.0 0.0))
(define bottom-right (make-vect upper-bound 0.0))
(define top-left (make-vect 0.0 upper-bound))
(define top-right (make-vect upper-bound upper-bound))

Next, an outline painter:

(define outline-painter
  (let ((bottom (make-segment bottom-left bottom-right))
        (left (make-segment bottom-left top-left))
        (right (make-segment bottom-right top-right))
        (top (make-segment top-left top-right)))
    (segments->painter
     (list bottom left right top))))

Next, an "X" painter:

(define x-painter
  (segments->painter
   (list
    (make-segment bottom-left top-right)
    (make-segment top-left bottom-right))))

These two procedures could be generalized to be applications of a procedure creating painters working on the corners of a frame.

A painter for a diamond whose corners are the midpoints of the frame (using hardcoded midpoints because we are working on the unit frame):

(define diamond-painter
  (let ((bottom (make-vect 0.5 0.0))
        (left (make-vect 0.0 0.5))
        (right (make-vect upper-bound 0.5))
        (top (make-vect 0.5 upper-bound)))
    (let ((bottom-left (make-segment bottom left))
          (left-top (make-segment left top))
          (top-right (make-segment top right))
          (right-bottom (make-segment right bottom)))
      (segments->painter
       (list bottom-left left-top top-right right-bottom)))))

Defining wave is simply a matter of supplying segments->painter with a more complex list of segments. I don’t think the amount to be learned here is worth the work it takes to find these edges based on the images in the book.

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