【SICP练习】79 练习2.51



练习2.51

通过书中前面的内容，我们知道below中来将框架分成上下两部分，而beside将框架分成左右两部分。因此，below定义如下：

(define (below painter1 painter2)

(let ((split-point (make-vect 0.0 0.5)))

(let ((paint-top (transform-painter painter2

split-point

(make-vect 1.0 0.5)

(make-vect 0.0 1.0)))

(paint-down (transform-painterpainter1

(make-vect 0.0 0.0)

(make-vect 1.0 0.0)

split-point)))

(lambda (frame)

(paint-top frame)

(paint-down frame)))))

另一种利用上一题中的rotate270等可以很容易的实现：

(define (below painter1 painter2)

(lambda (frame)

((flip-horiz

(rotate90

(beside

(rotate270

(flip-horizpainter1))

(rotate270

(flip-horizpainter2)))))

frame)))