SICP 1.3.3 二分法作为方程的根

这次所犯的错误是abs函数括号闭合错了，不懂得怎么调Bug，有点烦。

(define (search f neg-point pos-point)
(let ((mid-point (average neg-point pos-point)))
(if (close-enough? neg-point pos-point)
mid-point
(let ((test-value (f mid-point)))
(cond ((positive? test-value) (search f neg-point mid-point))
((negative? test-value) (search f mid-point pos-point))
(else mid-point))))))

(define (close-enough? x y)
(< (abs (- x y)) 0.001))

(define (average x y)
(/ (+ x y) 2))

(define (half-interval-method f a b)
(let ((a-value (f a))
(b-value (f b)))
(cond ((and (negative? a-value) (positive? b-value))
(search f a b))
((and (negative? b-value) (positive? a-value))
(search f b a))
(else
(error "Values are not of opposite sign" a b)))))

(newline)
(display (half-interval-method sin 2.0 4.0))

(newline)
(display (half-interval-method (lambda (x) (- (* x x x) (* 2 x) 3))
1.0
2.0))

(newline)
(display (half-interval-method sin 2.0 3.0))