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SICP练习题 1.7~1.8

h2. 1.7 mysqrt的改进 改善精度控制的细节, 判断如果连续两次迭代的猜测值相差很小, 就退出. 实验表明达到效果了, 大数和小数都得到了很大改善. 美中不足是我还不会用变量, get-guess-result方法被重复调用了. 
(define (mysqrt x) 

 	(define default-guess 1.0)
 	(define min-dx 0.000000001)

	(define (get-guess-result guess )
	 		(* guess guess))

	(define (improve guess )
		(/ (+ guess (/ x guess)) 2))

	(define (goodguess? guess old-gruess)
			(< (abs(- (/ (get-guess-result guess)
					 		old-gruess )
					1))
			 	min-dx))

	(define (sqrt-itor guess old-gruess)
			(if (goodguess? guess old-gruess)
				guess
				(sqrt-itor (improve guess )
				 	(get-guess-result guess ))))

	(sqrt-itor default-guess x))
h2. 1.8 求立方根 求立方根实际上只需要替换improve函数和get-guess-result函数的实现, 其他代码相同. 我为了共享代码中的相同部分, 先重构了上面的代码, 做了一些抽象: 
(define (gen-entire-surd-fun n )
 	;entire2/entire3

 	(define default-guess 1.0)
 	(define min-dx 0.000000001)

	(define get-guess-result
		(cond ((= n 2)
			(lambda (guess )
					(* guess guess)))
	 	((= n 3)
			(lambda (guess )
					(* guess guess guess )))))

	(define improve
  		(cond ((= n 2)
			(lambda (guess x)
				(/ (+ guess (/ x guess)) 2)))
	 	((= n 3)
			(lambda (guess x)
				(/ (+ 	(/ x (* guess guess))
						(* 2 guess)
					)
				   3)))))

	(define (goodguess? guess old-gruess)
			(< (abs(- (/ (get-guess-result guess)
					 		old-gruess )
					1))
			 	min-dx))

	(define (entire-itor x guess old-gruess)
			(if (goodguess? guess old-gruess)
				guess
				(entire-itor x (improve guess x )
				 	(get-guess-result guess ))))

	(lambda (x) (entire-itor x default-guess x)))

(define my-entire2 (gen-entire-surd-fun 2 ))
(define my-entire3 (gen-entire-surd-fun 3 ))

(write "hello, world, myentire sicp.1.8")
题外话, 熟悉了Lisp的写法, 感觉还是不错的, 简洁明了. 使用缩进能基本消除括号的烦恼. 进一步想, 如果直接使用缩进来控制代码结构, 那就变成Haskell和Python了. :D:D 但这样的风格似乎减弱了*表达式*和*求值*的思想.
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