scheme3.1.1 局部状态变量 练习(2)

练习3.5 蒙特卡洛求定积分

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(define (random-in-range low high)(let ((range (- high low)))(+ low (*(random) range)))) 
(define (monte-carlo trials experiment)(define (iter trials-remaining trials-passed)(cond ((= trials-remaining 0)(/ trials-passed trials))((experiment)(iter (- trials-remaining 1) (+ trials-passed 1)))(else(iter (- trials-remaining 1) trials-passed))))(iter trials 0))
(define (estimate-integral P x1 x2 y1 y2 trials)(define (experiment)(P (random-in-range x1 x2)(random-in-range y1 y2)))(let ((rt-area (* (abs (- x2 x1))(abs (- y2 y1)))))(* rt-area (monte-carlo trials experiment))))
(define (square x) (* x x))
(define (estimate-pi trials)(define (inside-circle? x y)(< (+ (square x)(square y)) 1.0))(estimate-integral inside-circle? -1 1 -1 1 trials))

练习3.6 生成相同随机数数列

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(define (rand-update x)(let ((a 1753)(b 2026)(m 3571))(modulo (+ (* a x) b) m)))
(define random-init 1)
(define rand(let ((x random-init))(lambda (m)(cond ((eq? m 'generate)(set! x (rand-update x))x)((eq? m 'reset)(lambda (new-value)(set! x new-value)))(else (error "Unknown request -- Rand" m))))))

练习3.7 创建共享账户

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(define (incorrect-password . args)"Incorrect password")(define (make-great-account balance password)(define (withdraw amount)(if (>= balance amount)(begin (set! balance (- balance amount))balance)("余额不足!")))(define (deposit amount)(begin (set! balance (+ balance amount)) balance))(define (incorrect-password . args)(display "密码错误!"))(define (dispath try-password m)(if (eq? try-password password)(cond ((eq? m 'withdraw) withdraw)((eq? m 'deposit) deposit)(else (error "Unknown request--Make account" m)))(incorrect-password)))dispath)(define (make-joint account-name password new-password)(define (check-password account password)(number? ((account password 'withdraw) 0)))(if (check-password account-name password)(lambda (try-password m)(if (eq? try-password new-password)(account-name password m)(incorrect-password)))(incorrect-password)))

练习3.8 外置变量

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(define f(let ((initial 0))(lambda (x)(let ((old initial))(set! initial x)old))))