SICP_3.5-3.6

  1 (#%require (only racket/base random))
  2 
  3 ;;;;;;;;;3.5
  4 (define (estimate-integral expriment x1 x2 y1 y2 trials)
  5   (let ((length (abs (- x2 x1)))
  6         (high (abs (- y2 y1))))
  7     (* (* high length) (monte-carlo trials expriment))))
  8 
  9 (define (monte-carlo trials expriment)
 10   (define (iter trials-remaining trials-passed)
 11     (cond ((= trials-remaining 0)
 12           (/ trials-passed trials))
 13           ((expriment)
 14            (iter (- trials-remaining 1) (+ trials-passed 1)))
 15           (else
 16            (iter (- trials-remaining 1) trials-passed))))
 17   (iter trials 0))
 18 
 19 ;(define p1
 20 ;  (let ((x (random-in-range -1 1))
 21 ;        (y (random-in-range -1 1)))
 22 ;    (<= (+ (square x) (square y)) 1)))
 23 (#%require (only racket/base random))
 24 
 25 ;;;;;;;;;3.5
 26 (define (estimate-integral expriment x1 x2 y1 y2 trials)
 27   (let ((length (abs (- x2 x1)))
 28         (high (abs (- y2 y1))))
 29     (* (* high length) (monte-carlo trials expriment))))
 30 
 31 (define (monte-carlo trials expriment)
 32   (define (iter trials-remaining trials-passed)
 33     (cond ((= trials-remaining 0)
 34           (/ trials-passed trials))
 35           ((expriment)
 36            (iter (- trials-remaining 1) (+ trials-passed 1)))
 37           (else
 38            (iter (- trials-remaining 1) trials-passed))))
 39   (iter trials 0))
 40 
 41 ;(define p1
 42 ;  (let ((x (random-in-range -1 1))
 43 ;        (y (random-in-range -1 1)))
 44 ;    (<= (+ (square x) (square y)) 1)))
 45 
 46 (define p
 47   (lambda ()
 48     (let ((x (random-in-range 0 2))
 49           (y (random-in-range 0 2)))
 50       (>= 1 (+ (square (- x 1)) (square (- y 1)))))))
 51 
 52 (define (random-in-range low high)
 53   (let ((range (- high low)))
 54     (+ low (random range))))
 55 
 56 (define square
 57   (lambda (x) (* x x)))
 58 
 59 (exact->inexact (estimate-integral p 0 2 0 2 1000))
 60 
 61 ;;;;;;;;3.6
 62 (define rand-init 1)
 63 
 64 (define rand-update
 65   (lambda (x)
 66     (+ x 1)))
 67 
 68 (define rand
 69   (let ((x rand-init))
 70     
 71     (define reset
 72       (lambda (value)
 73         (set!  x value)))
 74 
 75     (define generate
 76         (begin(set! x (rand-update x))
 77         x))
 78 
 79     (define (dispatch x)
 80       (cond ((eq? x 'generate) generate)
 81             ((eq? x 'reset) reset)
 82             (else (error "No such method -- rand" a))))
 83     dispatch))
 84 
 85 (define rand1 
 86    (let ((x rand-init)) 
 87      (define (dispatch message) 
 88        (cond ((eq? message 'generate) 
 89                (begin (set! x (rand-update x)) 
 90                       x)) 
 91              ((eq? message 'reset) 
 92                (lambda (new-value) (set! x new-value))))) 
 93      dispatch))
 94 
 95 (define rand2
 96   (let ((x rand-init))
 97     (define (generate)
 98       (set! x (rand-update x))
 99       x)
100     (define (reset n)
101       (set! x n))
102     (define (dispatch a)
103       (cond ((eq? 'generate a) (generate))
104             ((eq? 'reset a) reset)
105             (else (error "No such method -- rand" a))))
106     dispatch))
107 
108 
109 
110 ;(rand 'generate)
111 ;(rand 'generate)
112 ;(rand 'generate)
113 ;((rand 'reset) 3)
114 ;(rand 'generate)
115 
116 (rand2 'generate)
117 (rand2 'generate)
118 (rand2 'generate)
119 ((rand2 'reset) 3)
120 (rand2 'generate)

1.被注释掉的p 不是一个过程，而是一个值如果把rand-in-range 放后面会报错

2.也是同样的情况被注释掉的rand过程中我试图直接把generate做成一个值，但是总是得不到正确的结果，

我想可能是因为，当generate 为数值时，它的数值的取决于rand中的x，而在这里就会出现 x一直为

rand-init。如果把generate 做成一个无参过程，除了开始之外就不会依赖于rand 中的x了，从而每次都会update

自己的x了，可能和作用域有关

Yosoro