SICP_2.24-2.29

这一次的习题让我发现对cons和list的理解完全不是那么回事  参考资料

  1 #lang racket
  2 
  3 ;;;;;;;;;;;;;;;2.24
  4 
  5 (list 1 (list 2 (list 3 4)))
  6 
  7 ;;;;;;;;;;;;;;;2.25
  8 
  9 ;;;;;;;;;;;;（cdr （list 1 2）） 将产生list （car （list 1 2））
 10 ;;;;;;;;;;;;产生integer
 11 ;;;;;;;;;;;;(car '(1 . 2)) -> 1
 12 ;;;;;;;;;;;;(cdr '(1 . 2)) -> 2
 13 ;;;;;;;;;;;;注意序对和表的区别！！！
 14 (car (cdr (car (cdr (cdr '(1 2 (5 7) 9))))))
 15 (car (car '((7))))
 16 (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr
 17                                '(1 (2 (3 (4 (5 (6 7))))))
 18                                ))))))))))))
 19 
 20 ;;;;;;;;;;;;;;2.26
 21 (define x (list 1 2 3))
 22 (define y (list 4 5 6))
 23 
 24 (append x y)
 25 (cons x y)
 26 (list x y)
 27 ;;;;;;;;;;;;2.27
 28 (define (deep-reverse tree)
 29   (cond ((null? tree) '())
 30         ((not (pair? tree)) tree)
 31         (else (append (deep-reverse (cdr tree))
 32                       (list (deep-reverse (car tree)))))))
 33 
 34 (deep-reverse '('(3 4) '(1 2)))
 35 ;;;;;;;;;;;;;;2.28
 36 (define (fringe tree)
 37   (cond ((null? tree) '())
 38         ((not (pair? tree))  (list tree))
 39         (else (append (fringe (car tree))
 40                       (fringe (cdr tree))))))
 41 
 42 (define lis1 (list (list 1 2) (list 3 4)))
 43 
 44 ;;test
 45 (fringe lis1)
 46 (fringe (list lis1 lis1))
 47 
 48 ;;;;;;;;;;;;;;2.29
 49 (define (make-mobile left right)
 50   (list left right))
 51 
 52 (define (make-branch length structure)
 53   (list length structure))
 54 
 55 (define (left-branch mobile)
 56   (car mobile))
 57 (define (right-branch mobile)
 58   (car (cdr mobile)))
 59 
 60 (define (branch-length branch)
 61   (car branch))
 62 (define (branch-structure branch)
 63   (car (cdr branch)))
 64 ;;;;;;;;;;;;;;;;;;;;;;;;;;;思路不清晰时的写法
 65 ;(define (total-weight mobile)
 66  ; (cond ((null? mobile) 0)
 67   ;      ((not (pair? mobile)) mobile)
 68    ;     (else (+ (total-weight (branch-structure (left-branch mobile)))
 69     ;             (total-weight (branch-structure (right-branch mobile)))))))
 70 ;;;;;;;;;;;;;;;;;;;;;;;;;;;
 71 (define (total-weight2 mobile)
 72   (+ (branch-weight (left-branch mobile))
 73      (branch-weight (right-branch mobile))))
 74 
 75 (define (branch-weight branch)
 76   (if (have-mobile? branch)
 77       (total-weight2 (branch-structure branch))
 78       (branch-structure branch)))
 79 
 80 (define (have-mobile? branch)
 81   (pair? (branch-structure branch)))
 82 ;;test
 83 (define mobile (make-mobile (make-branch 10 2)
 84                             (make-branch 10 (make-mobile (make-branch 10 20)
 85                                                       (make-branch 10 20)))))
 86 
 87 (define mobile2 (make-mobile (make-branch 10 20)
 88                             (make-branch 11 20)))
 89 
 90 ;(total-weight2 mobile)
 91 ;(total-weight mobile)
 92 ;(total-weight2 mobile2)
 93 ;(total-weight mobile2)
 94 
 95 ;;;;;;;;;;torque
 96 (define (branch-torque branch)
 97   (* (branch-length branch)
 98      (branch-weight branch)))
 99 
100 (define (branch-balanced? branch)
101   (if (pair? (branch-structure branch))
102       (balanced? (branch-structure branch))
103       #t))
104 
105 (define (balanced? mobile)
106   (and (= (branch-torque (left-branch mobile))
107           (branch-torque (right-branch mobile)))
108        (branch-balanced? (left-branch mobile))
109        (branch-balanced? (right-branch mobile))))
110 ;;test
111 (define a (make-mobile (make-branch 2 2) (make-branch 2 2)))
112 (define b (make-mobile (make-branch 2 7) (make-branch 6 5)))
113 
114 (balanced? a)
115 (balanced? b)
116 
117 ;;;;;;;;;;;;;;;;;(define (make-mobile left right)
118 ;;;;;;;;;;;;;;;;;;        (cons left right))
119 ;;;;;;;;;;;;;;;;;(define (right-branch mobile)
120 ;;;;;;;;;;;;;;;;;;         (cdr branch))
121 ;;;;;;;;;;;;;;;;;;记住list和cons list总是有个nil的

 

posted @ 2017-02-14 23:56  lan126  阅读(164)  评论(0)  编辑  收藏  举报