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
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