SICP 2.25~2.29习题答案
程序员文章站
2022-07-14 10:49:30
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这一节难度开始加大,有一些答案是借鉴了别人才做出来的。
;; 2.25 (define list1 (list 1 3 (list 5 7) 9)) ;(car (cdr (car (cdr (cdr list1))))) (define list2 (list (list 7))) ;(car (car list2)) (define list3 (list 1 (list 2 (list 3 (list 4 (list 5 (list 6 7))))))) ;(car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr list3)))))))))))) ;; 2.26 (define x (list 1 2 3)) (define y (list 4 5 6)) ;(append x y) ;(cons x y) ;(list x y) ;; 2.27 (define (deep-reverse items) (if (null? items) nil (append (deep-reverse (cdr items)) (cons (if (pair? (car items)) (deep-reverse (car items)) (car items)) nil)))) (define x (list (list 1 2) (list 3 4))) ;(deep-reverse x) (define (count-leaves x) (cond ((null? x) 0) ((not (pair? x)) 1) (else (+ (count-leaves (car x)) (count-leaves (cdr x)))))) ;; 2.28 (define (fringe items) (cond ((null? items) nil) ((pair? items) (append (fringe (car items)) (fringe (cdr items)))) (else (list items)))) (define x (list (list 1 2) (list 3 4))) ;(fringe x) ;(fringe (list x x)) ;; 2.29 ;(define (make-mobile left right) (list left right)) ;(define (make-branch length structure) (list length structure)) ;(define (left-branch mobile) (car mobile)) ;(define (right-branch mobile) (car (cdr mobile))) ;(define (branch-length branch) (car branch)) ;(define (branch-structure branch) (car (cdr branch))) (define (make-mobile left right) (cons left right)) (define (make-branch length structure) (cons length structure)) (define (left-branch mobile) (car mobile)) (define (right-branch mobile) (cdr mobile)) (define (branch-length branch) (car branch)) (define (branch-structure branch) (cdr branch)) (define (mobile? structure) (pair? structure)) (define (branch-weight branch) (let ((structure (branch-structure branch))) (if (mobile? structure) (total-weight structure) structure))) (define (total-weight mobile) (+ (branch-weight (left-branch mobile)) (branch-weight (right-branch mobile)))) (define (weight branch) (* (branch-length branch) (branch-weight branch))) (define (balance? mobile) (let ((left (left-branch mobile)) (right (right-branch mobile))) (and (= (weight left) (weight right)) (if (mobile? (branch-structure left)) (balance? left) #t) (if (mobile? (branch-structure right)) (balance? right) #t))))