2.2.2.org

2.2.2 Hierarchical Structures

#lang sicp
(define x (cons (list 1 2) (list 3 4)))

(length x)
3

(count-leaves x)
;; 4

(list x x)
;; (((1 2) 3 4) ((1 2) 3 4))

(length (list x x))
;; 2

(count-leaves (list x x))
;; 8

(define (count-leaves x)
  (cond ((null? x) 0)
        ((not (pair? x)) 1)
        (else (+ (count-leaves (car x))
                 (count-leaves (cdr x))))))

(define (scale-tree tree factor)
  (cond ((null? tree) nil)
        ((not (pair? tree))
         (* tree factor))
        (else
         (cons (scale-tree (car tree)
                           factor)
               (scale-tree (cdr tree)
                           factor)))))

(scale-tree (list 1
                  (list 2 (list 3 4) 5)
                  (list 6 7))
            10)

(define (scale-tree tree factor)
  (map (lambda (sub-tree)
         (if (pair? sub-tree)
             (scale-tree sub-tree factor)
             (* sub-tree factor)))
       tree))

(define (map proc items)
  (if (null? items)
      nil
      (cons (proc (car items))
            (map proc (cdr items)))))