SICP q2.35

gistfile1.scm

(define nil '())

(define (accumulate op initial sequence)
  (if (null? sequence)
    initial
    (op (car sequence)
        (accumulate op initial (cdr sequence)))))

; 2.2.2
(define (count-leaves x)
  (cond ((null? x) 0)
        ((not (pair? x)) 1)
        (else (+ (count-leaves (car x))
                 (count-leaves (cdr x))))))

(define x (cons (list 1 2) (list 3 4)))
(print (count-leaves x))
(print (count-leaves (list x x)))

;----------------------------------------------------------------------------
; p.67で木の葉を数え上げる enumerate-tree を作った
; また、p.65で「cont-leaves 手続きに類似のsum-odd-squares」というヒントがあるので
; sum-odd-squaresとにたものになるはず
(define (enumerate-tree tree)
  (cond ((null? tree) nil)
        ((not (pair? tree)) (list tree))
        (else (append (enumerate-tree (car tree))
                      (enumerate-tree (cdr tree))))))

; つまり、
; enumerate-tree で木の葉一覧を作り、
; それをmapで全て1にし、
; それらを全て足せば木の葉の数が出る
(define (count-leaves t)
  (accumulate + 0 (map (lambda (x) 1) (enumerate-tree t))))

(print (count-leaves x))
(print (count-leaves (list x x)))