SICP 2.5 习题答案

通过数学的方法可以解决，假设乘积n不是偶数，那a肯定是0。

(define (power x n r)
  (if (< n 1) r
      (power x (- n 1) (* r x))))
(define (cons a b)
  (* (power 2 a 1) (power 3 b 1)))
(define (car n)
  (define (car-iter x counter)
    (if (not(even? x)) counter
        (car-iter (/ x 2) (+ counter 1))))
  (car-iter n 0))
(define (cdr n)
  (define (cdr-iter x counter)
    (cond ((= x 1) counter)
          ((even? x) (cdr-iter (/ x 2) counter))
          (else (cdr-iter (/ x 3) (+ counter 1)))))
  (cdr-iter n 0))