[sicp] 1.17, 1.18

#lang scheme
(define (even? n)
  (= (remainder n 2) 0))

(define (double a )
   (double-iter a 2 0))

(define (double-iter a counter product)
  (if (= counter 0)
      product
      (double-iter a (- counter 1) (+ a product))))

(define (mod a div)
  (- (/ a div) (/ 1 div)))

;; 1.18
(define (fast-multi-iter a counter product result )
  (cond ((= counter 1) (+ product  result ) )
        (( even? counter) (fast-multi-iter a (/ counter 2)  (double product) result ))
        (else (fast-multi-iter a (mod counter 2) (double product) (+ result product ) ) )))
 
(define (* a b)
  (fast-multi-iter a b a 0 )
  )

 

 

 

;; 1.17
(define (fast-multi b n)
  (cond ((= n 0) 0)
        ((even? n) (double (fast-multi b (/ n 2))))
        (else (+ b (fast-multi b (- n 1))))))

 

ps. 1.18 을 답을 보니 나만 틀렸네 ㅡㅡ

잘못된 방법으로 풀었음 .

역시 난 ㅠㅠ

영어 어려워서 책 질렀음