SICP 习题解答 1.11

Exercise 1.11.  A function f is defined by the rule that f(n) = n if n<3 and f(n) = f(n - 1) + 2f(n - 2) + 3f(n - 3) if n> 3. Write a procedure that computes f by means of a recursive process. Write a procedure that computes f by means of an iterative process.

my solution:

recursive process

(define (f n)  (if (< n 3)    n    (+ (f (- n 1)) (* 2 (f (- n 2))) (* 3 (f (- n 3))))))

iterative process

#lang sicp
(define (f n)  (define (f-iter s m l i)    (if (= i n)        l        (f-iter m l (+ l (* 2 m) (* 3 s)) (+ i 1))))  (if (< n 3)      n      (f-iter 0 1 2 2)))

多写了一对括号，检查半天才查出来，真要命。

error message 是准确的，为什么不盯着 error message 想呢，就算不能马上想起来，也比乱找好。

every mistake is a lesson and every lesson will make you better.