学而不思则罔

(defn lazy-foldr [f coll]

    (lazy-seq

        (if-let [[x & xs] coll]

            (cons x (lazy-foldr f xs)))))

详见Google Group
上...

引用笑来的一段话，完全说出了去年底看《思考的技术》的感受：

大约在1997年前后，二十五六岁的我，突然有一天发现自己竟然缺乏足够的逻辑思维能力，于是只好跑到图书馆里找答案。从“ thinking”这个词入手开始检索，发现有很多书的名字都以“critical thinking”为关键字——那之前我都不知道还有这样一个词组（可见我有多么孤陋寡闻）。于是，在一大堆名字里有“critical thi...

先是一样,(magnitude z)出错, 因为显然magnitude不能处理complex, DrScheme给的错是:
    magnitude: expects argument of type <number>; given (complex rectangular 3 . 4)
z是通过
    (define z (make-complex-from-real-imag 3 4))...

Exercise 2.30. Define a procedure square-tree analogous to the square-list procedure of
exercise 2.21. That is, square-list should behave as follows:
(square-tree
(list 1
(list 2 (list 3 4) 5)
(list 6 7)))
(1 (4 (9 16) 25)...

#lang scheme

; the solution for the SICP 2.33

(define (square x)
  (* x x))

(define (accumulate op initial seq)
  (if (null? seq)
      initial
   &n...

#lang scheme

; the solution for the SICP 2.21

(define (square x)
  (* x x))

(define (square-list items)
  (if (null? items)
      '()
     ...

2.17 返回一个list的最后元素的表

#lang scheme

(define (last-pair l)
  (if (= (length l) 1)
      l
      (last-pair (cdr l))))

(define l (list 1 2 3 4))
...

#lang scheme

; the solution for SICP 2.6

(define zero (lambda (f) (lambda (x) x)))
             
(define (add-1 n)
  (lambda (f) (lambda (x) (f ...

#lang scheme

;the solution for the SICP 2.1

(define (make-rat n d)
  (let ((g (gcd n d)))
    (cond ((and (> n 0) (< d 0)) (cons (/ (* -1 n) g) (/ (* -1 d) g)))
    &nbs...

1.40

#lang scheme

(define tolerance 0.0001)

(define (fixed-point f first-guess)
  (define (close-enough? v1 v2)
    (< (abs (- v1 v2)) tolerance))
  (define (try guess)
...

#lang scheme

; the solution for SICP 1.35

(define tolerance 0.000001)

(define (fixed-point f first-guess)
  (define (close-enough? v1 v2)
    (< (abs (- v1 v2)) tolerance))
  (...

1.32 accumulate的递归和迭代解
#lang scheme
; the solution for the SICP 1.32

(define (accumulate combiner null-value term a next b)
  (if (> a b)
      null-value
    &nb...

Google前两天发布了go语言

techcrunch也以Google’s Go: A New Programming Language That’s Python Meets C++报道,不过在我看来题目应该改成Google's Go: A New Progogramming Language That's Stackless Python Meets C, 原因如下:

1) Go语言的比较特别的地方是concurrenc...

1.29 simpson求定积分

#lang scheme

(define (cube x)
  (* x x x))

(define (sum term a next b)
  (if (> a b)
      0
      (+ (term a)
&...

1.25 使用fast-expt计算expmod的问题

(define (expmod base exp m)
  (cond ((= exp 0) 1)
        ((even? exp)
         (remainder (square (expmod base (/ e...