Add implementation of exercise 3-74

Author: ramsayleung

README.org

@@ -25,7 +25,7 @@
   习题完成情况:
   - 章节一: 43/46
   - 章节二: 88/97
-  - 章节三: 67/82
+  - 章节三: 68/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行
@@ -144,18 +144,22 @@
      #+end_src
 
      这里的蒙特卡罗实现真的是优雅
-*** 翻译错误
-    第2版，238页，模块化，对象和状态，中文翻译是
-    #+begin_quote
-    练习3.70
-    b) 所有正正数序对 (i,j) 的流，其中要求 i<=j, 而且这里的i或者j可以被2,3或者5整除
-    #+end_quote
-
-    原文是:
-    #+begin_quote
-    the stream of all pairs of positive integers ( i , j ) with i ≤ j , where neither i nor j is divisible by 2, 3, or 5,
-    #+end_quote
-
-    https://sarabander.github.io/sicp/html/3_002e5.xhtml#g_t3_002e5
+** 翻译错误
+   第2版，238页，模块化，对象和状态，中文翻译是
+   #+begin_quote
+   练习3.70
+   b) 所有正正数序对 (i,j) 的流，其中要求 i<=j, 而且这里的i或者j可以被2,3或者5整除
+   #+end_quote
+
+   原文是:
+   #+begin_quote
+   the stream of all pairs of positive integers ( i , j ) with i ≤ j , where neither i nor j is divisible by 2, 3, or 5,
+   #+end_quote
+
+   https://sarabander.github.io/sicp/html/3_002e5.xhtml#g_t3_002e5
 
-    i和j不可以被2,3,5整除，翻译成了可以被2,3或者5整除，意思完全相反，真的是服了。
+   i和j不可以被2,3,5整除，翻译成了可以被2,3或者5整除，意思完全相反，真的是服了。
+** 排版错误
+   第2版，240页，模块化，对象和状态
+
+   练习3.74 输入信号流和过零点信号流完全没有对齐

chapter3/exercise3-74.rkt

@@ -0,0 +1,33 @@
+#lang racket
+(require "stream.rkt")
+(require "exercise3-50.rkt")
+
+(define (sign-change-detector current prev)
+  (cond ((and (< prev 0) (>= current 0)) 1)
+        ((and (>= prev 0) (< current 0)) -1)
+        (else 0)))
+
+(define (make-zero-crossings input-stream last-value)
+  (if (stream-null? input-stream)
+      the-empty-stream
+      (cons-stream
+       (sign-change-detector (stream-car input-stream) last-value)
+       (make-zero-crossings (stream-cdr input-stream)
+                            (stream-car input-stream)))))
+
+(provide sign-change-detector make-zero-crossings)
+(define sense-data (list-to-stream '(1 2 1.5 1 0.5 -0.1 -2 -3 -2 -0.5 0.2 3 4)))
+
+(define zero-crossings (map-stream sign-change-detector (stream-cdr sense-data) sense-data))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for make-zero-crossings"
+             (define zero-crossings (make-zero-crossings sense-data 0))
+             (check-equal? (stream-take-n zero-crossings 13) '(0 0 0 0 0 -1 0 0 0 0 1 0 0))
+             )
+  (test-case "Test for zero-crossings"
+             (check-equal? (stream-take-n zero-crossings 11) '(0 0 0 0 -1 0 0 0 0 1 0))
+             )
+  )