Add implementation of exercise 3-73

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: 66/82
+  - 章节三: 67/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行
@@ -158,4 +158,4 @@
 
     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，意义完全相反，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。

chapter3/exercise3-73.rkt

@@ -0,0 +1,18 @@
+#lang racket
+(require "stream.rkt")
+(require "infinite-stream.rkt")
+
+(define (RC resistor capacitor time-step)
+  (lambda (input-stream initial-voltage)
+    (add-streams (integral (scale-stream input-stream (/ 1 capacitor)) initial-voltage time-step)
+                 (scale-stream input-stream resistor))))
+(define RC1 (RC 5 1 0.5))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for resistor-capacitor(RC)"
+             (define v-result (RC1 ones 0))
+             (check-equal? (stream-take-n v-result 5) '(5 5.5 6.0 6.5 7.0))
+  )
+)

chapter3/infinite-stream.rkt

@@ -45,7 +45,14 @@
                     (stream-cdr t))
         (pairs (stream-cdr s) (stream-cdr t))))))
 
-(provide add-streams scale-stream ones integers fibs interleave pairs)
+(define (integral integrand initial-value dt)
+  (define int
+    (cons-stream initial-value
+                 (add-streams (scale-stream integrand dt)
+                              int)))
+  int)
+
+(provide add-streams scale-stream ones integers fibs interleave pairs integral)
 
 (module+ test
   (require rackunit)
@@ -76,4 +83,9 @@
              (define int-pairs (stream-take-n (pairs s1 s2) 4))
              (check-equal? int-pairs '((1 1) (1 2) (2 2) (1 3)))
              )
+
+  (test-case "Test for integral"
+             (define result (integral ones 0 0.5))
+             (check-equal? (stream-take-n result 5) '(0 0.5 1.0 1.5 2.0))
+             )
   )