Add implementation of exercise 3-54

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: 53/82
+  - 章节三: 54/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-54.rkt

@@ -0,0 +1,27 @@
+#lang racket
+(require "stream.rkt")
+(require "exercise3-50.rkt")
+(require "infinite-stream.rkt")
+
+(define (mul-stream s1 s2)
+  (map-stream * s1 s2))
+
+(define factorials (cons-stream 1 (mul-stream (stream-cdr integers) factorials)))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for mul-stream"
+             (define s1 (list-to-stream '(2 3 4 5)))
+             (define s2 (list-to-stream '(3 4 5 6)))
+             (define s3 (mul-stream s1 s2))
+             (check-equal? (stream-ref s3 2) 20)
+             (check-equal? (stream-to-list s3 4) '(6 12 20 30))
+             )
+  (test-case "Test for factorials"
+             (check-equal? (stream-to-list factorials 5) '(1 2 6 24 120))
+             ;; n = 0, n+1=1 的阶乘是1
+             ;; n = 1, n+1=2 的阶乘是2
+             ;; n = 2, n+1=3 的阶乘是6
+             )
+  )

chapter3/infinite-stream.rkt

@@ -17,9 +17,17 @@
              (not (divisible? x (stream-car stream))))
            (stream-cdr stream)))))
 
+(define ones (cons-stream 1 ones))
+(define integers (cons-stream 1 (add-streams ones integers)))
+(define fibs (cons-stream 0
+                          (cons-stream 1
+                                       (add-streams (stream-cdr fibs)
+                                                    fibs))))
 (define (add-streams s1 s2)
   (map-stream + s1 s2))
 
+(provide add-streams ones integers fibs)
+
 (module+ test
   (require rackunit)
 
@@ -35,12 +43,6 @@
              )
 
   (test-case "Test for infinite stream"
-             (define ones (cons-stream 1 ones))
-             (define integers (cons-stream 1 (add-streams ones integers)))
-             (define fibs (cons-stream 0
-                                       (cons-stream 1
-                                                    (add-streams (stream-cdr fibs)
-                                                                 fibs))))
              (check-equal? (stream-ref fibs 6) 8)
              )
   )