Add implementation of exercise 3-33

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: 32/82
+  - 章节三: 33/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-33.rkt

@@ -0,0 +1,46 @@
+#lang racket
+(require "constraints.rkt")
+;;; c = (a + b) * 0.5
+
+(define (averager a b c)
+  (let ((sum (make-connector))
+        (half (make-connector))
+        )
+    (constant 0.5 half)
+    (adder a b sum)
+    (multiplier sum half c)
+    'ok))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for averager with a and b are set"
+             (define a (make-connector))
+             (define b (make-connector))
+             (define c (make-connector))
+             (averager a b c)
+             (probe "value a " a)
+             (probe "value b " b)
+             (probe "value c " c)
+             (check-false (has-value? c))
+             (set-value! a 2 'user)
+             (set-value! b 4 'user)
+             (check-equal? (get-value c) 3.0)
+             (check-exn (regexp "Contradiction") (lambda () (set-value! a 212 'user)))
+             )
+
+  (test-case "Test for averager with c and b are set"
+             (define a (make-connector))
+             (define b (make-connector))
+             (define c (make-connector))
+             (averager a b c)
+             (probe "value a " a)
+             (probe "value b " b)
+             (probe "value c " c)
+             (check-false (has-value? c))
+             (set-value! b 4 'user)
+             (set-value! c 0 'user)
+             (check-true (has-value? a))
+             (check-equal? (get-value a) -4)
+             )
+  )