0. #lang racket
1. "Exercise 50. If you copy and paste the
2. above function definition into the definitions
3. area of DrRacket and click RUN, DrRacket
4. highlights two of the three cond lines.
5. This coloring tells you that your test cases do
6. not cover the full cond itional. Add enough
7. tests to make DrRacket happy."
8.  
9. ; TrafficLight -> TrafficLight
10. ; yields the next state given current state s
11. (check-expect (traffic-light-next "red") "green")
12. (check-expect (traffic-light-next "green") "yellow")
13. (check-expect (traffic-light-next "yellow") "red")
14. (define (traffic-light-next s)
15.   (cond
16.     [(string=? "red" s) "green"]
17.     [(string=? "green" s) "yellow"]
18.     [(string=? "yellow" s) "red"]))
19.  
20. "Exercise 51. Design a big-bang program
21.  that simulates a traffic light for a
22. given duration. The program renders the state of
23. a traffic light as a solid circle of
24. the appropriate color, and it changes state
25. on every clock tick."
26.  
27. ; timer
28. (define (tock time)
29.    (cond
30.     [(string=? "red" time) "green"]
31.     [(string=? "green" time) "yellow"]
32.     [(string=? "yellow" time) "red"]))
33.  
34. ; the-end function
35. (define (the-end time)
36.   (< time 1))
37.  
38. ; width of the scene
39. (define WIDTH 100)
40.  
41. ; height of the scene
42. (define HEIGHT 100)
43.  
44. ; scene itself
45. (define traff-light-scene
46.   (empty-scene WIDTH HEIGHT))
47.  
48. ; circle radius
49. (define RADIUS 50)
50.  
51. ; traffic light
52. (define (traffic-light-render color)
53.   (place-image
54.    (circle RADIUS "solid"
55.            (cond
56.     [(string=? "red" color) "green"]
57.     [(string=? "green" color) "yellow"]
58.     [(string=? "yellow" color) "red"]))
59.    50 50
60.    traff-light-scene))
61.  
62. ; traffic-light-next program
63. (define (traff-light-next time)
64.   (big-bang time
65.             (on-tick tock)
66.             (to-draw traffic-light-render)
67.            ; (stop-when the-end)
68.             ))
69.  
70. ; for testing
71. (traff-light-next "red")
72. "This program works good but need stop function."

=>

"Exercise 50. If you copy and paste the \r\nabove function definition into the definitions \r\narea of DrRacket and click RUN, DrRacket \r\nhighlights two of the three cond lines. \r\nThis coloring tells you that your test cases do \r\nnot cover the full cond itional. Add enough \r\ntests to make DrRacket happy."

check-expect: undefined;

cannot reference an identifier before its definition

in module: 'm

check-expect: undefined;

cannot reference an identifier before its definition

in module: 'm

check-expect: undefined;

cannot reference an identifier before its definition

in module: 'm

"Exercise 51. Design a big-bang program\r\n that simulates a traffic light for a \r\ngiven duration. The program renders the state of \r\na traffic light as a solid circle of \r\nthe appropriate color, and it changes state \r\non every clock tick."

empty-scene: undefined;

cannot reference an identifier before its definition

in module: 'm

big-bang: undefined;

cannot reference an identifier before its definition

in module: 'm

"This program works good but need stop function."