WEBVTT

0
00:01.630 --> 00:02.330
Welcome!

1
00:02.560 --> 00:07.960
In this section, I'm going to create a program that animates a ball on a 2D surface.

2
00:08.020 --> 00:15.680
First of all, let me show you the final program. As you can see, the ball bounces inside an invisible board.

3
00:16.520 --> 00:20.880
The invisible board is actually a two dimensional slice.

4
00:20.990 --> 00:23.650
Your goal is creating this program.

5
00:23.660 --> 00:26.690
Let me show you how it works in detail.

6
00:26.690 --> 00:32.840
I'm using a multidimensional bool slice to keep track of the ball's position on the board.

7
00:32.870 --> 00:39.980
You can use it to render a ball like so. There is no ball on the board yet because all the elements of

8
00:39.980 --> 00:42.450
the inner slices are false now.

9
00:42.560 --> 00:47.310
So when an element is false you only need to print an empty cell.

10
00:47.450 --> 00:49.440
Let's bring the ball. To do that,

11
00:49.460 --> 00:56.330
I'm going to set the first element of the first inner slice to true like so. When I do that, the program

12
00:56.600 --> 00:59.420
prints the ball at the top left corner of the ball.

13
01:00.320 --> 01:05.870
So when an element becomes true, you need to print the ball on that position.

14
01:05.990 --> 01:10.260
By the way, the first index corresponds to the X position.

15
01:10.460 --> 01:16.690
The second index corresponds to the Y position. After the board and ball is ready

16
01:16.730 --> 01:23.950
the program starts animating the ball in a loop continuously like so. By the way,

17
01:23.980 --> 01:29.370
whenever the program sets an element to true, it sets the other elements to false.

18
01:29.400 --> 01:33.390
By doing so, it keeps rendering a single ball in a single cell.

19
01:34.020 --> 01:37.390
Otherwise it would render multiple balls.

20
01:37.410 --> 01:39.770
Finally the ball hits the bottom

21
01:39.810 --> 01:40.890
edge of the board.

22
01:40.890 --> 01:45.440
So now the program needs to reverse the vertical direction of the ball

23
01:45.510 --> 01:50.880
like so. As you can see, now the program reverses the vertical direction of the ball.

24
01:51.120 --> 01:54.890
That's why the Y position changes from 3 to 2.

25
01:54.960 --> 02:00.440
The x position also keeps increasing because the ball didn't hit the horizontal edges yet.

26
02:00.450 --> 02:05.900
Now the ball hits a horizontal edge, to the right border.

27
02:05.910 --> 02:14.300
So this time it needs the bounce back horizontally. As you can see, now the program reverses the horizontal

28
02:14.300 --> 02:15.800
direction of the ball.

29
02:15.800 --> 02:19.340
That's why the X position changes from 5 to 4.

30
02:19.370 --> 02:26.970
The Y position also keeps decreasing because the ball didn't hit the vertical edges yet. But now the ball

31
02:26.970 --> 02:33.150
hits the top edge, so it needs to bounce back vertically, so the program reverses the vertical direction

32
02:33.150 --> 02:33.740
of the ball.

33
02:33.750 --> 02:37.640
Now the Y position starts to decrease and it goes like this.

34
02:37.650 --> 02:43.050
So here is the overview of my algorithm. Your algorithm can be different.

35
02:43.050 --> 02:47.120
Of course, you are free to create the same program using different logic.

36
02:48.010 --> 02:53.320
OK first I create a two-dimensional bool slice for the board.

37
02:53.370 --> 02:56.940
After that I clear the screen using the screen package.

38
02:56.940 --> 03:02.410
If you don't know how to use it please watch the previous retro led clock project again.

39
03:02.460 --> 03:06.090
After that I create a loop for the animation. In the loop,

40
03:06.120 --> 03:09.110
first, I calculate the next position of the ball.

41
03:09.120 --> 03:14.940
After that, I create a buffer, and I draw the characters that I'm going to print into that buffer.

42
03:14.940 --> 03:21.690
It is because I cannot print a bool slice but I can print a rune slice by converting it to a string.

43
03:21.690 --> 03:25.030
Now I need to print the board and the ball. To do that,

44
03:25.110 --> 03:29.000
first, I move the cursor to the top left corner of the screen.

45
03:29.040 --> 03:32.250
I do so by using the screen package again.

46
03:32.250 --> 03:38.970
After that, I actually print the buffer by converting the rune buffer to a string value like so.

47
03:39.000 --> 03:43.670
And lastly, I wait before drawing the next step of the animation.

48
03:43.680 --> 03:46.920
This loop continues as long as I want.

49
03:47.040 --> 03:52.370
By the way, can find more information about this challenge in the next lecture. By the way,

50
03:52.380 --> 03:56.890
if you have a Twitter account, don't forget to tweet about your progress.

51
03:56.970 --> 04:02.760
You can do so after you complete your solution or when you start working on it. Just send it to my Twitter

52
04:02.790 --> 04:04.700
using my account name.

53
04:04.740 --> 04:06.770
You can even upload it to your own github.

54
04:07.110 --> 04:07.570
Okay.

55
04:07.590 --> 04:08.910
See you in the next lecture.

56
04:08.910 --> 04:09.460
Good luck.

57
04:09.510 --> 04:09.720
Bye!