WEBVTT

0
00:00.810 --> 00:02.080
Welcome back.

1
00:02.100 --> 00:06.450
You've learned what happens when a method receives a value.

2
00:06.450 --> 00:09.780
But what happens when it receives a pointer instead.

3
00:13.200 --> 00:17.840
First of all I'm going to reorganize this code a little bit more.

4
00:18.000 --> 00:25.080
I'm going to group and put each variable on a single line so I can show you more code without scrolling

5
00:25.200 --> 00:30.570
up and down OK better before talking about pointer receivers.

6
00:30.750 --> 00:32.780
Let's take a look at an example.

7
00:32.850 --> 00:40.350
As you can see you can run methods using a value for example you can print a book value using the print

8
00:40.350 --> 00:41.740
method here.

9
00:41.790 --> 00:46.770
Go automatically passes the book value to the print method.

10
00:46.770 --> 00:52.900
There is one more way to run a method since a method belongs to a type.

11
00:52.920 --> 00:55.830
You can also run it using its type.

12
00:55.830 --> 01:00.470
Remember game is not a value but a type. Voila!

13
01:00.600 --> 01:02.970
The print method is here.

14
01:02.970 --> 01:04.640
Let's run it.

15
01:04.690 --> 01:05.460
Hold on.

16
01:05.470 --> 01:12.870
It doesn't work because now the print method doesn't know which game value that it should use.

17
01:12.880 --> 01:21.280
When you run a method to using a type you need to send a value of that type to the method as the first

18
01:21.370 --> 01:22.250
argument.

19
01:22.300 --> 01:28.840
For example I can say use minecraft or I can say use Tetris.

20
01:28.840 --> 01:33.360
Let's run the print method through the book type with a book value.

21
01:33.490 --> 01:36.690
I'm going to say use Moby Dick.

22
01:36.760 --> 01:42.010
You run a method through a type it is called a method expression.

23
01:42.010 --> 01:45.220
So these are called Method expressions.

24
01:45.400 --> 01:49.320
Anyway let's try it it works.

25
01:49.390 --> 01:58.060
But why does it work? To understand that you need to learn how a method really works behind the scenes

26
01:58.370 --> 02:02.770
and method exactly looks and works like a function.

27
02:02.770 --> 02:10.680
For example the print method looks like this as you can see receivers are not magical entities.

28
02:10.990 --> 02:18.100
So you can run the print method on a book value like so you don't usually see this happening but behind

29
02:18.100 --> 02:20.580
the scenes go automatically passes

30
02:20.590 --> 02:28.480
Moby Dick variable to the method as the first argument or you can also manually sent the receiver value

31
02:28.570 --> 02:36.250
using the book type as you can see in this case you need to provide the book value as an argument by

32
02:36.250 --> 02:37.190
yourself.

33
02:37.330 --> 02:44.530
Therefore in both cases the printing method receives a copy of Moby Dick.

34
02:44.530 --> 02:52.840
You can run a method either way but we usually run it using a value I explained you this because

35
02:52.990 --> 03:01.870
I want you to deeply understand how a method really works in summary a method is a function that takes

36
03:01.870 --> 03:04.360
a receiver as the first argument.

37
03:04.480 --> 03:11.440
Technically there is only one difference between a method and a function. A method belongs to a type.

38
03:11.950 --> 03:20.570
However a function belongs to a package now that you understand what a method really is.

39
03:20.580 --> 03:22.280
I can remove this code.

40
03:22.880 --> 03:26.690
OK now we can talk about pointer receivers.

41
03:26.970 --> 03:31.500
Let's say you want to make a discount on game prices.

42
03:31.650 --> 03:35.680
To do that I'm going to add a discount method to the game type.

43
03:35.760 --> 03:44.520
It takes a discount ratio let's discount the game price like so as you can see I can use the game value

44
03:44.580 --> 03:48.560
inside the method because I've got it as a receiver.

45
03:48.570 --> 03:53.030
OK let's go to the main function and use the discount method there.

46
03:53.280 --> 03:57.910
Here I'm going to discount the Minecraft by 50 percent.

47
03:57.990 --> 03:59.850
Let's run it.

48
04:00.030 --> 04:01.850
Let's check out the Minecraft.

49
04:01.860 --> 04:04.170
The price hasn't changed.

50
04:04.170 --> 04:07.370
It is still 20 dollars not 10 dollars.

51
04:07.380 --> 04:09.240
Let's see what's going on.

52
04:09.270 --> 04:13.520
Let's go back to the game type here.

53
04:13.550 --> 04:18.740
We assume that we can access to the minecraft variable through the receiver.

54
04:19.280 --> 04:27.930
So let's print it and understand what's going on let's run it after 50 percent discount the game price

55
04:27.960 --> 04:29.720
becomes 10 dollars.

56
04:29.760 --> 04:33.340
However the discount is only applied here.

57
04:33.420 --> 04:39.560
It's because the receiver is a copy of the original Minecraft variable.

58
04:39.560 --> 04:42.130
Let me remove the debugging code from here.

59
04:45.560 --> 04:48.900
This is the discount method we wrote earlier.

60
04:48.950 --> 04:57.890
It has a value receiver let's run it using the Minecraft the Minecraft becomes a receiver in the discount

61
04:57.890 --> 04:59.050
method.

62
04:59.060 --> 05:01.280
The problem is that the variable G.

63
05:01.280 --> 05:02.870
Here is a copy.

64
05:03.470 --> 05:12.320
So changing the price of the copy won't change the price of the original Minecraft variable here is

65
05:12.320 --> 05:13.100
a quick question.

66
05:14.470 --> 05:21.870
What would you do if you want to change an input parameter inside a function you would use a pointer.

67
05:21.870 --> 05:22.710
Right?

68
05:22.770 --> 05:25.570
You learned that a method is just a function.

69
05:25.650 --> 05:31.490
Therefore I can fix the problem by changing the receiver type to a pointer.

70
05:31.650 --> 05:34.440
Let's rerun the discount method.

71
05:34.890 --> 05:42.870
Now the receiver is a pointer to the original Minecraft so the receiver variable G becomes a pointer

72
05:42.900 --> 05:51.970
to Minecraft changing the price field of the receiver will change the original Minecraft variable as

73
05:51.970 --> 05:53.860
well.

74
05:54.080 --> 06:03.880
Okay let's start by changing the discount method to a pointer receiver as you can see G is a pointer

75
06:04.000 --> 06:06.110
to a game value now.

76
06:06.250 --> 06:10.930
Okay let's try it nice now it works.

77
06:12.900 --> 06:20.140
But I didn't send a pointer to the discount method. So, how could it change the original Minecraft?

78
06:20.220 --> 06:22.920
How could it receive a pointer?

79
06:22.920 --> 06:24.260
Let me explain.

80
06:24.270 --> 06:31.950
Here Go sees that the discount method has a pointer receiver so it automatically takes the address

81
06:32.010 --> 06:36.930
of the Minecraft variable like so let me run it.

82
06:36.930 --> 06:44.700
So this is what happens behind the scenes when a method has a pointer receiver Go takes its address

83
06:44.790 --> 06:46.330
automatically.

84
06:46.350 --> 06:47.510
Let me take it back.

85
06:48.400 --> 06:51.350
Let me tell you one more thing here.

86
06:51.580 --> 06:59.310
When one of the methods takes a pointer a receiver it is better to convert all the methods to pointer

87
06:59.320 --> 07:00.610
receivers.

88
07:00.610 --> 07:05.030
It doesn't matter whether they need to change the original value or not.

89
07:05.190 --> 07:08.290
You will see why this is necessary to do so.

90
07:08.290 --> 07:15.250
When I talk about interfaces later in this section. So I'm going to change the print method to a pointer

91
07:15.250 --> 07:16.390
receiver as well.

92
07:19.900 --> 07:21.100
for the book type.

93
07:21.130 --> 07:27.520
You don't need to do anything else it doesn't change the book value so the print method doesn't have

94
07:27.520 --> 07:30.040
to be a pointer receiver.

95
07:30.040 --> 07:33.730
There is one more reason for using a pointer receiver.

96
07:33.760 --> 07:42.390
You learned that a method with a value receiver always copy is the receiver when it executes let me show

97
07:42.390 --> 07:46.320
you what happens when you receive a large value.

98
07:46.470 --> 07:48.690
First I'm going to create a new file

99
07:51.800 --> 07:54.760
and I'm going to create a new struct here.

100
07:55.070 --> 08:06.180
It will hold an array with 10 million games this data roughly corresponds to 200 megabytes let's create

101
08:06.240 --> 08:10.170
a method that gives us the address of the receiver.

102
08:10.200 --> 08:15.490
I'm just gonna print the address of the receiver.

103
08:15.530 --> 08:18.640
Let's go back to the main function here.

104
08:18.650 --> 08:25.490
I'm going to create a variable of the huge type this variable occupies around two hundred megabytes

105
08:25.490 --> 08:26.810
of memory.

106
08:26.810 --> 08:32.630
Now I'm going to make the situation more difficult by calling the addr method 10 times

107
08:36.400 --> 08:37.960
let's try it.

108
08:37.990 --> 08:45.580
I don't know whether it exists on your operating system but there is a program called the time it returns

109
08:45.580 --> 08:48.330
the total execution time of a command.

110
08:48.790 --> 08:55.880
So I'm going to run the program with the time tool. As you can see it is very slow.

111
08:55.880 --> 08:58.050
It takes seconds.

112
08:58.100 --> 09:05.060
Also note that each time I call the addr method it copies the h variable.

113
09:05.060 --> 09:11.120
Remember a method with a value receiver copies the received value.

114
09:11.120 --> 09:19.610
That's why the receiver's memory address changes 200 megabytes of data are being copied each time I call

115
09:19.610 --> 09:21.020
the method.

116
09:21.080 --> 09:25.490
Now let's change the value receiver to a pointer receiver.

117
09:25.700 --> 09:29.030
Now the age variable becomes a pointer.

118
09:29.120 --> 09:32.360
I don't have to take its address anymore.

119
09:32.390 --> 09:35.630
OK let's try it.

120
09:35.680 --> 09:36.340
Wow.

121
09:36.340 --> 09:39.340
Now it runs in a couple of milliseconds.

122
09:39.520 --> 09:43.300
It is way faster than the value receiver version.

123
09:43.300 --> 09:49.810
It's because every time it is called it only copies a tiny pointer data.

124
09:50.020 --> 09:56.430
As you can see the receiver address always points to the same memory location.

125
09:56.670 --> 10:05.100
In summary: Use a pointer receiver when you want to change the receiver variable or use a pointer receiver

126
10:05.220 --> 10:09.090
when the received value is very large.

127
10:09.090 --> 10:13.210
Picking the right receiver type confuses beginners.

128
10:13.470 --> 10:21.020
Most of the time they cannot decide whether to use a pointer receiver or a value receiver.

129
10:21.060 --> 10:27.570
Fortunately now you understand when the use a pointer receiver see you in the next lecture.