1
00:00:00,000 --> 00:00:03,080
>> SPEAKER 1: It turns out we can
successfully swap the values in two

2
00:00:03,080 --> 00:00:07,670
variables by passing them into a
function not by value or by copy, but

3
00:00:07,670 --> 00:00:10,390
by reference or by their addresses.

4
00:00:10,390 --> 00:00:12,740
In other words, we need to leverage
something known as a pointer.

5
00:00:12,740 --> 00:00:15,580
A pointer, really, is just the
address of some variable.

6
00:00:15,580 --> 00:00:19,660
And so if we provide a function called,
say, swap with the address of

7
00:00:19,660 --> 00:00:23,550
a variable and the address of another
variable, swap should be empowered to

8
00:00:23,550 --> 00:00:26,090
go to each of those addresses
and actually change the

9
00:00:26,090 --> 00:00:27,360
values that are there.

10
00:00:27,360 --> 00:00:28,890
>> Let's see this in context.

11
00:00:28,890 --> 00:00:31,360
Let's reimplement swap as follows.

12
00:00:31,360 --> 00:00:35,810
First, let's change a not to be an int
but to be a pointer to an int or the

13
00:00:35,810 --> 00:00:36,920
address of an int.

14
00:00:36,920 --> 00:00:40,820
Then let's do the same for b, changing
it from an int to be a pointer to an

15
00:00:40,820 --> 00:00:42,780
int or the address of an int.

16
00:00:42,780 --> 00:00:45,860
>> Then inside of swap, let's still
declare tmp so that we have a

17
00:00:45,860 --> 00:00:47,810
temporary place for a's value.

18
00:00:47,810 --> 00:00:52,430
But a's value is not a itself, because,
again, a is now the address

19
00:00:52,430 --> 00:00:53,270
of some int.

20
00:00:53,270 --> 00:00:57,320
So if we want to go to that address and
get int at that address, we have

21
00:00:57,320 --> 00:01:03,020
dereference this pointer, also by way of
the star operator, writing star a.

22
00:01:03,020 --> 00:01:05,470
>> Next, I don't want to change
the value of a.

23
00:01:05,470 --> 00:01:08,770
I want to change the value at
a, keeping in mind, again,

24
00:01:08,770 --> 00:01:10,350
that a is an address.

25
00:01:10,350 --> 00:01:14,050
So to do so, I again need
to say star a gets.

26
00:01:14,050 --> 00:01:18,360
And now I want to put in the value
that's at b, not the value of b, which

27
00:01:18,360 --> 00:01:19,720
also is an address.

28
00:01:19,720 --> 00:01:22,280
>> So again I say, star b.

29
00:01:22,280 --> 00:01:26,690
Then in my last line, I need to
overwrite what is at address b with

30
00:01:26,690 --> 00:01:28,970
what was at a's original location.

31
00:01:28,970 --> 00:01:32,910
To do that, I do star b gets tmp.

32
00:01:32,910 --> 00:01:34,820
>> Now at the end of the day, this
function is still just

33
00:01:34,820 --> 00:01:35,950
three lines of code.

34
00:01:35,950 --> 00:01:39,860
But because it's manipulating values
by way of their address and not the

35
00:01:39,860 --> 00:01:43,700
raw values that were passed into the
function, I claim that swap is now

36
00:01:43,700 --> 00:01:47,670
empowered to change the values that
are passed in via their addresses.

37
00:01:47,670 --> 00:01:49,510
>> But I need to make one change still.

38
00:01:49,510 --> 00:01:52,190
I can no longer pass in
x and y themselves.

39
00:01:52,190 --> 00:01:55,030
I need to pass in the addresses
of x and y.

40
00:01:55,030 --> 00:01:58,160
And to do that, I need some slightly
different notation up top.

41
00:01:58,160 --> 00:02:02,510
I want to swap x and y by passing in
the address of x, indicated by

42
00:02:02,510 --> 00:02:07,190
ampersand x, and the address of
y, indicated by ampersand y.

43
00:02:07,190 --> 00:02:10,570
>> Similarly, up top now do I need to
change the function's prototype to

44
00:02:10,570 --> 00:02:14,980
match the change that I've made, so that
a is, again, a pointer to an int.

45
00:02:14,980 --> 00:02:17,190
b is, again, a pointer to an int.

46
00:02:17,190 --> 00:02:18,770
And now I can save my file.

47
00:02:18,770 --> 00:02:20,680
And let's recompile and run it.

48
00:02:20,680 --> 00:02:25,330
>> Make swap dot slash swap.

49
00:02:25,330 --> 00:02:29,660
And this time, x and y are indeed now
swapped such that their values are not

50
00:02:29,660 --> 00:02:31,950
1 and 2, but 2 and 1.

51
00:02:31,950 --> 00:02:34,900