1
00:00:07,910 --> 00:00:12,975
Welcome back. So, supposing we have our point class from before,

2
00:00:12,975 --> 00:00:17,140
and we have a constructor _init_ and

3
00:00:17,140 --> 00:00:23,370
_str_ to represent how we represent any instance as a string.

4
00:00:23,370 --> 00:00:25,905
Now, suppose that we have two points as well,

5
00:00:25,905 --> 00:00:31,215
point one at negative five and 10 and point two at 15 and 20.

6
00:00:31,215 --> 00:00:36,175
So, suppose that we wanted the ability to be able to add points together.

7
00:00:36,175 --> 00:00:41,695
So, for example, if we take point one plus point two,

8
00:00:41,695 --> 00:00:46,160
then we might get something like point one, two.

9
00:00:46,160 --> 00:00:50,660
We might want to add points together by adding their x-coordinates.

10
00:00:50,660 --> 00:00:55,520
So, negative five plus 15 would give us 10 and then adding our y-coordinate.

11
00:00:55,520 --> 00:00:58,860
So, 10 plus 20 would give us 30.

12
00:00:58,860 --> 00:01:00,930
So, our new point P, one,

13
00:01:00,930 --> 00:01:03,705
two would be at 10 and 30.

14
00:01:03,705 --> 00:01:08,060
So, if we write in our code, just to say,

15
00:01:08,060 --> 00:01:12,840
print out the value of P1 plus P2,

16
00:01:12,840 --> 00:01:17,405
then we're going to get an error saying that we can't add points.

17
00:01:17,405 --> 00:01:21,935
In other words, when we tell Python that we want to add these two point instances,

18
00:01:21,935 --> 00:01:25,730
then Python doesn't know how to actually take these instances and

19
00:01:25,730 --> 00:01:29,885
produce something that represents their values added together.

20
00:01:29,885 --> 00:01:33,355
But we can override a method called

21
00:01:33,355 --> 00:01:38,765
_add_ that will tell Python how to actually add two points together.

22
00:01:38,765 --> 00:01:45,960
So, if we say, def _add_.

23
00:01:45,960 --> 00:01:48,585
So, add is going to take in self,

24
00:01:48,585 --> 00:01:53,720
but it's also going to take in what we're expecting to add to self.

25
00:01:53,720 --> 00:01:56,340
So, I'll call this otherPoint.

26
00:01:58,250 --> 00:02:03,675
So, for example, when we print out the value of P1 plus P2,

27
00:02:03,675 --> 00:02:11,265
then P1 gets passed in for self and P2 gets passed in for otherPoint.

28
00:02:11,265 --> 00:02:15,140
Now, if we want to take the x coordinates and add them together and

29
00:02:15,140 --> 00:02:18,670
then take the y coordinates and add them together and return a new point,

30
00:02:18,670 --> 00:02:21,010
then what we can say is,

31
00:02:21,010 --> 00:02:28,605
we want to return a new point whose x is equal to self.x,

32
00:02:28,605 --> 00:02:38,840
plus otherPoint.x, and whose y is equal to self.y plus otherPoint.y.

33
00:02:40,090 --> 00:02:43,990
Now, as soon as we define this add method,

34
00:02:43,990 --> 00:02:47,260
then when we print out the value of point one plus point two,

35
00:02:47,260 --> 00:02:51,280
then we're going to see that we get a new point at 10 and 30.

36
00:02:51,280 --> 00:02:59,625
Again, this is just 15 plus negative five gives us 10 and 10 plus 20 gives us 30.

37
00:02:59,625 --> 00:03:03,400
Beyond add, there are several other methods that we can override.

38
00:03:03,400 --> 00:03:07,160
So, for example, there is subtraction.

39
00:03:09,620 --> 00:03:17,055
So, for example, if we wanted to be able to print out the value of P1 minus P2,

40
00:03:17,055 --> 00:03:20,795
then we could define this subtraction method.

41
00:03:20,795 --> 00:03:23,810
Suppose that we wanted this to be defined

42
00:03:23,810 --> 00:03:32,850
as self.x minus otherPoint.x and self.y minus otherPoint.y,

43
00:03:32,850 --> 00:03:38,285
then now we have the ability to quote unquote subtract two points together.

44
00:03:38,285 --> 00:03:40,835
There are many other methods that we can override,

45
00:03:40,835 --> 00:03:43,550
but they follow the same pattern as add,

46
00:03:43,550 --> 00:03:47,465
sub, and they all start and end with underscores.

47
00:03:47,465 --> 00:03:50,700
That's all for now, until next time.