1
00:00:00,000 --> 00:00:02,639
[MUSIC]

2
00:00:02,639 --> 00:00:08,707
[SOUND] In this stage, we're going to
learn how to call methods on an object.

3
00:00:08,707 --> 00:00:12,770
To be clear, we're not talking about
passing an argument to a method.

4
00:00:12,770 --> 00:00:14,770
We mean taking a piece of data and

5
00:00:14,770 --> 00:00:18,790
calling a method that is only
available on that piece of data.

6
00:00:18,790 --> 00:00:22,480
For example, strings have one set
of methods available on them, and

7
00:00:22,480 --> 00:00:24,490
numbers have a different set of methods.

8
00:00:25,710 --> 00:00:29,720
Almost every type of data available in
Ruby has a large set of methods you

9
00:00:29,720 --> 00:00:30,860
can call on it.

10
00:00:30,860 --> 00:00:32,040
How large?

11
00:00:32,040 --> 00:00:36,780
Any given string is going to
have over 170 different methods.

12
00:00:36,780 --> 00:00:38,954
A number is going to have over 130.

13
00:00:39,990 --> 00:00:43,990
Don't worry, we're not going to try and
teach you all of those methods right now.

14
00:00:43,990 --> 00:00:47,540
You'll only use a small portion
of them on a regular basis.

15
00:00:47,540 --> 00:00:51,280
But it is important to understand how
to call methods on an object, and

16
00:00:51,280 --> 00:00:53,100
that's just what this stage will show you.

17
00:00:54,460 --> 00:00:58,570
When you call a method, it's important for
Ruby to know which object you're calling

18
00:00:58,570 --> 00:01:02,500
it on, because the behavior of the method
will vary based on the object.

19
00:01:02,500 --> 00:01:06,128
We type the dot operator following
an object to indicate that we're calling

20
00:01:06,128 --> 00:01:07,430
a method on it.

21
00:01:07,430 --> 00:01:10,280
So here you see a dot operator

22
00:01:10,280 --> 00:01:15,290
that indicates that we're calling
the length method on this string, "AA".

23
00:01:15,290 --> 00:01:19,488
Here's another dot operator that indicates
we're calling the length method on this

24
00:01:19,488 --> 00:01:20,371
string instead.

25
00:01:20,371 --> 00:01:24,155
And the result of the length method
varies for these two different strings,

26
00:01:24,155 --> 00:01:26,653
because obviously they
have different lengths.

27
00:01:26,653 --> 00:01:30,700
The first output's a length of 2,
the second one output's a length of 9.

28
00:01:30,700 --> 00:01:34,760
And here you see the data operator
being used with several numbers.

29
00:01:34,760 --> 00:01:38,450
So we call the odd method on the number 3,
and

30
00:01:38,450 --> 00:01:43,330
since 3 is odd and not even,
that returns true down here.

31
00:01:43,330 --> 00:01:48,245
By the way, question mark is a valid
character at the end of a method name,

32
00:01:48,245 --> 00:01:49,770
and so is exclamation point.

33
00:01:49,770 --> 00:01:53,410
These two symbols are valid only
at the very end of a method name.

34
00:01:53,410 --> 00:01:54,810
This feature is unique to Ruby.

35
00:01:54,810 --> 00:01:57,990
It's a little unusual for
programming languages.

36
00:01:57,990 --> 00:02:02,170
The question mark doesn't have any special
meaning to the Ruby language itself but

37
00:02:02,170 --> 00:02:05,590
by convention, a question mark usually
means that a method is going to return

38
00:02:05,590 --> 00:02:06,970
either true or false.

39
00:02:06,970 --> 00:02:09,810
The same is true for
this even method down here.

40
00:02:09,810 --> 00:02:12,060
Here, we call the even
method on the number 3.

41
00:02:12,060 --> 00:02:13,725
And since 3 is odd and not even,

42
00:02:13,725 --> 00:02:17,130
obviously that's going to return
false down here at the output.

43
00:02:18,135 --> 00:02:22,930
Likewise, if we do call dot
even on the number 4, well,

44
00:02:22,930 --> 00:02:25,540
4 is even so that is going to return true.

45
00:02:26,710 --> 00:02:30,900
In place of the object itself, we can use
a variable that refers to the object.

46
00:02:30,900 --> 00:02:35,890
So if we wanted,
instead of saying "AA".length,

47
00:02:35,890 --> 00:02:40,740
we could assign the string "AA"
to a variable named string,

48
00:02:40,740 --> 00:02:45,804
and then we can print the length of the
value that stored in the string variable.

49
00:02:47,460 --> 00:02:51,730
So let's do that in place of
put this "AA".length here, and

50
00:02:51,730 --> 00:02:56,580
get rid of all this, and
we can also do the same with a number.

51
00:02:56,580 --> 00:02:58,609
Let's create a variable named, number.

52
00:02:58,609 --> 00:03:03,923
We'll assign it number 4, and
we'll call the even method

53
00:03:03,923 --> 00:03:09,010
on the value that's stored
in the number variable.

54
00:03:09,010 --> 00:03:10,290
Save that, try running it.

55
00:03:13,077 --> 00:03:14,980
Oops, I made a typo, one second.

56
00:03:17,671 --> 00:03:20,020
You'll notice that it gave
me an error down here.

57
00:03:20,020 --> 00:03:25,420
Reporting an undefined method while
obviously there's no method name to pute.

58
00:03:25,420 --> 00:03:30,090
So we're gonna change that to puts,
try rerunning it again.

59
00:03:30,090 --> 00:03:31,060
There we go.

60
00:03:31,060 --> 00:03:36,120
And now, we get length of 2 for
the value stored in the string variable.

61
00:03:36,120 --> 00:03:38,910
And we get a value of true for

62
00:03:38,910 --> 00:03:43,090
calling the even method on the value
stored in the number variable.

63
00:03:43,090 --> 00:03:45,640
You can also chain method calls together.

64
00:03:45,640 --> 00:03:48,860
Let's suppose we wanna know whether
a string contains an even number of

65
00:03:48,860 --> 00:03:49,930
characters.

66
00:03:49,930 --> 00:03:52,100
Well, we can call length on the string,
and

67
00:03:52,100 --> 00:03:54,860
then call even on the number
return from length.

68
00:03:54,860 --> 00:03:55,980
Let's try that.

69
00:03:55,980 --> 00:03:59,110
For our first attempt here, we're going
to store everything in variables.

70
00:03:59,110 --> 00:04:06,042
So we'll say string = "AA", and
then we'll create a variable named length,

71
00:04:07,871 --> 00:04:13,749
And store the value returned from
calling a string.length in that.

72
00:04:13,749 --> 00:04:17,700
And then we can print
the result of calling,

73
00:04:20,611 --> 00:04:24,630
The even method on the value
that's stored in length.

74
00:04:24,630 --> 00:04:25,753
This is one way we can do it.

75
00:04:25,753 --> 00:04:30,208
Let's try running this, and
we get tricked because there are two

76
00:04:30,208 --> 00:04:35,090
characters in the string "AA", and
that is of course an even number.

77
00:04:35,090 --> 00:04:39,380
But there's a shorter way to do this,
we can simply print out the result of

78
00:04:39,380 --> 00:04:44,888
calling, string.length,
which will return a number.

79
00:04:44,888 --> 00:04:49,103
And then calling.even
on that return value.

80
00:04:49,103 --> 00:04:51,930
And then we can get rid
of all this other code.

81
00:04:51,930 --> 00:04:55,665
And if we try rerunning this, we'll
get the exact same result, it's true.

82
00:04:55,665 --> 00:05:01,250
string.length returns the value 2,
and 2 is an even number.

83
00:05:01,250 --> 00:05:04,550
You should only chain methods in
situations where you're sure the first

84
00:05:04,550 --> 00:05:08,790
method call won't fail, otherwise,
subsequent methods will return an error.

85
00:05:08,790 --> 00:05:11,110
But you'll often see this
done in Ruby code, so

86
00:05:11,110 --> 00:05:12,590
it's good to be aware of the technique.