1
00:00:04,970 --> 00:00:07,220
DOUG LLOYD: So if you've
seen our video on structures,

2
00:00:07,220 --> 00:00:10,950
you already know that we have the
ability to define our own data types.

3
00:00:10,950 --> 00:00:14,530
But if you've used structures, you know
that sometimes using those data types

4
00:00:14,530 --> 00:00:16,446
can actually be a little
cumbersome because we

5
00:00:16,446 --> 00:00:19,210
have to use the struct keyword
when we're working with them--

6
00:00:19,210 --> 00:00:21,257
struct car or struct student.

7
00:00:21,257 --> 00:00:23,090
That's a lot to type
if we just want to have

8
00:00:23,090 --> 00:00:24,830
something simple, like a structure.

9
00:00:24,830 --> 00:00:26,390
We want to have-- we want to
be able to work with things

10
00:00:26,390 --> 00:00:29,350
a little more shorthand way,
like int or char, something a lot

11
00:00:29,350 --> 00:00:31,240
more convenient to type.

12
00:00:31,240 --> 00:00:33,100
>> Fortunately, there's
a way to do this in C,

13
00:00:33,100 --> 00:00:37,150
with something called typedef, which is
a way to create shorthand or rewritten

14
00:00:37,150 --> 00:00:38,640
names for data types.

15
00:00:38,640 --> 00:00:40,720
You can rewrite data
types that already exist,

16
00:00:40,720 --> 00:00:43,970
or you can write data types of your own.

17
00:00:43,970 --> 00:00:46,890
>> The first thing you do is you
define a type in the normal way,

18
00:00:46,890 --> 00:00:52,271
and then you just alias it to something
else-- typedef old name new name.

19
00:00:52,271 --> 00:00:53,520
Pretty straightforward, right?

20
00:00:53,520 --> 00:00:56,240
So we could typedef for example,
the already existing data

21
00:00:56,240 --> 00:01:00,266
type of unsigned char as byte.

22
00:01:00,266 --> 00:01:03,140
And then, from now on, after we've
made this type definition-- again,

23
00:01:03,140 --> 00:01:07,570
which is usually going to be at the top
of our dot C files, or in a dot H file

24
00:01:07,570 --> 00:01:10,590
separately, we can just
use bite everywhere

25
00:01:10,590 --> 00:01:12,180
we would have used unsigned char.

26
00:01:12,180 --> 00:01:14,300
That's a lot shorter to type.

27
00:01:14,300 --> 00:01:19,280
>> In CS50's library, we do this--
we typedef char star as string.

28
00:01:19,280 --> 00:01:21,400
Because we abstracted
away the idea of pointers.

29
00:01:21,400 --> 00:01:25,072
And the fact that a string is really
a pointer to the first character--

30
00:01:25,072 --> 00:01:26,780
an array of characters,
because it's just

31
00:01:26,780 --> 00:01:29,863
so confusing to get your head around
that-- but in fact that's what we do.

32
00:01:29,863 --> 00:01:35,140
And this line of code actually exists
in CS50 dot H typedef char star string,

33
00:01:35,140 --> 00:01:39,021
just to make that a little less
cumbersome to have to deal with.

34
00:01:39,021 --> 00:01:41,520
Already you've seen probably a
little bit of the value here,

35
00:01:41,520 --> 00:01:44,160
but typedef becomes great
when combined with structures,

36
00:01:44,160 --> 00:01:45,780
as I alluded to earlier.

37
00:01:45,780 --> 00:01:48,550
Structures have a two-word type name.

38
00:01:48,550 --> 00:01:52,020
And so they can be really annoying
to create variables of that type,

39
00:01:52,020 --> 00:01:56,310
or [INAUDIBLE] of struct something, your
lines can just get really, really long.

40
00:01:56,310 --> 00:01:59,400
And so you can use typedef to come
up with something a lot shorter.

41
00:01:59,400 --> 00:02:04,650
>> So if I define a car as follows, I have
my definition of a car-- struct car,

42
00:02:04,650 --> 00:02:07,230
and then open curly brace, all
the fields of my structure,

43
00:02:07,230 --> 00:02:11,030
close curly brace, semi-colon--
after I've defined my data type,

44
00:02:11,030 --> 00:02:15,680
I can typedef struct
car as car underscore t.

45
00:02:15,680 --> 00:02:19,670
And then when I've done that, now every
time I would have otherwise used struct

46
00:02:19,670 --> 00:02:22,680
car, I can just use car underscore t.

47
00:02:22,680 --> 00:02:26,480
That's a lot shorter of a way to
express this idea of this structure

48
00:02:26,480 --> 00:02:28,530
that I just created.

49
00:02:28,530 --> 00:02:33,620
>> Alternatively, because structures
are so commonly used in typedef,

50
00:02:33,620 --> 00:02:37,980
there's a-- you can actually define the
type in between the start and the end.

51
00:02:37,980 --> 00:02:42,020
So the typedef structure again is
usually typedef, old name, new name,

52
00:02:42,020 --> 00:02:45,360
where the names are already
types that you've created.

53
00:02:45,360 --> 00:02:49,620
But you can actually define a structure
right in the middle of the typedef

54
00:02:49,620 --> 00:02:51,760
instead of having to define
it separately, and then

55
00:02:51,760 --> 00:02:52,990
do a typedef of it.

56
00:02:52,990 --> 00:02:55,780
And that would look just like
this-- typedef struct car,

57
00:02:55,780 --> 00:02:59,957
open curly brace, all of your field
definitions, close curly brace, car t.

58
00:02:59,957 --> 00:03:03,290
So the old name is all that stuff in the
red, you're just defining the structure

59
00:03:03,290 --> 00:03:05,610
and naming it at the same time.

60
00:03:05,610 --> 00:03:07,790
And then the new name
is car underscore t.

61
00:03:07,790 --> 00:03:10,150
And so if we start to use
this in code, previously I

62
00:03:10,150 --> 00:03:13,279
might have said, struct
car mycar semi-colon.

63
00:03:13,279 --> 00:03:14,820
Well I don't have to do that anymore.

64
00:03:14,820 --> 00:03:20,265
Now that I've used the typedef, I
can just say car underscore t, mycar.

65
00:03:20,265 --> 00:03:22,640
That's a lot shorter of a way
to do it, and in fact, it's

66
00:03:22,640 --> 00:03:24,515
going to be a lot more
convenient, especially

67
00:03:24,515 --> 00:03:27,730
as you start to use structures
a lot more in your code.

68
00:03:27,730 --> 00:03:28,810
>> I'm Doug Lloyd.

69
00:03:28,810 --> 00:03:33,810
This is CS50.