2
00:00:00,000 --> 00:00:02,470
>> SPEAKER 1: Let's write a program that
prompts the user for the names and

3
00:00:02,470 --> 00:00:03,990
houses of three students.

4
00:00:03,990 --> 00:00:07,300
But rather than just print out their
names and houses to the screen, let's

5
00:00:07,300 --> 00:00:09,660
instead save that information to disk.

6
00:00:09,660 --> 00:00:14,530
In other words, let's create, say, a CSV
file, for comma separated values,

7
00:00:14,530 --> 00:00:17,720
which is actually just a text file that
programs like Excel or Numbers

8
00:00:17,720 --> 00:00:21,690
can open, and effectively save those
names and houses permanently so that

9
00:00:21,690 --> 00:00:23,220
we can review them later.

10
00:00:23,220 --> 00:00:25,960
>> To do so, let's first take a look at
some boilerplate code that I've

11
00:00:25,960 --> 00:00:27,650
started with already.

12
00:00:27,650 --> 00:00:32,380
Notice first that among the headers
up top is now standard lib.h which

13
00:00:32,380 --> 00:00:36,710
happens to have some functions related
to file I/O, file input and output.

14
00:00:36,710 --> 00:00:39,560
Notice that I've also declared
a constant called STUDENTS--

15
00:00:39,560 --> 00:00:42,380
in all caps-- whose value
is hard coded as three.

16
00:00:42,380 --> 00:00:45,820
Notice now that inside of my main
program, I'm declaring an array of

17
00:00:45,820 --> 00:00:50,270
size three using that constant called
STUDENTS each of whose members is of

18
00:00:50,270 --> 00:00:51,650
type students.

19
00:00:51,650 --> 00:00:56,150
>> Recall now that a student we'll define
as having a name and a house both of

20
00:00:56,150 --> 00:01:00,410
which are strings as per this
declaration instructs, dot h.

21
00:01:00,410 --> 00:01:04,680
Now back in structs1.c, notice that I
have a for loop here that's going to

22
00:01:04,680 --> 00:01:06,750
iterate from zero up to three.

23
00:01:06,750 --> 00:01:10,020
It's going to prompt me for a student's
name and a student's house

24
00:01:10,020 --> 00:01:12,310
again and again and again.

25
00:01:12,310 --> 00:01:15,620
>> Then at the bottom of this program,
notice that I have another for loop

26
00:01:15,620 --> 00:01:19,970
that's going to free the ith student's
name and the ith student's house in a

27
00:01:19,970 --> 00:01:21,570
similarly constructed loop.

28
00:01:21,570 --> 00:01:24,480
To be sure, we could combine those loops
into just one, but I wanted to

29
00:01:24,480 --> 00:01:28,180
have some distinct segments of code
for the sake of discussion here.

30
00:01:28,180 --> 00:01:31,920
>> Now in between those for loops, let's
actually have another, and this one's

31
00:01:31,920 --> 00:01:35,210
purpose in life is to actually save all
of these names and houses that are

32
00:01:35,210 --> 00:01:39,810
currently in RAM to disk in the form
of comma separated values.

33
00:01:39,810 --> 00:01:42,080
To do so, we're going to use
three new functions--

34
00:01:42,080 --> 00:01:47,450
F open, F printf, and F close, which
open a file, print to a file, and

35
00:01:47,450 --> 00:01:48,440
close a file.

36
00:01:48,440 --> 00:01:49,690
We can use them as follows.

37
00:01:49,690 --> 00:01:52,110

38
00:01:52,110 --> 00:01:58,240
>> FILE, in all caps, which is somewhat of
a curiosity in C, FILE gets fopen

39
00:01:58,240 --> 00:02:03,020
quote unquote and now a name for the
file, say, students.CSV, but I could

40
00:02:03,020 --> 00:02:08,150
call it most anything, comma and now I'm
going to specify a single w inside

41
00:02:08,150 --> 00:02:09,390
of double quotes.

42
00:02:09,390 --> 00:02:13,290
W, as you might have guessed already,
means that fopen should open this file

43
00:02:13,290 --> 00:02:17,360
called Students.CSV for writing so
that we can actually save some

44
00:02:17,360 --> 00:02:19,370
contents to it.

45
00:02:19,370 --> 00:02:23,080
>> Let's next check if file
is not equal to null.

46
00:02:23,080 --> 00:02:25,860
Because if it is, something has probably
gone wrong in which case we

47
00:02:25,860 --> 00:02:28,340
should not proceed to try
to print anything to it.

48
00:02:28,340 --> 00:02:33,400
But if it's not null, then inside of the
curly braces I'm going to iterate

49
00:02:33,400 --> 00:02:38,030
from i equals 0 on up to STUDENTS,
and I'm going to

50
00:02:38,030 --> 00:02:40,180
increment i on each iteration.

51
00:02:40,180 --> 00:02:45,750
And inside of this loop, I am going to
fprintf so as to print to a file--

52
00:02:45,750 --> 00:02:47,940
specifically the one
I already opened--

53
00:02:47,940 --> 00:02:54,650
a string that looks like this %s,%s
backslash n close quote.

54
00:02:54,650 --> 00:02:58,790
>> And now I want to plug in to each of
those placeholders the actual values

55
00:02:58,790 --> 00:03:03,390
of a student's name and a house
using the dot operator.

56
00:03:03,390 --> 00:03:10,030
Students bracket i.name, students
bracket i.house close paren

57
00:03:10,030 --> 00:03:11,230
semi-colon.

58
00:03:11,230 --> 00:03:16,180
Now below this for loop, I'm simply
going to call fclose of file in order

59
00:03:16,180 --> 00:03:18,520
to ultimately close the file.

60
00:03:18,520 --> 00:03:21,360
>> Now when I run this program, I shouldn't
actually see anything on the

61
00:03:21,360 --> 00:03:25,010
screen, but I should have after running
this program a file called

62
00:03:25,010 --> 00:03:29,130
Students.CSV in the same directory that
I run the command in that should

63
00:03:29,130 --> 00:03:32,480
contain a comma separated
list of values.

64
00:03:32,480 --> 00:03:34,790
Let's take a look.

65
00:03:34,790 --> 00:03:41,690
>> Make structs 1./ structs1 student's
name, let's say David,

66
00:03:41,690 --> 00:03:43,140
he'll live in Mather.

67
00:03:43,140 --> 00:03:46,890
Student's name, let's say, Lauren,
she'll live in Leverett.

68
00:03:46,890 --> 00:03:50,800
Student's name, let's say, Rob,
he'll live in Kirkland.

69
00:03:50,800 --> 00:03:54,050
Now, again, as expected, nothing appears
to have happened, but let me

70
00:03:54,050 --> 00:03:58,790
go ahead at the command prompt and type
g edit students.CSV in hopes that

71
00:03:58,790 --> 00:04:00,850
that file indeed exists.

72
00:04:00,850 --> 00:04:07,010
>> Gedit students.CSV ENTER, and indeed,
notice that a file containing purely

73
00:04:07,010 --> 00:04:11,320
text, but text separated by commas
for each field, indeed exists.

74
00:04:11,320 --> 00:04:14,530
And if we were using, not the CS50
appliance, but a more familiar Mac or

75
00:04:14,530 --> 00:04:18,080
PC, it turns out that we could indeed
open this CSV file with a more

76
00:04:18,080 --> 00:04:20,400
familiar program like Excel or Numbers.

77
00:04:20,400 --> 00:04:22,906