1
00:00:00,000 --> 00:00:02,418
>> [MUSIC PLAYING]

2
00:00:02,418 --> 00:00:10,542

3
00:00:10,542 --> 00:00:12,000
ALLISON BUCHHOLTZ-AU: Hey everyone.

4
00:00:12,000 --> 00:00:15,640
Welcome to your first
official CS50 section.

5
00:00:15,640 --> 00:00:19,507
As you can, see this is
section CS50-like, just

6
00:00:19,507 --> 00:00:20,840
to pull up our agenda for today.

7
00:00:20,840 --> 00:00:23,990
So, who am I, as I'm sure
you're all wondering.

8
00:00:23,990 --> 00:00:25,514
I am your TF.

9
00:00:25,514 --> 00:00:27,930
I'm not just a random student
who's impersonating your TF.

10
00:00:27,930 --> 00:00:31,190
>> And I'm going to go through
kind of have sections will flow,

11
00:00:31,190 --> 00:00:34,900
expectations we have,
resources, so on and so forth.

12
00:00:34,900 --> 00:00:39,190
We're going to talk about arrays, ASCII
functions, command-line arguments,

13
00:00:39,190 --> 00:00:42,020
and it's not on here, but
I will also be helping

14
00:00:42,020 --> 00:00:45,054
you kind of think through
your next pset for this week.

15
00:00:45,054 --> 00:00:46,720
Which I am sure you will all appreciate.

16
00:00:46,720 --> 00:00:48,190

17
00:00:48,190 --> 00:00:53,390
>> So, first question-- who am I,
besides your exuberant TF here.

18
00:00:53,390 --> 00:00:55,390
I'm Alison Buchholtz-Au.

19
00:00:55,390 --> 00:00:57,970
This is my second year TFing CS50.

20
00:00:57,970 --> 00:01:00,570
I also TF CS51 in the Spring.

21
00:01:00,570 --> 00:01:03,930
You might see again there
if you decide to take it.

22
00:01:03,930 --> 00:01:07,370
I'm also a PAF, so any Freshmen--
and if you're not a freshman,

23
00:01:07,370 --> 00:01:09,980
this is my third year
as an advising fellow.

24
00:01:09,980 --> 00:01:13,630
I'm very well-versed in
advising you on life and courses

25
00:01:13,630 --> 00:01:16,549
within CS and not within CS.

26
00:01:16,549 --> 00:01:18,090
I am a Computer Science Concentrator.

27
00:01:18,090 --> 00:01:20,800
I'm a senior in Adams House, best house.

28
00:01:20,800 --> 00:01:22,430

29
00:01:22,430 --> 00:01:26,050
And before I switched to
CS my Sophomore Spring,

30
00:01:26,050 --> 00:01:27,640
I was actually a biomedical engineer.

31
00:01:27,640 --> 00:01:29,020
I was going to go to Med school.

32
00:01:29,020 --> 00:01:30,530
I was going to be a trauma surgeon.

33
00:01:30,530 --> 00:01:33,000
And that has completely
changed since I took CS50.

34
00:01:33,000 --> 00:01:34,250

35
00:01:34,250 --> 00:01:36,310
I took the course my Sophomore Fall.

36
00:01:36,310 --> 00:01:38,920
It was my first introduction to CS ever.

37
00:01:38,920 --> 00:01:44,104
I was one of the 78% of you who
had zero experience coming in,

38
00:01:44,104 --> 00:01:45,520
and it completely changed my life.

39
00:01:45,520 --> 00:01:49,820
And now I'm working at
Microsoft, and your lovely TF.

40
00:01:49,820 --> 00:01:53,500
And CS50 is probably one
of the best experiences

41
00:01:53,500 --> 00:01:56,210
that I've had here at
Harvard-- both taking the class

42
00:01:56,210 --> 00:01:59,040
and being able to help
teach students like you.

43
00:01:59,040 --> 00:02:01,410
>> So I'm really excited
that you're all here.

44
00:02:01,410 --> 00:02:04,270
In case you came in
late, there is candy,

45
00:02:04,270 --> 00:02:07,227
which you should feel
free to come grab, or send

46
00:02:07,227 --> 00:02:08,560
someone else to grab it for you.

47
00:02:08,560 --> 00:02:09,399
it's OK.

48
00:02:09,399 --> 00:02:10,440
I don't want to eat that.

49
00:02:10,440 --> 00:02:13,350
My room has enough chocolate, so
y'all should try and finish that.

50
00:02:13,350 --> 00:02:16,730
I know there are 100 pieces,
but like, 4 o'clock on a Monday,

51
00:02:16,730 --> 00:02:19,000
I think everyone could use some sugar.

52
00:02:19,000 --> 00:02:21,750
>> So all of you who are
officially in my section

53
00:02:21,750 --> 00:02:24,700
should have gotten an email
from me with my phone number,

54
00:02:24,700 --> 00:02:29,920
email address-- feel free to add me on
Gchat, feel free to add me on Facebook,

55
00:02:29,920 --> 00:02:33,360
and also for the rest of you,
you can email me right here.

56
00:02:33,360 --> 00:02:34,450
There are two H's.

57
00:02:34,450 --> 00:02:37,090
Everyone always does
like two L's or two C's.

58
00:02:37,090 --> 00:02:38,260
Two H's in the last name.

59
00:02:38,260 --> 00:02:41,800
Otherwise it's going to bounce, and
I'm not going to get your email.

60
00:02:41,800 --> 00:02:46,400
>> So feel free to email me,
to contact me at any time.

61
00:02:46,400 --> 00:02:51,440
I may not get back to
you within 24 minutes,

62
00:02:51,440 --> 00:02:53,760
but I promise to get back
to you within 24 hours.

63
00:02:53,760 --> 00:02:54,900

64
00:02:54,900 --> 00:02:58,067
If you call me half an hour before
your pset is due, you being like,

65
00:02:58,067 --> 00:02:59,650
I have no idea what I'm doing Allison.

66
00:02:59,650 --> 00:03:00,160
Help me.

67
00:03:00,160 --> 00:03:04,049
I'm going to calm you down,
but at the end of the day,

68
00:03:04,049 --> 00:03:07,340
if you're calling me half an hour before
your pset is due with nothing written,

69
00:03:07,340 --> 00:03:11,570
I'm going to be like, well, maybe
it's time to use that late day.

70
00:03:11,570 --> 00:03:15,215
So I will respond to all of your
requests in a very timely manner.

71
00:03:15,215 --> 00:03:16,482

72
00:03:16,482 --> 00:03:18,190
My phone is usually
attached to my hands.

73
00:03:18,190 --> 00:03:21,530
I typically respond much
quicker than 24 hours,

74
00:03:21,530 --> 00:03:25,650
but I can only guarantee
a 24 hour response.

75
00:03:25,650 --> 00:03:26,580
>> All right.

76
00:03:26,580 --> 00:03:28,410
So why are we here?

77
00:03:28,410 --> 00:03:32,160
Also, if you have questions at
any time, please let me know.

78
00:03:32,160 --> 00:03:32,780
I talk a lot.

79
00:03:32,780 --> 00:03:35,320
I talk fast, but please
feel free to interrupt me.

80
00:03:35,320 --> 00:03:36,980
It gives me breathing room as well.

81
00:03:36,980 --> 00:03:38,159

82
00:03:38,159 --> 00:03:40,200
So sections are a time
for us to really just dive

83
00:03:40,200 --> 00:03:44,270
in, get some hands-on experience, to
go through topics that we mentioned

84
00:03:44,270 --> 00:03:49,760
in class or in study materials that
we recommend to you guys online.

85
00:03:49,760 --> 00:03:52,980
And we'll actually go through
some of those resources in a bit.

86
00:03:52,980 --> 00:03:54,300

87
00:03:54,300 --> 00:03:57,400
>> So some notes on section support.

88
00:03:57,400 --> 00:04:00,250
CS50-- one of things that makes
it one of my favorite classes

89
00:04:00,250 --> 00:04:02,510
is the feeling that you're never alone.

90
00:04:02,510 --> 00:04:05,430
We have a staff of over 100
people who are here to help you.

91
00:04:05,430 --> 00:04:08,080
We have office hours
Monday through Thursday.

92
00:04:08,080 --> 00:04:13,230
So there are so many people who
love the class just as much as I do

93
00:04:13,230 --> 00:04:15,750
and who are really here
because they want to be here.

94
00:04:15,750 --> 00:04:19,390
Most of us are students, and this
is like a fifth class in addition

95
00:04:19,390 --> 00:04:20,390
to the rest of our work.

96
00:04:20,390 --> 00:04:22,723
>> And it's a lot of work, but
we do it because we love it,

97
00:04:22,723 --> 00:04:25,600
and we really love to teach you
and help share our excitement

98
00:04:25,600 --> 00:04:27,260
for this subject and this class.

99
00:04:27,260 --> 00:04:28,490
So please take advantage.

100
00:04:28,490 --> 00:04:29,370
Come talk to us.

101
00:04:29,370 --> 00:04:33,034
I get lonely when my students
don't talk to me, so if you want,

102
00:04:33,034 --> 00:04:33,950
come hang out with me.

103
00:04:33,950 --> 00:04:35,200
It'll be great.

104
00:04:35,200 --> 00:04:37,400
>> So section is obviously
one of your biggest things.

105
00:04:37,400 --> 00:04:41,770
We'll go through things
that you learned in lecture,

106
00:04:41,770 --> 00:04:44,100
do some short examples
when we have time,

107
00:04:44,100 --> 00:04:46,187
and generally kind of
get an idea about things

108
00:04:46,187 --> 00:04:48,270
you should be thinking
about for your problem set.

109
00:04:48,270 --> 00:04:49,540

110
00:04:49,540 --> 00:04:53,591
Shorts-- how many people actually
watched the video from your scratch

111
00:04:53,591 --> 00:04:54,090
short?

112
00:04:54,090 --> 00:04:55,460

113
00:04:55,460 --> 00:04:57,390
Anyone recognize me?

114
00:04:57,390 --> 00:04:59,720
So those are very great.

115
00:04:59,720 --> 00:05:01,210
You should definitely watch those.

116
00:05:01,210 --> 00:05:02,880
A lot of work has been put into them.

117
00:05:02,880 --> 00:05:06,690
And they're just meant to be
bite-size pieces for you to just watch

118
00:05:06,690 --> 00:05:09,760
for three or four minutes and get a
better understanding of a concept.

119
00:05:09,760 --> 00:05:11,147

120
00:05:11,147 --> 00:05:12,980
Walk throughs-- how
many people have watched

121
00:05:12,980 --> 00:05:14,880
the walk-throughs for previous ones?

122
00:05:14,880 --> 00:05:16,240
Zamyla is amazing, right?

123
00:05:16,240 --> 00:05:18,100
Like, I wish I were Zamyla sometimes.

124
00:05:18,100 --> 00:05:19,650

125
00:05:19,650 --> 00:05:21,640
So definitely use your walk-throughs.

126
00:05:21,640 --> 00:05:25,070
She will break it down into
small, bite-size pieces.

127
00:05:25,070 --> 00:05:28,079
And when you have these huge
specifications from your problem sets,

128
00:05:28,079 --> 00:05:30,870
it's going to be really important
to be able to just find somewhere

129
00:05:30,870 --> 00:05:32,600
to start and work slowly through it.

130
00:05:32,600 --> 00:05:33,610

131
00:05:33,610 --> 00:05:40,260
>> All right, we also have Study50, which
is study50.harvard.edu, I believe.

132
00:05:40,260 --> 00:05:42,270
You can just Google
study, and it'll come up.

133
00:05:42,270 --> 00:05:45,750
This is one of the
best resources we have.

134
00:05:45,750 --> 00:05:49,800
It is PowerPoints with notes
and practice problems for you

135
00:05:49,800 --> 00:05:52,430
with solutions that you
can actually walk through.

136
00:05:52,430 --> 00:05:57,000
So if you ever want more practice,
more than we do in sections or more

137
00:05:57,000 --> 00:06:00,700
than your problem sets, this is
really a place I encourage you to go.

138
00:06:00,700 --> 00:06:03,590
It was built last summer by
some of my really good friends.

139
00:06:03,590 --> 00:06:04,480
And it's amazing.

140
00:06:04,480 --> 00:06:05,540

141
00:06:05,540 --> 00:06:08,670
In fact, a lot of the slides
that I'll be using for Section

142
00:06:08,670 --> 00:06:10,190
will come from Study50.

143
00:06:10,190 --> 00:06:12,000
>> So a lot of the TFs use it.

144
00:06:12,000 --> 00:06:13,920
And finally, as I
mentioned, office hours.

145
00:06:13,920 --> 00:06:15,836
If you're having trouble
with homework, you're

146
00:06:15,836 --> 00:06:18,270
having trouble with a
concept, come to office hours.

147
00:06:18,270 --> 00:06:20,190
Go early in the week if you can.

148
00:06:20,190 --> 00:06:23,130
Get out to the quad,
because it is kind of far.

149
00:06:23,130 --> 00:06:24,780
No one likes to walk out there.

150
00:06:24,780 --> 00:06:27,090
But it is to your advantage,
because then you're

151
00:06:27,090 --> 00:06:29,580
going to have all these TFs,
[INAUDIBLE] surround you.

152
00:06:29,580 --> 00:06:31,920
And especially now,
just a tip, Thursdays

153
00:06:31,920 --> 00:06:35,310
are very chill right now in
Mather because your psets

154
00:06:35,310 --> 00:06:36,577
are due on Thursdays.

155
00:06:36,577 --> 00:06:38,410
And knowing wants to
use your late days yet.

156
00:06:38,410 --> 00:06:40,030
>> So if you're having
trouble with concepts,

157
00:06:40,030 --> 00:06:41,780
there are lots of TFs who
are there to help you.

158
00:06:41,780 --> 00:06:43,196
So come out to Mather on Thursday.

159
00:06:43,196 --> 00:06:45,280
If you want to see me,
I'm going to be there.

160
00:06:45,280 --> 00:06:48,870
I'm typically doing my own homework,
because no one wants my help.

161
00:06:48,870 --> 00:06:50,260
So come see us.

162
00:06:50,260 --> 00:06:51,460

163
00:06:51,460 --> 00:06:52,440
>> Meet us halfway.

164
00:06:52,440 --> 00:06:55,400
So how many people have attended
lecture or watched it online?

165
00:06:55,400 --> 00:06:56,570

166
00:06:56,570 --> 00:06:59,140
How many people went to
super section last week?

167
00:06:59,140 --> 00:07:00,171

168
00:07:00,171 --> 00:07:00,670
Cool.

169
00:07:00,670 --> 00:07:02,190
That's actually a fair number.

170
00:07:02,190 --> 00:07:05,425
How many of you have read your
spec for this week, for your pset?

171
00:07:05,425 --> 00:07:07,081

172
00:07:07,081 --> 00:07:08,461
Ooh, I'm proud of you guys!

173
00:07:08,461 --> 00:07:09,335
More candy for y'all.

174
00:07:09,335 --> 00:07:11,070

175
00:07:11,070 --> 00:07:13,280
>> Good, so what we mean
"meet us halfway," is

176
00:07:13,280 --> 00:07:16,440
that section is really only
going to be super useful to you

177
00:07:16,440 --> 00:07:19,810
if you come in having read your
specification for your pset.

178
00:07:19,810 --> 00:07:23,859
Because when I go through an overview
of things you should be looking out for,

179
00:07:23,859 --> 00:07:25,650
it's not going to make
as much sense if you

180
00:07:25,650 --> 00:07:28,358
don't know what your problem set
is going to be asking you to do.

181
00:07:28,358 --> 00:07:29,020

182
00:07:29,020 --> 00:07:32,590
>> If you don't come to section, obviously
I can't be that useful to you.

183
00:07:32,590 --> 00:07:36,600
I'm not going to take it personally if
you don't come to my section right now,

184
00:07:36,600 --> 00:07:38,570
but definitely you should.

185
00:07:38,570 --> 00:07:40,090
If you can't, watch them online.

186
00:07:40,090 --> 00:07:41,230

187
00:07:41,230 --> 00:07:42,519
They're there for a reason.

188
00:07:42,519 --> 00:07:43,560
Mine will be right there.

189
00:07:43,560 --> 00:07:44,187

190
00:07:44,187 --> 00:07:47,270
As you notice, we're being recorded,
so it'll be right there for you guys.

191
00:07:47,270 --> 00:07:49,110

192
00:07:49,110 --> 00:07:51,570
As well, going to
lectures-- that's obviously

193
00:07:51,570 --> 00:07:53,790
where you're getting the
start of your material here.

194
00:07:53,790 --> 00:07:57,430
So I will definitely try and
help you as much as I can,

195
00:07:57,430 --> 00:07:58,840
but I can only meet you so far.

196
00:07:58,840 --> 00:08:01,220
You have to kind of
meet us halfway there.

197
00:08:01,220 --> 00:08:04,050
>> Grading-- so, all of you
who got an email from me,

198
00:08:04,050 --> 00:08:05,445
you are my official section.

199
00:08:05,445 --> 00:08:06,470

200
00:08:06,470 --> 00:08:08,120
I will be grading your psets.

201
00:08:08,120 --> 00:08:12,870
And I just want to say, one thing that
you should really pay attention to

202
00:08:12,870 --> 00:08:13,820
are the comments.

203
00:08:13,820 --> 00:08:17,390
The comments are often more useful
than the actual score we give you.

204
00:08:17,390 --> 00:08:19,890
And the comments are actually
where I spend a lot of my time

205
00:08:19,890 --> 00:08:20,770
when I'm grading.

206
00:08:20,770 --> 00:08:22,700
So I would appreciate
it if you read them.

207
00:08:22,700 --> 00:08:26,990
And they're actually how you're going
to learn more about design and style

208
00:08:26,990 --> 00:08:29,860
and things that are a
little less cut and dry.

209
00:08:29,860 --> 00:08:31,770
>> So really pay attention
to those comments.

210
00:08:31,770 --> 00:08:34,720
If you have questions about them,
or questions about your score,

211
00:08:34,720 --> 00:08:37,620
please come talk to me
either before section,

212
00:08:37,620 --> 00:08:41,039
I'll probably be hanging out
in the lobby, or afterwards.

213
00:08:41,039 --> 00:08:43,559
If you want to schedule
one-on-one meetings about how

214
00:08:43,559 --> 00:08:46,100
you can help improve later
problem sets, just let me know.

215
00:08:46,100 --> 00:08:47,520

216
00:08:47,520 --> 00:08:49,750
>> And then just a couple
of tips for you guys.

217
00:08:49,750 --> 00:08:53,110
So one of the biggest things
I always stress to my section

218
00:08:53,110 --> 00:08:57,700
when you're learning how to code is
to write things out on paper first.

219
00:08:57,700 --> 00:09:01,080
If you have a game plan for
where your code needs to go

220
00:09:01,080 --> 00:09:03,230
and what it needs to
do and it's broken down

221
00:09:03,230 --> 00:09:06,000
into little bits of pseudocode
code you've written out,

222
00:09:06,000 --> 00:09:09,780
you're going to be less
likely to make syntax errors

223
00:09:09,780 --> 00:09:12,355
or create an if loop that
doesn't have an else.

224
00:09:12,355 --> 00:09:14,064

225
00:09:14,064 --> 00:09:15,980
If you know where you're
going overall, you're

226
00:09:15,980 --> 00:09:19,764
less likely to make these tiny
mistakes that will sometimes take you

227
00:09:19,764 --> 00:09:22,555
hours to fix, because you're like,
where am I missing this bracket?

228
00:09:22,555 --> 00:09:23,580

229
00:09:23,580 --> 00:09:27,420
>> On that note, please use Style50.

230
00:09:27,420 --> 00:09:28,820

231
00:09:28,820 --> 00:09:31,590
Especially when you're
going to office hours,

232
00:09:31,590 --> 00:09:34,850
if your code is all
switched over to one side,

233
00:09:34,850 --> 00:09:37,740
it is course policy
that we can say, fix it

234
00:09:37,740 --> 00:09:41,990
so that it looks like Style50 says
it should, and then we'll help you.

235
00:09:41,990 --> 00:09:43,830
So it'll make your life easier.

236
00:09:43,830 --> 00:09:45,030
It'll make our lives easier.

237
00:09:45,030 --> 00:09:46,000
Everyone's happier.

238
00:09:46,000 --> 00:09:47,290
Everyone gets better grades.

239
00:09:47,290 --> 00:09:48,760
Isn't that what we all want?

240
00:09:48,760 --> 00:09:50,420

241
00:09:50,420 --> 00:09:53,860
>> So write things out on paper before
you ever touch your computer.

242
00:09:53,860 --> 00:09:57,540
Talk things out at a high level, and
make sure you know where you're going.

243
00:09:57,540 --> 00:09:59,940
And if you're unsure,
sit down with someone

244
00:09:59,940 --> 00:10:03,340
and walk them through step by step
what your code is supposed to do.

245
00:10:03,340 --> 00:10:07,940
And nine times out of 10, you'll be
like, oh, I forgot an if condition

246
00:10:07,940 --> 00:10:12,420
or I forgot a semicolon here or
I'm updating this variable wrong.

247
00:10:12,420 --> 00:10:15,740
So those are my tips for success.

248
00:10:15,740 --> 00:10:19,820
>> So since about half of you look
like you attended super section,

249
00:10:19,820 --> 00:10:24,080
I'm just going to very
briefly go through loops,

250
00:10:24,080 --> 00:10:26,650
which weren't on our original agenda.

251
00:10:26,650 --> 00:10:28,190
But they are really important.

252
00:10:28,190 --> 00:10:30,250
And so I'm going to kind
of speed through those

253
00:10:30,250 --> 00:10:32,860
before we get into our actual section.

254
00:10:32,860 --> 00:10:36,269
Before I do that, are there
any questions-- logistically,

255
00:10:36,269 --> 00:10:38,060
personally, is there
anything else you want

256
00:10:38,060 --> 00:10:40,990
to know about me or about
section or class in general?

257
00:10:40,990 --> 00:10:43,195

258
00:10:43,195 --> 00:10:43,880
All good?

259
00:10:43,880 --> 00:10:45,300
OK, cool.

260
00:10:45,300 --> 00:10:46,781

261
00:10:46,781 --> 00:10:47,280
Lovely.

262
00:10:47,280 --> 00:10:50,071
>> So loops-- you guys should all
recognize these pieces from scratch.

263
00:10:50,071 --> 00:10:51,310

264
00:10:51,310 --> 00:10:55,220
So loops are basically just
a way for us to do something

265
00:10:55,220 --> 00:10:58,900
some number of times, some repeated
action based on some conditional.

266
00:10:58,900 --> 00:11:00,140

267
00:11:00,140 --> 00:11:02,090
So we have three different types.

268
00:11:02,090 --> 00:11:04,860
We have for loop, while, and do-while.

269
00:11:04,860 --> 00:11:06,430

270
00:11:06,430 --> 00:11:11,320
>> So for loops-- we just have a very
general layout here of a for loop.

271
00:11:11,320 --> 00:11:16,110
And this is great for when you know how
many times something needs to execute.

272
00:11:16,110 --> 00:11:17,040

273
00:11:17,040 --> 00:11:18,790
When we talk about the
other loops, you'll

274
00:11:18,790 --> 00:11:20,650
see why that's an important distinction.

275
00:11:20,650 --> 00:11:22,530
But for loops are for something set.

276
00:11:22,530 --> 00:11:24,590
You know you can either
calculate the number

277
00:11:24,590 --> 00:11:29,230
or you know the number of times you
want this repeated at the beginning.

278
00:11:29,230 --> 00:11:32,220
>> So if you see here, we
have just a general kind

279
00:11:32,220 --> 00:11:34,480
of skeleton framework
for a for loop here.

280
00:11:34,480 --> 00:11:38,080
So for-initialization, this is where
your variables are initialized.

281
00:11:38,080 --> 00:11:41,915
With Mario, I'm sure you guys did
something like int i equals 0.

282
00:11:41,915 --> 00:11:42,920

283
00:11:42,920 --> 00:11:44,710
That's where that would happen in blue.

284
00:11:44,710 --> 00:11:48,290
You have your condition, which
is what's checked every time.

285
00:11:48,290 --> 00:11:52,410
If this condition is true, then
the rest of the code executes.

286
00:11:52,410 --> 00:11:54,749
Then it'll run again-- and ask.

287
00:11:54,749 --> 00:11:57,290
And then we have update, where
you're updating your variable.

288
00:11:57,290 --> 00:12:01,410
So, again, with like Mario, I'm sure
you guys did something like i plus plus.

289
00:12:01,410 --> 00:12:04,640
So every time the loop
ran, i got updated so

290
00:12:04,640 --> 00:12:09,370
that when we were checking it against
some condition, it was changing.

291
00:12:09,370 --> 00:12:12,570
>> Because if you just
have a static variable,

292
00:12:12,570 --> 00:12:15,567
if it executes the first time, it's
just going to execute infinitely.

293
00:12:15,567 --> 00:12:18,025
So you have to make sure you
update your variable properly.

294
00:12:18,025 --> 00:12:19,350

295
00:12:19,350 --> 00:12:23,890
And we also have just a
visual representation there.

296
00:12:23,890 --> 00:12:24,930
Everyone good?

297
00:12:24,930 --> 00:12:25,630
For loops.

298
00:12:25,630 --> 00:12:27,540
Should have seen these in your pset.

299
00:12:27,540 --> 00:12:28,840
Cool.

300
00:12:28,840 --> 00:12:31,870
>> So here's just an easy example.

301
00:12:31,870 --> 00:12:32,860
Print This is CS50!

302
00:12:32,860 --> 00:12:33,360
10 times.

303
00:12:33,360 --> 00:12:34,850

304
00:12:34,850 --> 00:12:37,906
And so we have our initialization,
as we see there, with int i

305
00:12:37,906 --> 00:12:41,280
equals 0, for i is less
than 10, and i plus plus.

306
00:12:41,280 --> 00:12:43,090

307
00:12:43,090 --> 00:12:44,540
And it'll print that 10 times.

308
00:12:44,540 --> 00:12:46,230

309
00:12:46,230 --> 00:12:56,781
>> So while loops-- while loops are
great when you don't necessarily

310
00:12:56,781 --> 00:12:59,530
have to know how many times it's
going to update in the beginning.

311
00:12:59,530 --> 00:13:00,640

312
00:13:00,640 --> 00:13:02,580
You just have some
condition that's checked.

313
00:13:02,580 --> 00:13:09,550
And this could be something like while--
let's take an example from your pset.

314
00:13:09,550 --> 00:13:12,540
If Mario, you try to
input a negative number.

315
00:13:12,540 --> 00:13:13,270
Right?

316
00:13:13,270 --> 00:13:15,460
You were supposed to
re-prompt your user.

317
00:13:15,460 --> 00:13:20,890
So you can say, well, if the user inputs
something less than zero, re-prompt

318
00:13:20,890 --> 00:13:21,390
them.

319
00:13:21,390 --> 00:13:23,890
And I'm sure that might have
been something that some of you

320
00:13:23,890 --> 00:13:24,660
used in your code.

321
00:13:24,660 --> 00:13:26,270
>> So it's a simple thing.

322
00:13:26,270 --> 00:13:28,490
You have while, some
conditional that is checked

323
00:13:28,490 --> 00:13:30,460
every time the code goes to execute.

324
00:13:30,460 --> 00:13:32,660
If it evaluates to true, we run it.

325
00:13:32,660 --> 00:13:33,820
Otherwise we don't.

326
00:13:33,820 --> 00:13:35,650
And what's really
important-- something I

327
00:13:35,650 --> 00:13:38,155
think that David talked about
in lecture-- are the braces.

328
00:13:38,155 --> 00:13:40,760

329
00:13:40,760 --> 00:13:42,930
Whatever's within the
braces is what's executed.

330
00:13:42,930 --> 00:13:45,850
If you forget those
braces, it's only going

331
00:13:45,850 --> 00:13:48,800
to be the line directly
after the while was executed.

332
00:13:48,800 --> 00:13:51,040
>> So if you have three
things that are supposed

333
00:13:51,040 --> 00:13:54,020
to happen when this
condition evaluates to true,

334
00:13:54,020 --> 00:13:58,040
and you don't have those braces, only
the first thing is going to happen.

335
00:13:58,040 --> 00:14:01,510
So be very cognizant of
where you put your braces.

336
00:14:01,510 --> 00:14:04,880
If you stick with Style50,
this will definitely help you.

337
00:14:04,880 --> 00:14:06,220

338
00:14:06,220 --> 00:14:06,760
Cool.

339
00:14:06,760 --> 00:14:10,835
>> So this is a countdown from 10 to zero.

340
00:14:10,835 --> 00:14:12,380

341
00:14:12,380 --> 00:14:16,210
And as you see here, we initialize
some counter outside of it.

342
00:14:16,210 --> 00:14:19,250
One thing that's different is
we're not initializing our variable

343
00:14:19,250 --> 00:14:20,300
within our while loop.

344
00:14:20,300 --> 00:14:22,150
It's initialized outside of it.

345
00:14:22,150 --> 00:14:25,480
We are simply just putting the
condition in for our while.

346
00:14:25,480 --> 00:14:28,630
So in this case, it's while
count is greater than zero.

347
00:14:28,630 --> 00:14:35,134
And we print out what our count is,
and then we decrement our variable.

348
00:14:35,134 --> 00:14:36,800
And that's also another thing to notice.

349
00:14:36,800 --> 00:14:40,440
Our update doesn't happen within
that first part of the while loop.

350
00:14:40,440 --> 00:14:44,255
It will actually happen within
the braces, the body of your text.

351
00:14:44,255 --> 00:14:46,820

352
00:14:46,820 --> 00:14:51,860
>> So do-while loops-- do-while loops
are great for user validation.

353
00:14:51,860 --> 00:14:54,820
So some of you might have
also used this in your pset.

354
00:14:54,820 --> 00:14:59,960
You can say, do, like,
ask the user for input.

355
00:14:59,960 --> 00:15:04,690
And then while, like, the
input is less than some number.

356
00:15:04,690 --> 00:15:10,700
>> So for an explicit case with Mario, it
would be do printf, enter an integer,

357
00:15:10,700 --> 00:15:13,820
and then some integer equals getint.

358
00:15:13,820 --> 00:15:17,060
And then it'll actually
execute that code first.

359
00:15:17,060 --> 00:15:19,120
You'll actually have
some sort of integer.

360
00:15:19,120 --> 00:15:23,380
And then you could say, while
that integer is less than zero.

361
00:15:23,380 --> 00:15:26,120
So what it's going to do is it's
going to execute at least once.

362
00:15:26,120 --> 00:15:27,536
It's going to check the condition.

363
00:15:27,536 --> 00:15:29,690
If the condition is
true, it'll run again.

364
00:15:29,690 --> 00:15:32,900
>> So do-while loops are
great for user validation,

365
00:15:32,900 --> 00:15:36,770
because you know the code
is going to execute at least

366
00:15:36,770 --> 00:15:39,720
once, whereas with
while loops, you're not

367
00:15:39,720 --> 00:15:41,630
guaranteed that it's
going to execute once.

368
00:15:41,630 --> 00:15:45,330
It's going to check the condition
first and then decide to execute it,

369
00:15:45,330 --> 00:15:48,350
while a do-while will
execute the code first

370
00:15:48,350 --> 00:15:51,110
and then check to see whether
you need to repeat it.

371
00:15:51,110 --> 00:15:52,985
Does that distinction
make sense to everyone?

372
00:15:52,985 --> 00:15:53,680

373
00:15:53,680 --> 00:15:54,180
OK.

374
00:15:54,180 --> 00:15:55,651

375
00:15:55,651 --> 00:15:56,150
Cool.

376
00:15:56,150 --> 00:15:57,380

377
00:15:57,380 --> 00:15:59,800
>> So in this case, this is kind
of what I was talking about,

378
00:15:59,800 --> 00:16:02,210
this re-prompts until you
get a positive number.

379
00:16:02,210 --> 00:16:05,060
So we know that printf
"enter a positive number"

380
00:16:05,060 --> 00:16:08,720
and actually asking for that
input will happen at least once.

381
00:16:08,720 --> 00:16:12,650
If the user is evil and keeps
entering a negative number, who

382
00:16:12,650 --> 00:16:14,210
knows how many times it'll execute.

383
00:16:14,210 --> 00:16:17,500
But this code is guaranteed
to execute at least once.

384
00:16:17,500 --> 00:16:20,040
And that's why it's great
for validating input.

385
00:16:20,040 --> 00:16:22,300
And you will use that quite a bit.

386
00:16:22,300 --> 00:16:23,870

387
00:16:23,870 --> 00:16:25,830
>> All right, any questions so far?

388
00:16:25,830 --> 00:16:27,080

389
00:16:27,080 --> 00:16:28,030
We're all good?

390
00:16:28,030 --> 00:16:29,513
Am I talking too fast?

391
00:16:29,513 --> 00:16:30,520

392
00:16:30,520 --> 00:16:31,130
We're good?

393
00:16:31,130 --> 00:16:31,630
OK.

394
00:16:31,630 --> 00:16:32,960

395
00:16:32,960 --> 00:16:33,800
Awesome

396
00:16:33,800 --> 00:16:35,841
>> So we're going to go ahead
and talk about arrays.

397
00:16:35,841 --> 00:16:36,660

398
00:16:36,660 --> 00:16:37,600
Cool.

399
00:16:37,600 --> 00:16:40,700
So arrays are basically
just data structures

400
00:16:40,700 --> 00:16:43,260
that allow us to store
things of the same type.

401
00:16:43,260 --> 00:16:47,920
So if you ever have an array, it's
either just going to have ints

402
00:16:47,920 --> 00:16:51,590
or it's just going to have floats
or it's just going to have chars.

403
00:16:51,590 --> 00:16:55,440
You're not going to have an int with
a char with a float with a double.

404
00:16:55,440 --> 00:16:56,850
One thing.

405
00:16:56,850 --> 00:17:00,810
Arrays are just one size,
or they're just one type.

406
00:17:00,810 --> 00:17:05,679
>> So here we have an array of size
three with three integers in it.

407
00:17:05,679 --> 00:17:07,970
They could floats, but we're
going to say they're ints.

408
00:17:07,970 --> 00:17:09,310

409
00:17:09,310 --> 00:17:16,970
So one thing to realize is that arrays
are a set size when you initialize

410
00:17:16,970 --> 00:17:22,285
them, and they are not easily--
since you all are less comfortable,

411
00:17:22,285 --> 00:17:25,034
you should just think of them as
not being able to extend in size.

412
00:17:25,034 --> 00:17:26,119

413
00:17:26,119 --> 00:17:28,740
However big you set out
your array in the beginning,

414
00:17:28,740 --> 00:17:31,070
that's the size it's going
to stay, because arrays

415
00:17:31,070 --> 00:17:33,070
are continuous blocks of memory.

416
00:17:33,070 --> 00:17:34,007

417
00:17:34,007 --> 00:17:35,840
And when you guys get
into a little bit more

418
00:17:35,840 --> 00:17:40,820
of how memory's actually laid out on
disks and in the heap and the stack,

419
00:17:40,820 --> 00:17:42,220
it'll make a little more sense.

420
00:17:42,220 --> 00:17:48,310
>> But you could just think of like, it's
just a row of spaces on your disk.

421
00:17:48,310 --> 00:17:52,540
And you can't guarantee that there's
going to be free space after it.

422
00:17:52,540 --> 00:17:55,290
You could initialize an array
of three and then maybe you

423
00:17:55,290 --> 00:17:59,190
initialize another array of five
later, and it's right after that.

424
00:17:59,190 --> 00:18:04,000
So if you were to go past spot
three in that first array,

425
00:18:04,000 --> 00:18:06,370
you would be writing
over something else.

426
00:18:06,370 --> 00:18:10,390
So arrays are-- for you guys, just
think of them as a fixed size.

427
00:18:10,390 --> 00:18:12,700

428
00:18:12,700 --> 00:18:16,890
>> So creating an array-- you're going
to need to do this quite a bit.

429
00:18:16,890 --> 00:18:18,240

430
00:18:18,240 --> 00:18:22,080
So in the same way that we have a
general structure for our for loops,

431
00:18:22,080 --> 00:18:24,910
we have a nice general
structure for our array.

432
00:18:24,910 --> 00:18:28,430
Because they are of one type, all the
elements in an array are of one type,

433
00:18:28,430 --> 00:18:30,950
you need to initialize
what that type is.

434
00:18:30,950 --> 00:18:33,804
>> So, as you see here, we have a
nice little bracket data type.

435
00:18:33,804 --> 00:18:35,970
So if we're creating an int
array, that will be int.

436
00:18:35,970 --> 00:18:38,830
If we're creating a char
array, it'll be char.

437
00:18:38,830 --> 00:18:41,890
If we're creating a string
array, it'll be string.

438
00:18:41,890 --> 00:18:45,140
And then the name of your array,
whatever you'd like it to be.

439
00:18:45,140 --> 00:18:51,750
So maybe it's test scores or maybe
it's students or maybe it's candy.

440
00:18:51,750 --> 00:18:54,440
Whatever you decide to name your
array, that's what it'll be.

441
00:18:54,440 --> 00:18:57,260
>> And then in brackets, you'll
have the size that you want.

442
00:18:57,260 --> 00:19:01,350
So, are we storing 10 students or
are we storing 15 types of candy?

443
00:19:01,350 --> 00:19:03,200

444
00:19:03,200 --> 00:19:03,920
What not.

445
00:19:03,920 --> 00:19:06,830
So in our example here,
we're creating an array

446
00:19:06,830 --> 00:19:10,100
of size three, which you guys
see right here on the right.

447
00:19:10,100 --> 00:19:11,350

448
00:19:11,350 --> 00:19:15,990
And when we first initialize
it, everything is set to zero.

449
00:19:15,990 --> 00:19:18,970
So it's just thought
of like a blank slate.

450
00:19:18,970 --> 00:19:22,550
We have all these spaces, we have all
these boxes we could put our data into,

451
00:19:22,550 --> 00:19:25,530
but they're just blank
for the time being.

452
00:19:25,530 --> 00:19:28,700
>> So if we want to actually
assign them these values,

453
00:19:28,700 --> 00:19:33,050
we do so as right under here shows.

454
00:19:33,050 --> 00:19:39,170
So you have whatever the name of your
array is and then what index you want.

455
00:19:39,170 --> 00:19:43,070
So the index just refers to,
like, what slot we're looking at.

456
00:19:43,070 --> 00:19:47,830
And an important thing to notice
is that arrays are zero-indexed.

457
00:19:47,830 --> 00:19:53,160
So if we want the first space in memory
of our array, it's going to be zero.

458
00:19:53,160 --> 00:19:54,740
If we want the second, it'll be one.

459
00:19:54,740 --> 00:19:56,198
If we want the third, it'll be two.

460
00:19:56,198 --> 00:19:57,820
So on and so forth.

461
00:19:57,820 --> 00:20:02,130
>> Which is also why, conventionally,
when we do for loops-- I'm sure

462
00:20:02,130 --> 00:20:05,030
you guys were wondering, why
do we start at 0 versus 1?

463
00:20:05,030 --> 00:20:09,909
And that's because when we transition
into using arrays, it maps correctly.

464
00:20:09,909 --> 00:20:11,700
So if you want to
iterate into an array, it

465
00:20:11,700 --> 00:20:14,830
makes a lot more sense to
do i equals 0, because we

466
00:20:14,830 --> 00:20:17,940
know that will correspond
to the first spot in memory.

467
00:20:17,940 --> 00:20:18,990

468
00:20:18,990 --> 00:20:19,990
Everyone good with that?

469
00:20:19,990 --> 00:20:21,060

470
00:20:21,060 --> 00:20:21,560
Cool.

471
00:20:21,560 --> 00:20:26,240
>> And then on the bottom here is just
another way to initialize an array.

472
00:20:26,240 --> 00:20:29,590
You still have your
data type and the name,

473
00:20:29,590 --> 00:20:31,960
but instead of actually
putting a size in there,

474
00:20:31,960 --> 00:20:33,830
you can just do empty brackets.

475
00:20:33,830 --> 00:20:36,350
And then with these curly
braces at the bottom,

476
00:20:36,350 --> 00:20:40,270
you can just input the data that
you want separated by commas.

477
00:20:40,270 --> 00:20:43,070
And that will automatically
say, OK, I see

478
00:20:43,070 --> 00:20:47,300
that you have three
things in these braces.

479
00:20:47,300 --> 00:20:51,410
So I know that I need to allocate three
blocks of memory and then store those.

480
00:20:51,410 --> 00:20:55,300

481
00:20:55,300 --> 00:21:01,670
>> So the first version you might use
if you're asking your user to input

482
00:21:01,670 --> 00:21:04,770
values so that you can
iterate through the array

483
00:21:04,770 --> 00:21:07,660
and ask-- get some int to input them.

484
00:21:07,660 --> 00:21:10,500
If you know the values beforehand,
it makes a lot more sense

485
00:21:10,500 --> 00:21:12,110
to use the second way.

486
00:21:12,110 --> 00:21:17,270
But in most cases, you might not know
what those values are going to be.

487
00:21:17,270 --> 00:21:19,060

488
00:21:19,060 --> 00:21:19,560
Cool.

489
00:21:19,560 --> 00:21:20,393
Any other questions?

490
00:21:20,393 --> 00:21:21,206

491
00:21:21,206 --> 00:21:21,705
Alright.

492
00:21:21,705 --> 00:21:25,960

493
00:21:25,960 --> 00:21:30,370
>> So accessing elements-- so one
of the great things about arrays

494
00:21:30,370 --> 00:21:33,310
is that they are random
access, meaning that you

495
00:21:33,310 --> 00:21:36,010
don't have to look through every block.

496
00:21:36,010 --> 00:21:38,150
If you know that you
want what's in block two,

497
00:21:38,150 --> 00:21:40,820
you can just say, give me block two.

498
00:21:40,820 --> 00:21:44,160
And that's why these
indices are so important,

499
00:21:44,160 --> 00:21:46,220
and that's how we actually access them.

500
00:21:46,220 --> 00:21:50,660
So in this case, as we saw before when
we were assigning values, in the one

501
00:21:50,660 --> 00:21:55,360
before, we had the name and the
index we wanted to access, right?

502
00:21:55,360 --> 00:21:59,370
So in the same way, that's all we
do to actually pull that data out.

503
00:21:59,370 --> 00:22:02,940
We have the name and we
have the index that we want.

504
00:22:02,940 --> 00:22:06,710
>> So in this case, the for loop
down here at the bottom, anyone

505
00:22:06,710 --> 00:22:09,060
know what it's doing,
what it would print out?

506
00:22:09,060 --> 00:22:13,771

507
00:22:13,771 --> 00:22:14,270
Mmhmm?

508
00:22:14,270 --> 00:22:17,790

509
00:22:17,790 --> 00:22:18,520
Exactly.

510
00:22:18,520 --> 00:22:20,130
So yeah, it's just iterating through.

511
00:22:20,130 --> 00:22:23,410
i is equal to zero-- we can walk
through the code just quickly.

512
00:22:23,410 --> 00:22:26,680
i is equal to zero, i is less
than three at this point, right?

513
00:22:26,680 --> 00:22:27,780
So that checks out.

514
00:22:27,780 --> 00:22:33,370
And we say, OK, print f
whatever is in temperature i.

515
00:22:33,370 --> 00:22:37,990
i is zero right here when we first
iterate, so we go to this first spot,

516
00:22:37,990 --> 00:22:40,840
and we say, OK, 65 is the
number we want to print out.

517
00:22:40,840 --> 00:22:43,570
So it'll print out 65
and then do a new line.

518
00:22:43,570 --> 00:22:46,550
i will update, so it prints 87.

519
00:22:46,550 --> 00:22:48,780
It updates again, and it'll print 30.

520
00:22:48,780 --> 00:22:49,810

521
00:22:49,810 --> 00:22:50,630
Everyone cool?

522
00:22:50,630 --> 00:22:51,630

523
00:22:51,630 --> 00:22:52,130
Awesome.

524
00:22:52,130 --> 00:22:54,340

525
00:22:54,340 --> 00:22:54,840
>> All right.

526
00:22:54,840 --> 00:22:57,710
So here's kind of one
thing I was saying how

527
00:22:57,710 --> 00:23:00,020
you can keep track of
someone's score and why

528
00:23:00,020 --> 00:23:03,300
you would use the first
way of initializing it

529
00:23:03,300 --> 00:23:04,890
instead of that second way.

530
00:23:04,890 --> 00:23:06,620
And this just goes through.

531
00:23:06,620 --> 00:23:08,945
And notice we have a class size of 30.

532
00:23:08,945 --> 00:23:10,690

533
00:23:10,690 --> 00:23:15,770
And we're initializing this
array of ints that is of size 30.

534
00:23:15,770 --> 00:23:18,070
And then we are iterating
through and we're

535
00:23:18,070 --> 00:23:20,910
asking the user to input
scores for each of these

536
00:23:20,910 --> 00:23:23,700
and then assigning it to
a specific place in memory

537
00:23:23,700 --> 00:23:24,860
somewhere in that array.

538
00:23:24,860 --> 00:23:26,700

539
00:23:26,700 --> 00:23:27,890
Cool?

540
00:23:27,890 --> 00:23:29,650
Does that make sense to everyone?

541
00:23:29,650 --> 00:23:30,150
Mmhmm?

542
00:23:30,150 --> 00:23:32,910

543
00:23:32,910 --> 00:23:40,360
>> So hashtag define class size
30 is a preprocessor directive,

544
00:23:40,360 --> 00:23:43,780
which just means it gets-- it has
to do like the compiling process.

545
00:23:43,780 --> 00:23:47,595
You can think of it
as a global variable.

546
00:23:47,595 --> 00:23:49,260

547
00:23:49,260 --> 00:23:53,030
The way we do it is
typically-- it allows your code

548
00:23:53,030 --> 00:23:54,770
to be more easily changed.

549
00:23:54,770 --> 00:23:59,480
So let's say that our class size
suddenly goes from 30 to 15,

550
00:23:59,480 --> 00:24:03,930
if I hadn't defined it this way, I would
have to go through my entire program

551
00:24:03,930 --> 00:24:07,420
and change every instance of 30 to 15.

552
00:24:07,420 --> 00:24:11,590
But with this, I get to change one
spot, and everything else changes.

553
00:24:11,590 --> 00:24:14,930
>> If you ever want to do
a hash define in a case

554
00:24:14,930 --> 00:24:19,020
where you're keeping track of some
set number of scores for a class

555
00:24:19,020 --> 00:24:23,250
or you're using a number
that will be used, like,

556
00:24:23,250 --> 00:24:27,740
throughout a very long program, it's
better to define that at the beginning

557
00:24:27,740 --> 00:24:34,440
so that if ever it changes, you get
to change one spot instead of 100.

558
00:24:34,440 --> 00:24:34,940
Yes?

559
00:24:34,940 --> 00:24:39,830
>> STUDENT: Between doing that and just
declaring [INAUDIBLE] over at the top.

560
00:24:39,830 --> 00:24:42,470
>> ALLISON BUCHHOLTZ-AU: So it
has to do with efficient--

561
00:24:42,470 --> 00:24:45,460
it's kind of outside the scope of
what we can cover in this section.

562
00:24:45,460 --> 00:24:49,236
It has to do more with efficiency
and how things actually

563
00:24:49,236 --> 00:24:50,485
work in the compiling process.

564
00:24:50,485 --> 00:24:52,590

565
00:24:52,590 --> 00:24:55,006
If you want to really know
about it, I'm happy to send you

566
00:24:55,006 --> 00:24:56,450
an email with resources about it.

567
00:24:56,450 --> 00:24:57,810

568
00:24:57,810 --> 00:25:03,880
Hash define tends to be
preferred for things.

569
00:25:03,880 --> 00:25:06,380
And as you code more, you
kind of learn the nuances

570
00:25:06,380 --> 00:25:09,135
of when you should use a
global versus the hash define.

571
00:25:09,135 --> 00:25:11,010
But for the time being,
you don't really have

572
00:25:11,010 --> 00:25:13,290
to worry about it is
that the short answer.

573
00:25:13,290 --> 00:25:15,360

574
00:25:15,360 --> 00:25:16,360
Everyone good with that?

575
00:25:16,360 --> 00:25:18,151
And also, if you want
to use a hash define,

576
00:25:18,151 --> 00:25:21,120
it's really important to notice
that the name should be in all caps.

577
00:25:21,120 --> 00:25:22,210

578
00:25:22,210 --> 00:25:24,674
We're not just doing
CLASS SIZE to be dramatic.

579
00:25:24,674 --> 00:25:26,090
It should actually be in all caps.

580
00:25:26,090 --> 00:25:27,880

581
00:25:27,880 --> 00:25:28,610
Cool.

582
00:25:28,610 --> 00:25:30,130
Anything else there?

583
00:25:30,130 --> 00:25:31,190
We're good?

584
00:25:31,190 --> 00:25:32,220
Lovely.

585
00:25:32,220 --> 00:25:32,720
Welcome.

586
00:25:32,720 --> 00:25:34,240

587
00:25:34,240 --> 00:25:38,495
>> OK, so I want you guys to take a look
at this and see if you can find the bug.

588
00:25:38,495 --> 00:25:45,580

589
00:25:45,580 --> 00:25:46,507
I'll give you a hint.

590
00:25:46,507 --> 00:25:47,840
It's somewhere in that for-loop.

591
00:25:47,840 --> 00:25:53,070

592
00:25:53,070 --> 00:25:53,665
Mmhmm?

593
00:25:53,665 --> 00:25:55,704
>> STUDENT: Should be less than equal to 2.

594
00:25:55,704 --> 00:25:56,920

595
00:25:56,920 --> 00:26:01,094
>> ALLISON: So it could be less than or
equal to 2, or it could be less than 3.

596
00:26:01,094 --> 00:26:02,510
And what's the reasoning for that?

597
00:26:02,510 --> 00:26:05,135
>> STUDENT: The [INAUDIBLE], 0, 1, 2.

598
00:26:05,135 --> 00:26:06,080
>> ALLISON: Exactly.

599
00:26:06,080 --> 00:26:12,180
So in an array of size n, we
only have indices of n minus 1.

600
00:26:12,180 --> 00:26:13,700

601
00:26:13,700 --> 00:26:14,200
Cool.

602
00:26:14,200 --> 00:26:15,091

603
00:26:15,091 --> 00:26:17,840
And then we can get really crazy
and get multi-dimensional arrays.

604
00:26:17,840 --> 00:26:19,340

605
00:26:19,340 --> 00:26:23,960
One of the problems when I took it in my
year required multi-dimensional arrays,

606
00:26:23,960 --> 00:26:28,720
and I think one of them might require
it this year, so be comfortable.

607
00:26:28,720 --> 00:26:30,140
Wrap your head around it now.

608
00:26:30,140 --> 00:26:33,087
It will come back to haunt
you, but in a cool way.

609
00:26:33,087 --> 00:26:35,420
So you can really just think
of multi-dimensional arrays

610
00:26:35,420 --> 00:26:36,870
as arrays of arrays.

611
00:26:36,870 --> 00:26:38,060

612
00:26:38,060 --> 00:26:43,809
So you can kind of think of this top
row as the first chunk of memory.

613
00:26:43,809 --> 00:26:45,600
And this one is the
second chunk of memory,

614
00:26:45,600 --> 00:26:47,570
and the last row is the
third chunk of memory.

615
00:26:47,570 --> 00:26:50,740
And within that, there's an array.

616
00:26:50,740 --> 00:26:53,250
But of course, it's easier
to depict like this.

617
00:26:53,250 --> 00:26:55,610
>> So you initialize it the same way.

618
00:26:55,610 --> 00:26:57,290

619
00:26:57,290 --> 00:27:01,610
This is a character
board of three by three.

620
00:27:01,610 --> 00:27:04,180
So you have three rows
and three columns.

621
00:27:04,180 --> 00:27:06,760
We're representing it this way.

622
00:27:06,760 --> 00:27:11,800
And you would access it the
same way, column by row.

623
00:27:11,800 --> 00:27:14,840
And so 1,1 as we see here.

624
00:27:14,840 --> 00:27:18,730
We assign a zero, zero up there.

625
00:27:18,730 --> 00:27:21,900
2,0 and 0,2.

626
00:27:21,900 --> 00:27:26,900
So you would just access them-- if
anyone's ever done linear algebra,

627
00:27:26,900 --> 00:27:31,580
the same way you access an element in
a matrice, it's the same idea here.

628
00:27:31,580 --> 00:27:33,030
So you can relate it back to math.

629
00:27:33,030 --> 00:27:34,130

630
00:27:34,130 --> 00:27:36,380
You don't have to worry too
much about this right now.

631
00:27:36,380 --> 00:27:38,671
It's good to have exposure,
to know that you can do it.

632
00:27:38,671 --> 00:27:42,700
You can create some
crazy number-- you can

633
00:27:42,700 --> 00:27:44,750
create crazy arrays is
all I'm going to say.

634
00:27:44,750 --> 00:27:45,250
[INAUDIBLE]

635
00:27:45,250 --> 00:27:46,585

636
00:27:46,585 --> 00:27:48,460
It gets a little crazy,
but it's really cool.

637
00:27:48,460 --> 00:27:50,020

638
00:27:50,020 --> 00:27:50,670
Awesome.

639
00:27:50,670 --> 00:27:52,550
>> And then, so we have an example here.

640
00:27:52,550 --> 00:27:54,165
It calculates a string length.

641
00:27:54,165 --> 00:27:55,210

642
00:27:55,210 --> 00:27:59,670
So how many people knew that
the strings that you're using

643
00:27:59,670 --> 00:28:01,265
are just arrays of characters?

644
00:28:01,265 --> 00:28:03,670

645
00:28:03,670 --> 00:28:04,655
OK, yeah.

646
00:28:04,655 --> 00:28:08,000
So you guys may think that you haven't
used arrays before, but any time

647
00:28:08,000 --> 00:28:10,740
you use getstring in the
CSView library, you're

648
00:28:10,740 --> 00:28:14,690
actually just asking for
an array of characters.

649
00:28:14,690 --> 00:28:17,450
And we're taking care of all
that in the back-end for you.

650
00:28:17,450 --> 00:28:20,184
But you have been using
arrays since you started.

651
00:28:20,184 --> 00:28:21,350
You just didn't know it yet.

652
00:28:21,350 --> 00:28:22,420

653
00:28:22,420 --> 00:28:28,390
>> And whenever you have a character array
or an array that's storing a string,

654
00:28:28,390 --> 00:28:32,050
the last thing is always what's
called a null terminator, which

655
00:28:32,050 --> 00:28:33,830
is this right here.

656
00:28:33,830 --> 00:28:37,300
And that is at the end of
every word that you're storing.

657
00:28:37,300 --> 00:28:41,170
So if we want to figure out the
length of a string, we can say,

658
00:28:41,170 --> 00:28:45,320
well, you know, the
contents of that block

659
00:28:45,320 --> 00:28:46,867
is not equal to our null terminator.

660
00:28:46,867 --> 00:28:49,450
That means that there is some
character there that we actually

661
00:28:49,450 --> 00:28:51,540
care about that's part of the word.

662
00:28:51,540 --> 00:28:52,665
You increase your length.

663
00:28:52,665 --> 00:28:55,540
And then when we actually get to
the end of the word, it'll terminate

664
00:28:55,540 --> 00:28:58,790
and it'll return our length for us.

665
00:28:58,790 --> 00:29:00,098
Mmhmm?

666
00:29:00,098 --> 00:29:02,906
>> STUDENT: Does the space
count as the null terminator?

667
00:29:02,906 --> 00:29:04,780
ALLISON: So a space is
not a null terminator.

668
00:29:04,780 --> 00:29:10,590
So if you have multiple-- a space
is actually a specific ASCII value.

669
00:29:10,590 --> 00:29:11,719

670
00:29:11,719 --> 00:29:13,552
STUDENT: What's the
exclamation equal again?

671
00:29:13,552 --> 00:29:17,080
ALLISON: So, this is what you refer to.

672
00:29:17,080 --> 00:29:21,220
If you ever hear me in office hours,
I always call it, like, bang equals.

673
00:29:21,220 --> 00:29:23,520
So bang is not.

674
00:29:23,520 --> 00:29:25,370
So this is not equals.

675
00:29:25,370 --> 00:29:28,480

676
00:29:28,480 --> 00:29:32,290
So if you're trying to
see if something's false,

677
00:29:32,290 --> 00:29:35,915
you know always do, bang whatever
the variable is, and if it's false,

678
00:29:35,915 --> 00:29:38,540
it evaluates to true and you
can do cool stuff with that.

679
00:29:38,540 --> 00:29:39,830

680
00:29:39,830 --> 00:29:40,675
More on that later.

681
00:29:40,675 --> 00:29:42,090

682
00:29:42,090 --> 00:29:42,590
Cool.

683
00:29:42,590 --> 00:29:44,260
Everything good there?

684
00:29:44,260 --> 00:29:44,760
Awesome.

685
00:29:44,760 --> 00:29:48,045
>> So now it's your guys' time to
work, since I've been talking.

686
00:29:48,045 --> 00:29:49,220

687
00:29:49,220 --> 00:29:53,200
So I want you to just create an array
with the integers one, two, and three,

688
00:29:53,200 --> 00:29:54,660
and then have them printed out.

689
00:29:54,660 --> 00:29:58,050
You don't have to do, like,
main, blah, blah, blah, whatever.

690
00:29:58,050 --> 00:30:00,840
I just want you to initialize
the array and then create

691
00:30:00,840 --> 00:30:04,790
a for loop to print them out--
or a while loop, up to you.

692
00:30:04,790 --> 00:30:07,600
>> I'll just give you a couple
minutes to work on that.

693
00:30:07,600 --> 00:30:09,320
I'm going to rest my voice.

694
00:30:09,320 --> 00:30:13,290
If you have any questions, I'm happy
to come around and talk to you guys.

695
00:30:13,290 --> 00:30:15,290
Feel free to talk with each other.

696
00:30:15,290 --> 00:30:16,125
Get more candy.

697
00:30:16,125 --> 00:30:17,181

698
00:30:17,181 --> 00:30:18,930
In fact, I'll just
walk around with candy.

699
00:30:18,930 --> 00:30:19,600
How's that?

700
00:30:19,600 --> 00:30:22,974

701
00:30:22,974 --> 00:30:23,938
>> Do you want any?

702
00:30:23,938 --> 00:30:34,703

703
00:30:34,703 --> 00:30:36,675
Anyone else in this room want candy?

704
00:30:36,675 --> 00:30:47,030

705
00:30:47,030 --> 00:30:49,206
You can also take more than one, guys.

706
00:30:49,206 --> 00:30:50,330
Take a handful if you want.

707
00:30:50,330 --> 00:30:52,690

708
00:30:52,690 --> 00:30:53,190
May as well.

709
00:30:53,190 --> 00:30:54,929

710
00:30:54,929 --> 00:30:55,720
Everyone else good?

711
00:30:55,720 --> 00:30:57,561

712
00:30:57,561 --> 00:30:58,060
OK.

713
00:30:58,060 --> 00:31:02,750

714
00:31:02,750 --> 00:31:07,290
>> Also, I'm going to create
an anonymous Google Form,

715
00:31:07,290 --> 00:31:09,830
and you guys can just submit
feedback after every section

716
00:31:09,830 --> 00:31:14,115
if there's something you want to
improve upon or something you want done.

717
00:31:14,115 --> 00:31:17,135
If I'm a little too peppy
for you, I can tone it down.

718
00:31:17,135 --> 00:31:18,510

719
00:31:18,510 --> 00:31:20,885
I'll create that and send that
out to you all afterwards.

720
00:31:20,885 --> 00:32:39,420

721
00:32:39,420 --> 00:32:40,020
>> All right.

722
00:32:40,020 --> 00:32:41,050
So let's start small.

723
00:32:41,050 --> 00:32:42,740
How would we initialize our array?

724
00:32:42,740 --> 00:32:49,690

725
00:32:49,690 --> 00:32:51,670
What's the type of our array?

726
00:32:51,670 --> 00:32:52,410
An int, right?

727
00:32:52,410 --> 00:32:54,740
OK, so what do you want
to call your array?

728
00:32:54,740 --> 00:32:57,880

729
00:32:57,880 --> 00:32:59,230
Int array, cool.

730
00:32:59,230 --> 00:33:04,200
All right, so we have int int array
equals, and what do we have after that?

731
00:33:04,200 --> 00:33:07,009

732
00:33:07,009 --> 00:33:08,259
STUDENT: [INAUDIBLE] brackets.

733
00:33:08,259 --> 00:33:09,140
ALLISON: Braces.

734
00:33:09,140 --> 00:33:10,330

735
00:33:10,330 --> 00:33:11,775
And then inside the braces?

736
00:33:11,775 --> 00:33:14,360

737
00:33:14,360 --> 00:33:16,830
One comma two comma three.

738
00:33:16,830 --> 00:33:17,330
Cool.

739
00:33:17,330 --> 00:33:18,660
So that's all right.

740
00:33:18,660 --> 00:33:21,480
So now we have our for loop.

741
00:33:21,480 --> 00:33:24,180
So in the first part of our
for loop, what do we have?

742
00:33:24,180 --> 00:33:25,796

743
00:33:25,796 --> 00:33:27,420
>> STUDENT: i equals 0?

744
00:33:27,420 --> 00:33:31,400
>> ALLISON: So int i equals 0,
and then what is our condition?

745
00:33:31,400 --> 00:33:33,750

746
00:33:33,750 --> 00:33:36,120
What's i going to be less than?

747
00:33:36,120 --> 00:33:39,510
Less than three, and
how we do we update i?

748
00:33:39,510 --> 00:33:41,630
i plus plus, updating it by one.

749
00:33:41,630 --> 00:33:46,150
And then we're going to have
some printf of the integer,

750
00:33:46,150 --> 00:33:49,530
and what is that last
part that's actually

751
00:33:49,530 --> 00:33:51,370
going to say what we should be printing?

752
00:33:51,370 --> 00:33:55,610

753
00:33:55,610 --> 00:33:59,120
It would be the name of the
array, which is int array, right?

754
00:33:59,120 --> 00:34:01,090
And what's in the brackets of int array?

755
00:34:01,090 --> 00:34:02,540

756
00:34:02,540 --> 00:34:03,640
i.

757
00:34:03,640 --> 00:34:06,550
[? So I ?] called my
example, but there you go.

758
00:34:06,550 --> 00:34:07,939

759
00:34:07,939 --> 00:34:08,480
Not that bad.

760
00:34:08,480 --> 00:34:09,250
Everyone good?

761
00:34:09,250 --> 00:34:10,220

762
00:34:10,220 --> 00:34:10,719
Cool.

763
00:34:10,719 --> 00:34:12,949

764
00:34:12,949 --> 00:34:14,340
>> So we're done with the arrays.

765
00:34:14,340 --> 00:34:15,190
Congrats.

766
00:34:15,190 --> 00:34:18,274
You managed to iterate
through all the-- yes?

767
00:34:18,274 --> 00:34:19,705
>> STUDENT: [INAUDIBLE]

768
00:34:19,705 --> 00:34:20,659
>> ALLISON: Yes.

769
00:34:20,659 --> 00:34:26,383

770
00:34:26,383 --> 00:34:27,798
>> STUDENT: I have a question.

771
00:34:27,798 --> 00:34:30,288
Are you supposed to indent the braces?

772
00:34:30,288 --> 00:34:34,150
>> ALLISON: So the braces should line up
with the for loop, and then everything

773
00:34:34,150 --> 00:34:35,699
inside the braces should be indented.

774
00:34:35,699 --> 00:34:37,824
>> STUDENT: OK, should the
for loop be indented?

775
00:34:37,824 --> 00:34:41,159
>> ALLISON: The for loop does not
need to be indented at this point.

776
00:34:41,159 --> 00:34:46,750
If you were in main, if we
actually had a main function here,

777
00:34:46,750 --> 00:34:48,929
it would be indented from main.

778
00:34:48,929 --> 00:34:52,590
But in this case, it's fine.

779
00:34:52,590 --> 00:34:53,354
Yes, question.

780
00:34:53,354 --> 00:34:55,687
>> STUDENT: Do you need to have
the brackets after example?

781
00:34:55,687 --> 00:34:59,050

782
00:34:59,050 --> 00:35:01,260
>> ALLISON: Yes, if you're
initializing it that way.

783
00:35:01,260 --> 00:35:03,430

784
00:35:03,430 --> 00:35:06,860
So remember, this is the second
way of initializing an array where

785
00:35:06,860 --> 00:35:10,680
we have the braces and then our actual
data separated by commas within.

786
00:35:10,680 --> 00:35:14,700

787
00:35:14,700 --> 00:35:17,281
>> STUDENT: I thought there were
brackets for that example.

788
00:35:17,281 --> 00:35:19,010
>> ALLISON: No, they're braces.

789
00:35:19,010 --> 00:35:20,039
They're braces.

790
00:35:20,039 --> 00:35:22,330
If you're initializing it
that second way, it's braces.

791
00:35:22,330 --> 00:35:28,780
If we were to say, int example-- if
we just wanted a blank array for ints,

792
00:35:28,780 --> 00:35:32,270
it would be int example brackets three.

793
00:35:32,270 --> 00:35:34,670
The brackets represent the size.

794
00:35:34,670 --> 00:35:36,720
When you have braces,
it's the actual data

795
00:35:36,720 --> 00:35:39,244
you're putting into it in this way.

796
00:35:39,244 --> 00:35:40,535
We can scroll back really fast.

797
00:35:40,535 --> 00:35:42,370

798
00:35:42,370 --> 00:35:48,010
>> So in this one, this is just our
initial array, initialization.

799
00:35:48,010 --> 00:35:51,640
And here, we are individually
assigning spots to them,

800
00:35:51,640 --> 00:35:56,730
so this represents the index of our
array, which is why we have brackets.

801
00:35:56,730 --> 00:36:01,630
But here, if you notice, we've
left our brackets without a size,

802
00:36:01,630 --> 00:36:05,969
and we initialize it with the
actual data all-in-one with braces.

803
00:36:05,969 --> 00:36:08,260
STUDENT: So why don't we have
brackets in this example?

804
00:36:08,260 --> 00:36:11,030
ALLISON: So, in which part?

805
00:36:11,030 --> 00:36:14,645
STUDENT: Wouldn't we
say, int example brackets

806
00:36:14,645 --> 00:36:19,710
equals braces [INAUDIBLE]
brackets for example.

807
00:36:19,710 --> 00:36:20,900
>> ALLISON: Oh, sorry.

808
00:36:20,900 --> 00:36:21,560
You're right.

809
00:36:21,560 --> 00:36:22,690
We do have brackets there.

810
00:36:22,690 --> 00:36:23,486
Sorry guys, my bad.

811
00:36:23,486 --> 00:36:25,150

812
00:36:25,150 --> 00:36:27,560
Yes, you should have
brackets after example.

813
00:36:27,560 --> 00:36:28,850
You're absolutely right.

814
00:36:28,850 --> 00:36:29,750
>> STUDENT: [INAUDIBLE] not doing it.

815
00:36:29,750 --> 00:36:31,270
>> ALLISON: No, you have to have
brackets, because otherwise it's

816
00:36:31,270 --> 00:36:32,810
not going to declare an array.

817
00:36:32,810 --> 00:36:33,685
STUDENT: [INAUDIBLE].

818
00:36:33,685 --> 00:36:34,200
Sorry about that.

819
00:36:34,200 --> 00:36:36,200
>> ALLISON: Sorry, you need
brackets after example.

820
00:36:36,200 --> 00:36:36,620

821
00:36:36,620 --> 00:36:37,120
Typo.

822
00:36:37,120 --> 00:36:40,650

823
00:36:40,650 --> 00:36:43,160
Good catch, gold star for you.

824
00:36:43,160 --> 00:36:47,730
Also, if you are asking a question, if
you guys would just tell me your names,

825
00:36:47,730 --> 00:36:48,570
I'd love that.

826
00:36:48,570 --> 00:36:50,630
I'd love to be able to
know all your names.

827
00:36:50,630 --> 00:36:53,187
I'm not going to cold
call you, I actually

828
00:36:53,187 --> 00:36:54,520
do just want to know your names.

829
00:36:54,520 --> 00:36:57,530
So please actually tell me your names.

830
00:36:57,530 --> 00:36:58,426
>> LEAH: Leah.

831
00:36:58,426 --> 00:36:59,340
>> ALLISON: Leah.

832
00:36:59,340 --> 00:37:03,960
OK, so functions-- I know in brief
they talked about this during lecture.

833
00:37:03,960 --> 00:37:07,590
So functions are kind of just
like these little bite-size things

834
00:37:07,590 --> 00:37:11,525
where you pass in inputs, something
magical happens, and you get outputs.

835
00:37:11,525 --> 00:37:12,680

836
00:37:12,680 --> 00:37:13,180
Cool.

837
00:37:13,180 --> 00:37:15,013
So you actually used a
lot of these already.

838
00:37:15,013 --> 00:37:15,970

839
00:37:15,970 --> 00:37:18,840
Get int, get string, print f.

840
00:37:18,840 --> 00:37:21,790
These are all functions
where you just call them,

841
00:37:21,790 --> 00:37:24,370
there's lots of magical things
going on in the background

842
00:37:24,370 --> 00:37:27,390
that you don't necessarily see,
and you get out what you want.

843
00:37:27,390 --> 00:37:30,280
Or at least you get
what you hope you want.

844
00:37:30,280 --> 00:37:31,920

845
00:37:31,920 --> 00:37:35,620
>> And basically the point of functions,
and one of the main themes of CS,

846
00:37:35,620 --> 00:37:38,700
is to break your code
into manageable pieces.

847
00:37:38,700 --> 00:37:41,520
When you start writing
these really long programs,

848
00:37:41,520 --> 00:37:44,970
or in Scratch when you had
this grand idea for a game,

849
00:37:44,970 --> 00:37:48,070
you need to be able to break it
down to, like, OK, how do I start?

850
00:37:48,070 --> 00:37:50,470
What are the little pieces that I need?

851
00:37:50,470 --> 00:37:53,320
Oh, I need to ask the
user for something.

852
00:37:53,320 --> 00:37:54,830
Now I need to print something.

853
00:37:54,830 --> 00:37:56,590
Oh, I need to calculate this value.

854
00:37:56,590 --> 00:38:00,050
>> And learning how to break up your
code and the large problems you

855
00:38:00,050 --> 00:38:02,740
have into these small pieces
and creating functions

856
00:38:02,740 --> 00:38:05,330
is actually one of the
big cornerstones of CS.

857
00:38:05,330 --> 00:38:06,440

858
00:38:06,440 --> 00:38:11,740
So you can think of a function just as
like a black box, a magical black box,

859
00:38:11,740 --> 00:38:15,030
that you put things into
and you get some output.

860
00:38:15,030 --> 00:38:16,290

861
00:38:16,290 --> 00:38:19,460
And the rest of the program
doesn't need to necessarily know

862
00:38:19,460 --> 00:38:21,150
what's going on within that black box.

863
00:38:21,150 --> 00:38:24,190
All it cares about is what
goes in and what comes out.

864
00:38:24,190 --> 00:38:26,230

865
00:38:26,230 --> 00:38:26,730
Cool.

866
00:38:26,730 --> 00:38:27,820
>> So why functions?

867
00:38:27,820 --> 00:38:33,240
Organization-- as I said, when you're
dealing with very large code bases,

868
00:38:33,240 --> 00:38:37,357
how you organize your code will be
much easier if you use functions.

869
00:38:37,357 --> 00:38:39,190
Because you'll be able
to be, like, OK, this

870
00:38:39,190 --> 00:38:41,849
is what this function does and
here's what another one does.

871
00:38:41,849 --> 00:38:43,890
And you can easily see
how they all fit together.

872
00:38:43,890 --> 00:38:45,590

873
00:38:45,590 --> 00:38:48,210
So breaking it up into all
these manageable subparts.

874
00:38:48,210 --> 00:38:51,860
>> So simplification-- I'm sure you guys
all saw this, as I said, with Scratch.

875
00:38:51,860 --> 00:38:53,230

876
00:38:53,230 --> 00:38:56,790
You have this grand idea, and
you're like, how does all this work?

877
00:38:56,790 --> 00:38:59,710
But if you approach it
piecemeal, you say, OK,

878
00:38:59,710 --> 00:39:03,650
how do I make one sprite
float across the screen?

879
00:39:03,650 --> 00:39:04,805
That's a little bit easier.

880
00:39:04,805 --> 00:39:06,000

881
00:39:06,000 --> 00:39:09,590
So good use of function makes
your code much easier to read.

882
00:39:09,590 --> 00:39:13,800
It makes it easier to debug which as
you get into your later problem sets,

883
00:39:13,800 --> 00:39:15,900
you're going to really
want to be able to do.

884
00:39:15,900 --> 00:39:17,900
And they're also easier
to design and implement.

885
00:39:17,900 --> 00:39:21,100
You can code up a small
function relatively quickly

886
00:39:21,100 --> 00:39:25,260
and make sure it works versus trying
to create this whole long program

887
00:39:25,260 --> 00:39:28,960
and then kind of go through and
see what's working and what's not.

888
00:39:28,960 --> 00:39:30,300
>> And then reusability.

889
00:39:30,300 --> 00:39:33,380
So functions only need
to be written once,

890
00:39:33,380 --> 00:39:35,586
and then you can use them
as many times as possible.

891
00:39:35,586 --> 00:39:37,210
So it's, like, eco-friendly in a sense.

892
00:39:37,210 --> 00:39:39,660

893
00:39:39,660 --> 00:39:42,070
If you had things like
print f, where you

894
00:39:42,070 --> 00:39:47,420
had to write out the magic that goes
on behind print f every single time

895
00:39:47,420 --> 00:39:51,040
you wanted to print something, you
would be pretty sick and tired of it

896
00:39:51,040 --> 00:39:51,650
by the end.

897
00:39:51,650 --> 00:39:52,660

898
00:39:52,660 --> 00:39:55,292
>> One of the things that you'll
learn in later CS classes,

899
00:39:55,292 --> 00:39:57,000
or one of the best
pieces of advice I get

900
00:39:57,000 --> 00:40:00,870
is, if you are copying and pasting
code, it should probably be a function.

901
00:40:00,870 --> 00:40:01,880

902
00:40:01,880 --> 00:40:06,007
If you have the exact same
lines all throughout your code,

903
00:40:06,007 --> 00:40:08,090
if you factored them out,
your code would probably

904
00:40:08,090 --> 00:40:11,860
be, like, five times shorter
and be much more easy to read.

905
00:40:11,860 --> 00:40:15,230
And instead of trying to troubleshoot
all these different places where things

906
00:40:15,230 --> 00:40:18,021
might go wrong, you have one function
that you get to troubleshoot.

907
00:40:18,021 --> 00:40:19,230

908
00:40:19,230 --> 00:40:21,890
And I promise, a lot of this
might seem kind of abstract now,

909
00:40:21,890 --> 00:40:24,620
but as you get into later
and later problem sets,

910
00:40:24,620 --> 00:40:28,950
it'll make a lot more sense
and really be driven home.

911
00:40:28,950 --> 00:40:31,775
Are there any questions
about functions so far?

912
00:40:31,775 --> 00:40:32,650
Why we're using them?

913
00:40:32,650 --> 00:40:34,775
I know we haven't gotten
into the nitty gritty yet.

914
00:40:34,775 --> 00:40:40,610
So defining a function-- just like
arrays, we need some sort of-- this

915
00:40:40,610 --> 00:40:41,860
is just the general output.

916
00:40:41,860 --> 00:40:45,110
So this is a function that's
just going to cube some input.

917
00:40:45,110 --> 00:40:50,120
And on the next page, actually, we have
all these awesome little things here.

918
00:40:50,120 --> 00:40:52,420
So, can everyone read
that, out of curiosity?

919
00:40:52,420 --> 00:40:55,050
I know the purple and black
might be a little hard.

920
00:40:55,050 --> 00:41:00,730
>> But big things to know-- so the first
one right here is our return type.

921
00:41:00,730 --> 00:41:05,270
So this is talking about the output
of this function when, in this case,

922
00:41:05,270 --> 00:41:08,990
we put in some number, what we're
getting is that number cubed.

923
00:41:08,990 --> 00:41:11,090
So it should be an int in this case.

924
00:41:11,090 --> 00:41:14,055
Maybe it would be a double or
something else later, but in this case,

925
00:41:14,055 --> 00:41:14,555
it's an int.

926
00:41:14,555 --> 00:41:15,620

927
00:41:15,620 --> 00:41:19,440
>> With c, you always need a return type.

928
00:41:19,440 --> 00:41:20,267
It'll be an int.

929
00:41:20,267 --> 00:41:20,975
It'll be a float.

930
00:41:20,975 --> 00:41:22,180
It'll be a double.

931
00:41:22,180 --> 00:41:25,370
But you have to specify what
this function is going to return.

932
00:41:25,370 --> 00:41:26,480

933
00:41:26,480 --> 00:41:29,170
Otherwise it will yell at
you, and it won't compile.

934
00:41:29,170 --> 00:41:30,462
You'll be sad, and I'll be sad.

935
00:41:30,462 --> 00:41:31,420
And it's just not good.

936
00:41:31,420 --> 00:41:31,940
OK.

937
00:41:31,940 --> 00:41:34,290
>> And then we have our function name.

938
00:41:34,290 --> 00:41:38,440
And as you can see here, with c
there's this very consistent paradigm.

939
00:41:38,440 --> 00:41:42,630
What's your type, what's the name,
and then some other thing at the end.

940
00:41:42,630 --> 00:41:45,550
So we have our return type,
our function name, and then

941
00:41:45,550 --> 00:41:49,320
we have our header with
our parameter list.

942
00:41:49,320 --> 00:41:52,080
>> So the parameter list is, what is
this function going to take in?

943
00:41:52,080 --> 00:41:56,360
A parameter list is simply a
synonym for, what are our inputs?

944
00:41:56,360 --> 00:42:00,230
And in the same way that we
have to define our function

945
00:42:00,230 --> 00:42:03,210
and give it a return
type, each of our inputs

946
00:42:03,210 --> 00:42:05,350
needs to have a type associated with it.

947
00:42:05,350 --> 00:42:08,850
So we know what our function
can actually work with.

948
00:42:08,850 --> 00:42:11,290
>> So in this case, we have some int input.

949
00:42:11,290 --> 00:42:14,660
So again, it'll be the type
and what you're calling it.

950
00:42:14,660 --> 00:42:16,500

951
00:42:16,500 --> 00:42:19,040
And then, as you see
here, we have our body.

952
00:42:19,040 --> 00:42:23,600
So we have some int output, that
is just our input times itself

953
00:42:23,600 --> 00:42:25,810
times itself, which just cubes it.

954
00:42:25,810 --> 00:42:27,610
And then we return that output.

955
00:42:27,610 --> 00:42:32,140
>> So as you see here, we have an
int times an int times an int,

956
00:42:32,140 --> 00:42:35,420
so it returns an int, which
has been declared there.

957
00:42:35,420 --> 00:42:38,240
So everything is cohesive.

958
00:42:38,240 --> 00:42:39,210
Everything's happy.

959
00:42:39,210 --> 00:42:40,250
Your function will run.

960
00:42:40,250 --> 00:42:41,192

961
00:42:41,192 --> 00:42:42,650
And this is just the general thing.

962
00:42:42,650 --> 00:42:46,750
So always have return type,
name, and your parameter list.

963
00:42:46,750 --> 00:42:48,830
Each thing in your
parameter list, or input,

964
00:42:48,830 --> 00:42:51,410
needs to have a type associated with it.

965
00:42:51,410 --> 00:42:54,210
And then you have your
body here with whatever

966
00:42:54,210 --> 00:42:55,590
you want to do with your input.

967
00:42:55,590 --> 00:42:57,390

968
00:42:57,390 --> 00:43:00,410
>> And then obviously you
want to return something.

969
00:43:00,410 --> 00:43:02,630
Sometimes functions will just return.

970
00:43:02,630 --> 00:43:06,090
They don't actually return
something for you to use.

971
00:43:06,090 --> 00:43:08,660
But you have to return in some way.

972
00:43:08,660 --> 00:43:12,900
And when you're making
your own functions,

973
00:43:12,900 --> 00:43:15,430
we can get into that a little deeper.

974
00:43:15,430 --> 00:43:18,880
Personally, if you want, there are a lot
of different things you could do there.

975
00:43:18,880 --> 00:43:21,350

976
00:43:21,350 --> 00:43:22,140
Everyone good?

977
00:43:22,140 --> 00:43:25,900
Anything on this list that you want me
to go over, that you didn't understand?

978
00:43:25,900 --> 00:43:27,204

979
00:43:27,204 --> 00:43:28,120
Everyone's good there?

980
00:43:28,120 --> 00:43:29,071

981
00:43:29,071 --> 00:43:29,570
Cool.

982
00:43:29,570 --> 00:43:31,350

983
00:43:31,350 --> 00:43:32,120
Awesome.

984
00:43:32,120 --> 00:43:34,700
>> OK, so we're putting
all this together now.

985
00:43:34,700 --> 00:43:35,720

986
00:43:35,720 --> 00:43:41,201
So we have some int cube input, so
this is a complete program here.

987
00:43:41,201 --> 00:43:42,950
Up until now, I've
kind of been giving you

988
00:43:42,950 --> 00:43:45,699
guys snippets that might
be going within a program.

989
00:43:45,699 --> 00:43:47,240
We've just been looking at functions.

990
00:43:47,240 --> 00:43:48,448
But here's an entire program.

991
00:43:48,448 --> 00:43:49,400

992
00:43:49,400 --> 00:43:53,690
So how many of you remember the
word prototype from lecture?

993
00:43:53,690 --> 00:43:54,750

994
00:43:54,750 --> 00:43:55,300
Cool.

995
00:43:55,300 --> 00:43:55,890
We've got one.

996
00:43:55,890 --> 00:43:57,160

997
00:43:57,160 --> 00:43:58,110
What's your name?

998
00:43:58,110 --> 00:43:58,850
>> STEPH: Steph.

999
00:43:58,850 --> 00:43:59,475
>> ALLISON: Steph?

1000
00:43:59,475 --> 00:44:01,030
OK, awesome.

1001
00:44:01,030 --> 00:44:03,310
So, do you remember what a prototype is?

1002
00:44:03,310 --> 00:44:04,651

1003
00:44:04,651 --> 00:44:10,230
>> STUDENT: You say [INAUDIBLE]
before you actually deal with it.

1004
00:44:10,230 --> 00:44:11,700
>> ALLISON: Do you remember why?

1005
00:44:11,700 --> 00:44:13,158
>> STUDENT: No.

1006
00:44:13,158 --> 00:44:14,620
>> ALLISON: OK.

1007
00:44:14,620 --> 00:44:16,090
Gold star.

1008
00:44:16,090 --> 00:44:19,470
So yes, a prototype we have
beforehand, because otherwise,

1009
00:44:19,470 --> 00:44:21,260
our compiler is going to yell at us.

1010
00:44:21,260 --> 00:44:24,294
It's going to say, OK,
what is this cube function?

1011
00:44:24,294 --> 00:44:26,460
Like, you literally have
told me nothing about this.

1012
00:44:26,460 --> 00:44:28,110
It's like when you
walk into a classroom,

1013
00:44:28,110 --> 00:44:29,320
and someone's like,
there's a quiz today.

1014
00:44:29,320 --> 00:44:31,380
And you're like, you
never told me about this.

1015
00:44:31,380 --> 00:44:32,930
I'm not happy with us.

1016
00:44:32,930 --> 00:44:34,300

1017
00:44:34,300 --> 00:44:37,020
The prototype is basically like
your syllabus saying, look.

1018
00:44:37,020 --> 00:44:37,900
Heads up.

1019
00:44:37,900 --> 00:44:39,570
There's going to be a quiz on this day.

1020
00:44:39,570 --> 00:44:41,420
Don't freak out when you get to it.

1021
00:44:41,420 --> 00:44:42,690
You're going to be fine.

1022
00:44:42,690 --> 00:44:48,170
>> So all the prototype does is tell
main, I'm going to use this function.

1023
00:44:48,170 --> 00:44:51,010
I promise I'm defining it later.

1024
00:44:51,010 --> 00:44:52,260
Don't freak out at me.

1025
00:44:52,260 --> 00:44:54,750
Just compile and do what I tell you to.

1026
00:44:54,750 --> 00:44:58,880
So we have the prototype there
just to make our compiler happy.

1027
00:44:58,880 --> 00:45:04,360
And it's basically a promise that
you have defined this function later

1028
00:45:04,360 --> 00:45:07,960
and that you aren't just calling
this random thing that it

1029
00:45:07,960 --> 00:45:10,070
doesn't know what you're
going to be doing.

1030
00:45:10,070 --> 00:45:11,910
>> So in this case, we have main here.

1031
00:45:11,910 --> 00:45:14,660
We initialize some integer x.

1032
00:45:14,660 --> 00:45:15,620
That's two.

1033
00:45:15,620 --> 00:45:17,510
We're going to print out what x is.

1034
00:45:17,510 --> 00:45:18,800
We're going to cube x.

1035
00:45:18,800 --> 00:45:21,520
As you see, we have our
function declaration

1036
00:45:21,520 --> 00:45:23,770
down here that we
talked about previously.

1037
00:45:23,770 --> 00:45:28,260
It'll cube x, and then, if we
remember, the cube function actually

1038
00:45:28,260 --> 00:45:31,340
returns an integer to
us, which is stored

1039
00:45:31,340 --> 00:45:35,860
in x again so that we can print
out eight, or cube x right now.

1040
00:45:35,860 --> 00:45:37,170

1041
00:45:37,170 --> 00:45:38,920
Does that make sense to everyone?

1042
00:45:38,920 --> 00:45:40,360
We're good?

1043
00:45:40,360 --> 00:45:41,160
All right.

1044
00:45:41,160 --> 00:45:41,660
Awesome.

1045
00:45:41,660 --> 00:45:45,540

1046
00:45:45,540 --> 00:45:46,840
>> All right.

1047
00:45:46,840 --> 00:45:50,879
How many of you guys
remember This so this

1048
00:45:50,879 --> 00:45:52,670
is basically just your
stack and your heap,

1049
00:45:52,670 --> 00:45:56,510
just a visualization of
how memory is stored here.

1050
00:45:56,510 --> 00:45:58,200

1051
00:45:58,200 --> 00:46:01,400
So we just want to make
sure that you understand

1052
00:46:01,400 --> 00:46:03,680
how these are represented in memory.

1053
00:46:03,680 --> 00:46:07,670
If you take classes like
CS61 and stuff later,

1054
00:46:07,670 --> 00:46:10,570
you get to learn this far more
in depth, and it's really cool.

1055
00:46:10,570 --> 00:46:11,730
I highly recommend it.

1056
00:46:11,730 --> 00:46:12,740

1057
00:46:12,740 --> 00:46:14,720
But for now, I'll give
you the broad overview

1058
00:46:14,720 --> 00:46:16,950
so you don't have to
know the nitty gritty.

1059
00:46:16,950 --> 00:46:20,820
>> So the top just a text segment which
contains the actual zeros and ones,

1060
00:46:20,820 --> 00:46:23,740
the binary for that.

1061
00:46:23,740 --> 00:46:27,510
And this is used for storing
global variables if you have any.

1062
00:46:27,510 --> 00:46:28,870

1063
00:46:28,870 --> 00:46:32,240
As you move down, we
have, as you see here,

1064
00:46:32,240 --> 00:46:34,670
initialized data, uninitialized
data, and then heap.

1065
00:46:34,670 --> 00:46:35,800

1066
00:46:35,800 --> 00:46:38,584
>> So we don't really talk
about the heap right now.

1067
00:46:38,584 --> 00:46:39,500
We'll get to it later.

1068
00:46:39,500 --> 00:46:40,762

1069
00:46:40,762 --> 00:46:42,970
For now, I'm just going to
wave my hands and be like,

1070
00:46:42,970 --> 00:46:44,553
you don't need to know about this now.

1071
00:46:44,553 --> 00:46:46,100

1072
00:46:46,100 --> 00:46:49,060
But we will talk a little
bit about the stack.

1073
00:46:49,060 --> 00:46:53,050
So the stack is where-- we have zoom in.

1074
00:46:53,050 --> 00:46:58,590
This is actually how the program
we just looked at occurs in memory.

1075
00:46:58,590 --> 00:47:01,630
>> So what happens is, every
time we call a function,

1076
00:47:01,630 --> 00:47:04,180
we get what's called a stack
frame, which is one of these.

1077
00:47:04,180 --> 00:47:05,870
So main's parameters.

1078
00:47:05,870 --> 00:47:07,944
So those are the things
that we pass into main.

1079
00:47:07,944 --> 00:47:11,110
So they're right here at the bottom,
because that's the first thing we call.

1080
00:47:11,110 --> 00:47:13,401
And then we get to main's
locals, and when we say that,

1081
00:47:13,401 --> 00:47:15,970
we mean the local variables
that are stored within main.

1082
00:47:15,970 --> 00:47:19,950

1083
00:47:19,950 --> 00:47:23,620
So locals here would be, like,
x is equal to two in this case.

1084
00:47:23,620 --> 00:47:26,350
Because that's localized to main.

1085
00:47:26,350 --> 00:47:29,745
Does everyone remember scope,
going over that in lecture?

1086
00:47:29,745 --> 00:47:31,517

1087
00:47:31,517 --> 00:47:32,017
OK.

1088
00:47:32,017 --> 00:47:36,110
So, just the variables that
are initialized within main.

1089
00:47:36,110 --> 00:47:38,550
So that's why we have main [? vocals. ?]

1090
00:47:38,550 --> 00:47:40,390
>> And then within main, we call cube.

1091
00:47:40,390 --> 00:47:41,000
Right?

1092
00:47:41,000 --> 00:47:44,510
So we get another frame
with cube's parameters.

1093
00:47:44,510 --> 00:47:47,050
So in this case, cube's
parameters are now

1094
00:47:47,050 --> 00:47:50,800
the x that we passed in,
the two that we passed in.

1095
00:47:50,800 --> 00:47:55,320
And then cube's locals, which is
where the actual cubing happens.

1096
00:47:55,320 --> 00:47:56,450
And then it returns.

1097
00:47:56,450 --> 00:48:02,120
>> So what happens is as cube actually does
what it's supposed to do, it returns.

1098
00:48:02,120 --> 00:48:07,020
When it returns, this frame leaves,
and its returned down to main.

1099
00:48:07,020 --> 00:48:09,160
And then within main, we
can actually print it.

1100
00:48:09,160 --> 00:48:10,840

1101
00:48:10,840 --> 00:48:14,500
So when you're returning something,
when your function returns,

1102
00:48:14,500 --> 00:48:19,900
it's like passing on those values to
the frame below it and then leaving.

1103
00:48:19,900 --> 00:48:22,200
And things have to execute in order.

1104
00:48:22,200 --> 00:48:25,550
>> And when you get to bigger programs,
we can make cooler and more complicated

1105
00:48:25,550 --> 00:48:26,470
diagrams.

1106
00:48:26,470 --> 00:48:29,070
But for now, this is
just a general overview

1107
00:48:29,070 --> 00:48:31,110
so you have kind of an
understanding of what

1108
00:48:31,110 --> 00:48:35,160
happens when you're calling a function
and how that actually looks in memory.

1109
00:48:35,160 --> 00:48:37,230

1110
00:48:37,230 --> 00:48:37,770
Cool?

1111
00:48:37,770 --> 00:48:38,353
Everyone good?

1112
00:48:38,353 --> 00:48:39,920

1113
00:48:39,920 --> 00:48:40,420
Awesome.

1114
00:48:40,420 --> 00:48:42,070

1115
00:48:42,070 --> 00:48:50,575
>> So this is one that is
just trying to swap things.

1116
00:48:50,575 --> 00:48:52,260

1117
00:48:52,260 --> 00:48:54,930
As we see here, we have
our function prototype

1118
00:48:54,930 --> 00:48:56,900
so that our compiler doesn't yell at us.

1119
00:48:56,900 --> 00:48:59,220
We have some main, and we
want to switch x and y.

1120
00:48:59,220 --> 00:49:00,320

1121
00:49:00,320 --> 00:49:04,129
They haven't done this demo
in lecture yet, have they?

1122
00:49:04,129 --> 00:49:04,670
They haven't?

1123
00:49:04,670 --> 00:49:05,900
OK.

1124
00:49:05,900 --> 00:49:08,450
So we're going to go
over this very briefly.

1125
00:49:08,450 --> 00:49:12,210
You'll get into this example more
in depth, I think, this week.

1126
00:49:12,210 --> 00:49:15,680
And then next week we can really
dive into why this doesn't work.

1127
00:49:15,680 --> 00:49:18,810
>> So we have this void
function here-- swap.

1128
00:49:18,810 --> 00:49:20,720
So void just means that
nothing is returned.

1129
00:49:20,720 --> 00:49:22,000

1130
00:49:22,000 --> 00:49:24,110
And we have swap int a and int b.

1131
00:49:24,110 --> 00:49:27,046
And we have some temporary
variable that's a.

1132
00:49:27,046 --> 00:49:30,050
a gets assigned to b, and then
b gets assigned to the temp

1133
00:49:30,050 --> 00:49:32,235
so that a and b's
values are now switched.

1134
00:49:32,235 --> 00:49:33,510

1135
00:49:33,510 --> 00:49:36,030
>> But, plot twist, this doesn't work.

1136
00:49:36,030 --> 00:49:37,390

1137
00:49:37,390 --> 00:49:42,380
And part of it actually has
to do with the fact that a

1138
00:49:42,380 --> 00:49:48,800
and b here, the ones that get passed in
here, are actually copies of x and y.

1139
00:49:48,800 --> 00:49:53,220
So when the function actually returns,
it switches the copies but not

1140
00:49:53,220 --> 00:49:55,960
the actual x and y's.

1141
00:49:55,960 --> 00:50:02,370
So one way to think about it is
that-- pretend these are swap.

1142
00:50:02,370 --> 00:50:03,130
OK?

1143
00:50:03,130 --> 00:50:05,390
So in main, we have x and y initialized.

1144
00:50:05,390 --> 00:50:06,530

1145
00:50:06,530 --> 00:50:10,760
But when we actually go up
to these frames with swap,

1146
00:50:10,760 --> 00:50:15,740
we're passing the values over
to it, and they're initialized.

1147
00:50:15,740 --> 00:50:18,570
>> And they only ever live right here.

1148
00:50:18,570 --> 00:50:19,796
So a and b live here.

1149
00:50:19,796 --> 00:50:20,670
And they get swapped.

1150
00:50:20,670 --> 00:50:21,870

1151
00:50:21,870 --> 00:50:26,190
But when we return, we don't
do anything with a and b. a

1152
00:50:26,190 --> 00:50:28,980
and b leave with our function.

1153
00:50:28,980 --> 00:50:30,850
And so x and y stay the same.

1154
00:50:30,850 --> 00:50:31,960

1155
00:50:31,960 --> 00:50:39,665
You'll get more into how to fix that and
how we actually deal with that later.

1156
00:50:39,665 --> 00:50:40,750

1157
00:50:40,750 --> 00:50:43,740
>> But it's just one thing
to kind of keep in mind.

1158
00:50:43,740 --> 00:50:45,270
Use it for the future.

1159
00:50:45,270 --> 00:50:48,050
Don't worry if that didn't make
all the sense in the world.

1160
00:50:48,050 --> 00:50:49,510

1161
00:50:49,510 --> 00:50:51,440
They are copies is the biggest thing.

1162
00:50:51,440 --> 00:50:54,580
If you're going to take anything
away from that, you passed in copies.

1163
00:50:54,580 --> 00:50:56,410
So the originals stay the same.

1164
00:50:56,410 --> 00:50:58,170

1165
00:50:58,170 --> 00:50:58,770
Everyone good?

1166
00:50:58,770 --> 00:51:00,280

1167
00:51:00,280 --> 00:51:00,930
Cool.

1168
00:51:00,930 --> 00:51:02,295
>> So command-line arguments.

1169
00:51:02,295 --> 00:51:03,520

1170
00:51:03,520 --> 00:51:07,630
I'm sure in the beginning you guys all
had those great, like, int main voids.

1171
00:51:07,630 --> 00:51:09,517
And you're like, OK cool.

1172
00:51:09,517 --> 00:51:10,350
I don't really care.

1173
00:51:10,350 --> 00:51:12,360
This is just what I have to write.

1174
00:51:12,360 --> 00:51:16,349
But in your new programs,
especially in this pset,

1175
00:51:16,349 --> 00:51:17,890
and why is there chalk on the ground?

1176
00:51:17,890 --> 00:51:19,640

1177
00:51:19,640 --> 00:51:22,680
>> With your next pset, you're
going to be seeing this.

1178
00:51:22,680 --> 00:51:26,130
Int main, int arg c,
string arc v, brackets.

1179
00:51:26,130 --> 00:51:28,770
So, from what we just
learned today, what

1180
00:51:28,770 --> 00:51:32,190
do we think that second parameter
or that second element is here?

1181
00:51:32,190 --> 00:51:34,830

1182
00:51:34,830 --> 00:51:35,824
It's an array.

1183
00:51:35,824 --> 00:51:36,615
What type of array?

1184
00:51:36,615 --> 00:51:38,430

1185
00:51:38,430 --> 00:51:40,690
String array, yes.

1186
00:51:40,690 --> 00:51:41,980
Cool.

1187
00:51:41,980 --> 00:51:45,320
So that's how you're going
to be declaring these now.

1188
00:51:45,320 --> 00:51:48,570
>> Does anyone remember
what these stand for?

1189
00:51:48,570 --> 00:51:52,551

1190
00:51:52,551 --> 00:51:53,050
No?

1191
00:51:53,050 --> 00:51:54,020

1192
00:51:54,020 --> 00:51:54,520
Hmm?

1193
00:51:54,520 --> 00:51:55,640
>> STUDENT: arg c.

1194
00:51:55,640 --> 00:51:56,961

1195
00:51:56,961 --> 00:51:58,950
>> ALLISON: So arg c keeps a counter.

1196
00:51:58,950 --> 00:51:59,450
It's an int.

1197
00:51:59,450 --> 00:52:00,615
It's a number, right?

1198
00:52:00,615 --> 00:52:02,240
So what do you think that number is of?

1199
00:52:02,240 --> 00:52:08,260

1200
00:52:08,260 --> 00:52:08,760
Yeah.

1201
00:52:08,760 --> 00:52:11,450
So arg c is the number of strings
that make up the command line.

1202
00:52:11,450 --> 00:52:12,560

1203
00:52:12,560 --> 00:52:18,499
So if we were to do-- actually,
there are examples after this,

1204
00:52:18,499 --> 00:52:19,790
so I won't get ahead of myself.

1205
00:52:19,790 --> 00:52:21,299

1206
00:52:21,299 --> 00:52:23,840
It's a number of strings that
just make up your command line.

1207
00:52:23,840 --> 00:52:35,080
>> So when you do, like, dot slash Mario,
that's one string that makes it up.

1208
00:52:35,080 --> 00:52:38,520
In this piece, you'll actually be
feeding things into the command line,

1209
00:52:38,520 --> 00:52:41,750
as I'm sure you guys who
have read the spec saw.

1210
00:52:41,750 --> 00:52:44,470
So in those cases, maybe you'll
have two or three arguments.

1211
00:52:44,470 --> 00:52:45,652

1212
00:52:45,652 --> 00:52:48,360
It's going to be a useful thing to use.

1213
00:52:48,360 --> 00:52:51,500
>> And then arg v, as we said,
is just a string array.

1214
00:52:51,500 --> 00:52:57,000
So that actually stores what
you input into the command line.

1215
00:52:57,000 --> 00:52:59,960
So we have these.

1216
00:52:59,960 --> 00:53:02,850
You have some dot slash
copy infile outfile.

1217
00:53:02,850 --> 00:53:06,670
So, if arg c is the number of strings
that we're passing into the command

1218
00:53:06,670 --> 00:53:09,080
line, what is our arg c in this case?

1219
00:53:09,080 --> 00:53:14,520

1220
00:53:14,520 --> 00:53:15,850
Three.

1221
00:53:15,850 --> 00:53:16,690
Exactly.

1222
00:53:16,690 --> 00:53:20,086
>> So what's arg v of zero?

1223
00:53:20,086 --> 00:53:21,710
So what's the first thing we've stored?

1224
00:53:21,710 --> 00:53:24,640

1225
00:53:24,640 --> 00:53:26,290
Dot slash copy, exactly.

1226
00:53:26,290 --> 00:53:28,065
And then the second would be infile.

1227
00:53:28,065 --> 00:53:29,290

1228
00:53:29,290 --> 00:53:30,930
The third would be outfile.

1229
00:53:30,930 --> 00:53:32,390
So what about arg v three?

1230
00:53:32,390 --> 00:53:35,810

1231
00:53:35,810 --> 00:53:39,370
It would be null, because that's
the end of our array, right?

1232
00:53:39,370 --> 00:53:40,600

1233
00:53:40,600 --> 00:53:41,460
Cool.

1234
00:53:41,460 --> 00:53:44,500
>> And then what about the sixth one?

1235
00:53:44,500 --> 00:53:45,900
It's kind of a trick question.

1236
00:53:45,900 --> 00:53:46,400
Ish.

1237
00:53:46,400 --> 00:53:49,027

1238
00:53:49,027 --> 00:53:49,943
Do we know what it is?

1239
00:53:49,943 --> 00:53:50,960

1240
00:53:50,960 --> 00:53:51,690
It's undefined.

1241
00:53:51,690 --> 00:53:53,170
We have no idea what that could be.

1242
00:53:53,170 --> 00:53:57,000
It's whatever is right
after the array in memory,

1243
00:53:57,000 --> 00:54:00,220
and we have no clue what that is.

1244
00:54:00,220 --> 00:54:03,500
And it's dangerous to touch those
things, because for all you know,

1245
00:54:03,500 --> 00:54:07,080
it's some part of memory that you
shouldn't be accessing or null.

1246
00:54:07,080 --> 00:54:09,000
And it can do crazy things.

1247
00:54:09,000 --> 00:54:11,690
It's called over-indexing
your bound to your array.

1248
00:54:11,690 --> 00:54:14,785
Don't go outside the bounds of your
array, or bad things can happen.

1249
00:54:14,785 --> 00:54:16,274

1250
00:54:16,274 --> 00:54:18,940
You come back and, like, the laws
of physics have been destroyed

1251
00:54:18,940 --> 00:54:19,481
or something.

1252
00:54:19,481 --> 00:54:21,021

1253
00:54:21,021 --> 00:54:21,520
Cool.

1254
00:54:21,520 --> 00:54:22,895
Does that make sense to everyone?

1255
00:54:22,895 --> 00:54:23,810

1256
00:54:23,810 --> 00:54:25,025
Not too bad.

1257
00:54:25,025 --> 00:54:28,926
>> So now, everyone's
favorite part, pset review.

1258
00:54:28,926 --> 00:54:29,426
Yay!

1259
00:54:29,426 --> 00:54:30,530

1260
00:54:30,530 --> 00:54:31,030
OK.

1261
00:54:31,030 --> 00:54:33,830
So for those of you who
haven't read the pset spec,

1262
00:54:33,830 --> 00:54:36,790
you are doing some really
cool stuff with cryptography.

1263
00:54:36,790 --> 00:54:39,360
You're going to create a Ceasar
Cipher and a Vigenere Cipher.

1264
00:54:39,360 --> 00:54:42,000

1265
00:54:42,000 --> 00:54:45,480
You should definitely read the
spec to see how those work.

1266
00:54:45,480 --> 00:54:49,650
And if you're having any trouble about
what it should actually be doing,

1267
00:54:49,650 --> 00:54:52,920
please come talk to me,
email me or text me.

1268
00:54:52,920 --> 00:54:53,420
I'm around.

1269
00:54:53,420 --> 00:54:54,780

1270
00:54:54,780 --> 00:54:57,070
>> So there are three main
things here that we

1271
00:54:57,070 --> 00:55:00,440
want to talk about-- just kind
of an extension of lecture.

1272
00:55:00,440 --> 00:55:03,645
Things that you might not know
about, helpful hints and tools.

1273
00:55:03,645 --> 00:55:05,520
So we're going to do a
quick review of ASCII,

1274
00:55:05,520 --> 00:55:10,420
because that's going to be super
important for Vigenere's Cipher.

1275
00:55:10,420 --> 00:55:11,809

1276
00:55:11,809 --> 00:55:14,100
We're going to conversion of
command line inputs, which

1277
00:55:14,100 --> 00:55:17,160
will be very helpful for Caesar Cipher.

1278
00:55:17,160 --> 00:55:18,133
And then modulo.

1279
00:55:18,133 --> 00:55:20,220

1280
00:55:20,220 --> 00:55:21,210
Cool.

1281
00:55:21,210 --> 00:55:24,005
>> So, ASCII maps characters to numbers.

1282
00:55:24,005 --> 00:55:25,270

1283
00:55:25,270 --> 00:55:26,365
This is a great chart.

1284
00:55:26,365 --> 00:55:28,115
You should have this
bookmarked somewhere.

1285
00:55:28,115 --> 00:55:29,310

1286
00:55:29,310 --> 00:55:33,250
You will want it for
your first mid-term.

1287
00:55:33,250 --> 00:55:36,320
I'm pretty sure everyone has this
chart on their mid-term sheet.

1288
00:55:36,320 --> 00:55:37,810
So learn it.

1289
00:55:37,810 --> 00:55:38,410
Love it.

1290
00:55:38,410 --> 00:55:39,740
Keep it handy.

1291
00:55:39,740 --> 00:55:41,210
It'll be useful.

1292
00:55:41,210 --> 00:55:45,630
>> And all it is is an encoding
that maps alphabetic, numeric,

1293
00:55:45,630 --> 00:55:47,740
and other characters to
numbers for our computer.

1294
00:55:47,740 --> 00:55:49,947
Because of course, in the
end, everything we store

1295
00:55:49,947 --> 00:55:52,030
is going to get converted
down to zeroes and ones,

1296
00:55:52,030 --> 00:55:55,200
so we need some way to represent the
text and characters that we're all

1297
00:55:55,200 --> 00:55:59,105
used to seeing as some sort of number.

1298
00:55:59,105 --> 00:56:00,370

1299
00:56:00,370 --> 00:56:05,880
>> So as we see here, we have
uppercase A, which is right there.

1300
00:56:05,880 --> 00:56:06,555
It's 65.

1301
00:56:06,555 --> 00:56:07,570

1302
00:56:07,570 --> 00:56:09,660
And lowercase A is 97.

1303
00:56:09,660 --> 00:56:13,170
So you can figure out--
as I said earlier,

1304
00:56:13,170 --> 00:56:16,532
if you had array of multiple
strings, what each of them

1305
00:56:16,532 --> 00:56:17,490
have a null terminator.

1306
00:56:17,490 --> 00:56:18,480
It would be a space.

1307
00:56:18,480 --> 00:56:22,581
Space has its own special--
I forget where it is here.

1308
00:56:22,581 --> 00:56:23,080
Ah.

1309
00:56:23,080 --> 00:56:24,700
32 is the space.

1310
00:56:24,700 --> 00:56:27,090
So everything maps to it.

1311
00:56:27,090 --> 00:56:28,680

1312
00:56:28,680 --> 00:56:29,740
>> So we have ASCII math.

1313
00:56:29,740 --> 00:56:31,080

1314
00:56:31,080 --> 00:56:34,290
Pro tip-- in Vigenere's,
you might be tempted

1315
00:56:34,290 --> 00:56:39,480
to convert your numbers to integers,
but it's actually better practice

1316
00:56:39,480 --> 00:56:43,860
to be able to use the
characters like this

1317
00:56:43,860 --> 00:56:45,820
when you're actually manipulating them.

1318
00:56:45,820 --> 00:56:48,950
So if you want to use numbers, you can.

1319
00:56:48,950 --> 00:56:55,260
But a better way, or a way that
we tend to like you guys to do it,

1320
00:56:55,260 --> 00:56:57,830
is this way where you're
actually subtracting characters.

1321
00:56:57,830 --> 00:56:58,840

1322
00:56:58,840 --> 00:57:02,830
>> So I want you guys to
kind of figure these out.

1323
00:57:02,830 --> 00:57:05,070
Why don't you try every other one?

1324
00:57:05,070 --> 00:57:07,580

1325
00:57:07,580 --> 00:57:09,830
So do the first one, the
third one, and the fifth one.

1326
00:57:09,830 --> 00:57:11,160

1327
00:57:11,160 --> 00:57:15,000
Because I want to make sure that we talk
about everything we need to talk about.

1328
00:57:15,000 --> 00:57:17,410

1329
00:57:17,410 --> 00:57:20,259
I'm just going to say, one of the
important things to-- oh wait,

1330
00:57:20,259 --> 00:57:21,550
you guys haven't seen this one.

1331
00:57:21,550 --> 00:57:23,710
OK so do the first three.

1332
00:57:23,710 --> 00:57:24,430
Let's do that.

1333
00:57:24,430 --> 00:57:25,971
Because we have to talk about modulo.

1334
00:57:25,971 --> 00:57:27,810

1335
00:57:27,810 --> 00:57:28,310
I know.

1336
00:57:28,310 --> 00:57:28,851
Math is hard.

1337
00:57:28,851 --> 00:57:29,870

1338
00:57:29,870 --> 00:57:30,980
You can use a calculator.

1339
00:57:30,980 --> 00:57:31,580
It's OK.

1340
00:57:31,580 --> 00:57:37,464

1341
00:57:37,464 --> 00:57:40,380
Or pull up an ASCII table, because
you're probably going to want that.

1342
00:57:40,380 --> 00:58:03,660

1343
00:58:03,660 --> 00:58:04,820
>> Cool.

1344
00:58:04,820 --> 00:58:07,960
So I will quickly walk
you guys through these.

1345
00:58:07,960 --> 00:58:11,120
So people have ASCII tables pulled up?

1346
00:58:11,120 --> 00:58:15,645
What is our numeric
number for lowercase A?

1347
00:58:15,645 --> 00:58:16,270
STUDENT: Seven.

1348
00:58:16,270 --> 00:58:19,420

1349
00:58:19,420 --> 00:58:24,550
>> ALLISON: So lowercase A is
97 and uppercase A is 65.

1350
00:58:24,550 --> 00:58:26,390
So 97 minus 65?

1351
00:58:26,390 --> 00:58:27,782

1352
00:58:27,782 --> 00:58:28,570
>> AUDIENCE: 32?

1353
00:58:28,570 --> 00:58:33,260

1354
00:58:33,260 --> 00:58:35,630
>> ALLISON: 62, yeah.

1355
00:58:35,630 --> 00:58:38,675
So in this case, what
would it print out?

1356
00:58:38,675 --> 00:58:40,360
That first one?

1357
00:58:40,360 --> 00:58:43,830
If we have percent d,
what would that indicate?

1358
00:58:43,830 --> 00:58:45,980

1359
00:58:45,980 --> 00:58:46,730
STUDENT: A number.

1360
00:58:46,730 --> 00:58:48,604
ALLISON: We're printing
out an actual number.

1361
00:58:48,604 --> 00:58:50,770
So we're actually going
to print out 32 here.

1362
00:58:50,770 --> 00:58:56,360
And if this were percent c,
32 would give us a space.

1363
00:58:56,360 --> 00:58:57,700

1364
00:58:57,700 --> 00:59:01,670
So understanding that
characters can be printed

1365
00:59:01,670 --> 00:59:06,140
both as numbers and as
the actual characters

1366
00:59:06,140 --> 00:59:11,080
is really important, and paying
attention to the actual types

1367
00:59:11,080 --> 00:59:12,040
that we're doing here.

1368
00:59:12,040 --> 00:59:14,520

1369
00:59:14,520 --> 00:59:15,020
Cool.

1370
00:59:15,020 --> 00:59:18,862
So for every other one of these,
what are we going to be printing?

1371
00:59:18,862 --> 00:59:19,825
>> STUDENT: A character.

1372
00:59:19,825 --> 00:59:20,830
>> ALLISON: A character.

1373
00:59:20,830 --> 00:59:21,330
Cool.

1374
00:59:21,330 --> 00:59:24,260
So if you guys want to know, you
can work these out on your own.

1375
00:59:24,260 --> 00:59:25,900
If you're having trouble, email me.

1376
00:59:25,900 --> 00:59:28,950
But the second one will
print out a lowercase b.

1377
00:59:28,950 --> 00:59:32,910
The third one will print out an
uppercase B. The fourth one will print

1378
00:59:32,910 --> 00:59:36,410
out an uppercase C, and the
last one will be a lowercase A.

1379
00:59:36,410 --> 00:59:42,530
And the last one-- we're actually going
to get into what that crazy percent

1380
00:59:42,530 --> 00:59:45,020
sign even means in a couple slides.

1381
00:59:45,020 --> 00:59:46,667
So try those on your own.

1382
00:59:46,667 --> 00:59:48,500
If you have trouble,
please come talk to me.

1383
00:59:48,500 --> 00:59:49,655

1384
00:59:49,655 --> 00:59:53,290
If you're typically in Adams D hall,
you'll probably find me around.

1385
00:59:53,290 --> 00:59:54,760

1386
00:59:54,760 --> 00:59:55,760
>> So, atoi.

1387
00:59:55,760 --> 00:59:59,660
How many of you have seen this
function or heard of it at all?

1388
00:59:59,660 --> 01:00:00,380

1389
01:00:00,380 --> 01:00:00,880
Anyone?

1390
01:00:00,880 --> 01:00:02,100

1391
01:00:02,100 --> 01:00:02,600
Cool.

1392
01:00:02,600 --> 01:00:06,250
So what it actually stands
for is ASCII to integer.

1393
01:00:06,250 --> 01:00:13,150
So what you can do is, with Caesar,
for those who read the spec,

1394
01:00:13,150 --> 01:00:16,460
you're going to do dot slash Caesar
after you write your program,

1395
01:00:16,460 --> 01:00:18,690
and then you're going to
input some number that you

1396
01:00:18,690 --> 01:00:24,780
want to encode your secret message with.

1397
01:00:24,780 --> 01:00:30,700
>> But, if we remember, all of our
inputs are stored as strings.

1398
01:00:30,700 --> 01:00:31,390
Right?

1399
01:00:31,390 --> 01:00:35,160
We have an arg v array
that is all type string.

1400
01:00:35,160 --> 01:00:37,870
So if you just try to
pull that one, it would

1401
01:00:37,870 --> 01:00:43,240
think that that one or whatever number
you used is actually a character.

1402
01:00:43,240 --> 01:00:45,490
So you're going to get
some crazy results.

1403
01:00:45,490 --> 01:00:48,200
So if you actually want to
turn this into an integer

1404
01:00:48,200 --> 01:00:52,060
that you can use to manipulate
your word or your message,

1405
01:00:52,060 --> 01:00:53,366
you'll want to use atoi.

1406
01:00:53,366 --> 01:00:56,320
atoi just converts
your string to an int.

1407
01:00:56,320 --> 01:01:04,530
>> So if we have a string of 12, if we call
atoi on 12, or whatever that input is,

1408
01:01:04,530 --> 01:01:07,370
it will actually return
to you the integer.

1409
01:01:07,370 --> 01:01:10,610
Not the character or the string 12.

1410
01:01:10,610 --> 01:01:14,230
Which, when you start
to add that to numbers,

1411
01:01:14,230 --> 01:01:17,710
will be very different,
because the string 12 is

1412
01:01:17,710 --> 01:01:21,210
some crazy number in
ASCII, but the integer 12

1413
01:01:21,210 --> 01:01:23,390
is actually 12, which is what you want.

1414
01:01:23,390 --> 01:01:25,630
So you want to make sure to use atoi.

1415
01:01:25,630 --> 01:01:29,203
>> You're going to want this
in Caesar, because you

1416
01:01:29,203 --> 01:01:32,520
need the int supplied by the
user in the command line.

1417
01:01:32,520 --> 01:01:35,230
But when they put it in the command
line, it's stored as a string

1418
01:01:35,230 --> 01:01:35,813
to begin with.

1419
01:01:35,813 --> 01:01:36,950

1420
01:01:36,950 --> 01:01:38,210
Does that make sense?

1421
01:01:38,210 --> 01:01:40,590
You don't necessarily
need this for Vigenere.

1422
01:01:40,590 --> 01:01:43,180
With Vigenere, as I said
before, you should try and use

1423
01:01:43,180 --> 01:01:46,600
ASCII math that looks more like
this, where you're actually

1424
01:01:46,600 --> 01:01:49,320
using the chars that we're given to you.

1425
01:01:49,320 --> 01:01:50,970

1426
01:01:50,970 --> 01:01:51,470
Cool.

1427
01:01:51,470 --> 01:01:52,970

1428
01:01:52,970 --> 01:01:54,170
Everyone good there?

1429
01:01:54,170 --> 01:01:56,530

1430
01:01:56,530 --> 01:01:57,480
Awesome.

1431
01:01:57,480 --> 01:01:59,030
>> So modulo.

1432
01:01:59,030 --> 01:02:01,950
So what if you're given
this huge number for Caesar?

1433
01:02:01,950 --> 01:02:03,720

1434
01:02:03,720 --> 01:02:07,970
You have this idea that
if you're at Z and you're

1435
01:02:07,970 --> 01:02:11,020
given a number two,
that means you need--

1436
01:02:11,020 --> 01:02:15,130
Z becomes the second
letter after itself, right?

1437
01:02:15,130 --> 01:02:20,800
So you need to somehow wrap around,
and modulo is the way to do that.

1438
01:02:20,800 --> 01:02:23,310
So all it does is it
gives you the remainder

1439
01:02:23,310 --> 01:02:25,660
of the division of the
first number by the second.

1440
01:02:25,660 --> 01:02:28,660
And we have some examples to
make that a little more concrete.

1441
01:02:28,660 --> 01:02:34,390
But basically, you use modulo when you
want to make something wrap around.

1442
01:02:34,390 --> 01:02:36,950
>> So if you only want the
numbers one through eight,

1443
01:02:36,950 --> 01:02:39,670
you can use modulo on any other
number, and it will always

1444
01:02:39,670 --> 01:02:42,960
return a number from zero to eight.

1445
01:02:42,960 --> 01:02:48,750
So some examples-- if
we have 55 modulo 10,

1446
01:02:48,750 --> 01:02:55,350
it just gives you the remainder of
55 divided by 10, which would be 5.

1447
01:02:55,350 --> 01:02:56,440

1448
01:02:56,440 --> 01:03:02,470
And then three modulo five,
anyone guess what that would be?

1449
01:03:02,470 --> 01:03:03,800

1450
01:03:03,800 --> 01:03:04,300
Three.

1451
01:03:04,300 --> 01:03:05,570

1452
01:03:05,570 --> 01:03:10,910
>> So if you have a smaller number before
the modulo, it can't go in evenly.

1453
01:03:10,910 --> 01:03:11,520
It's zero.

1454
01:03:11,520 --> 01:03:14,090
So it just returns the number itself.

1455
01:03:14,090 --> 01:03:15,920
So eight modulo eight would be?

1456
01:03:15,920 --> 01:03:16,603
>> STUDENT: Zero.

1457
01:03:16,603 --> 01:03:18,350
>> ALLISON: Zero.

1458
01:03:18,350 --> 01:03:20,160
Because it goes in evenly.

1459
01:03:20,160 --> 01:03:22,095
16 modulo 15?

1460
01:03:22,095 --> 01:03:23,660
>> AUDIENCE: One.

1461
01:03:23,660 --> 01:03:24,700
>> ALLISON: Cool.

1462
01:03:24,700 --> 01:03:29,280
And then this last one is just to
show you-- you might be wondering, OK,

1463
01:03:29,280 --> 01:03:31,800
what's the order of operations here?

1464
01:03:31,800 --> 01:03:33,100
Do we divide first?

1465
01:03:33,100 --> 01:03:34,570
Do we modulo first?

1466
01:03:34,570 --> 01:03:38,730
So modulo holds the same precedence
as division or multiplication,

1467
01:03:38,730 --> 01:03:39,940
and its left associative.

1468
01:03:39,940 --> 01:03:41,730
So it's in the same way.

1469
01:03:41,730 --> 01:03:46,520
You would always do parentheses,
then multiplication, division,

1470
01:03:46,520 --> 01:03:49,570
and modulo in order from left to right.

1471
01:03:49,570 --> 01:03:51,050
So standard rules.

1472
01:03:51,050 --> 01:03:55,260
Just put it in the same category
as division and multiplication.

1473
01:03:55,260 --> 01:03:59,040
>> So in this case, we would
have 1 plus 2 gives us 3.

1474
01:03:59,040 --> 01:04:02,210
We multiply that by 2, so we get 6.

1475
01:04:02,210 --> 01:04:05,050
We modulo that by 2, which gives us?

1476
01:04:05,050 --> 01:04:06,042
>> STUDENT: 0.

1477
01:04:06,042 --> 01:04:07,430
>> ALLISON: 0.

1478
01:04:07,430 --> 01:04:10,270
And then we add 2, so we
get 2 in this last case.

1479
01:04:10,270 --> 01:04:12,080

1480
01:04:12,080 --> 01:04:16,360
So modulo-- you're definitely going to
be thinking about ways to incorporate

1481
01:04:16,360 --> 01:04:19,520
that when you're wrapping
around the alphabet.

1482
01:04:19,520 --> 01:04:24,210
If you're at Z and you need to move
forward three spaces to get to C,

1483
01:04:24,210 --> 01:04:26,230
there's that whole concept
of wrapping around.

1484
01:04:26,230 --> 01:04:28,790
So I will leave it to
you guys to figure out

1485
01:04:28,790 --> 01:04:31,940
how exactly you're going to be using it.

1486
01:04:31,940 --> 01:04:35,660
But definitely a useful tool
for your pset this week.

1487
01:04:35,660 --> 01:04:36,910

1488
01:04:36,910 --> 01:04:37,730
I really like this.

1489
01:04:37,730 --> 01:04:39,480
This is one of my favorite psets.

1490
01:04:39,480 --> 01:04:41,230
Then after you do it,
if you have friends,

1491
01:04:41,230 --> 01:04:44,146
you can, like, send each other secret
messages and make sure it works.

1492
01:04:44,146 --> 01:04:45,695
Because it'll decrypt it or whatever.

1493
01:04:45,695 --> 01:04:46,710

1494
01:04:46,710 --> 01:04:47,390
Lots of fun.

1495
01:04:47,390 --> 01:04:48,840

1496
01:04:48,840 --> 01:04:51,220
>> And that is the end of section.

1497
01:04:51,220 --> 01:04:52,430

1498
01:04:52,430 --> 01:04:54,040
I finished early.

1499
01:04:54,040 --> 01:04:57,540
I still have 15 minutes with
you guys, so if there's anything

1500
01:04:57,540 --> 01:05:02,180
that you would like to go over
further, I'd be happy to do that.

1501
01:05:02,180 --> 01:05:04,340
Any other questions on
your pset for those of you

1502
01:05:04,340 --> 01:05:06,355
who have started or read the spec.

1503
01:05:06,355 --> 01:05:07,870

1504
01:05:07,870 --> 01:05:12,060
Anything that we've talked
about in the last hour

1505
01:05:12,060 --> 01:05:16,880
and 15 minutes that you'd like me
to kind of rehash, I'd be happy to.

1506
01:05:16,880 --> 01:05:20,790
Or we can call it quits, and you can
all leave and take more candy with you

1507
01:05:20,790 --> 01:05:21,650
as you go.

1508
01:05:21,650 --> 01:05:23,719

1509
01:05:23,719 --> 01:05:26,260
But if there are any lingering
questions, please let me know.

1510
01:05:26,260 --> 01:05:28,467
You can also come up and
talk to me afterwards.

1511
01:05:28,467 --> 01:05:29,425
I promise I don't bite.

1512
01:05:29,425 --> 01:05:32,140

1513
01:05:32,140 --> 01:05:32,795
Anything else?

1514
01:05:32,795 --> 01:05:33,890

1515
01:05:33,890 --> 01:05:35,240
Everyone's good?

1516
01:05:35,240 --> 01:05:37,540
Everyone's feeling like
they can handle this pset?

1517
01:05:37,540 --> 01:05:38,820

1518
01:05:38,820 --> 01:05:42,405
You're going to be fine guys.

1519
01:05:42,405 --> 01:05:43,905
Office hours are there for a reason.

1520
01:05:43,905 --> 01:05:45,930

1521
01:05:45,930 --> 01:05:46,460
Cool.

1522
01:05:46,460 --> 01:05:47,020
Alright.

1523
01:05:47,020 --> 01:05:49,680
>> Well, in that case, thank
you all so much for coming.

1524
01:05:49,680 --> 01:05:50,940
I hope to see you next week.

1525
01:05:50,940 --> 01:05:52,120
There will be more candy.

1526
01:05:52,120 --> 01:05:53,495
There might be other cool things.

1527
01:05:53,495 --> 01:05:55,010

1528
01:05:55,010 --> 01:05:59,090
And I look forward to getting
to know all of you this year.

1529
01:05:59,090 --> 01:06:00,320