1
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[ Silence ]

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00:00:09,076 --> 00:00:09,716
>> [Sound effects] Be it
known through all in summary

3
00:00:09,886 --> 00:00:19,456
that Ralph Parker is here
by appointed a member

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00:00:19,456 --> 00:00:22,086
of the Little Orphan Annie
secret circle and is entitled

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to all the honors and
benefits occurring thereto.

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>> Signed Little Orphan Annie
countersigned Pierre Andre

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in ink.

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>> Honors and benefits
already at the age of nine.

9
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[ Sound effects ]

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00:00:41,216 --> 00:00:42,416
>> Come on let's get on with it.

11
00:00:42,626 --> 00:00:44,846
I don't need all that jazz
about smugglers and pirates.

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[Sound effects]

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00:00:44,996 --> 00:00:48,986
>> Listen tomorrow night
for the concluding adventure

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of the Black Pirate Ship.

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00:00:51,246 --> 00:00:53,876
Now it's time for
Annie's secret message

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for you members of
the secret circle.

17
00:00:57,256 --> 00:00:59,156
Remember kids, only members

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of Annie's secret circle can
decode Annie's secret message.

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00:01:03,566 --> 00:01:06,856
Remember Annie is
depending on you.

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[Background sound effects]
Set your pens to B2.

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Here is the message 12, 11 --

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>> [Background sound effects] I
attend my first secret meeting.

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>> 14, 11, 18,16 --

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>> [Background sound
effects] Pierre was

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in great voice tonight.

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I could tell that tonight's
message was really important.

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>> [Background music] 3, 25.

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That's a message
from Annie herself.

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Remember don't tell anyone.

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>> 90 seconds later I'm in
the only room in the house

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where a boy of 9 could
sit in privacy and decode.

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[Laughter] Ah hah, B
and on to the next,

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E the first word is be.

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S, it was coming easier now.

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U

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>> All come on Ralphie.

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[Background sound effects] I'll
be right down Ma, gee whiz.

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>> [Background music] T,O.

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00:02:09,026 --> 00:02:10,566
Be sure to what?

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What was Little Orphan
Annie trying to say?

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00:02:13,456 --> 00:02:13,906
Be sure to what?

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00:02:13,906 --> 00:02:16,696
[Background sound effects]
Alright mom, I'll be right out.

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>> [Background music] I
was getting closer now.

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The tension was terrible.

45
00:02:24,246 --> 00:02:25,226
What was it?

46
00:02:25,226 --> 00:02:27,076
The fate of the planet
may hang in the balance.

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[Sound effects]

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00:02:27,356 --> 00:02:29,706
>> I'll be right out
for crying out loud.

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00:02:30,486 --> 00:02:32,446
>> [Background music]
Almost there.

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00:02:33,486 --> 00:02:35,416
My fingers flew.

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My mind was a steel trap.

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Every pore vibrated.

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It was almost clear.

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Yes, yes, yes, yes.

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00:02:46,156 --> 00:02:49,506
>> Be sure to drink
your Ovaltine.

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00:02:55,416 --> 00:03:01,046
Ovaltine. A crummy commercial.

57
00:03:01,046 --> 00:03:05,576
[Background music]
Son of a bitch.

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[Laughter]

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00:03:05,643 --> 00:03:06,516
>> This is CS50.

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Welcome to the start of week 2.

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So this excerpt from A Christmas
Story, which is a movie

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that plays incessantly
around Christmas time

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in this country,
pretty much 24-7.

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Is actually an interesting
example of cryptography or code.

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And this is one of
the real world domains

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that we'll introduce
this week for the sake

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of motivation not only for the
material of the week but also

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for problems that
too will be going

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into your hands later this week.

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So what little Ralphie had
there was a secret decoder ring

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of sorts.

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Essentially it had the alphabet,
A through Z on one side

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of that circle and then
A through Z on another.

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And what the guy on the
radio told them was how

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to line those two circles
up so that A might map to M

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and B might map to another
letter, so it was a code.

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And he was intent on
actually decoding that.

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So what you saw was
actually an instance

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of what geeks have known for
some time is rot13, r o t 13.

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This is a rotational cipher and
if you're increasingly geekified

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00:04:10,016 --> 00:04:12,666
by courses such as this you
will understand the irony

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00:04:13,016 --> 00:04:14,156
of a quote like this.

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00:04:15,306 --> 00:04:15,976
Let's see if you do.

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00:04:16,516 --> 00:04:22,666
[ Sound effects ]

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00:04:23,166 --> 00:04:24,496
>> So rot13 to be clear is
when you take the alphabet

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00:04:24,496 --> 00:04:26,786
and you take an input
and you rotate all

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of the letters 13 places.

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[Laughter] There we go.

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If it's kind of one of
these moments right now,

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just ask the person next to you.

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00:04:38,356 --> 00:04:42,226
So this was-- this is sort of a
age old joke but it's an example

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of something more generally
known as a Caesar cipher.

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00:04:44,746 --> 00:04:46,256
And a Caesar cipher's
something we'll look

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at in more academic
detail on Wednesday.

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But the joke here is of
course, I'll leave it

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00:04:51,436 --> 00:04:53,386
to the astute thinker there.

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00:04:53,386 --> 00:04:54,246
So a couple of announcement --

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00:04:54,246 --> 00:04:57,096
one new handout for today the
printout of the source code

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00:04:57,096 --> 00:04:58,176
that we'll be using
for the week.

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00:04:58,176 --> 00:05:00,066
All of this stuff is
also available online.

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00:05:00,316 --> 00:05:02,276
And also if you weren't
paying attention last week

102
00:05:02,276 --> 00:05:04,486
or weren't here,
do realize that so

103
00:05:04,486 --> 00:05:06,616
that you are freed
intellectually not to scribble

104
00:05:06,616 --> 00:05:08,886
down most anything
that is said in class.

105
00:05:09,106 --> 00:05:11,186
Do realize that we've been
putting up scribe notes thanks

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00:05:11,186 --> 00:05:13,546
to one of the teaching fellows,
which are an authoritative set

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of notes so to speak in
PDF format on the website.

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00:05:17,086 --> 00:05:20,476
And also know too with
regard to problem sets,

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so problem set 1
went out on Friday.

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And from some of the
e-mails we got we know some

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of you dove into it.

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00:05:25,586 --> 00:05:28,346
But it's okay if normal
people are waiting

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00:05:28,346 --> 00:05:29,666
until later this week to start.

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00:05:29,866 --> 00:05:31,416
But do realize per the syllabus

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00:05:31,416 --> 00:05:32,666
that the course offers
you a number

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00:05:32,666 --> 00:05:34,326
of late days for problem sets.

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00:05:34,576 --> 00:05:39,426
Nine specifically, which means
you can give yourself a 24 hour

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00:05:39,426 --> 00:05:41,736
extension for instance at any
point in the semester simply

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00:05:41,736 --> 00:05:43,256
by cashing in one
of your late days.

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00:05:43,566 --> 00:05:45,826
Once you know who your teaching
fellow is we simply expect

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00:05:45,826 --> 00:05:47,596
that you advise him or
her that you're doing it,

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00:05:47,846 --> 00:05:49,606
so they don't start
grading and then realize

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00:05:49,606 --> 00:05:51,146
that oh you're not
actually in their pile.

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00:05:51,396 --> 00:05:53,406
But you can spend one
late day, nine late days,

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00:05:53,406 --> 00:05:56,076
we would encourage you just
from our own experience

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00:05:56,076 --> 00:05:58,386
to be conservative up front
because inevitably things come

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00:05:58,386 --> 00:05:59,296
up later in the semester.

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00:05:59,586 --> 00:06:02,066
But also to mitigate
that inevitable Pset

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00:06:02,066 --> 00:06:04,756
that you just completely
screw up for whatever personal

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00:06:04,966 --> 00:06:08,166
or academic reason, we will
also drop your lowest Pset score

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00:06:08,426 --> 00:06:09,426
at the end of the semester.

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00:06:09,426 --> 00:06:11,196
So realize that support
structure is there.

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00:06:11,466 --> 00:06:15,886
Super sections are taking place
today, tomorrow and Wednesday,

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00:06:15,886 --> 00:06:17,956
check the courses website
for the Google calendar

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00:06:17,956 --> 00:06:19,346
that will show you when
and where they are.

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00:06:19,566 --> 00:06:22,456
So these are just sections but
they're just a few sections

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that anyone and everyone can go

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00:06:23,806 --> 00:06:25,566
to irrespective of
comfort level.

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So if you are in -- among those
less comfortable let's say,

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those in between these are
apps specifically targeted

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00:06:31,666 --> 00:06:33,636
at that demographic this
week so that you feel

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00:06:33,636 --> 00:06:35,226
that you do have a
support structure in place

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00:06:35,546 --> 00:06:36,746
for the first problem set.

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00:06:36,806 --> 00:06:39,506
One of our teaching fellows
Marta led the first walkthrough

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last night, it was filmed

146
00:06:41,166 --> 00:06:42,676
and that video will
go online as well.

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00:06:42,676 --> 00:06:45,396
And the walkthroughs are meant
to be an authoritative how

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00:06:45,396 --> 00:06:47,426
to on how to begin the Pset

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00:06:47,426 --> 00:06:50,686
where sections will focus not
only on some related piece

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00:06:50,686 --> 00:06:53,546
of material but also
on lecture material

151
00:06:53,616 --> 00:06:56,326
and the stuff you should be
getting more comfortable with.

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00:06:56,496 --> 00:06:58,086
Late mid-week will you find

153
00:06:58,086 --> 00:06:59,856
out via e-mail what
your assigned section

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00:06:59,856 --> 00:07:02,786
and teaching fellow is and in
a class this size inevitably we

155
00:07:02,786 --> 00:07:03,876
need to shuffle people around,

156
00:07:03,876 --> 00:07:06,426
that's okay we'll fix
any mistakes that we

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00:07:06,426 --> 00:07:07,596
or your schedules have made.

158
00:07:07,966 --> 00:07:10,646
Office hours and virtual
office hours have also begun.

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00:07:11,166 --> 00:07:13,946
Office hours are an opportunity
to drop down into the basement

160
00:07:13,946 --> 00:07:14,616
of the science center.

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00:07:14,616 --> 00:07:15,526
You go down those stairs,

162
00:07:15,526 --> 00:07:17,126
you take a left there's
some white boards

163
00:07:17,126 --> 00:07:19,366
and that's generally where
the TFs and CAs hang out.

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00:07:19,636 --> 00:07:21,726
If you've got a question you
write your name on the board.

165
00:07:21,726 --> 00:07:24,046
You generally write down the
number of the computer station

166
00:07:24,046 --> 00:07:25,916
that you're at, they're
all numbered with stickers

167
00:07:26,176 --> 00:07:28,166
and then we the staff
go through the list top

168
00:07:28,166 --> 00:07:29,096
to bottom, left to right.

169
00:07:29,246 --> 00:07:33,276
And then eventually right to
left, bottom to top going back

170
00:07:33,276 --> 00:07:34,896
and forth through the
list crossing names off

171
00:07:34,896 --> 00:07:36,876
and then plucking off
people's questions

172
00:07:36,876 --> 00:07:37,926
so realize that that exists.

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00:07:38,176 --> 00:07:40,106
And then also this
little technology,

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00:07:40,106 --> 00:07:43,876
so sometimes it just doesn't
make all that much sense to go

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00:07:43,876 --> 00:07:47,356
into -- say trek across
campus to actually get

176
00:07:47,456 --> 00:07:50,326
to the science center to just
ask like a 1 minute question.

177
00:07:50,326 --> 00:07:52,566
And sometimes e-mail's
not perfect for it

178
00:07:52,566 --> 00:07:54,566
because sometimes you want
the staff member to see what's

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00:07:54,566 --> 00:07:56,306
on your screen, so we've offered

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00:07:56,306 --> 00:07:59,396
for some time now what we call
virtual office hours using a

181
00:07:59,616 --> 00:08:01,776
virtual terminal
room in contrast

182
00:08:01,776 --> 00:08:03,476
with the science center's
physical terminal room.

183
00:08:03,786 --> 00:08:06,836
And essentially we do this by
a program called Elluminate.

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00:08:07,156 --> 00:08:09,666
It's special software that's
free for all of us to use,

185
00:08:09,666 --> 00:08:12,346
it's cross platform, works
Windows, MACs, Linux computers.

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00:08:12,606 --> 00:08:14,256
And essentially when you want

187
00:08:14,256 --> 00:08:17,576
to attend virtual office hours
you'll click a button like this

188
00:08:17,576 --> 00:08:20,406
under VTR, you will
then log in for now

189
00:08:20,406 --> 00:08:22,446
with you FAS user
name and password.

190
00:08:22,446 --> 00:08:25,886
My connection's being a bit
slow but once you do so,

191
00:08:26,506 --> 00:08:30,466
you will then be led
hopefully after very few,

192
00:08:30,466 --> 00:08:34,136
if any technical hiccups to a
window that's going to pop up.

193
00:08:34,136 --> 00:08:35,186
The first time you use it,

194
00:08:35,186 --> 00:08:37,266
it's going to install
some special Java software

195
00:08:37,476 --> 00:08:40,406
and then even if it doesn't
come up right now that's fine.

196
00:08:40,406 --> 00:08:42,376
You will essentially have
a glorified chat room

197
00:08:42,376 --> 00:08:44,296
so there's going to be a
little portion of the screen

198
00:08:44,296 --> 00:08:46,426
where you can type back and
forth with the teaching staff.

199
00:08:46,706 --> 00:08:50,656
But more compellingly there
is a white board so to speak,

200
00:08:50,706 --> 00:08:52,636
which takes up most of the
screen and the neat thing

201
00:08:52,636 --> 00:08:54,326
with that is that
the teaching fellows

202
00:08:54,326 --> 00:08:58,176
and the CAs can take control of
or share control of your screen,

203
00:08:58,416 --> 00:08:59,596
your keyboard and mouse.

204
00:08:59,596 --> 00:09:01,456
And actually help you
troubleshoot something that is

205
00:09:01,516 --> 00:09:03,876
on your screen at
that moment and time.

206
00:09:03,876 --> 00:09:05,556
And if you're a little
freaked out by that idea,

207
00:09:06,106 --> 00:09:08,266
realize we won't
necessarily have the ability

208
00:09:08,266 --> 00:09:09,786
to see your Facebook windows,

209
00:09:09,786 --> 00:09:11,476
your personal chat
windows and all of that.

210
00:09:11,656 --> 00:09:15,076
It's very much an opt in thing,
it's not either or so I'll let

211
00:09:15,076 --> 00:09:17,886
that load in the background
and pull it up at some point.

212
00:09:18,166 --> 00:09:20,496
But for now a couple
of final notes.

213
00:09:20,496 --> 00:09:23,126
We will eventually
have a bulletin board

214
00:09:23,126 --> 00:09:25,596
on the courses website once
we have enrollment data

215
00:09:25,596 --> 00:09:27,636
and can actually
authenticate people properly.

216
00:09:27,916 --> 00:09:29,486
But for now if you
have any questions

217
00:09:29,906 --> 00:09:33,916
that are appropriately e-mailed
just drop a note to cs50.net

218
00:09:33,916 --> 00:09:36,926
and do get into the habit of
being as clear and as precise

219
00:09:36,926 --> 00:09:38,296
and as technical as possible.

220
00:09:38,606 --> 00:09:41,476
Sending us e-mails whatever
the hour with comments like,

221
00:09:41,916 --> 00:09:43,746
hi I'm having some
trouble with my code.

222
00:09:43,746 --> 00:09:44,626
Do you have any advice?

223
00:09:45,486 --> 00:09:47,916
Not really, you gotta be
more specific than that.

224
00:09:47,916 --> 00:09:49,996
Include your source code,
specify your problems.

225
00:09:49,996 --> 00:09:51,126
Tell us what you've done.

226
00:09:51,126 --> 00:09:53,236
What did work, did
not work and so forth.

227
00:09:53,356 --> 00:09:55,726
Help us help you, because
realize we get a lot

228
00:09:55,726 --> 00:09:57,016
of such e-mails.

229
00:09:57,186 --> 00:09:59,696
So we like to pluck off the
ones that are really clear

230
00:09:59,696 --> 00:10:00,816
and really straight forward.

231
00:10:01,306 --> 00:10:03,436
Sensor boards, if you did
hacker edition on your way

232
00:10:03,436 --> 00:10:04,386
out today we've got a couple

233
00:10:04,386 --> 00:10:06,026
of cardboard boxes,
just drop them off.

234
00:10:06,086 --> 00:10:09,176
But on that slip of paper
write your name, ID number

235
00:10:09,176 --> 00:10:11,556
and FAS user name so we
can cross you off the list.

236
00:10:11,866 --> 00:10:14,356
And then finally, this.

237
00:10:14,666 --> 00:10:19,856
So Harvard, not so
rich anymore but thanks

238
00:10:19,856 --> 00:10:23,326
to some wonderfully supportive
alumni who have fond memories

239
00:10:23,326 --> 00:10:25,006
of CS50 and computer
science here.

240
00:10:25,296 --> 00:10:26,546
We will be fortunately able

241
00:10:26,546 --> 00:10:28,686
to continue a little
tradition we began last year,

242
00:10:28,686 --> 00:10:34,006
which was lunch with David and
team or et al whereby we hang

243
00:10:34,006 --> 00:10:36,136
out with 10, 20, 30 students.

244
00:10:36,136 --> 00:10:37,566
Whoever's free on
Friday afternoons.

245
00:10:37,806 --> 00:10:39,556
We go to lunch somewhere,
sit down, chit chat,

246
00:10:39,556 --> 00:10:40,296
talk about the course.

247
00:10:40,296 --> 00:10:42,516
Talk about past experiences
at Harvard,

248
00:10:42,516 --> 00:10:44,286
whatever the case may be
and it's just a chance

249
00:10:44,536 --> 00:10:46,636
to make a big class
a lot more intimate.

250
00:10:46,636 --> 00:10:49,296
So if you're interested in
hanging out with me and some

251
00:10:49,296 --> 00:10:53,256
of the TFs and CAs over lunch
this Friday, 1:15 p.m. just go

252
00:10:53,366 --> 00:10:57,526
to cs50.net/rsvp and we'll
let you know where and all

253
00:10:57,526 --> 00:10:58,436
of the logistics there.

254
00:10:58,516 --> 00:10:59,746
So it's a nice tradition
that thanks

255
00:10:59,806 --> 00:11:02,246
to some generous alumni we
will begin, we will be able

256
00:11:02,246 --> 00:11:03,266
to continue this year.

257
00:11:03,766 --> 00:11:05,636
So I went back and
did my homework.

258
00:11:05,766 --> 00:11:11,306
I re-downloaded our surveys
and I made this integrated

259
00:11:11,306 --> 00:11:13,056
into the courses website
using Google maps,

260
00:11:13,176 --> 00:11:15,446
something you'll be able to
do later in the semester.

261
00:11:15,446 --> 00:11:17,696
It was actually a good sanity
check because we've got a couple

262
00:11:17,696 --> 00:11:18,856
of notes from students saying,

263
00:11:19,236 --> 00:11:22,056
"Hi my country is
not listed here,

264
00:11:22,056 --> 00:11:23,376
my city is not listed here.

265
00:11:23,376 --> 00:11:25,326
Did I actually set my
problem set correctly?"

266
00:11:25,626 --> 00:11:27,896
Sometimes the answer was yes,
sometimes the answer was no,

267
00:11:27,896 --> 00:11:30,076
so it was actually a nice
little way of checking as much.

268
00:11:30,406 --> 00:11:33,846
But this is again just a
mash up so to speak of all

269
00:11:33,846 --> 00:11:36,086
of the interesting
data that we collected.

270
00:11:36,146 --> 00:11:37,656
So more on that in the future.

271
00:11:37,916 --> 00:11:39,936
Do take notes if you
haven't already then

272
00:11:39,936 --> 00:11:42,296
on the problem sets page
there has been for a week,

273
00:11:42,586 --> 00:11:44,906
galleries not only
from the past two years

274
00:11:44,906 --> 00:11:45,966
but from this past year.

275
00:11:46,196 --> 00:11:48,386
So it's one thing to actually
do your scratch project,

276
00:11:48,386 --> 00:11:49,916
upload it and then be done.

277
00:11:50,086 --> 00:11:52,596
But if you get some time or just
want to procrastinate some night

278
00:11:52,596 --> 00:11:55,546
by all means flip through
the 300 plus scratch projects

279
00:11:55,576 --> 00:11:56,636
that your classmates made.

280
00:11:56,636 --> 00:11:58,906
Because there's some
really nice projects

281
00:11:58,976 --> 00:12:00,336
that got uploaded there.

282
00:12:00,726 --> 00:12:04,396
And then finally, let's see
if this actually came up.

283
00:12:04,396 --> 00:12:06,956
Yes. So what is -- has just
popped up on the screen

284
00:12:06,956 --> 00:12:09,446
with a slight delay there
is Elluminate the virtual

285
00:12:09,446 --> 00:12:10,126
terminal room.

286
00:12:10,126 --> 00:12:12,106
And this is again this
glorified chat room.

287
00:12:12,106 --> 00:12:14,326
So at the top left it looks

288
00:12:14,416 --> 00:12:17,896
like teaching fellow Andrew
Sellergren has perfectly timed

289
00:12:17,896 --> 00:12:20,226
this demonstration
to say hello to us.

290
00:12:20,226 --> 00:12:22,546
So he is chatting
with us down there.

291
00:12:23,016 --> 00:12:26,016
If I dare to indulge
something like this.

292
00:12:26,156 --> 00:12:32,066
Hello Andrew, you are in
Sanders right now to see

293
00:12:32,066 --> 00:12:33,206
if he's actually
paying attention.

294
00:12:33,206 --> 00:12:35,756
At the top left you see who's
logged in, both me and Andrew.

295
00:12:35,756 --> 00:12:37,326
Bottom left is obviously
the chat window.

296
00:12:37,596 --> 00:12:39,596
The right hand side is
where we can share screens

297
00:12:39,596 --> 00:12:41,796
but the workflow you will
find is that if you log

298
00:12:41,796 --> 00:12:43,606
into this thing,
you've got a question,

299
00:12:43,686 --> 00:12:46,066
you click this little
icon here at the top left.

300
00:12:46,426 --> 00:12:47,966
The little hand icon, click that

301
00:12:48,306 --> 00:12:50,456
and then notice [Sound
effects] a little 1 goes

302
00:12:50,456 --> 00:12:51,316
up next to my name.

303
00:12:51,316 --> 00:12:53,676
Which means David is first in
line, which hopefully means

304
00:12:53,676 --> 00:12:56,606
that Andrew the teaching fellow
will pull me into a private --

305
00:12:57,386 --> 00:13:01,226
dare I turn around
[Laughter] he'll pull me

306
00:13:01,226 --> 00:13:04,496
into a private chat room where
I can then share my screen just

307
00:13:04,496 --> 00:13:07,036
with that TF or CA and so forth.

308
00:13:07,156 --> 00:13:10,306
So it's really neat,
works reasonably well.

309
00:13:10,306 --> 00:13:17,996
Doesn't work so well over
dial-up and we're done.

310
00:13:17,996 --> 00:13:20,176
[Sound effects] But realize it's
a nice alternative to trucking

311
00:13:20,176 --> 00:13:23,126
to the science center when it's
say snowing in November so just

312
00:13:23,126 --> 00:13:25,056
to ask that little
quick question.

313
00:13:25,376 --> 00:13:27,286
So, why are you all here?

314
00:13:27,286 --> 00:13:33,006
So, we had 365 plus
folks register

315
00:13:33,006 --> 00:13:34,786
for CS50, which is fantastic.

316
00:13:34,786 --> 00:13:38,126
We of course asked folks, why
did you decide to take this CS50

317
00:13:38,176 --> 00:13:39,536
in that Pset 0 survey?

318
00:13:39,536 --> 00:13:42,106
I thought I would give you
a couple of choice excerpts.

319
00:13:42,406 --> 00:13:43,636
We love the serious ones

320
00:13:43,686 --> 00:13:46,746
but frankly the funny ones are
more fun to indulge in lecture.

321
00:13:47,096 --> 00:13:49,366
So the first one was an
answer to question 13.

322
00:13:49,366 --> 00:13:51,626
Why did you decide to take CS50?

323
00:13:51,836 --> 00:13:52,436
Quote, unquote.

324
00:13:52,816 --> 00:13:54,836
"I feel like an idiot
when I use computers."

325
00:13:55,696 --> 00:13:57,116
[Laughter] So fair enough.

326
00:13:57,386 --> 00:13:59,436
This one I kind of
had to read twice

327
00:13:59,436 --> 00:14:02,746
because I wasn't sure really why
one would answer in this way.

328
00:14:03,116 --> 00:14:05,666
Question 13, Why did
you decide to take CS50?

329
00:14:05,666 --> 00:14:08,166
Answer, I've never taken CS50.

330
00:14:09,886 --> 00:14:12,366
[Laughter] Thank you.

331
00:14:13,236 --> 00:14:18,096
Some of these can only
be shown, not read also

332
00:14:18,096 --> 00:14:19,216
in answer to the same.

333
00:14:19,616 --> 00:14:25,106
[Laughter] So it seems we're not
the ones doing all of the damage

334
00:14:25,106 --> 00:14:26,626
because clearly some
of the folks are coming

335
00:14:26,626 --> 00:14:27,786
into the course this way.

336
00:14:27,786 --> 00:14:30,236
And then finally
this one was fun

337
00:14:30,236 --> 00:14:31,736
if only for the all capitals.

338
00:14:32,826 --> 00:14:33,326
There you go.

339
00:14:36,206 --> 00:14:39,146
[Laughter] So thank you
for those choice responses.

340
00:14:39,246 --> 00:14:43,596
On a more serious note
for just a moment.

341
00:14:43,846 --> 00:14:46,076
So this course has a lot of fun.

342
00:14:46,076 --> 00:14:47,366
It does have a lot of work.

343
00:14:47,606 --> 00:14:50,666
And it is nonetheless an
independent sort of work

344
00:14:50,666 --> 00:14:52,746
but very much a shared
experience,

345
00:14:52,746 --> 00:14:55,386
very much an opportunity in
office hours and sections

346
00:14:55,386 --> 00:14:57,766
and just in your dorm rooms
to talk to other students

347
00:14:57,766 --> 00:14:59,866
about the problems that --
what you're struggling with,

348
00:14:59,866 --> 00:15:01,176
material related to it.

349
00:15:01,426 --> 00:15:04,676
But realize there is
a line between talking

350
00:15:04,676 --> 00:15:06,656
about the problem set, reasoning
through how you're going

351
00:15:06,656 --> 00:15:08,706
to approach it and
the act of saying,

352
00:15:09,116 --> 00:15:10,956
"Hey why don't you just
look at what I've done

353
00:15:10,956 --> 00:15:12,596
or hey here's my code."

354
00:15:12,596 --> 00:15:15,006
Or in an e-mail hey
here is an attachment.

355
00:15:15,006 --> 00:15:17,376
So realize the course's
syllabus and repeat it

356
00:15:17,376 --> 00:15:19,976
on every problem set
second page is the policy

357
00:15:19,976 --> 00:15:23,006
on academic honesty is
very clear over the years

358
00:15:23,006 --> 00:15:24,776
as to what's appropriate,
what is not.

359
00:15:24,776 --> 00:15:27,656
Frankly the rule of thumb is
talk all you want in English

360
00:15:27,696 --> 00:15:31,136
but as soon as the conversation
devolves into specific C code,

361
00:15:31,476 --> 00:15:34,046
you probably are
starting to come close to

362
00:15:34,046 --> 00:15:35,236
or crossing that line.

363
00:15:35,556 --> 00:15:37,976
Let me make the message
ever so clear with this--

364
00:15:38,516 --> 00:15:49,976
[ Sound effects ]

365
00:15:50,476 --> 00:15:52,426
>> So in the past two
years this is the number

366
00:15:52,426 --> 00:15:54,406
of students this
course had aborted.

367
00:15:54,456 --> 00:15:56,166
It gives us no pleasure
to do so.

368
00:15:56,386 --> 00:15:59,426
But do you realize when problem
sets are submitted we cross

369
00:15:59,426 --> 00:16:01,106
compare every problem
set against every other.

370
00:16:01,106 --> 00:16:03,136
We have archives of years
worth of problem sets,

371
00:16:03,136 --> 00:16:04,366
we cross compare
all of that stuff.

372
00:16:04,686 --> 00:16:06,886
It's not hard, we have
computers with which to help us.

373
00:16:07,166 --> 00:16:09,186
So please let me make
one personal plea

374
00:16:09,186 --> 00:16:10,216
at the start of this semester.

375
00:16:10,326 --> 00:16:14,206
Please do not put your time at
this wonderful place at risk

376
00:16:14,596 --> 00:16:15,946
by doing something stupid.

377
00:16:15,946 --> 00:16:18,886
We are very good at what we do
and we take again no pleasure

378
00:16:18,886 --> 00:16:21,886
in doing so but this does
happen and we will do it.

379
00:16:22,286 --> 00:16:23,726
So please meet us halfway.

380
00:16:23,726 --> 00:16:24,916
We're happy to have
a lot of fun.

381
00:16:25,076 --> 00:16:26,586
We're happy to do a lot of work

382
00:16:26,586 --> 00:16:28,426
but do realize those
constraints.

383
00:16:28,676 --> 00:16:31,306
So, let me try to
take the tension off

384
00:16:31,496 --> 00:16:33,286
and do some fun facts
about CS50 now.

385
00:16:33,586 --> 00:16:34,986
So we looked at Pset
just because we

386
00:16:34,986 --> 00:16:35,756
like playing with data.

387
00:16:35,756 --> 00:16:37,016
We did this already
with the Google map.

388
00:16:37,156 --> 00:16:39,826
What are you guys carrying
[Sound effects] Pre is not doing

389
00:16:39,826 --> 00:16:40,696
so well it seems.

390
00:16:40,696 --> 00:16:43,516
This was the Palm phone that
recently came out but a lot

391
00:16:43,516 --> 00:16:45,306
of you have normal
phones or smart phones.

392
00:16:45,306 --> 00:16:46,696
A lot of Blackberry's
and iPhones.

393
00:16:46,926 --> 00:16:48,506
Who are you guys using if only

394
00:16:48,506 --> 00:16:50,616
because it's an interesting
tidbit.

395
00:16:50,756 --> 00:16:53,026
Nobody's using Nextel but
that's really my fault

396
00:16:53,026 --> 00:16:54,276
because I forgot they
merged with Sprint

397
00:16:54,346 --> 00:16:55,256
so that kind of makes sense.

398
00:16:57,636 --> 00:16:59,476
[Laughter] Verizon
though is quite popular,

399
00:16:59,476 --> 00:17:00,906
as an aside I'll point out --

400
00:17:00,906 --> 00:17:02,766
and some Googling should
answer this for you.

401
00:17:02,766 --> 00:17:04,796
So Harvard actually has some
pretty good discounts on a lot

402
00:17:04,796 --> 00:17:07,036
of these carriers, so if you do
pay your own cell phone bill --

403
00:17:07,286 --> 00:17:11,286
don't e-mail cs50.net for this
but Google around and you'll see

404
00:17:11,286 --> 00:17:13,696
that you can get like 10, 19
percent off some of these guys.

405
00:17:13,836 --> 00:17:15,476
Completely irrelevant to CS50.

406
00:17:15,766 --> 00:17:16,316
Who are you?

407
00:17:16,556 --> 00:17:18,356
So historically you're
mostly sophomores

408
00:17:18,356 --> 00:17:21,126
but we do have a pretty
good range of folks here.

409
00:17:21,126 --> 00:17:24,126
The sample size for all of
these charts is roughly 350,

410
00:17:24,126 --> 00:17:27,066
400 people because we are also
offering this course via the

411
00:17:27,066 --> 00:17:28,596
extension school as
computer science E52.

412
00:17:28,596 --> 00:17:31,606
Hello to those of you
tuning in from afar and it's

413
00:17:31,606 --> 00:17:34,366
to the extension school that
we owe the very nice videos

414
00:17:34,366 --> 00:17:36,566
that are produced
each week as well

415
00:17:36,566 --> 00:17:37,566
as one of our own producers.

416
00:17:37,906 --> 00:17:39,166
Oh, what else we got?

417
00:17:39,486 --> 00:17:41,226
So, dorms, mess hall.

418
00:17:41,456 --> 00:17:43,396
So I couldn't remember
are students still living

419
00:17:43,396 --> 00:17:43,986
in mess hall?

420
00:17:44,266 --> 00:17:45,166
Or did they boot them all?

421
00:17:45,406 --> 00:17:47,226
Yes? But not here.

422
00:17:47,586 --> 00:17:50,176
Okay, so there's a few of
them just not here today.

423
00:17:51,156 --> 00:17:54,756
So Apley is understandably--
Grays is a pretty damn big dorm

424
00:17:54,756 --> 00:17:57,776
and no one from Grays
is apparently taking 50.

425
00:17:58,066 --> 00:18:00,636
But the houses, we have
this demographic here

426
00:18:00,856 --> 00:18:02,356
and this is complete
coincidence.

427
00:18:02,356 --> 00:18:04,626
So the most popular
house in terms

428
00:18:04,626 --> 00:18:07,046
of taking CS50 this year is
no longer Levritt [Assumed

429
00:18:07,356 --> 00:18:10,816
spelling] it is in fact
Mather, which is perhaps --

430
00:18:13,796 --> 00:18:15,186
[Applause] which is
perhaps not surprising

431
00:18:15,186 --> 00:18:18,766
because on this same survey when
asked what house would you want

432
00:18:18,906 --> 00:18:21,666
to live in if given the
choice everyone said,

433
00:18:21,746 --> 00:18:22,646
"Mather" actually.

434
00:18:23,286 --> 00:18:28,146
[Applause] So what else,
random little facts.

435
00:18:28,146 --> 00:18:28,916
What are you using?

436
00:18:28,916 --> 00:18:30,976
Well there's diversity
even in operating systems.

437
00:18:30,976 --> 00:18:32,356
A bunch of you have
clearly upgraded

438
00:18:32,356 --> 00:18:34,116
to Snow Leopard,
some of you have not.

439
00:18:34,116 --> 00:18:38,026
The most common answer in other
is MAC OS10.4, which is fine

440
00:18:38,026 --> 00:18:39,966
but I think you can even
get upgrades from Harvard

441
00:18:39,966 --> 00:18:41,146
as well from the helpdesk.

442
00:18:41,366 --> 00:18:42,576
Why are you taking CS50?

443
00:18:42,756 --> 00:18:46,176
So certainly allots for
concentration credit.

444
00:18:46,386 --> 00:18:48,966
But what's really neat to see is
just how many folks honestly are

445
00:18:48,966 --> 00:18:52,876
taking it as a pure elective and
then also these days for GenEd

446
00:18:52,876 --> 00:18:54,186
or formerly core credits.

447
00:18:54,536 --> 00:18:55,096
Who are you?

448
00:18:55,096 --> 00:18:57,616
So this one is actually
more useful I think to know,

449
00:18:57,846 --> 00:18:59,986
to reassure perhaps at
this point in the semester.

450
00:18:59,986 --> 00:19:02,166
So this is consistent
with last year's number.

451
00:19:02,166 --> 00:19:05,486
There's absolutely some people
in here, maybe one of the people

452
00:19:05,486 --> 00:19:08,356
or you know maybe a tenth of
the person sitting next to you

453
00:19:08,356 --> 00:19:11,776
who knows more than you do,
if not significantly so.

454
00:19:12,046 --> 00:19:12,676
And that's fine.

455
00:19:12,746 --> 00:19:14,636
That's why we have the
hacker editions of Psets.

456
00:19:14,696 --> 00:19:17,536
That's why we push the
envelope sometimes in lecture

457
00:19:17,536 --> 00:19:21,056
with some slightly more
advanced material but realize

458
00:19:21,116 --> 00:19:24,596
that the meat, the sweet
spot here is those in between

459
00:19:24,876 --> 00:19:26,046
and those less comfortable.

460
00:19:26,046 --> 00:19:28,706
So if you still have this
mentality that this person

461
00:19:28,756 --> 00:19:31,026
and this person both know
more than you in this room,

462
00:19:31,426 --> 00:19:34,396
probably not the case just
based on pure statistics alone.

463
00:19:34,716 --> 00:19:36,966
Prior courses, just too
really hammer this through.

464
00:19:37,186 --> 00:19:39,156
There's this misconception
on campus for years

465
00:19:39,156 --> 00:19:41,816
that almost everyone who takes
CS50 has prior course work

466
00:19:41,816 --> 00:19:43,916
in computer science,
AP computer science.

467
00:19:44,356 --> 00:19:46,846
Not so. What are you
all whispering about?

468
00:19:46,846 --> 00:19:48,136
This is all of your
data you know,

469
00:19:48,776 --> 00:19:52,406
so zero courses is the
most popular answer here.

470
00:19:53,206 --> 00:19:54,896
So, welcome to CS50.

471
00:19:54,896 --> 00:19:58,346
That is who you are and we hope
you enjoy your stay with us.

472
00:19:58,746 --> 00:20:02,736
So how do you write a program
in languages other than C just

473
00:20:02,736 --> 00:20:05,316
so that you're clear
that this is not a course

474
00:20:05,316 --> 00:20:08,276
on C. This is not going to be
a course on PHP or JavaScript.

475
00:20:08,276 --> 00:20:10,826
It's an introductory computer
science course that just

476
00:20:10,826 --> 00:20:12,766
so happens to use
those several languages

477
00:20:12,796 --> 00:20:15,496
to explore what are ultimately
some really interesting

478
00:20:15,496 --> 00:20:16,976
and really fundamental topics.

479
00:20:17,246 --> 00:20:20,366
Well you don't have to
necessarily use C to go

480
00:20:20,366 --> 00:20:21,936
about implementing programs.

481
00:20:21,936 --> 00:20:25,756
The canonical program for
instance in Java is something

482
00:20:25,756 --> 00:20:26,906
that might look like this.

483
00:20:26,956 --> 00:20:30,156
If I want to write hello
dot Java I can use --

484
00:20:30,296 --> 00:20:32,176
actually I'll use
the program familiar.

485
00:20:32,176 --> 00:20:33,246
Hello dot Java.

486
00:20:33,246 --> 00:20:35,096
So Java has different
conventions,

487
00:20:35,096 --> 00:20:36,726
usually capitalize
the file name.

488
00:20:36,986 --> 00:20:40,046
It ends in dot Java and
frankly this is where languages

489
00:20:40,076 --> 00:20:43,726
like Java get a little annoying
though Cs not much better.

490
00:20:43,726 --> 00:20:46,556
First we need to say class
hello, whatever that means.

491
00:20:46,946 --> 00:20:50,986
Then we say public,
static, void, main,

492
00:20:50,986 --> 00:20:53,026
string, bracket, args.

493
00:20:53,026 --> 00:20:54,786
But there's some shared
features there right.

494
00:20:54,786 --> 00:20:56,236
There's this main keyword.

495
00:20:56,236 --> 00:20:57,366
There's the square brackets.

496
00:20:57,426 --> 00:21:00,856
Arg sound like argv so there's
definitely some similarities.

497
00:21:01,076 --> 00:21:02,396
And now if I want to go ahead

498
00:21:02,396 --> 00:21:06,196
and say print something
it can be printf

499
00:21:06,336 --> 00:21:08,846
because there is a
printf function in C

500
00:21:09,096 --> 00:21:11,976
but it's usually system
dot, out dot, print line

501
00:21:11,976 --> 00:21:16,316
and then hello world,
exclamation point, semi colon.

502
00:21:16,556 --> 00:21:17,376
It's pretty similar.

503
00:21:17,606 --> 00:21:19,516
So I'm going to go
ahead and hot control X

504
00:21:19,586 --> 00:21:21,966
to save this, Y to save yes.

505
00:21:21,966 --> 00:21:23,996
And now I'm going to run Java C,

506
00:21:24,256 --> 00:21:26,956
which is the Java
compiler on hello dot Java.

507
00:21:27,406 --> 00:21:29,156
It seems to take a
moment then I have

508
00:21:29,156 --> 00:21:31,716
in my directory here not only
today's other source code,

509
00:21:31,716 --> 00:21:33,416
for which you have
printouts but notice

510
00:21:33,496 --> 00:21:36,916
from this file came a file
called hello dot class.

511
00:21:37,196 --> 00:21:39,916
Because in the world of Java
you say Java and then the name

512
00:21:39,916 --> 00:21:42,916
of the file without that
file extension enter viola.

513
00:21:43,066 --> 00:21:44,506
So my first program in Java.

514
00:21:44,506 --> 00:21:47,816
So just realize when we
introduce certain features here

515
00:21:47,816 --> 00:21:50,366
if they're not necessarily
specific to C

516
00:21:50,556 --> 00:21:53,196
and frankly the only -- as
I may have said 2 weeks ago,

517
00:21:53,196 --> 00:21:56,026
the only language --
actually I said this outside

518
00:21:56,026 --> 00:21:56,776
of class to someone.

519
00:21:56,976 --> 00:21:59,376
The only languages I think
I was ever taught quote,

520
00:21:59,376 --> 00:22:01,056
unquote was C in CS50.

521
00:22:01,056 --> 00:22:04,046
A little bit of C plus
plus and Lisp in CS51

522
00:22:04,046 --> 00:22:07,126
and then pretty much after
that everything I've learned

523
00:22:07,126 --> 00:22:08,316
since has been on my own.

524
00:22:08,316 --> 00:22:10,986
PHP, JavaScript, I never took
courses in these languages.

525
00:22:10,986 --> 00:22:14,026
PHP I literally learned by
IMing a friend back and forth

526
00:22:14,026 --> 00:22:16,266
over the course of a weekend
asking him how do I do this,

527
00:22:16,266 --> 00:22:20,396
how do I do this and started to
finally RTFM the manual myself

528
00:22:20,766 --> 00:22:22,436
when he advised me to do so.

529
00:22:23,106 --> 00:22:25,946
[Laughter] So realize that
what you'll get is this ability

530
00:22:25,946 --> 00:22:28,206
to parlay a lot of what
you learn in this course

531
00:22:28,446 --> 00:22:29,906
to other context all together.

532
00:22:29,906 --> 00:22:32,436
Lisp is another language
scheme, something similar.

533
00:22:32,596 --> 00:22:35,876
You'll be introduced to those
in different courses but Pearl,

534
00:22:36,186 --> 00:22:38,496
PHP these are languages
so similar to C

535
00:22:38,756 --> 00:22:41,286
that you'll probably pick
them up fairly easily.

536
00:22:41,896 --> 00:22:46,386
So bugs, it turns out and
actually this is a bit of a myth

537
00:22:46,386 --> 00:22:48,746
or white lie but it
turns out the first bug

538
00:22:48,746 --> 00:22:51,156
in a computer program
was in fact a bug.

539
00:22:51,156 --> 00:22:54,086
There was a computer at
Harvard called the Mark 2.

540
00:22:54,086 --> 00:22:55,556
Grace Hopper was the engineer

541
00:22:55,556 --> 00:22:58,256
who was among the people
responsible for this machine

542
00:22:58,256 --> 00:23:00,246
and as the story goes
there was a problem

543
00:23:00,246 --> 00:23:01,916
with the Mark 1 computer
at one point.

544
00:23:02,196 --> 00:23:05,326
And sure enough when some humans
went inside what was literally

545
00:23:05,326 --> 00:23:07,066
inside of the computer,
it was in fact

546
00:23:07,066 --> 00:23:08,196
that large they found a moth

547
00:23:08,526 --> 00:23:10,266
that was wrecking
some havoc inside.

548
00:23:10,506 --> 00:23:14,216
And now supposedly bug had
been used for years prior

549
00:23:14,216 --> 00:23:16,636
to describe problems with
machines and so forth.

550
00:23:16,786 --> 00:23:20,026
But to debug is supposedly
perhaps one

551
00:23:20,026 --> 00:23:22,446
of the treasures we're left
with from that particular story

552
00:23:22,446 --> 00:23:25,956
because the act of debugging
this computer was literally the

553
00:23:25,956 --> 00:23:28,156
act of debugging this computer.

554
00:23:28,206 --> 00:23:30,206
But with that said
let's actually look

555
00:23:30,206 --> 00:23:33,756
at what's a more modern type
of bug in this program here.

556
00:23:33,756 --> 00:23:37,316
So you have this printout, this
is buggy 1 dot C. Let me ask

557
00:23:37,756 --> 00:23:40,136
that you turn to the person
next to you if you don't --

558
00:23:40,276 --> 00:23:45,136
turn to a person next to you
whom you do not know, say hello

559
00:23:45,296 --> 00:23:46,546
and then together figure

560
00:23:46,546 --> 00:23:48,376
out what the buggy
is in this program.

561
00:23:48,446 --> 00:23:48,976
Take about a minute.

562
00:23:49,516 --> 00:25:07,056
[ Sound effects ]

563
00:25:07,556 --> 00:25:09,236
>> Okay, I've been waiting
to do that for like a minute.

564
00:25:09,236 --> 00:25:12,376
Alright, so what is the bug?

565
00:25:12,376 --> 00:25:12,906
Anyone who found it.

566
00:25:12,906 --> 00:25:12,973
[Inaudible background
discussion]

567
00:25:12,973 --> 00:25:15,556
So it prints 11 stars
instead of the 10.

568
00:25:15,556 --> 00:25:17,776
And it's simply because
programmers tend to count

569
00:25:17,776 --> 00:25:20,786
from 0, which this person
has done but they're counting

570
00:25:20,836 --> 00:25:24,736
up to an equal to 10 and if
you actually do the math 0

571
00:25:24,736 --> 00:25:27,766
through 10 inclusive
is in fact 11 stars.

572
00:25:27,766 --> 00:25:29,016
So there's the bug there.

573
00:25:29,016 --> 00:25:30,926
Excellent, that was some
pretty good chatter.

574
00:25:30,926 --> 00:25:32,366
If you weren't among
those who kind

575
00:25:32,456 --> 00:25:33,876
of realized what
trajectory we're

576
00:25:33,876 --> 00:25:36,336
on this second one is
maybe a little easier.

577
00:25:36,336 --> 00:25:37,606
And let's just see
if you can spot it.

578
00:25:38,666 --> 00:25:39,766
Don't shout it out just yet.

579
00:25:40,686 --> 00:25:41,526
Let everyone flip.

580
00:25:42,476 --> 00:25:44,416
And now what's the bug?

581
00:25:44,446 --> 00:25:44,636
Yes?

582
00:25:44,636 --> 00:25:44,703
>> [Inaudible background
discussion]

583
00:25:44,703 --> 00:25:50,116
>> Sorry. Can you say
it a little louder,

584
00:25:50,546 --> 00:25:53,966
just a little loud back here.

585
00:25:54,506 --> 00:25:56,256
>> Prints 11 vertically.

586
00:25:56,256 --> 00:25:59,916
>> So it prints 11 vertically
but it doesn't, it supposed to.

587
00:25:59,916 --> 00:26:01,326
Why -- what's the problem here?

588
00:26:01,326 --> 00:26:01,393
>> [Inaudible background
discussion]

589
00:26:01,393 --> 00:26:06,106
>> Yeah so I've forgotten
the curly braces.

590
00:26:06,106 --> 00:26:08,756
So if you notice the program
kind of looks okay right,

591
00:26:08,806 --> 00:26:10,996
here's a loop and
yes it does prints --

592
00:26:11,096 --> 00:26:13,236
okay 11 again so
we can fix that.

593
00:26:13,236 --> 00:26:16,296
So same bug, we didn't
actually fix that problem here.

594
00:26:16,476 --> 00:26:19,986
But a quick fix there gets rid
of that but if this is supposed

595
00:26:19,986 --> 00:26:22,716
to print 10 stars, 1 per line
-- let me go ahead and check.

596
00:26:22,716 --> 00:26:28,706
So GCC dash o buggy 2 and
then buggy 2 dot C is again my

597
00:26:28,706 --> 00:26:29,616
compiler statement.

598
00:26:29,616 --> 00:26:34,126
Now I can run buggy 2 enter,
they're all on the same line.

599
00:26:34,126 --> 00:26:35,226
Not one per line and yeah

600
00:26:35,476 --> 00:26:37,076
so remember even
though you might indent

601
00:26:37,076 --> 00:26:38,696
because it looks pretty,
you've been told it's a matter

602
00:26:38,696 --> 00:26:40,576
of good style, the
computer doesn't care.

603
00:26:40,576 --> 00:26:43,046
So again this was that subtle
thing that now encasing this

604
00:26:43,046 --> 00:26:45,506
in curly braces means don't
just execute the next line,

605
00:26:45,706 --> 00:26:48,816
execute the next bunch
of lines and this should

606
00:26:48,816 --> 00:26:51,476
if I rerun it fix that.

607
00:26:53,276 --> 00:26:53,716
[Sound effects] Okay, right.

608
00:26:53,896 --> 00:26:58,126
So again here to common goof
early on, I need to recompile.

609
00:26:58,126 --> 00:27:01,366
Same exact command, now I
can run buggy 2 and I do

610
00:27:01,366 --> 00:27:02,866
in fact get 1 per line.

611
00:27:02,866 --> 00:27:03,426
Excellent.

612
00:27:03,426 --> 00:27:06,456
So that's a little
warm up exercise,

613
00:27:06,456 --> 00:27:07,896
they'll get a little
more sophisticated.

614
00:27:08,036 --> 00:27:09,426
Let's introduce a
new capability.

615
00:27:09,426 --> 00:27:11,456
So we knew from the
very first week of class

616
00:27:11,856 --> 00:27:14,356
that we have this
ability to represent data

617
00:27:14,356 --> 00:27:15,616
in the computer, numbers.

618
00:27:15,996 --> 00:27:18,366
And we do it with this,
turn on the switch.

619
00:27:18,366 --> 00:27:19,256
Turn off the switch.

620
00:27:19,476 --> 00:27:22,246
You have some means of storing
bits inside of a computer.

621
00:27:22,246 --> 00:27:24,116
From where does that
come, that capability?

622
00:27:24,416 --> 00:27:26,656
Well computers are generally
plugged into the wall

623
00:27:26,786 --> 00:27:29,256
or they have batteries,
so there's some aspect

624
00:27:29,256 --> 00:27:31,666
of electricity underlying
any computer.

625
00:27:31,906 --> 00:27:33,606
For today's purposes
you can assume

626
00:27:33,606 --> 00:27:36,226
that with electricity can
you represent two states

627
00:27:36,226 --> 00:27:37,056
very easily.

628
00:27:37,426 --> 00:27:38,456
On or off.

629
00:27:38,566 --> 00:27:42,276
There's either some electricity
there flowing or there is not.

630
00:27:42,396 --> 00:27:45,046
Or technically there's some
electricity that's tripped a

631
00:27:45,046 --> 00:27:48,296
transistor, a little switch,
a tiny, tiny little switch

632
00:27:48,546 --> 00:27:49,716
that turns a state on.

633
00:27:49,976 --> 00:27:53,216
Or no such electricity flow
to the transistor stayed off.

634
00:27:53,216 --> 00:27:56,216
So very simple ultimately to
represent two different states

635
00:27:56,216 --> 00:27:58,226
in computers, not that useful

636
00:27:58,226 --> 00:28:00,706
if all you can express
though are 0s and 1s.

637
00:28:00,886 --> 00:28:03,286
But thankfully we had our
bytes of volunteers come up

638
00:28:03,286 --> 00:28:06,756
and by raising or lowering their
hands those 8 bits represented

639
00:28:06,756 --> 00:28:07,346
a byte.

640
00:28:07,346 --> 00:28:11,536
But that byte, a number could
be mapped to a letter by way

641
00:28:11,536 --> 00:28:16,606
of that little coding scheme
called ASCII, A-S-C-I-I.

642
00:28:16,646 --> 00:28:18,546
Just a standard the world
came up with years ago

643
00:28:18,546 --> 00:28:23,416
that just says capital A is 65.

644
00:28:23,416 --> 00:28:25,306
Random trivia but
actually useful

645
00:28:25,306 --> 00:28:27,336
because from there can you
bootstrap yourself to most

646
00:28:27,336 --> 00:28:30,196
of the other capital letters
and 97 is lower case A.

647
00:28:30,506 --> 00:28:33,436
So there's clearly a way
of representing letters

648
00:28:33,536 --> 00:28:36,796
with numbers but how do you
the programmer get access

649
00:28:36,866 --> 00:28:38,096
to that kind of capability?

650
00:28:38,346 --> 00:28:39,836
Well through this
ability called typecasting

651
00:28:39,836 --> 00:28:42,766
and we talked briefly last
week about typecasting.

652
00:28:42,766 --> 00:28:45,526
But we casted things
like ints to floats

653
00:28:45,526 --> 00:28:47,746
so that we could actually
use a decimal point.

654
00:28:48,066 --> 00:28:50,656
But if you know, you the
computer scientists know

655
00:28:50,656 --> 00:28:53,086
that this character,
the capital A,

656
00:28:53,326 --> 00:28:55,196
I know this is just
the number 65.

657
00:28:55,196 --> 00:28:56,446
How do I get at that number?

658
00:28:56,676 --> 00:29:00,226
Well you can tell the compiler,
"Give me the numeric value

659
00:29:00,226 --> 00:29:02,516
of this character" by
way of typecasting.

660
00:29:02,556 --> 00:29:06,006
To typecast one type to
another means to change it

661
00:29:06,046 --> 00:29:09,616
if it's possible conceptually
from one type to another.

662
00:29:09,846 --> 00:29:11,896
So here's two lines of code
clearly out of context,

663
00:29:11,976 --> 00:29:14,606
clearly not a real program
because we have no main function

664
00:29:14,606 --> 00:29:15,456
or anything like that.

665
00:29:15,816 --> 00:29:18,646
But here's two lines
of code, int I is going

666
00:29:18,646 --> 00:29:22,976
to get the integral value of
quote-unquote A and by contrast

667
00:29:22,976 --> 00:29:26,586
or conversely C, the
little c variable is going

668
00:29:26,586 --> 00:29:29,876
to get the character
value of 65.

669
00:29:29,876 --> 00:29:31,496
So this is just showing
you one direction

670
00:29:31,736 --> 00:29:34,516
and then the other direction
but notice the single quotes.

671
00:29:34,516 --> 00:29:37,166
We reserve double quotes
last week for strings

672
00:29:37,466 --> 00:29:40,036
and a string is anything
that has,

673
00:29:40,286 --> 00:29:42,566
for simplicity let's say
two or more characters.

674
00:29:42,566 --> 00:29:45,276
But you can have strings that
just have 1 character but more

675
00:29:45,276 --> 00:29:46,266
on that later this week.

676
00:29:46,436 --> 00:29:49,776
But single characters like this,
like A are just single quotes.

677
00:29:49,776 --> 00:29:52,746
So let's actually make
use of this information.

678
00:29:52,746 --> 00:29:55,686
Here's a little demo,
ASCII 1 that's going

679
00:29:55,686 --> 00:29:57,036
to show me the mapping.

680
00:29:57,036 --> 00:29:59,296
So that's actually -- I'm kind
of overwhelmed by the code there

681
00:29:59,296 --> 00:30:04,536
so let me go ahead and go
back and do GCC dash O ASCII 1

682
00:30:04,826 --> 00:30:09,426
and then S. That's a
little gap there, sorry.

683
00:30:09,486 --> 00:30:11,976
[Laughter] ASCII 1 and
now let me go ahead

684
00:30:11,976 --> 00:30:14,206
and run ASCII 1 enter.

685
00:30:14,646 --> 00:30:17,846
Okay, interesting a little
overwhelming but okay,

686
00:30:17,846 --> 00:30:19,876
looks like this is just
a stupid little program

687
00:30:19,876 --> 00:30:22,396
that prints a chart of
alphabetical letters

688
00:30:22,396 --> 00:30:25,786
and then their corresponding
ASCII integral values.

689
00:30:25,786 --> 00:30:26,916
So how would I do this?

690
00:30:27,126 --> 00:30:29,596
Well hopefully I didn't
copy and paste the same line

691
00:30:29,596 --> 00:30:32,176
of code 26 or 52 times right.

692
00:30:32,176 --> 00:30:34,696
Hopefully there's a more
intelligent way and there is,

693
00:30:34,696 --> 00:30:36,926
just from last week when
we introduced loops.

694
00:30:37,186 --> 00:30:40,756
So we have this ability
here, we're declaring I just

695
00:30:40,756 --> 00:30:42,386
because I wanted a
variable up front

696
00:30:42,386 --> 00:30:43,616
that I wanted to use twice.

697
00:30:43,616 --> 00:30:45,016
So I said, "Let's
put it up front."

698
00:30:45,406 --> 00:30:47,926
I didn't bother commenting
it because to put a comment

699
00:30:47,926 --> 00:30:50,156
for int I, you know
you could comment it

700
00:30:50,186 --> 00:30:51,426
but there's a line
there somewhere

701
00:30:51,426 --> 00:30:52,666
where it's a little
excessive maybe

702
00:30:52,666 --> 00:30:53,966
to comment something like I.

703
00:30:54,346 --> 00:30:55,256
But let's see.

704
00:30:55,316 --> 00:30:56,136
What am I doing here?

705
00:30:56,136 --> 00:30:57,306
I've got comments telling me.

706
00:30:57,306 --> 00:31:03,236
So let's see, four I gets
65, I is less than 65 plus 26

707
00:31:03,236 --> 00:31:05,006
and then I plus, plus.

708
00:31:05,076 --> 00:31:06,856
But remember the order
of operations there,

709
00:31:06,856 --> 00:31:09,216
so with the four loop the
initialization happens,

710
00:31:09,526 --> 00:31:12,646
the condition gets checked
then the stuff happens.

711
00:31:12,956 --> 00:31:15,986
Then the increment happens
and then you repeat except

712
00:31:15,986 --> 00:31:18,756
for the initialization, which
as the word implies only happens

713
00:31:18,756 --> 00:31:19,956
once, when you initialize.

714
00:31:20,286 --> 00:31:22,776
So it looks like this
loop starts I at 65,

715
00:31:23,166 --> 00:31:26,636
it prints its character
and integral value how?

716
00:31:26,636 --> 00:31:27,796
Well let's see.

717
00:31:27,796 --> 00:31:30,236
Printf here is saying
print a character

718
00:31:30,236 --> 00:31:33,136
with this little format code,
then print a colon literally,

719
00:31:33,136 --> 00:31:34,236
then a space literally,

720
00:31:34,236 --> 00:31:37,026
then a decimal integer
and then a new line.

721
00:31:37,176 --> 00:31:39,386
Well what do I want to swap
into each of those places?

722
00:31:39,546 --> 00:31:43,436
Well not just I but the
character equivalent of I.

723
00:31:43,686 --> 00:31:47,316
So I typecast I but then at the
end here I just want I itself

724
00:31:47,716 --> 00:31:49,146
so I just string it in.

725
00:31:49,146 --> 00:31:50,496
Alright, so pretty
straight forward and then

726
00:31:50,496 --> 00:31:53,766
in fact the stuff down below
is identical except I started

727
00:31:53,766 --> 00:31:55,606
from 97 and went on up.

728
00:31:55,866 --> 00:32:00,146
Now there's no need, realize to
hard code things like 65 and 97.

729
00:32:00,146 --> 00:32:02,906
This particular example is just
meant to demonstrate the mapping

730
00:32:03,166 --> 00:32:04,876
because we can actually
generalize this.

731
00:32:04,976 --> 00:32:06,646
But all this is doing
is showing me

732
00:32:06,786 --> 00:32:10,166
that in fact what we preached
2 weeks ago is clearly the case

733
00:32:10,276 --> 00:32:11,186
underneath the hood.

734
00:32:11,396 --> 00:32:13,416
Well let's look at a
slightly different example.

735
00:32:13,416 --> 00:32:17,246
So this is the second version,
ASCII 2 and I got a little fancy

736
00:32:17,246 --> 00:32:19,416
because what I didn't
like a moment ago was

737
00:32:19,416 --> 00:32:22,026
when I ran ASCII 1,
it's just unmanageable.

738
00:32:22,026 --> 00:32:24,606
The big font size, small font
size it just doesn't fit.

739
00:32:24,936 --> 00:32:27,666
Let me see if I can start
using my formatting strings

740
00:32:27,696 --> 00:32:29,586
to lay things out a
little more cleverly.

741
00:32:29,936 --> 00:32:33,396
So let's see this time I
just have 1 loop from 65

742
00:32:33,396 --> 00:32:35,126
up to 26 places later.

743
00:32:35,516 --> 00:32:37,186
And then okay I got a
little creative here,

744
00:32:37,186 --> 00:32:40,656
so I have character -- a
character, some space, an int,

745
00:32:41,016 --> 00:32:44,176
a bunch more space and then 3 D

746
00:32:44,276 --> 00:32:47,006
so there's no decimal point
there, so what might it mean

747
00:32:47,186 --> 00:32:54,736
in a format code to say 3
and then int, the little D?

748
00:32:54,986 --> 00:33:01,826
It's not specifying number of
decimal places but -- Yeah?

749
00:33:01,826 --> 00:33:02,446
>> [Inaudible background
discussion]

750
00:33:02,446 --> 00:33:04,316
>> Yeah. Exactly.

751
00:33:04,316 --> 00:33:06,686
So the numbers that
we're going to be playing

752
00:33:06,686 --> 00:33:08,566
with might actually over --

753
00:33:08,656 --> 00:33:12,336
if we're starting at 97
for lower case 97, 98, 99,

754
00:33:12,336 --> 00:33:15,866
100 which is 3 digits
whereas 97 is just 2.

755
00:33:16,066 --> 00:33:18,036
So if we're iterating
over this loop but I want

756
00:33:18,036 --> 00:33:19,456
to exercise some control --

757
00:33:19,456 --> 00:33:22,256
problem with playing
music during class,

758
00:33:22,346 --> 00:33:23,736
just hung up on FASIT.

759
00:33:23,736 --> 00:33:26,276
[Laughter] That was my phone

760
00:33:26,276 --> 00:33:27,516
if you didn't realize
what was ringing.

761
00:33:27,796 --> 00:33:29,376
Apparently I'm the
only one anyway.

762
00:33:29,436 --> 00:33:32,186
No comment about the
phone in the future.

763
00:33:32,356 --> 00:33:33,716
[Laughter] So what I want

764
00:33:33,716 --> 00:33:37,036
to do is keeps this formatting
pretty, so let me see.

765
00:33:37,036 --> 00:33:41,666
So this going to say put an
int here but reserve 3 places

766
00:33:41,666 --> 00:33:44,366
for it not just the
standard 1 or 2 or 3.

767
00:33:44,586 --> 00:33:46,666
So what am I plugging in
after that F character?

768
00:33:46,666 --> 00:33:49,506
So I've got 4 format
codes, the first one's going

769
00:33:49,506 --> 00:33:52,416
to take the character equivalent
of I, the second just I.

770
00:33:52,846 --> 00:33:55,096
Oh this is interesting,
I kind of did some math

771
00:33:55,096 --> 00:33:56,056
and realized you know what?

772
00:33:56,256 --> 00:33:59,026
The lowercase letters are
always the same distance

773
00:33:59,296 --> 00:34:04,396
from the uppercase 97 is
32 places away from 65

774
00:34:04,466 --> 00:34:05,586
if you just do the arithmetic.

775
00:34:05,956 --> 00:34:06,426
So you know what?

776
00:34:06,426 --> 00:34:08,906
I don't need a separate loop
to do the lowercase letters,

777
00:34:08,906 --> 00:34:11,416
I can simply do both
at once just

778
00:34:11,416 --> 00:34:13,526
by using a little
clever arithmetic.

779
00:34:13,526 --> 00:34:16,646
An offset if you will because
the first time I print I's

780
00:34:16,706 --> 00:34:21,146
character and then I but the
second time I print I plus 32

781
00:34:21,186 --> 00:34:24,196
and then its character
equivalent so it feels

782
00:34:24,196 --> 00:34:26,766
like what I'm going to get
here is a slightly more

783
00:34:26,766 --> 00:34:27,686
interesting chart.

784
00:34:27,686 --> 00:34:30,096
So let me go ahead
and run ASCII 2,

785
00:34:30,096 --> 00:34:34,286
I'm going to compile its
code here, thought C, okay,

786
00:34:34,286 --> 00:34:37,176
compiles okay, ASCII
2, enter, okay.

787
00:34:37,526 --> 00:34:39,846
So baby step toward something
a little more interesting

788
00:34:39,846 --> 00:34:41,646
but definitely more
useful, cleaner.

789
00:34:41,836 --> 00:34:44,956
And notice now what I was
getting at with the format code.

790
00:34:44,956 --> 00:34:48,356
97 just takes 2 places,
then there's 98, 99,

791
00:34:48,566 --> 00:34:50,446
everything kind of
lined up nicely.

792
00:34:50,666 --> 00:34:55,216
What if I had not done so, let
me go back in here and sort

793
00:34:55,216 --> 00:34:57,446
of not realize this capability.

794
00:34:57,506 --> 00:35:00,536
Change that to what I know
from last week, recompile,

795
00:35:01,446 --> 00:35:03,976
recompile as -- rerun ASCII 2

796
00:35:04,376 --> 00:35:06,626
and [Sound effects]
it kind of gets messy.

797
00:35:06,626 --> 00:35:09,856
It gets ugly quickly and if you
think now about how you're going

798
00:35:09,856 --> 00:35:12,886
to present any textual
information for problem set 1,

799
00:35:13,216 --> 00:35:14,976
you can think of
your terminal window.

800
00:35:15,056 --> 00:35:17,396
This SSH window for
historical reasons is kind

801
00:35:17,396 --> 00:35:18,716
of a terminal window

802
00:35:19,006 --> 00:35:21,946
and by default it's
typically 80 characters wide

803
00:35:22,226 --> 00:35:23,866
and 24 characters tall.

804
00:35:24,076 --> 00:35:27,466
Now on 30 inch monitors, 20 inch
monitors whatever these days,

805
00:35:27,466 --> 00:35:29,296
you can clearly drag
a putty window

806
00:35:29,496 --> 00:35:31,536
or drag a MAC OS terminal
window to be much,

807
00:35:31,536 --> 00:35:32,696
much larger than that.

808
00:35:32,726 --> 00:35:33,986
So this is somewhat historical

809
00:35:34,216 --> 00:35:37,476
but for the first problem
set we specifically

810
00:35:37,476 --> 00:35:39,246
for the Mario pyramid
will tell you

811
00:35:39,246 --> 00:35:41,496
to assume only a
certain screen size

812
00:35:41,736 --> 00:35:43,866
so that you can actually
present the information

813
00:35:43,896 --> 00:35:45,516
in a predictable way.

814
00:35:45,806 --> 00:35:47,806
So I would argue that
this is just ugly,

815
00:35:47,806 --> 00:35:50,006
you know it's not wrong
but we can do better.

816
00:35:50,006 --> 00:35:51,706
And that's again
something so simple.

817
00:35:51,706 --> 00:35:53,766
The format code instead
of doing point

818
00:35:53,926 --> 00:35:55,086
and then a number then d --

819
00:35:55,086 --> 00:35:58,656
rather point, a number then
F. We just do a number and D

820
00:35:58,756 --> 00:36:01,976
and that just gives you the
width of the place that you want

821
00:36:01,976 --> 00:36:05,046
to reserve from some --
for some variable's value.

822
00:36:05,266 --> 00:36:07,646
Alright let's take a look
at a third version here,

823
00:36:07,876 --> 00:36:11,386
so this one is slightly
more clever.

824
00:36:11,386 --> 00:36:14,446
I don't need it to do so
much math in the second line

825
00:36:14,446 --> 00:36:16,176
and I can kind of
integrate it all above.

826
00:36:16,466 --> 00:36:16,806
You know what?

827
00:36:16,806 --> 00:36:20,116
If characters are just numbers,
well why am I doing this back

828
00:36:20,306 --> 00:36:21,526
and forth and back and forth?

829
00:36:21,526 --> 00:36:24,176
Why don't I just iterate over
the characters themselves?

830
00:36:24,206 --> 00:36:27,136
Now arguably the first
approach was pretty darn clear,

831
00:36:27,406 --> 00:36:30,566
so realize this too is meant
more for instructional purposes

832
00:36:30,566 --> 00:36:32,846
than for here's how you
should have done this.

833
00:36:33,166 --> 00:36:34,606
But let's see what we can do

834
00:36:34,606 --> 00:36:37,356
because C does give us
this low level access

835
00:36:37,356 --> 00:36:39,136
to characters representation.

836
00:36:39,366 --> 00:36:42,596
So a 4 loop is initializing
a character C to quote,

837
00:36:42,596 --> 00:36:46,446
unquote A. Turns out because C
is just a number you can say,

838
00:36:46,446 --> 00:36:49,656
"Is this character less
than or equal to Z", quote,

839
00:36:49,656 --> 00:36:50,806
unquote, single quotes.

840
00:36:51,096 --> 00:36:52,136
And then just to be ever

841
00:36:52,136 --> 00:36:54,576
so clear I did show
the typecasting here.

842
00:36:54,886 --> 00:36:56,556
On each iteration you don't have

843
00:36:56,736 --> 00:36:58,896
to do something like
I plus, plus.

844
00:36:59,296 --> 00:37:01,356
What you can really do
is a whole expression

845
00:37:01,356 --> 00:37:04,736
and I will push the limits
of my handwriting here.

846
00:37:04,996 --> 00:37:10,686
This I plus, plus is the
same as doing I equals I,

847
00:37:11,146 --> 00:37:15,416
plus 1 so I could put
either of this or this

848
00:37:15,416 --> 00:37:17,446
in the update field [Inaudible].

849
00:37:17,766 --> 00:37:20,536
It's just the world and myself
prefers the top left one just

850
00:37:20,536 --> 00:37:22,516
because it's a lot
sexier, it's a lot simpler,

851
00:37:22,516 --> 00:37:24,016
it's more compact, I plus, plus.

852
00:37:24,316 --> 00:37:26,196
But realize I'm taking advantage

853
00:37:26,196 --> 00:37:28,346
of the latter approach
down here.

854
00:37:28,476 --> 00:37:30,376
I'm saying C gets what?

855
00:37:30,556 --> 00:37:31,406
Well what do I want to do?

856
00:37:31,406 --> 00:37:34,576
I want to first cast C to an
int to get it to a number.

857
00:37:34,856 --> 00:37:36,506
Then I want to add 1 to it,

858
00:37:36,756 --> 00:37:38,166
okay but I don't want
a number now I have

859
00:37:38,166 --> 00:37:40,096
to cast the whole
thing back to a char

860
00:37:40,346 --> 00:37:44,436
because I'm assigning it again
to the variable C. Now it turns

861
00:37:44,436 --> 00:37:47,236
out that I actually don't
need all this typecasting.

862
00:37:47,236 --> 00:37:49,276
This example has
it just for clarity

863
00:37:49,276 --> 00:37:52,366
as to what the casting --
what casting is going on.

864
00:37:52,596 --> 00:37:54,586
But a lot of this
typecasting can

865
00:37:54,636 --> 00:37:57,456
and will happen implicitly
even if you leave it off.

866
00:37:57,776 --> 00:37:59,316
But again the takeaways
for now are just

867
00:37:59,356 --> 00:38:01,196
that there is this equivalence.

868
00:38:01,256 --> 00:38:02,446
But let's do something

869
00:38:02,736 --> 00:38:07,796
that makes this slightly more
compelling, so Battleship.

870
00:38:08,426 --> 00:38:11,436
So at first glance we clearly
have some more code here,

871
00:38:11,436 --> 00:38:14,596
looks a little cryptic but
again I see nothing on the board

872
00:38:14,596 --> 00:38:16,486
that we haven't seen in
other context already.

873
00:38:16,486 --> 00:38:17,766
Some printf's, some for loops,

874
00:38:18,026 --> 00:38:21,236
some variables and
really that's it.

875
00:38:21,386 --> 00:38:24,246
So a common, you know
motivation in a course is --

876
00:38:24,306 --> 00:38:27,036
you know I don't want to just
write hello world, I don't want

877
00:38:27,316 --> 00:38:30,436
to just convert ASCII to
integers and vice versa.

878
00:38:30,436 --> 00:38:32,926
Let me make a game or just
like you did with Scratch,

879
00:38:32,926 --> 00:38:34,576
many of you wrote games not

880
00:38:34,576 --> 00:38:36,236
because it was the only
thing you could do but just

881
00:38:36,236 --> 00:38:39,016
because it's more interesting
than just doing you know,

882
00:38:39,016 --> 00:38:40,896
a cat bouncing back and forth.

883
00:38:41,126 --> 00:38:43,136
Well if we needed
to implement a game

884
00:38:43,136 --> 00:38:45,456
in this context of
C like Battleship.

885
00:38:45,586 --> 00:38:48,396
Well one of the things we
might do is actually represent

886
00:38:48,446 --> 00:38:49,046
a board.

887
00:38:49,266 --> 00:38:52,046
So let me go ahead and compile
Battleship and I'm going

888
00:38:52,046 --> 00:38:54,846
to start doing it
the quick approach.

889
00:38:54,846 --> 00:38:57,346
So Battleship, I'm just going
to type make and I'm going

890
00:38:57,346 --> 00:38:59,566
to let the computer figure
out what the command is

891
00:38:59,836 --> 00:39:01,446
so now I can run Battleship.

892
00:39:01,666 --> 00:39:03,996
And let me make one note
about security actually,

893
00:39:04,066 --> 00:39:06,166
since some of you might
have realized this already.

894
00:39:06,476 --> 00:39:10,476
A typical computer scientist
or at least Linux geek

895
00:39:10,476 --> 00:39:12,726
or MAC OS geek will
write commands.

896
00:39:12,806 --> 00:39:14,036
If they've written a program

897
00:39:14,076 --> 00:39:15,666
that they know is
currently stored

898
00:39:15,666 --> 00:39:18,736
in their current directory
they will be ever so explicit

899
00:39:18,736 --> 00:39:22,436
and say, "Run the program
that's in my current directory,

900
00:39:22,546 --> 00:39:26,026
which is symbolized as
dot, slash and then Foo

901
00:39:26,026 --> 00:39:27,536
if Foo is the name
of the program.

902
00:39:27,946 --> 00:39:31,196
And this is actually a security
thing so that when you connect

903
00:39:31,326 --> 00:39:34,376
to a system with SSH, if
you just write something

904
00:39:34,376 --> 00:39:37,496
like Foo thinking, "Oh Foo is
a program in, my directory."

905
00:39:37,676 --> 00:39:40,036
Well what if Foo is
actually a malicious command

906
00:39:40,036 --> 00:39:41,796
that some bad guy
put on the system?

907
00:39:42,006 --> 00:39:44,466
They just tricked you into
executing that version

908
00:39:44,466 --> 00:39:46,076
of Foo instead of your own.

909
00:39:46,546 --> 00:39:49,366
But for now in the course you
can just write your program's

910
00:39:49,366 --> 00:39:52,386
name at the prompt whether
it's ISBN or Mario just

911
00:39:52,386 --> 00:39:55,076
because that sort of adds
an unnecessary complication.

912
00:39:55,076 --> 00:39:56,976
But when I'm doing
it, it's just a matter

913
00:39:56,976 --> 00:39:58,156
of habit and good practice.

914
00:39:58,636 --> 00:39:59,716
So there's the game
of Battleship.

915
00:39:59,966 --> 00:40:02,186
So this was that little
childhood game where each --

916
00:40:02,186 --> 00:40:04,206
you and your friend have
this little laptop type thing

917
00:40:04,206 --> 00:40:06,866
with a bunch of plastic
holes and you say,

918
00:40:06,976 --> 00:40:09,626
"A2" and then they
put a puzzle piece in

919
00:40:09,626 --> 00:40:10,666
and then go [Sound effects]

920
00:40:10,706 --> 00:40:12,296
when there actually
a ship in the hole.

921
00:40:12,456 --> 00:40:14,666
Well if I want to represent
this here I've got A through J

922
00:40:14,896 --> 00:40:18,146
on the left, 1 through 10
at the top and then I went

923
00:40:18,146 --> 00:40:20,746
with the ASCII Art
as it's called.

924
00:40:20,986 --> 00:40:22,926
So if you really want to
geek out Google something

925
00:40:22,926 --> 00:40:26,836
like ASCII Art and the world
has wasted so many hours

926
00:40:26,836 --> 00:40:29,716
over the decades writing
pictures, writing --

927
00:40:30,846 --> 00:40:34,256
depicting people in photographs
just using the characters

928
00:40:34,256 --> 00:40:35,016
on the keyboard.

929
00:40:35,016 --> 00:40:36,776
So this is a little
bit of ASCII Art

930
00:40:37,106 --> 00:40:39,286
but there's clearly some
complexity there at least

931
00:40:39,286 --> 00:40:42,496
for out purposes today, because
I've gotten letters being

932
00:40:42,526 --> 00:40:44,736
incremented on the left,
that's kind of interesting

933
00:40:44,736 --> 00:40:46,926
but I also have numbers
incrementing on the right.

934
00:40:47,216 --> 00:40:48,846
Everything is formatted nicely

935
00:40:48,846 --> 00:40:50,606
so let's just tease
this apart slightly.

936
00:40:50,906 --> 00:40:55,396
This is Battleship dot
oops, what did I do wrong?

937
00:40:55,396 --> 00:40:55,596
>> [Inaudible background
discussion]

938
00:40:55,596 --> 00:40:58,966
>> Right. So I tried changing
accidentally the binary,

939
00:40:58,966 --> 00:41:01,326
the 0s and 1s version
not the source code.

940
00:41:01,366 --> 00:41:02,466
So here's the source code.

941
00:41:02,466 --> 00:41:04,776
Let me scroll down and
just think how would you

942
00:41:04,776 --> 00:41:05,386
approach this?

943
00:41:05,666 --> 00:41:07,976
Well if the goal
for a problem set,

944
00:41:07,976 --> 00:41:10,786
if this is how you're beginning
is to implement something

945
00:41:10,786 --> 00:41:12,986
like this, that you have
a picture in front of you

946
00:41:12,986 --> 00:41:15,436
or you have the game in front of
you, you want to implement this.

947
00:41:15,756 --> 00:41:17,686
Just think through how
you would implement this.

948
00:41:17,686 --> 00:41:20,966
Well this is a screen that
clearly for the past week

949
00:41:20,966 --> 00:41:24,836
and a half has only let me send
output to the screen from left

950
00:41:24,836 --> 00:41:26,576
to right and then
from top to bottom.

951
00:41:26,576 --> 00:41:29,386
Thus far I have not been able
to jump back and fix things.

952
00:41:29,386 --> 00:41:31,586
Once I print a new
line, that's it I'm

953
00:41:31,586 --> 00:41:33,726
on the new line just
like a typewriter.

954
00:41:34,296 --> 00:41:36,026
So what could I do first?

955
00:41:36,336 --> 00:41:38,466
Well what's the first thing
I should probably print then

956
00:41:38,466 --> 00:41:41,326
in my program, if I
want to replicate this?

957
00:41:42,036 --> 00:41:44,016
So probably the numbers
on top, right.

958
00:41:44,016 --> 00:41:45,386
And that's probably
not that hard.

959
00:41:45,386 --> 00:41:47,606
Even if you haven't even
dived into problem set 1

960
00:41:47,606 --> 00:41:50,146
yet you probably know that
if you copied and pasted some

961
00:41:50,146 --> 00:41:52,386
of the code from past
examples, the main line

962
00:41:52,386 --> 00:41:55,286
and then the curly braces, you
could probably write the 4 loop

963
00:41:55,286 --> 00:41:59,176
that iterates from I equals
1 up to and equal to 10

964
00:41:59,176 --> 00:42:01,076
and just print that
thing on the top row.

965
00:42:01,326 --> 00:42:02,636
What about the next row though?

966
00:42:02,736 --> 00:42:05,096
Well if we ignore the
letters it's really damn easy

967
00:42:05,096 --> 00:42:07,576
to print the grid of holes
in this board, right.

968
00:42:07,576 --> 00:42:09,076
You just iterate
from left to right

969
00:42:09,076 --> 00:42:10,726
and print 10 lower cases --

970
00:42:10,776 --> 00:42:13,946
O's followed by a little
white space after them.

971
00:42:14,316 --> 00:42:17,356
But the interesting part seems
to be the letters on the side.

972
00:42:17,416 --> 00:42:18,636
So let's focus then on that.

973
00:42:18,996 --> 00:42:22,076
The top part here as the
comment implies, print top row

974
00:42:22,076 --> 00:42:24,496
of numbers, there's
really no magic there.

975
00:42:24,736 --> 00:42:25,886
It's boring at this point.

976
00:42:26,146 --> 00:42:28,156
But now this part's a
little more interesting

977
00:42:28,156 --> 00:42:30,826
because it turns out you
can nest in loops in C. Just

978
00:42:30,826 --> 00:42:32,996
like some of you might
have nested forever loops

979
00:42:33,076 --> 00:42:35,826
or loop puzzle pieces
in Scratch.

980
00:42:36,126 --> 00:42:37,136
So here we have it.

981
00:42:37,136 --> 00:42:40,616
Let's see for I, I want 10
holes anyway so I'm going

982
00:42:40,616 --> 00:42:42,476
to go from 0 up to 10.

983
00:42:42,756 --> 00:42:46,086
I could go 1 through 10,

984
00:42:46,236 --> 00:42:49,246
really just as two different
approaches to the same problem.

985
00:42:49,246 --> 00:42:50,306
Now what do I want to do?

986
00:42:50,426 --> 00:42:52,746
Well the first thing I
want to do on every one

987
00:42:52,746 --> 00:42:56,676
of these 10 iterations because
I want 10 rows of little O's,

988
00:42:57,026 --> 00:42:58,306
I want to print out the letter.

989
00:42:58,686 --> 00:43:00,766
So lets see, character
and then some space.

990
00:43:00,766 --> 00:43:01,546
What do I want to print?

991
00:43:01,736 --> 00:43:04,236
Well I want to start
with A but you know what,

992
00:43:04,346 --> 00:43:05,766
I want to just do an offset.

993
00:43:05,846 --> 00:43:08,666
I don't need to hard code in
A, B, C, D. Let me just start

994
00:43:08,666 --> 00:43:14,186
with A and then add to A the
value of I, but if I do A plus I

995
00:43:14,186 --> 00:43:16,346
where I is 0 what do I get back?

996
00:43:17,286 --> 00:43:18,036
Just A right.

997
00:43:18,036 --> 00:43:21,326
So actually it makes sense that
I started with I equals 0 here

998
00:43:21,326 --> 00:43:24,196
because I'm intentionally using
it not as an increment yet

999
00:43:24,196 --> 00:43:27,796
but as an offset from the
capital letter A. So this line

1000
00:43:27,796 --> 00:43:29,976
of code here, if I just do this.

1001
00:43:29,976 --> 00:43:35,436
Let me go and remove
everything except that line

1002
00:43:35,436 --> 00:43:36,576
and the new line below it.

1003
00:43:36,576 --> 00:43:37,996
Let me go ahead and recompile

1004
00:43:37,996 --> 00:43:41,046
and now notice the compilers
saying, "Whoa you didn't use J."

1005
00:43:41,096 --> 00:43:42,996
That's okay because I
just removed the code

1006
00:43:43,086 --> 00:43:46,396
that might have used J. Let
me re-run Battleship, okay.

1007
00:43:46,466 --> 00:43:48,636
So that's a nice baby
step and this is not meant

1008
00:43:48,636 --> 00:43:50,076
to belittle the process
of coding.

1009
00:43:50,236 --> 00:43:52,666
This is precisely the approach
that someone should take

1010
00:43:52,666 --> 00:43:54,796
in this class especially
those less comfortable.

1011
00:43:54,796 --> 00:43:57,216
When you're trying to tackle
a problem set don't try

1012
00:43:57,216 --> 00:43:59,046
and implement the whole
damn program at once,

1013
00:43:59,266 --> 00:44:00,716
just get something
working right.

1014
00:44:00,716 --> 00:44:02,456
And we did, the top
row was so easy.

1015
00:44:02,746 --> 00:44:04,966
Second row took a little
bit of you know, thought,

1016
00:44:04,966 --> 00:44:07,066
might have taken 10 minutes,
20 minutes, half an hour

1017
00:44:07,066 --> 00:44:10,036
to get it right but now
we have almost everything.

1018
00:44:10,266 --> 00:44:13,056
We just need to do a little
more on that second loop

1019
00:44:13,056 --> 00:44:15,966
because we need to fill the
rest of the row with O's.

1020
00:44:16,026 --> 00:44:16,996
So how did I do that?

1021
00:44:16,996 --> 00:44:19,466
Well let me put my code
back and if what I'm doing

1022
00:44:19,466 --> 00:44:21,586
in the outer loop
with the variable I,

1023
00:44:21,586 --> 00:44:23,966
is going from row to
row to row to row.

1024
00:44:24,156 --> 00:44:26,946
Well if I need to
simultaneously, before going

1025
00:44:26,946 --> 00:44:29,756
to the next row quickly run
from left to right spiting

1026
00:44:29,756 --> 00:44:32,336
out some 0's well that just
calls for another loop.

1027
00:44:32,526 --> 00:44:37,046
So here it is, for J equals 1,
J up through and equal to 10,

1028
00:44:37,046 --> 00:44:39,836
J plus, plus you know what
this was actually pretty easy.

1029
00:44:39,896 --> 00:44:43,086
Just print out the lowercase
O followed by 2 spaces

1030
00:44:43,086 --> 00:44:45,016
because I decided
aesthetically that looked nice.

1031
00:44:45,356 --> 00:44:48,306
And in the end I put a new line,
another new line at the very end

1032
00:44:48,306 --> 00:44:49,476
to make it all very pretty.

1033
00:44:49,736 --> 00:44:54,786
And if I re-run the compiler
and then re-run Battleship,

1034
00:44:54,786 --> 00:44:57,766
in fact I have this
entire board here.

1035
00:44:59,156 --> 00:45:01,586
Any questions about this?

1036
00:45:02,216 --> 00:45:03,866
Well you can't help
but get excited

1037
00:45:03,866 --> 00:45:05,976
when the next topic somehow
involves Homer Simpson.

1038
00:45:05,976 --> 00:45:07,986
But let's go ahead and take
our 5 minute break here.

1039
00:45:08,136 --> 00:45:09,006
We'll resume in 5.

1040
00:45:14,096 --> 00:45:15,656
[Sound effects] So one
quick announcement.

1041
00:45:15,656 --> 00:45:17,236
The Harvard Computer Society,

1042
00:45:17,236 --> 00:45:19,476
two of whose members you
met last week are going

1043
00:45:19,476 --> 00:45:22,616
to be offering one of
several seminars tomorrow,

1044
00:45:22,616 --> 00:45:24,596
Tuesday at 7:00 p.m.
in Maxwell dorm

1045
00:45:24,596 --> 00:45:25,756
in the computer science
building.

1046
00:45:26,006 --> 00:45:28,456
The specifics are on the course
website's homepage right now.

1047
00:45:28,526 --> 00:45:32,196
CS50, net but this is
entitled demystifying Linux.

1048
00:45:32,566 --> 00:45:35,356
And this is meant as just
a free optional seminar.

1049
00:45:35,746 --> 00:45:38,496
If you would like to
join those folks tomorrow

1050
00:45:38,576 --> 00:45:41,636
and just learn a little
bit more about Linux.

1051
00:45:41,796 --> 00:45:43,756
This course does not
assume that you know Linux.

1052
00:45:43,756 --> 00:45:46,216
We will provide everything
you need to know in the form

1053
00:45:46,216 --> 00:45:48,586
of sections and the problem
sets PDF's themselves.

1054
00:45:48,826 --> 00:45:50,866
But if you like this kind
of stuff, you want to learn

1055
00:45:50,866 --> 00:45:54,846
from some particularly geeky
folks how you can do yet more

1056
00:45:54,846 --> 00:45:57,856
at the Linux command line do
feel free to check that out.

1057
00:45:58,186 --> 00:46:00,566
But for now our focus
will be on Homer here.

1058
00:46:00,566 --> 00:46:03,126
So this is perhaps one of
the most annoying songs ever.

1059
00:46:03,276 --> 00:46:06,566
There's this classic
Simpson's episode, which I --

1060
00:46:06,946 --> 00:46:09,136
if ever there was a Youtube
moment it would be this one.

1061
00:46:09,336 --> 00:46:12,556
Where Otto is actually driving
the bus listening to 99 bottles

1062
00:46:12,556 --> 00:46:13,416
of beer on the wall and then all

1063
00:46:13,416 --> 00:46:14,576
of the sudden you
realize it's only

1064
00:46:14,576 --> 00:46:15,846
in his headphones
for some reason.

1065
00:46:16,256 --> 00:46:17,836
But the motivation here is

1066
00:46:17,836 --> 00:46:20,256
that annoying though
this particular song is,

1067
00:46:20,656 --> 00:46:23,096
there's a pattern to it.

1068
00:46:23,096 --> 00:46:26,196
And it suggests that there's an
opportunity to just deploy some

1069
00:46:26,196 --> 00:46:28,806
of these basic building blocks
we've been looking at like loops

1070
00:46:28,806 --> 00:46:30,116
and variables and so forth.

1071
00:46:30,376 --> 00:46:33,656
And so let me go ahead and
open up one possible approach

1072
00:46:33,656 --> 00:46:35,176
to implementing this song.

1073
00:46:35,456 --> 00:46:38,216
Right. The most naive approach
would be to write a text file

1074
00:46:38,216 --> 00:46:40,796
that contains the entire
song four lines at a time.

1075
00:46:41,106 --> 00:46:42,786
Copy, paste, copy,
paste, copy paste

1076
00:46:42,786 --> 00:46:44,936
and just change the
number manually each time.

1077
00:46:45,226 --> 00:46:48,186
But finally now do we have
some programming chops

1078
00:46:48,186 --> 00:46:48,926
with which to do this.

1079
00:46:48,956 --> 00:46:51,186
So the programs are going to
start getting a little longer

1080
00:46:51,506 --> 00:46:53,836
but not because they're
fundamentally more complicated,

1081
00:46:53,836 --> 00:46:55,826
they just need to do
a little bit more.

1082
00:46:55,826 --> 00:46:59,536
So here's my main routine, I
want to print out the 99 bottles

1083
00:46:59,536 --> 00:47:01,076
of beer on the wall song.

1084
00:47:01,366 --> 00:47:03,266
Well let me go ahead
and first ask just

1085
00:47:03,266 --> 00:47:06,726
because hard coding 99
is even less interesting,

1086
00:47:06,946 --> 00:47:08,926
let me at least ask the
user for some input.

1087
00:47:08,926 --> 00:47:11,036
How many bottles of beer
will there be on the wall?

1088
00:47:11,286 --> 00:47:13,786
So I'm going to use
the CS50 library.

1089
00:47:14,026 --> 00:47:16,816
I'm calling get int, I'm
putting the return value into N.

1090
00:47:16,816 --> 00:47:19,256
And how do I use
the CS50 library?

1091
00:47:19,256 --> 00:47:22,086
Well again besides using
standard IO, which you need

1092
00:47:22,086 --> 00:47:26,496
for print F we use CS50 dot
H for functions like these.

1093
00:47:26,716 --> 00:47:28,556
Alright so the start of this
program's pretty straight

1094
00:47:28,556 --> 00:47:30,216
forward just ask
the user for an int,

1095
00:47:30,216 --> 00:47:33,386
I could have put any sentence
here I just chose to state it

1096
00:47:33,386 --> 00:47:34,336
in the form of a question.

1097
00:47:34,636 --> 00:47:36,546
And notice the silly
little aesthetics,

1098
00:47:36,546 --> 00:47:38,666
I put a space character
at the end just

1099
00:47:38,666 --> 00:47:41,036
so that the blinking prompt
would stay at the same line

1100
00:47:41,036 --> 00:47:42,666
and not arbitrarily
go to the next one.

1101
00:47:42,746 --> 00:47:43,806
But these are just aesthetics.

1102
00:47:44,066 --> 00:47:46,426
Well here is an example
of error checking.

1103
00:47:46,986 --> 00:47:49,756
Thus far we've just assumed
that the user's going

1104
00:47:49,756 --> 00:47:52,646
to be cooperative but in
general that's a very bad thing.

1105
00:47:52,646 --> 00:47:55,266
And most always when you
are using your own MAC or PC

1106
00:47:55,436 --> 00:47:59,526
and something goes wrong,
a program crashes, it hangs

1107
00:47:59,526 --> 00:48:01,466
or just something
weird happens that's

1108
00:48:01,516 --> 00:48:03,246
because a programmer somewhere

1109
00:48:03,246 --> 00:48:06,286
in an office somewhere far
away made some mistake.

1110
00:48:06,286 --> 00:48:08,366
Made maybe some stupid
mistake because he

1111
00:48:08,366 --> 00:48:11,666
or she did not anticipate
that you might hit that key

1112
00:48:11,666 --> 00:48:13,216
on the keyboard at
that moment and time.

1113
00:48:13,446 --> 00:48:16,316
Or even though you were
told to enter a number,

1114
00:48:16,316 --> 00:48:19,436
like a positive number, you know
you accidentally make a typo

1115
00:48:19,436 --> 00:48:22,316
and some punctuation goes in
or you are a malicious user

1116
00:48:22,476 --> 00:48:25,166
and you're intentionally
typing in a word instead

1117
00:48:25,166 --> 00:48:26,236
of a number when prompted.

1118
00:48:26,236 --> 00:48:28,166
Well you the programmer
now for this course,

1119
00:48:28,166 --> 00:48:29,296
for the problem sets and just

1120
00:48:29,296 --> 00:48:33,116
in general absolutely
have to code defensively.

1121
00:48:33,316 --> 00:48:36,066
And you have to assume that
the user is either an idiot

1122
00:48:36,066 --> 00:48:37,466
or just outright malicious

1123
00:48:37,706 --> 00:48:39,716
because only then can
you genuinely write code

1124
00:48:39,716 --> 00:48:42,076
that would stand just
ordinary use, right.

1125
00:48:42,076 --> 00:48:45,066
Even in this course, right,
300 plus students we had

1126
00:48:45,066 --> 00:48:46,766
on the little survey
for problem set 0.

1127
00:48:47,056 --> 00:48:49,096
Put your FAS user name here,

1128
00:48:49,276 --> 00:48:52,176
omit the at fast at
harvard.edu right.

1129
00:48:52,176 --> 00:48:53,966
If their sample size is
long enough you're going

1130
00:48:53,996 --> 00:48:55,816
to have someone like
doofus at FAS

1131
00:48:55,816 --> 00:48:58,376
at harvard.edu input
it into that field.

1132
00:48:58,376 --> 00:48:59,376
It happens right.

1133
00:48:59,716 --> 00:49:02,156
So fortunately we have
programming seals, we can strip

1134
00:49:02,156 --> 00:49:03,156
out that kind of text.

1135
00:49:03,436 --> 00:49:05,446
But again this speaks
to just the need

1136
00:49:05,446 --> 00:49:09,046
to code defensively your own,
you will be happier for it

1137
00:49:09,046 --> 00:49:11,346
because it's now we the
humans who have to go

1138
00:49:11,396 --> 00:49:12,696
and clean up that data.

1139
00:49:12,696 --> 00:49:16,266
Much better if we had been able
to defend against that with --

1140
00:49:16,786 --> 00:49:19,416
by rejecting that kind
of input at the get go.

1141
00:49:19,416 --> 00:49:20,536
But because we outsourced

1142
00:49:20,536 --> 00:49:22,956
to SurveyMonkey we
didn't have that ability.

1143
00:49:23,206 --> 00:49:24,366
So here's a little sanity check.

1144
00:49:24,526 --> 00:49:26,056
A little bit of error checking.

1145
00:49:26,336 --> 00:49:30,226
If N -- if you're that person
doofus at FAS please stay

1146
00:49:30,226 --> 00:49:33,276
on the course, we're not
trying to pick on you.

1147
00:49:33,356 --> 00:49:36,346
[Laughter] So if N is less than
1, it's so easy to check some

1148
00:49:36,346 --> 00:49:37,146
of these inputs right.

1149
00:49:37,366 --> 00:49:39,676
Get int last week, we
already saw will reject

1150
00:49:39,816 --> 00:49:41,226
non-numeric input.

1151
00:49:41,466 --> 00:49:45,306
Non-integral input because
we the CS50 staff made sure

1152
00:49:45,306 --> 00:49:48,616
that it prompts you to retry,
retry, retry if the user

1153
00:49:48,616 --> 00:49:51,046
as we saw tries typing in
monkey instead of a number.

1154
00:49:51,326 --> 00:49:55,066
So I don't have to check against
-- for word input or punctuation

1155
00:49:55,346 --> 00:49:56,946
but numeric is one thing.

1156
00:49:56,946 --> 00:49:59,956
If I actually care about the
sign of that value, positive,

1157
00:49:59,956 --> 00:50:02,336
negative, 0 I gotta
do that myself.

1158
00:50:02,336 --> 00:50:05,266
So if N is less than 1 I'm
going to yell at the user,

1159
00:50:05,266 --> 00:50:06,496
sorry that makes no sense

1160
00:50:06,496 --> 00:50:09,016
and then I'm actually
going to return 1.

1161
00:50:09,306 --> 00:50:10,856
So this is a sneak preview

1162
00:50:10,856 --> 00:50:13,566
of the capability I
promised existed -- oops.

1163
00:50:13,896 --> 00:50:16,616
There is the ability
in C for functions

1164
00:50:16,946 --> 00:50:19,346
like get int to return values.

1165
00:50:20,176 --> 00:50:24,146
Similarly can main the default
function that every program has,

1166
00:50:24,286 --> 00:50:26,116
that's the function that
just gets run by default.

1167
00:50:26,316 --> 00:50:29,386
So does main in fact have
what's called a return value

1168
00:50:29,386 --> 00:50:30,856
and even though we've
been sort of taking it

1169
00:50:30,856 --> 00:50:32,046
on blind faith thus far,

1170
00:50:32,276 --> 00:50:35,106
I've always been writing
the word int before main,

1171
00:50:35,396 --> 00:50:38,046
which means this main function
that again I've been copying

1172
00:50:38,046 --> 00:50:40,716
and pasting thus far does
in fact return a value.

1173
00:50:40,956 --> 00:50:42,716
It's just I'm not
doing anything with it.

1174
00:50:42,946 --> 00:50:44,486
But now think back
to the real world.

1175
00:50:44,486 --> 00:50:47,126
If you've ever been
using a MAC or a PC

1176
00:50:47,376 --> 00:50:49,826
and something goes
wrong, often, not always

1177
00:50:49,826 --> 00:50:52,886
but often you get a little
prompt saying an error occurred

1178
00:50:52,886 --> 00:50:54,686
or system error occurred
or whatever.

1179
00:50:54,816 --> 00:50:56,096
And then there's
usually a number

1180
00:50:56,146 --> 00:50:58,176
like a numeric identifier,
maybe it's negative,

1181
00:50:58,176 --> 00:51:00,546
maybe it's positive but
it's usually useless

1182
00:51:00,596 --> 00:51:03,186
to you the human unless you
actually get on the phone

1183
00:51:03,186 --> 00:51:06,406
or on e-mail and report
that error to the company.

1184
00:51:06,596 --> 00:51:08,056
And even then they
probably don't care

1185
00:51:08,056 --> 00:51:09,446
from an individual user.

1186
00:51:09,806 --> 00:51:14,246
So that was probably a
return value or an error code

1187
00:51:14,246 --> 00:51:16,236
of some sort and even you and C,

1188
00:51:16,236 --> 00:51:18,456
even with the simplest
programs have this ability

1189
00:51:18,456 --> 00:51:21,546
to return error codes
such as this one.

1190
00:51:21,546 --> 00:51:23,276
So because this is
the first error

1191
00:51:23,456 --> 00:51:25,626
that my program might
encounter I arbitrarily

1192
00:51:25,796 --> 00:51:27,096
but conventionally decided

1193
00:51:27,096 --> 00:51:29,266
that the first error
is going to return 1.

1194
00:51:29,506 --> 00:51:32,336
Now I won't see that, 1 will
not get printed to the screen.

1195
00:51:32,336 --> 00:51:32,956
This isn't printf.

1196
00:51:32,956 --> 00:51:36,326
This is return so only a
particularly sophisticated

1197
00:51:36,326 --> 00:51:39,366
person or programmer could
actually see that number.

1198
00:51:39,586 --> 00:51:40,706
But you a couple of weeks

1199
00:51:40,706 --> 00:51:43,756
from now will be using a
special tool called a debugger.

1200
00:51:43,966 --> 00:51:47,586
GDB, Visual Studio you might
have used in other classes

1201
00:51:47,886 --> 00:51:50,436
but this is a program that
can see this information

1202
00:51:50,436 --> 00:51:51,816
and you'll find that
it's quite useful.

1203
00:51:52,066 --> 00:51:53,986
So for now it's just a
convention that I'm adopting

1204
00:51:53,986 --> 00:51:55,496
on blind faith, more to come.

1205
00:51:55,826 --> 00:51:57,176
So now we've done the setup,

1206
00:51:57,176 --> 00:51:59,566
half of my program really
has just been devoted just

1207
00:51:59,566 --> 00:52:01,076
to the setup portion here.

1208
00:52:01,346 --> 00:52:04,246
Here's the juicy part of the
program, sing the annoying song.

1209
00:52:04,436 --> 00:52:06,126
Well first aesthetically
I decided

1210
00:52:06,126 --> 00:52:09,106
to print the new line right
here, not that interesting.

1211
00:52:09,456 --> 00:52:10,576
Now what am I doing?

1212
00:52:10,806 --> 00:52:14,196
Well here's the for loop in
which I'm counting down just

1213
00:52:14,196 --> 00:52:16,586
because that conceptually was
easier for me than counting

1214
00:52:16,586 --> 00:52:17,686
up for a song like this.

1215
00:52:17,966 --> 00:52:22,186
I'm initializing I to N, I'm
going to keep singing as long

1216
00:52:22,186 --> 00:52:25,646
as I is greater than 0 and
then just as I can increment

1217
00:52:25,706 --> 00:52:27,996
so can I decrement
as we've seen before.

1218
00:52:28,306 --> 00:52:30,286
Now what do I want to do on
each of these iterations?

1219
00:52:30,556 --> 00:52:33,016
I want to sing those stanzas,
so what do I want to do?

1220
00:52:33,196 --> 00:52:35,876
Printf. So how many
bottles and now I'm cheating

1221
00:52:35,876 --> 00:52:37,496
on the grammar right
because once I get

1222
00:52:37,536 --> 00:52:39,546
to like 1 I don't want
my grammar to be wrong.

1223
00:52:39,546 --> 00:52:42,996
So I parenthesis the S as sort
of a corner cutting here of beer

1224
00:52:42,996 --> 00:52:46,066
on the wall backslash N and
then I plug in the value.

1225
00:52:46,266 --> 00:52:47,626
So that's old school now.

1226
00:52:47,626 --> 00:52:48,466
That's pretty easy.

1227
00:52:48,676 --> 00:52:49,936
Fortunately the next line

1228
00:52:49,936 --> 00:52:52,386
of code is pretty much the same
thing verbatim so it's a copy,

1229
00:52:52,386 --> 00:52:53,776
paste with a change
in the wording.

1230
00:52:54,156 --> 00:52:56,276
Take one down pass it
around, that's just a string

1231
00:52:56,276 --> 00:52:57,206
that always get printed

1232
00:52:57,206 --> 00:52:59,376
so it only gets a little
interesting at the last line.

1233
00:52:59,586 --> 00:53:02,576
But again I know arithmetic, I
know I is a variable so print

1234
00:53:02,576 --> 00:53:05,136
out I minus 1 because
the last line

1235
00:53:05,136 --> 00:53:08,066
of the song per the little
canonical form here is

1236
00:53:08,096 --> 00:53:10,126
to say 98 bottles
of beer on the wall.

1237
00:53:10,336 --> 00:53:12,846
And now I repeat, add
nauseam until at least

1238
00:53:12,846 --> 00:53:14,596
until I bottom out at 0.

1239
00:53:14,916 --> 00:53:18,366
So if I go ahead and close
this, let me make beer 1,

1240
00:53:18,366 --> 00:53:21,356
let me go ahead and run beer 1.

1241
00:53:21,666 --> 00:53:24,516
How many bottles,
let's say 99, enter.

1242
00:53:25,246 --> 00:53:26,596
It's pretty damn fast right.

1243
00:53:26,596 --> 00:53:28,646
That's kind of the neat thing
when you have 2 gigahertz,

1244
00:53:28,646 --> 00:53:31,886
3 gigahertz computers
that allot really quickly

1245
00:53:31,886 --> 00:53:32,946
at least in this case.

1246
00:53:33,166 --> 00:53:35,346
But it seems to have
worked correctly

1247
00:53:35,776 --> 00:53:37,816
but that's not the only way
we could implement this.

1248
00:53:37,856 --> 00:53:39,376
Let's take a look at
a slight variance.

1249
00:53:39,376 --> 00:53:40,756
So this is version 2.

1250
00:53:41,126 --> 00:53:44,436
Let's see this looks the same
as before -- oh interesting.

1251
00:53:44,876 --> 00:53:46,386
So this time the --

1252
00:53:46,386 --> 00:53:49,146
I the programmer decided [Sound
effects] I like while loops.

1253
00:53:49,146 --> 00:53:49,996
I can do the same thing

1254
00:53:49,996 --> 00:53:51,356
in a while loop, let
me prove as much.

1255
00:53:51,356 --> 00:53:55,336
So here I said, "Sing the song
while N is greater than 0."

1256
00:53:55,466 --> 00:53:57,486
And maybe that's you
kind of speaks true

1257
00:53:57,486 --> 00:53:58,656
or it just makes more sense.

1258
00:53:58,656 --> 00:54:01,686
Do the following while
there are positive numbers

1259
00:54:01,686 --> 00:54:02,866
of beer on the wall.

1260
00:54:03,066 --> 00:54:05,586
So a while loop is perfectly
fine, the only thing you have

1261
00:54:05,626 --> 00:54:07,136
to remember to do is 1,

1262
00:54:07,406 --> 00:54:11,836
make sure that you manually
decrement N at the end there.

1263
00:54:11,836 --> 00:54:14,126
And now I am doing
something slightly different.

1264
00:54:14,346 --> 00:54:18,086
In version 1 whose value was
I changing on every iteration?

1265
00:54:18,646 --> 00:54:22,536
So I, I was doing minus, minus.

1266
00:54:22,946 --> 00:54:25,496
Here I'm kind of being
quote, unquote destructive.

1267
00:54:25,716 --> 00:54:28,836
I'm taking the users input
proffered at the very top

1268
00:54:28,836 --> 00:54:32,206
of the program by a get int,
storing it at N but at the end

1269
00:54:32,206 --> 00:54:34,846
of this program I have
lost track of what?

1270
00:54:34,846 --> 00:54:38,806
Like what that original
value was.

1271
00:54:38,956 --> 00:54:41,646
Now maybe that's a problem
if especially at the end

1272
00:54:41,646 --> 00:54:44,006
of the song you want to
say something like thanks

1273
00:54:44,106 --> 00:54:46,846
for playing with
99 bottles of beer.

1274
00:54:47,096 --> 00:54:50,756
But in this case where I don't
ever use that value again,

1275
00:54:51,066 --> 00:54:52,286
the short answer is who cares.

1276
00:54:52,416 --> 00:54:53,866
I had a variable, I chose

1277
00:54:53,866 --> 00:54:56,586
to decrement it destructively
thereby losing track

1278
00:54:56,586 --> 00:54:58,666
of its original value
but again who cares,

1279
00:54:58,666 --> 00:54:59,676
it doesn't really feel wrong

1280
00:54:59,676 --> 00:55:01,096
because I never need
that value again.

1281
00:55:01,096 --> 00:55:04,196
But if I did then there would
be a problem and I might need

1282
00:55:04,196 --> 00:55:05,526
to introduce another variable.

1283
00:55:05,836 --> 00:55:07,146
And one quick word on style,

1284
00:55:07,146 --> 00:55:10,906
so as much as I preached already
using well named variables

1285
00:55:11,076 --> 00:55:12,606
like we did with
previous programs.

1286
00:55:12,806 --> 00:55:16,186
It is completely
reasonable and conventional

1287
00:55:16,446 --> 00:55:18,806
to use basic place holders

1288
00:55:18,806 --> 00:55:22,346
like N whenever you have
a total number of things.

1289
00:55:22,486 --> 00:55:25,966
People like N, I is almost
always used as an increment.

1290
00:55:25,966 --> 00:55:28,526
If you've used I already
as we did for Battleship

1291
00:55:28,526 --> 00:55:32,896
because it's nested I went with
J, then you go with K, then L,

1292
00:55:32,896 --> 00:55:35,066
then if you're nesting that
deep you're probably doing

1293
00:55:35,066 --> 00:55:35,996
something foolishly.

1294
00:55:36,186 --> 00:55:39,666
At which point, yes you could
go to M but things just start

1295
00:55:39,666 --> 00:55:41,066
to feel a little weird there.

1296
00:55:41,196 --> 00:55:43,496
But actually let me roll back
to that just to be clear.

1297
00:55:43,496 --> 00:55:44,856
This was Battleship, recall.

1298
00:55:45,346 --> 00:55:47,816
Notice bad things
would have happened

1299
00:55:47,816 --> 00:55:51,046
if I had used or
reused I here, no.

1300
00:55:52,236 --> 00:55:55,546
Like if I had been taught
in class, you know use I.

1301
00:55:55,666 --> 00:55:57,316
So I say for I gets that.

1302
00:55:57,766 --> 00:55:58,606
I is that.

1303
00:55:58,896 --> 00:55:59,736
I is that.

1304
00:56:00,496 --> 00:56:03,326
Okay that seems fine
so let me go ahead

1305
00:56:03,326 --> 00:56:04,876
and recompile Battleship.

1306
00:56:05,386 --> 00:56:07,856
Okay unused variable J but
that was to be expected.

1307
00:56:07,856 --> 00:56:10,266
Battleship and [Sound
effects] broken.

1308
00:56:10,956 --> 00:56:12,716
Right. So just to realize--

1309
00:56:12,716 --> 00:56:15,426
just realize that because
I was declared already

1310
00:56:15,636 --> 00:56:17,396
if you're incrementing
it in this outer loop

1311
00:56:17,396 --> 00:56:20,506
in Battleship you better make
sure you know what you're doing

1312
00:56:20,506 --> 00:56:23,356
if you're also going to
increment that in a nested loop

1313
00:56:23,356 --> 00:56:26,546
because clearly bead things
or unintended things happen.

1314
00:56:26,546 --> 00:56:28,906
And that's why I just
whipped out J instead of I

1315
00:56:29,216 --> 00:56:30,456
for that particular program.

1316
00:56:30,716 --> 00:56:34,006
But in beer 2 the only
fundamental difference here was

1317
00:56:34,006 --> 00:56:34,936
that I used a while loop.

1318
00:56:35,816 --> 00:56:37,536
Why? [Sound effects]
It's different.

1319
00:56:37,596 --> 00:56:38,256
It's another way.

1320
00:56:38,586 --> 00:56:39,626
Well there's yet more ways.

1321
00:56:39,676 --> 00:56:41,326
Let's take a look at beer 3.

1322
00:56:41,876 --> 00:56:45,466
Alright, so beer 3 okay,
on intro -- oh interesting.

1323
00:56:45,696 --> 00:56:48,726
Little teaser here , I seem
to be about to use strings.

1324
00:56:48,786 --> 00:56:50,016
Let me see in what context.

1325
00:56:50,616 --> 00:56:53,696
Fast forward, oh I got a
little anal in this version.

1326
00:56:53,886 --> 00:56:55,616
So I wanted to use
proper grammar,

1327
00:56:55,616 --> 00:56:58,736
which is not unreasonable in
fact if you have the ability now

1328
00:56:58,736 --> 00:57:01,816
to fix what people generally
punt on in software just

1329
00:57:01,816 --> 00:57:03,106
because it's easy
to cut corners.

1330
00:57:03,376 --> 00:57:04,116
Why not do it?

1331
00:57:04,116 --> 00:57:05,846
And in fact I introduced
this here just

1332
00:57:05,846 --> 00:57:07,956
because it introduces
a neat piece of syntax

1333
00:57:08,126 --> 00:57:09,026
that you don't have to use.

1334
00:57:09,236 --> 00:57:12,006
But the more comfortable you get
with this stuff the sexier lines

1335
00:57:12,006 --> 00:57:15,016
of code like this tend to
feel, at least to some of us.

1336
00:57:15,406 --> 00:57:18,636
So it looks like S1 and S2
were strings because we saw

1337
00:57:18,636 --> 00:57:22,156
that up above, they were both
declared up here as stings.

1338
00:57:22,356 --> 00:57:24,186
And remember a string, trivia,

1339
00:57:24,186 --> 00:57:25,676
is equivalent to
what in reality?

1340
00:57:26,686 --> 00:57:27,686
Charstar right.

1341
00:57:27,686 --> 00:57:29,436
And eventually we'll take
that training wheel off

1342
00:57:29,486 --> 00:57:31,616
but the CS50 library
kind of hides that now

1343
00:57:31,616 --> 00:57:32,466
and calls it a string.

1344
00:57:32,756 --> 00:57:35,406
So what does S1 equal?

1345
00:57:35,856 --> 00:57:36,796
S1 equals, hmm.

1346
00:57:37,196 --> 00:57:41,126
I equal, equals 1 question
mark, bottle, bottles,

1347
00:57:41,126 --> 00:57:44,206
this is a little weird and
then where am I using this.

1348
00:57:44,206 --> 00:57:45,856
Okay S1 is here.

1349
00:57:46,026 --> 00:57:46,766
S1 is here.

1350
00:57:46,866 --> 00:57:47,666
S2 is there, alright.

1351
00:57:47,976 --> 00:57:50,256
It's a little confusing at
first so let me go ahead

1352
00:57:50,256 --> 00:57:53,536
and save and make beer 3.

1353
00:57:53,996 --> 00:57:54,736
Compiles, okay.

1354
00:57:54,736 --> 00:57:57,706
Let me go ahead and run
beer 3 and lets do it

1355
00:57:57,706 --> 00:58:00,036
with a few numbers so we
can actually see everything

1356
00:58:00,036 --> 00:58:00,846
on the screen at once.

1357
00:58:01,736 --> 00:58:06,616
Oh, okay. So again bottles of
-- 3 bottles of beer, 2 bottles.

1358
00:58:06,616 --> 00:58:09,106
Oh here's where it actually
seems to have made a difference,

1359
00:58:09,106 --> 00:58:12,636
1 bottle of beer but 0 bottles.

1360
00:58:12,936 --> 00:58:16,336
So apparently I need to have
both S1 and S2 because even

1361
00:58:16,336 --> 00:58:19,716
within the same chorus
I need to spit

1362
00:58:19,716 --> 00:58:22,716
out a slightly different
version of the word potentially.

1363
00:58:23,166 --> 00:58:24,076
So what's happening?

1364
00:58:24,556 --> 00:58:27,996
Well if in this sentence here,
the very first where I spit

1365
00:58:27,996 --> 00:58:30,836
out S1, something bottles
of beer on the wall.

1366
00:58:31,006 --> 00:58:35,086
Under what cases do I want
to say bottle versus bottles?

1367
00:58:35,896 --> 00:58:37,316
When do I want to say bottle?

1368
00:58:38,576 --> 00:58:41,046
So only when the number
of bottles of beer

1369
00:58:41,046 --> 00:58:42,226
on the wall is 1 right.

1370
00:58:42,226 --> 00:58:43,446
That's what's grammatically
correct.

1371
00:58:43,616 --> 00:58:45,486
1 bottle of beer on the wall.

1372
00:58:45,786 --> 00:58:47,406
So what we can infer now,

1373
00:58:47,406 --> 00:58:49,186
what this new piece
of syntax is doing.

1374
00:58:49,186 --> 00:58:52,046
So this is technically
called a ternary operator.

1375
00:58:52,346 --> 00:58:54,896
You've seen binary operators
before even though we didn't

1376
00:58:54,896 --> 00:58:55,936
slap that label on them

1377
00:58:56,146 --> 00:58:59,746
so X plus Y [Sound effects] plus
is apparently a binary operator.

1378
00:58:59,746 --> 00:59:01,966
It takes something on the left
and something on the right.

1379
00:59:02,306 --> 00:59:04,716
This is a fancy thing
called the ternary operator

1380
00:59:04,846 --> 00:59:06,096
because it takes
something on the left,

1381
00:59:06,196 --> 00:59:07,976
something in the middle
and something on the right.

1382
00:59:08,296 --> 00:59:09,576
But for now that's just jargon.

1383
00:59:09,826 --> 00:59:11,716
But apparently what
this is doing is kind

1384
00:59:11,716 --> 00:59:15,216
of like a condition, it's
doing it all on one line.

1385
00:59:15,246 --> 00:59:16,566
S1 is being assigned
the variable --

1386
00:59:16,566 --> 00:59:20,576
the string bottle if
I equals, equals 1.

1387
00:59:20,926 --> 00:59:23,956
Otherwise, else it's
being assigned bottles.

1388
00:59:24,286 --> 00:59:26,586
So it turns out this slick piece

1389
00:59:26,586 --> 00:59:29,016
of syntax is actually
equivalent to this.

1390
00:59:29,156 --> 00:59:37,916
If I equals, equals 1, S1 gets
bottle else S1 gets bottles.

1391
00:59:38,466 --> 00:59:41,116
So this is now equivalent
to this first line.

1392
00:59:41,116 --> 00:59:43,176
And then the second line
is the same exact code

1393
00:59:43,176 --> 00:59:44,686
but with S2 instead of S1.

1394
00:59:44,686 --> 00:59:45,576
Why did I do it?

1395
00:59:45,576 --> 00:59:47,556
Well again it's just
a matter of style.

1396
00:59:47,556 --> 00:59:49,626
This -- these four lines
[Sound effects] they just take

1397
00:59:49,626 --> 00:59:51,376
up four lines for
no good reason.

1398
00:59:51,376 --> 00:59:54,186
I can kind of collapse this into
something much more elegant,

1399
00:59:54,226 --> 00:59:56,356
not strictly necessary
but that's all this is.

1400
00:59:56,466 --> 00:59:58,796
So if you ever see this
operator in any context,

1401
00:59:58,796 --> 01:00:02,526
book or staff code it's just the
same thing as an if and an else.

1402
01:00:02,746 --> 01:00:07,106
And technically yes, if your
mind is already racing you can

1403
01:00:07,106 --> 01:00:10,096
nest these things so you can
effectively implement if, else,

1404
01:00:10,146 --> 01:00:11,896
if, else, if, else,
like don't cross

1405
01:00:11,896 --> 01:00:14,026
that line then it
becomes unreadable.

1406
01:00:14,316 --> 01:00:17,006
Do it for one, an if and an
else and leave it at that.

1407
01:00:17,006 --> 01:00:18,446
So what's the end result?

1408
01:00:18,446 --> 01:00:19,846
Well again, how many bottles?

1409
01:00:19,916 --> 01:00:23,426
99. It's the same exact song
but I fixed the grammar.

1410
01:00:23,786 --> 01:00:26,216
Now just to think outside
the box for a moment,

1411
01:00:26,446 --> 01:00:28,706
why might you not
want to do this?

1412
01:00:29,846 --> 01:00:33,886
What's a downside of adding this
check, in version 3 as opposed

1413
01:00:33,886 --> 01:00:36,386
to just doing bottle
parenthesis, S,

1414
01:00:36,566 --> 01:00:38,826
close parenthesis as I did
in the first two versions?

1415
01:00:39,406 --> 01:00:46,316
Why might you not
want to do this?

1416
01:00:46,526 --> 01:00:46,766
[Sound effects] Sorry.

1417
01:00:46,766 --> 01:00:48,436
>> [Inaudible background
discussion]

1418
01:00:48,436 --> 01:00:49,136
>> A little louder.

1419
01:00:49,136 --> 01:00:50,716
>> [Inaudible background
discussion]

1420
01:00:50,716 --> 01:00:52,246
>> So if I started with 1 --

1421
01:00:52,246 --> 01:00:55,306
okay if I started with 1 I
think it will still work.

1422
01:00:55,306 --> 01:00:56,426
How many bottles of beer?

1423
01:00:56,646 --> 01:00:58,856
So it still works but I've

1424
01:00:58,856 --> 01:01:01,086
at least generalized
the code enough

1425
01:01:01,086 --> 01:01:03,746
that it will still work
even in the case of input 1.

1426
01:01:03,976 --> 01:01:06,736
Even though again it's not
really doing much there

1427
01:01:06,736 --> 01:01:08,066
because I've just
chosen one number.

1428
01:01:08,066 --> 01:01:08,616
[Inaudible background
discussion]

1429
01:01:08,616 --> 01:01:12,306
>> Yeah, so that's
kind of the rub here.

1430
01:01:12,306 --> 01:01:15,806
So even though we won't worry so
much about this in this course

1431
01:01:15,806 --> 01:01:18,726
because again even my
laptop is 1.6 gigahertz,

1432
01:01:18,726 --> 01:01:20,506
the servers we have
are like 3 gigahertz.

1433
01:01:20,506 --> 01:01:23,316
I mean even though we have
really fast computers these days

1434
01:01:23,316 --> 01:01:25,886
-- and even though these things
are executed as we've seen

1435
01:01:25,886 --> 01:01:27,256
in the blink of an eye.

1436
01:01:27,796 --> 01:01:31,136
You know each of these two
lines of code do require

1437
01:01:31,236 --> 01:01:34,666
that the Intel CPU or whatever's
inside your computer do some

1438
01:01:34,666 --> 01:01:35,966
thinking and do some checks.

1439
01:01:36,436 --> 01:01:37,636
And the sad thing is

1440
01:01:37,826 --> 01:01:40,106
if you literally run this
program a hundred times

1441
01:01:40,106 --> 01:01:43,686
for 99 bottles of beer on down
to 0 you're executing both

1442
01:01:43,686 --> 01:01:46,196
of those if conditions
again and again and again

1443
01:01:46,196 --> 01:01:49,726
and they only change the
behavior of the program once

1444
01:01:50,196 --> 01:01:51,756
out of those 99 times.

1445
01:01:52,326 --> 01:01:53,986
Now that's again not
a fundamental problem

1446
01:01:54,366 --> 01:01:56,466
but it's a tradeoff in
terms of performance.

1447
01:01:56,526 --> 01:01:59,106
So if later in the course
you start working on things

1448
01:01:59,106 --> 01:02:01,956
like cell phone applications
for final projects.

1449
01:02:01,956 --> 01:02:05,116
Or after this course you start
writing code for little devices

1450
01:02:05,116 --> 01:02:07,326
in cars or robots
or little things

1451
01:02:07,476 --> 01:02:09,866
where you don't have the
luxury of many, many gigahertz

1452
01:02:09,866 --> 01:02:12,186
of power, all of
these little cycles,

1453
01:02:12,186 --> 01:02:14,786
all of these additional
statements start to add up.

1454
01:02:15,146 --> 01:02:18,746
So realize that you know maybe
this isn't the best approach

1455
01:02:18,746 --> 01:02:22,206
for certain context but clearly
here where we do have cycles

1456
01:02:22,236 --> 01:02:25,276
to spend it seems a very
reasonable tradeoff.

1457
01:02:25,566 --> 01:02:28,516
Well lets take a look at a
forth version of this here.

1458
01:02:28,806 --> 01:02:30,396
Just to see how it might differ.

1459
01:02:30,816 --> 01:02:31,546
Oh interesting.

1460
01:02:31,936 --> 01:02:34,266
I've gotten rid of
the chorus altogether.

1461
01:02:34,716 --> 01:02:37,326
So this is this one new
piece we'll introduce today.

1462
01:02:37,326 --> 01:02:40,716
Anytime you sense that there's
some redundancy in your code

1463
01:02:40,956 --> 01:02:44,156
or there's this ability where
you could outsource some piece

1464
01:02:44,156 --> 01:02:47,686
of functionality just like we've
been doing for printf right.

1465
01:02:47,716 --> 01:02:50,436
You and we in class have
not written a function

1466
01:02:50,436 --> 01:02:51,656
that prints stuff to the screen.

1467
01:02:51,656 --> 01:02:54,556
I don't frankly even know how I
would start to print characters

1468
01:02:54,556 --> 01:02:57,076
in a grid that's 80
by 24 characters wide.

1469
01:02:57,236 --> 01:02:59,466
I just want to take for
granted that someone else solved

1470
01:02:59,466 --> 01:03:01,686
that problem so I can do
something more interesting

1471
01:03:01,686 --> 01:03:01,926
with it.

1472
01:03:02,166 --> 01:03:04,586
Same thing for get int as
you'll see in later weeks

1473
01:03:04,586 --> 01:03:09,006
when we peel back the
layer of the CS50 library

1474
01:03:09,006 --> 01:03:10,306
and you see ho we
implemented it.

1475
01:03:10,306 --> 01:03:12,676
You'll see that [Sound effects]
so many lines of code just

1476
01:03:12,676 --> 01:03:15,536
to get an integer from the
user, really takes the fun

1477
01:03:15,536 --> 01:03:16,816
out of programming potentially.

1478
01:03:16,816 --> 01:03:18,496
And that's why you'll
see throughout the course

1479
01:03:18,766 --> 01:03:21,586
and computer science programming
that there's a reason

1480
01:03:21,586 --> 01:03:23,726
that people have
developed libraries and all

1481
01:03:23,726 --> 01:03:24,596
of these header files.

1482
01:03:24,806 --> 01:03:26,476
Because why did we
invent the wheel,

1483
01:03:26,476 --> 01:03:29,036
if you can leverage someone
else's work like printf

1484
01:03:29,036 --> 01:03:32,126
and get this and build you
own far more interesting stuff

1485
01:03:32,126 --> 01:03:32,856
on top of it.

1486
01:03:33,036 --> 01:03:35,636
And that's related to
this tidbit here right.

1487
01:03:35,636 --> 01:03:38,596
It seems to be an opportunity
for this stupid chorus

1488
01:03:38,666 --> 01:03:41,136
that the kids might sing,
let me just outsource that.

1489
01:03:41,546 --> 01:03:44,556
Let me just let someone else
generate all of the sentences

1490
01:03:44,556 --> 01:03:46,846
that pertain to the chorus
and the only thing I'm going

1491
01:03:46,846 --> 01:03:50,276
to tell this black box, this
little Oracle is the number

1492
01:03:50,276 --> 01:03:52,066
of bottles of beer
currently on the wall.

1493
01:03:52,066 --> 01:03:54,626
And I want that entity,
that black box

1494
01:03:54,946 --> 01:03:56,106
to do the printing for me.

1495
01:03:56,416 --> 01:03:58,066
So in fact that's
exactly what I do

1496
01:03:58,066 --> 01:04:01,076
and I simultaneously introduce
just a couple of new tricks just

1497
01:04:01,076 --> 01:04:01,936
for the sake of syntax.

1498
01:04:02,536 --> 01:04:05,946
So it turns out in a Boolean
expression you don't have

1499
01:04:05,986 --> 01:04:09,396
to say while N is
greater than 0 as I did

1500
01:04:09,396 --> 01:04:10,976
in a previous version right.

1501
01:04:10,976 --> 01:04:13,396
It's equivalent to
just say while N

1502
01:04:13,856 --> 01:04:17,726
because if N keeps getting
decremented so N is 99, then 98,

1503
01:04:17,726 --> 01:04:21,426
then, dot, dot, dot, then
1 then 0 at some point

1504
01:04:21,426 --> 01:04:23,856
in the story you
will have while 0.

1505
01:04:24,186 --> 01:04:27,406
But 0 is false by nature so even
though you don't have the less

1506
01:04:27,486 --> 01:04:30,236
than or the greater than
sign in there it's equivalent

1507
01:04:30,416 --> 01:04:31,996
to just terminating the loop.

1508
01:04:32,166 --> 01:04:34,476
So that's why I can get away
with this cleverness here

1509
01:04:34,666 --> 01:04:37,946
and then this cleverness
is arguably confusing here

1510
01:04:37,946 --> 01:04:39,256
but lets introduce the syntax.

1511
01:04:39,706 --> 01:04:43,096
Chorus is a function so
just as main is a function,

1512
01:04:43,626 --> 01:04:44,996
which you kind of have to write

1513
01:04:45,256 --> 01:04:47,196
because otherwise your
program generally is not going

1514
01:04:47,196 --> 01:04:47,866
to do anything.

1515
01:04:48,156 --> 01:04:50,076
You can write your
own functions just

1516
01:04:50,076 --> 01:04:51,836
as someone else wrote
printf and just

1517
01:04:51,836 --> 01:04:54,306
as the staff wrote
get int and get float.

1518
01:04:54,466 --> 01:04:56,856
I or the person who
wrote this code decided

1519
01:04:56,856 --> 01:05:00,116
to call the function chorus, so
apparently there's a function

1520
01:05:00,116 --> 01:05:01,706
in this world now called chorus

1521
01:05:02,066 --> 01:05:04,396
that if you pass it
an integer it's going

1522
01:05:04,396 --> 01:05:06,146
to presumably print the chorus.

1523
01:05:06,486 --> 01:05:08,666
But you what I also just
want to be clever here.

1524
01:05:08,896 --> 01:05:12,426
I didn't really love
this code you know,

1525
01:05:12,426 --> 01:05:14,486
again a little anal I feel

1526
01:05:14,486 --> 01:05:18,146
like this is [Sound effects]
just why the curly braces,

1527
01:05:18,146 --> 01:05:19,026
why the additional line?

1528
01:05:19,026 --> 01:05:20,146
Now it's perfectly clear

1529
01:05:20,376 --> 01:05:22,156
and it's you know,
100 percent credit.

1530
01:05:22,156 --> 01:05:25,096
Were not preaching that this is
bad code, bad design, bad style.

1531
01:05:25,406 --> 01:05:27,986
Perfectly acceptable,
but realize especially

1532
01:05:27,986 --> 01:05:30,476
when it comes time to read
other people's code whether it's

1533
01:05:30,476 --> 01:05:33,866
in a book or in a class that
this has the same effect.

1534
01:05:34,506 --> 01:05:37,176
So this is where that big
ugly table of precedence

1535
01:05:37,176 --> 01:05:41,006
and operators comes in handy
as a reference in the long run.

1536
01:05:41,006 --> 01:05:43,106
You can determine using
that or just trial

1537
01:05:43,106 --> 01:05:44,156
and error, what's happening?

1538
01:05:44,156 --> 01:05:46,926
So in fact what happens in
this context is the value

1539
01:05:46,926 --> 01:05:50,226
of N gets passed into
chorus, the function just

1540
01:05:50,226 --> 01:05:51,946
as a variable would
get passed into printf.

1541
01:05:52,476 --> 01:05:55,106
And then after that function
is done doing its thing,

1542
01:05:55,416 --> 01:05:57,876
then N simultaneously
gets decremented.

1543
01:05:58,436 --> 01:06:01,106
So it's the same as the code
I just wrote and then deleted.

1544
01:06:01,316 --> 01:06:02,746
So what in the world is chorus?

1545
01:06:02,746 --> 01:06:04,566
Well here's how you
write a method.

1546
01:06:04,566 --> 01:06:06,106
Wow that code actually
looks familiar.

1547
01:06:06,366 --> 01:06:09,606
I apparently ripped out the
code from main, put it later

1548
01:06:09,606 --> 01:06:12,626
in the file down below
and I simply wrote this.

1549
01:06:13,176 --> 01:06:15,536
So I wrote chorus and
then what did I do?

1550
01:06:15,536 --> 01:06:17,896
Well chorus has to
be parameterized,

1551
01:06:18,036 --> 01:06:20,226
it needs to take
input and inputs

1552
01:06:20,266 --> 01:06:23,136
to functions are called
parameters or arguments.

1553
01:06:23,526 --> 01:06:24,886
So B, what is B?

1554
01:06:24,886 --> 01:06:25,986
B just denotes bottle.

1555
01:06:26,026 --> 01:06:29,106
I could give this
parameter any name I want,

1556
01:06:29,206 --> 01:06:31,196
so it's essentially
a local variable.

1557
01:06:31,416 --> 01:06:33,916
So anytime a function takes
input you gotta tell the

1558
01:06:33,916 --> 01:06:36,176
compiler what do want
to call that input?

1559
01:06:36,346 --> 01:06:38,276
So that you the programmer
can actually use it.

1560
01:06:38,536 --> 01:06:39,726
What is the type of the input?

1561
01:06:39,726 --> 01:06:43,526
An int. So long story short,
just as printf is a function

1562
01:06:43,526 --> 01:06:45,246
that takes a format
string on the left

1563
01:06:45,246 --> 01:06:48,426
and then a comma separated
list of variables on the right.

1564
01:06:48,826 --> 01:06:51,776
So does chorus, apparently
take one argument,

1565
01:06:52,206 --> 01:06:54,266
just an int so that's past in.

1566
01:06:54,676 --> 01:06:57,876
So once it's passe in I can
refer to it as B. What do I do?

1567
01:06:57,876 --> 01:07:00,596
Well this code is just ripped
off from the previous example.

1568
01:07:00,596 --> 01:07:02,396
I declare a couple
of strings up here,

1569
01:07:02,576 --> 01:07:05,016
then I assign them using my
fancy little notation here.

1570
01:07:05,246 --> 01:07:07,526
This code is the same as
before, it's the same code.

1571
01:07:07,756 --> 01:07:09,156
I've just factored it out.

1572
01:07:09,456 --> 01:07:12,406
I've hierarchically decomposed,
fancy way of saying it

1573
01:07:12,406 --> 01:07:13,326
that you'll see in the textbook.

1574
01:07:13,586 --> 01:07:16,716
Decomposed my code into
now this separate function

1575
01:07:16,716 --> 01:07:19,076
because you know what, it
just feels a little cleaner.

1576
01:07:19,286 --> 01:07:21,546
There was this big ugly
block of code in my program,

1577
01:07:21,966 --> 01:07:24,146
feels perfectly reasonable
to factor that out

1578
01:07:24,176 --> 01:07:27,846
because it really
collectively, the seven lines

1579
01:07:28,026 --> 01:07:31,436
of real code they
accomplish a single goal

1580
01:07:31,436 --> 01:07:33,476
that can be well
summarized in a sentence.

1581
01:07:33,626 --> 01:07:35,626
Sings about specified
numbers of bottles.

1582
01:07:35,826 --> 01:07:37,826
So anytime there's this
opportunity to factor

1583
01:07:37,826 --> 01:07:41,276
out a piece of code, a
function is the direction

1584
01:07:41,276 --> 01:07:43,036
that most programmers
would go in.

1585
01:07:43,296 --> 01:07:46,116
Now there's one last piece
of detail here, void.

1586
01:07:46,116 --> 01:07:47,256
What do you think it means

1587
01:07:47,256 --> 01:07:50,426
if the word before the
function name here is void?

1588
01:07:50,746 --> 01:07:55,066
It just means it
has no return value.

1589
01:07:55,156 --> 01:07:58,906
Get int by nature returns a
value right, that's a good thing

1590
01:07:58,906 --> 01:08:01,266
because you use it to get
a number from the user.

1591
01:08:01,506 --> 01:08:04,656
But chorus does not need to
get any input from the user.

1592
01:08:04,816 --> 01:08:06,896
It just has this quote,
unquote side effect

1593
01:08:07,336 --> 01:08:09,146
of printing something
to the screen.

1594
01:08:09,396 --> 01:08:10,476
So for now you can kind

1595
01:08:10,476 --> 01:08:12,116
of simplify the world
into two buckets.

1596
01:08:12,116 --> 01:08:14,426
Functions that have side
effects, just print stuff

1597
01:08:14,426 --> 01:08:15,806
to the screen that if
you weren't watching

1598
01:08:15,806 --> 01:08:17,556
that you would never
know they did anything.

1599
01:08:17,756 --> 01:08:19,846
And then functions that
have return values.

1600
01:08:20,076 --> 01:08:22,516
So return value here is
void as in this case.

1601
01:08:22,776 --> 01:08:25,386
Return value here is
int, or string, or char,

1602
01:08:25,386 --> 01:08:26,926
or whatever the case may be.

1603
01:08:26,976 --> 01:08:28,546
We'll see we can do
anything we want.

1604
01:08:28,726 --> 01:08:32,306
So this is why up here
I just call chorus just

1605
01:08:32,306 --> 01:08:33,206
like I call printf.

1606
01:08:33,636 --> 01:08:36,346
I don't have to assign its
return value to a variable

1607
01:08:36,596 --> 01:08:39,486
because it's not actually
returning a variable at all.

1608
01:08:39,846 --> 01:08:42,736
So let me go ahead and
print out the results

1609
01:08:42,856 --> 01:08:44,356
of these statements here.

1610
01:08:44,356 --> 01:08:47,536
Let me go ahead and quit.

1611
01:08:47,796 --> 01:08:50,736
Let me go ahead an make beer 4.

1612
01:08:51,586 --> 01:08:54,216
Let me go ahead and
run beer 4 here.

1613
01:08:54,766 --> 01:08:59,036
99 bottles of beer and viola
the same exact example.

1614
01:08:59,766 --> 01:09:01,516
So why don't we end
on that note today

1615
01:09:01,516 --> 01:09:04,736
and we'll see you
again on Wednesday.

1616
01:09:05,236 --> 01:09:12,610
[Applause]