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[ Silence ]

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>>[background noise]
Alright, well thank you.

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Alright, welcome back.

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00:00:15,676 --> 00:00:16,856
My name is David Malan,

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for those of you joining
us for the first time.

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And this is CS50 and this is
the continuation of week zero.

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For those of you who may
have missed Wednesday,

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notice that we still
have some syllabi.

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No more cake but if you
did in fact miss Wednesday,

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do notice that on the courses
website is as will always be

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>>This is not your typical high
school computer science course

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>> So that there is what
you missed on Wednesday.

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So in the future you will find

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that the course makes available
its videos in flash formats,

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quick time formats as
well as in MP3 formats

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on the courses website so
do take a look at that.

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So we've been hard at work
over the past couple of days,

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particularly one of our CA's
who actually is an alumnus,

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he lives in New York, his
name is Welly [phonetic].

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You may meet him in the
virtual terminal room over time

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and we've implemented what we're
now calling Shuttleboy voice

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so you may recall
and you may in fact

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as an upper classman
have used before,

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this program called Shuttleboy.

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It's a good 12 plus
years old now.

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It lives via this link on
CS50 zone websites here

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and let me actually
grind to a halt here.

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Alright, let's change to
this, oh, so what you will see

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in a moment, oh, alright
[laughter] my name is David

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Malan and this is CS50.

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Welcome back to week
zero continued.

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We will crop off of the video
everything that just happened.

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If you missed Wednesday's
lecture video,

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know that it is available
online, I won't belabor

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that particular point but
I will draw your attention

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to a little application that
we've been working on so

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for sometime there's existed
this program called Shuttleboy.

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I'm, say, at Memorial Hall.

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I need to get back after class
to the quad so I click two links

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and wala, [phonetic] there are
the next several shuttles I can

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get to.

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Well we thought it would
be fun to take things

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to the next level this year and
to implement Shuttleboy voice,

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the new and improved
version of the same idea.

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As you may recall from
Wednesday we were lucky enough

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to share a secure C17 bug CS50.

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It's not quite yet
in production.

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This is really just
a sneak preview

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but in a moment I'm going
to turn this microphone on

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and we're going to make
a phone call together

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and see who here picks up.

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So I'm going to go ahead and
turn on my speaker phone.

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We're going to dial and
see what happens here.

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>>This is CS50 for Shuttleboy.

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Press 1 for Quinton
[phonetic] dining.

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Press 2 to start over.

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Press zero

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>>I'll press 1 for Shuttleboy.

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[noise]

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>>What is your origin for quad?

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Press 1, Mather.

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Press 2, Boylston
[phonetic] Press 3, Lamont.

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Press 4, Mem Hall.

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Press 5 to start over.

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[noise] What is your
destination for quad?

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Press 1, the next shuttle
leaves in 5 minutes at 11:00 pm

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and then at 11:10 pm,
11:20 pm and 11:30 pm.

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[laughter]

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>>So you're going to
be here a while today.

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It's perhaps the media takeaway

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but [applause] so
that's not a bug.

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That's a feature.

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So we just dummied it up with
some data for demonstration sake

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but it should debut for all
of you this coming week.

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And notice we've optimized
for the most popular stop

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so if you want to get on,
for instance, at Pound Hall,

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it's not gonna happen
with Shuttleboy.

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Boys Quad, Mather,
Mem Hall, Lamont,

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all the most popular stops will
be optimized in that interface.

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So, more to come.

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Why do something like that?

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Well frankly sometimes,
as you'll see perhaps

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in this course or beyond, you
do things just because you can.

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And by courses end will you be
able to do that and much more?

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So today is ultimately

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about laying the
foundation for the semester.

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And this semester is
about Computer Science,

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it's about Programming
but that sort

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of begs the question
perhaps on day 1,

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what is the latter,
what is Programming?

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Can someone in laymen's
terms give us an answer

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to this question?

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What is Programming?

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You don't want to be down here

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because I can see you
much more easily, yes?

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>>Writing a set of
instructions to a computer

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>>Okay so writing a set of
instructions to a computer.

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So a computer is just
this fairly dumb device.

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It's a mechanical device

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that literally can
only do what it is told

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so programming is ultimately

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about telling a computer
what to do.

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Now how do you tell a
computer what to do?

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Well in this course and in
programming in computer science,

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we don't mean you just point
and click and choose this

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from the menu and at
that high user level

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but rather how do you actually
go in underneath the hood

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and make all of that possible?

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Well you do this with
programming languages.

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C is the one we'll focus
on in this course and PHP

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and JavaScript and C Sharp
and a whole bunch of others.

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History is littered with dozens,

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hundreds of programming
languages,

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some of which are very
much still in vogue,

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some of which have
partly died off,

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some of which really never
saw the light of day.

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So today we're going to focus on
programming ultimately but not

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by way of C, not by way of Java,

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not by way of these
textual languages

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that while powerful
ultimately and the languages

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that we'll use throughout
the semester,

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they're not quite
the easiest thing

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to get excited about
at term start.

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In fact what I'm going to
do here in just a moment

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on my machine, is pull up
a little program that comes

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with Mac OS, you can do
something similar in Windows

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and you'll see how to do this
on your own computer before long

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but this is a terminal and what
it ultimately gives me access

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to is the files, the programs,
underneath the hood on a Mac

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which is essentially a
unit spaced computer.

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So what I'm going to do just
to underwhelm you here on day 1

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of 2 so far, I'm going to go
ahead and write hello dot C.

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So this is a text editor.

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This is like a geeks version
of Notepad or Microsoft Word

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but much simpler than
the latter certainly.

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And what I'm going to do
is something like this

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so include standard IO.H,
whatever that means.

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I'm gonna say [inaudible].

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I'm gonna say main.

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I'm gonna say [inaudible]
a couple

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of brackets and a parenthesis.

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And then in here I'm gonna
say print F. Let's say hello,

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world and then whatever this
backslash N thing denotes

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and then a semicolon.

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So it's really hard
to get excited

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if this is what you've
got in store

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for you throughout the semester
and plus what is [inaudible],

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what is [inaudible], what is
char, why all this syntax?

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Well it's a very fair question.

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It's really not intellectually
all that interesting.

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We do ultimately
need conventions.

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We need a language.

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We need some basic definitions
of how you express yourself

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to a computer that's
really not interesting here

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in week zero thus far.

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And so we'll quickly move away
from this but just to again,

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as promised, underwhelm you, now
I have a file called hello .C,

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just like .text or .dock.

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The .C is just a convention.

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I'm gonna run a program called
GCC which is a compiler.

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If you're familiar with Visual
Studio, something like that,

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it's the same type of program

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but in a Unix or
Lynx environment.

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And I'm gonna say

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to the computer compile
the stuff I just wrote.

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In other words, turn
it into zeros and ones.

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Now the fact that no messages
got printed is actually a

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good thing.

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So weekends when you're actually
playing with C for yourself

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for the first time,
if you get a message

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at that very first point, damn,
means you've messed something

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up already but that's okay
because much of the process

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of learning and in doing
interesting things, frankly,

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will be tripping over
some basics over time.

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Fortunately I have not put my
own foot in my mouth here yet.

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But there's now this file.

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It's cryptically named a.out,

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just a stupid convention
someone came up with years ago.

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We can change this ultimately
and we will but inside

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of this file are a
bunch of zeros and ones

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that ultimately this
computer, this Mac,

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this Intel Mac can understand.

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So I'm gonna go ahead
and hit enter and wala,

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you're very first
computer science program.

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Thank you.

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[laughter and applause]
Alright so not terribly exciting

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and the thing is we don't
need to focus so much

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on just these basic things like
parenthesis and semicolons.

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All of these are
completely uninteresting

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and we'll get there but you'll
find that it's really not

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such a big deal and it begins
to come more naturally.

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But today we focus, for
just this week zero,

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on programming itself
and what it means

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to make computers do what
you want but doing it

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in a much more intuitive way.

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And we do this, as
promised on Wednesday,

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by way of this graphical
language developed

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by MIT's media lab a
couple of years ago.

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It happens to be called Scratch

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and what we'll see
here is another demo

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of what you saw briefly
on Wednesday

202
00:08:45,236 --> 00:08:51,296
and this one I will give you
not one that I myself have made

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00:08:51,296 --> 00:08:53,106
but a former teaching
fellow of the course.

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00:08:53,446 --> 00:08:57,516
I give you by way of
this demo, raining men.

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[laughter]

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00:08:58,016 --> 00:10:29,000
[music and singing]

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So hopefully much more
interesting than hello world

208
00:10:34,026 --> 00:10:34,746
at the command prompt.

209
00:10:34,746 --> 00:10:37,186
Now grant it that was made
by a former teaching fellow,

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00:10:37,376 --> 00:10:39,336
in fact one of the stars
of the football team.

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00:10:39,336 --> 00:10:41,926
So realize we do get
all types here in CS50,

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00:10:41,926 --> 00:10:44,726
not those that I might have
recalled from my own high school

213
00:10:44,886 --> 00:10:47,696
but this one is clearly
more presentation oriented

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00:10:47,696 --> 00:10:50,546
than it is interactive and a
lot of what using computers is

215
00:10:50,546 --> 00:10:53,136
about is taking input and
then producing output,

216
00:10:53,136 --> 00:10:54,666
actually interacting
with the human.

217
00:10:54,916 --> 00:10:55,966
So why don't we do just that.

218
00:10:55,966 --> 00:10:57,816
And in fact, lest
we send the message

219
00:10:57,816 --> 00:10:59,676
that only teaching
fellows can make things

220
00:10:59,676 --> 00:11:01,936
that are this much more
interesting than hello world,

221
00:11:02,066 --> 00:11:03,226
let me show you something
that was made

222
00:11:03,226 --> 00:11:07,466
by a former CS50 student but
for this we need one volunteer

223
00:11:07,466 --> 00:11:10,726
to come on up here and
stand before my keyboard.

224
00:11:11,796 --> 00:11:14,016
Yep, in the back on the
edge there, come on down.

225
00:11:15,306 --> 00:11:16,756
And you do have to be
comfortable, again,

226
00:11:16,756 --> 00:11:20,206
being on camera since what you
don't see happen is the signing

227
00:11:20,206 --> 00:11:22,686
of legal forms off to
the stage once we're done

228
00:11:23,266 --> 00:11:24,716
with demonstrations like this.

229
00:11:24,716 --> 00:11:26,946
So what I'm going to do is
bring up a demonstration.

230
00:11:26,946 --> 00:11:28,506
This one was by Andrew Berry.

231
00:11:28,506 --> 00:11:30,286
This next one is by
Ann Shea [phonetic]

232
00:11:30,466 --> 00:11:33,386
and this one, no, come on.

233
00:11:33,676 --> 00:11:34,616
No turning back now.

234
00:11:35,866 --> 00:11:37,186
[laughter] May look familiar

235
00:11:37,186 --> 00:11:42,026
if you've played a little
something like this before.

236
00:11:42,076 --> 00:11:46,576
[laughter] So, some of you might
have played DDR or these kinds

237
00:11:46,576 --> 00:11:48,236
of coordination games,
maybe not,

238
00:11:48,236 --> 00:11:49,746
if not it's better for the demo.

239
00:11:50,376 --> 00:11:52,266
So in a moment, and
what is your name?

240
00:11:52,586 --> 00:11:53,196
>>Yonason [phonetic]

241
00:11:53,196 --> 00:11:55,296
>>Yonason, David,
nice to meet you.

242
00:11:55,296 --> 00:11:58,316
So we're running on
hit space to begin

243
00:11:58,316 --> 00:12:00,736
and what's gonna happen is
music is gonna start playing

244
00:12:00,736 --> 00:12:02,666
and as the instructions
here say, you're gonna have

245
00:12:02,666 --> 00:12:05,346
to hit the arrows in lock
step with stuff that's flowing

246
00:12:05,346 --> 00:12:07,406
up the screen so that
it overlaps the shapes

247
00:12:07,626 --> 00:12:10,066
and then we'll keep track of
your score and just how awesome,

248
00:12:10,066 --> 00:12:12,506
cool, good or boo you are.

249
00:12:13,006 --> 00:12:13,426
Ready to go?

250
00:12:13,486 --> 00:12:14,966
Arrow keys down here,
here we are.

251
00:12:15,516 --> 00:13:22,916
[music and noise]

252
00:13:23,416 --> 00:13:25,916
[background music] Alright,
how a big round of applause?

253
00:13:26,896 --> 00:13:31,906
That was pretty damn good.

254
00:13:32,216 --> 00:13:37,326
So how does one go about
writing this stuff?

255
00:13:37,326 --> 00:13:40,776
Well the interesting question
today, legal form, there we go.

256
00:13:40,946 --> 00:13:43,026
[laughter] So the interesting
thing today is going to be

257
00:13:43,026 --> 00:13:45,116
to try and dissect,
intellectually,

258
00:13:45,116 --> 00:13:45,976
what is going on, right.

259
00:13:45,976 --> 00:13:48,046
These things might be fun
and they might look silly

260
00:13:48,326 --> 00:13:50,116
but there's really some
interesting stuff going

261
00:13:50,116 --> 00:13:51,436
on underneath the hood.

262
00:13:51,626 --> 00:13:53,296
There's some randomness
going on there.

263
00:13:53,296 --> 00:13:56,106
The game is not always the
same in this particular case.

264
00:13:56,366 --> 00:13:57,326
In the previous case

265
00:13:57,326 --> 00:13:59,426
with Raining Men there was
clearly synchronization,

266
00:13:59,426 --> 00:14:01,756
there was lots of stuff
going on at the same time

267
00:14:01,986 --> 00:14:03,986
and if you think back just
a moment ago at what I did

268
00:14:03,986 --> 00:14:06,236
at the command prompt,
the textual interface,

269
00:14:06,536 --> 00:14:09,386
it's hard to imagine how you
could possibly coordinate all

270
00:14:09,386 --> 00:14:10,736
of those several things going

271
00:14:10,736 --> 00:14:13,086
on when I've completely
underwhelmed you thus far

272
00:14:13,086 --> 00:14:16,286
by just printing a simple
instruction to the screen.

273
00:14:16,516 --> 00:14:20,266
And I'll give you a quick
teaser too, in a little bit,

274
00:14:20,536 --> 00:14:23,836
of one that I wrote that we'll
use for the sake of discussion

275
00:14:23,836 --> 00:14:26,576
if only cause I understand
what's underneath the hood

276
00:14:26,576 --> 00:14:29,906
of that one best and that one,
we'll see, is called Oscar Time.

277
00:14:29,906 --> 00:14:33,286
But first let's consider
outside the context

278
00:14:33,286 --> 00:14:34,456
of this thing called Scratch,

279
00:14:34,456 --> 00:14:36,896
what really programming
is all about.

280
00:14:36,896 --> 00:14:38,506
Well let's pick something
completely silly.

281
00:14:38,796 --> 00:14:41,696
So waking up in the morning and
putting on one's socks, right,

282
00:14:41,696 --> 00:14:44,176
you've probably, hopefully,
never really thought hard

283
00:14:44,176 --> 00:14:45,326
about this particular problem.

284
00:14:45,326 --> 00:14:46,016
You just do it.

285
00:14:46,246 --> 00:14:48,706
But suppose you were trying to
instruct a computer to do it

286
00:14:48,706 --> 00:14:51,776
or maybe more concretely a
robot to do it, you kind of have

287
00:14:51,966 --> 00:14:53,526
to think a little harder.

288
00:14:53,896 --> 00:14:56,796
Now perhaps I've raised the
bar to volunteers a bit higher

289
00:14:56,796 --> 00:15:00,186
but I need one volunteer
for this thing as well.

290
00:15:02,286 --> 00:15:07,476
Yes, come on down and you too
will be on camera and shoeless.

291
00:15:09,016 --> 00:15:10,826
[laughter] So I went
shopping beforehand,

292
00:15:11,886 --> 00:15:14,586
turns out you can buy some
amazing propaganda at the Coupe.

293
00:15:14,586 --> 00:15:19,686
This is ultimately going to
be about this algorithm here,

294
00:15:19,686 --> 00:15:21,366
oh come prepared,
what is your name?

295
00:15:21,616 --> 00:15:21,796
>>Josh

296
00:15:22,006 --> 00:15:25,156
>>Josh, alright, come on over,
come on over to your chair here.

297
00:15:25,156 --> 00:15:26,696
We'll need you to
take off your socks.

298
00:15:27,196 --> 00:15:29,046
I kind of splurged here.

299
00:15:29,046 --> 00:15:31,316
We went with a few
different styles here today

300
00:15:31,396 --> 00:15:33,876
so they're all clean though.

301
00:15:33,876 --> 00:15:35,916
I thought for a moment maybe
I'd just go home for this

302
00:15:35,966 --> 00:15:37,676
but I thought that might
not go over so well.

303
00:15:37,876 --> 00:15:40,206
So we have some H
branded socks here,

304
00:15:40,206 --> 00:15:42,496
which we'll put, say
in the sock pile.

305
00:15:42,496 --> 00:15:44,726
I also got some more
sporty socks,

306
00:15:44,726 --> 00:15:47,866
if this is what your
preference so we have these sort

307
00:15:48,086 --> 00:15:50,306
of low rise type
socks stained crimson.

308
00:15:50,306 --> 00:15:53,476
And then these were actually
my favorite so [laughter]

309
00:15:54,206 --> 00:15:58,406
because you know there's some
tourists out there walking

310
00:15:58,406 --> 00:16:01,886
around in these somewhere so
we also have a pair of these.

311
00:16:01,886 --> 00:16:05,046
So an algorithm ultimately,
as we'll see in just a moment,

312
00:16:05,046 --> 00:16:08,346
is a procedure, it's a program
but it's not necessarily written

313
00:16:08,346 --> 00:16:10,536
in a specific language and
in fact what we see here

314
00:16:10,536 --> 00:16:13,036
on the board is what
we'll call sudo code.

315
00:16:13,036 --> 00:16:14,096
It's not quite code.

316
00:16:14,096 --> 00:16:15,176
It's not programming code.

317
00:16:15,176 --> 00:16:17,926
It's certainly not zeros and
ones but it's kind of a cross

318
00:16:17,986 --> 00:16:20,066
between something that's
very succinct and logical

319
00:16:20,306 --> 00:16:22,506
and it's also nonetheless
very human readable.

320
00:16:22,506 --> 00:16:25,246
And so throughout the
semester before we sit down

321
00:16:25,246 --> 00:16:27,976
and start typing things away
at a command prompt like I did

322
00:16:28,206 --> 00:16:31,126
or even dragging and dropping,
it's often worthwhile thinking

323
00:16:31,306 --> 00:16:34,366
about how could you express
this program in just English

324
00:16:34,366 --> 00:16:37,056
or English like syntax
because then you could work

325
00:16:37,056 --> 00:16:38,486
through the logic,
then you could think

326
00:16:38,486 --> 00:16:40,696
through hopefully some of the
bugs, some of the corner cases

327
00:16:40,966 --> 00:16:43,206
and then proceed to
implement that algorithm.

328
00:16:43,476 --> 00:16:45,526
So we're gonna go ahead and
use Josh here for the sake

329
00:16:45,526 --> 00:16:47,326
of execution and we'll see

330
00:16:47,326 --> 00:16:50,356
if we can't tease apart a
few programmatic fundamentals

331
00:16:50,356 --> 00:16:53,056
that will then recur in much
more traditional contexts

332
00:16:53,216 --> 00:16:53,936
than this one.

333
00:16:54,486 --> 00:16:57,966
So Josh if you would, perhaps,
think to yourself the following,

334
00:16:57,966 --> 00:17:00,286
let socks on feet, all one word.

335
00:17:00,286 --> 00:17:02,316
I seemed to have used this
underscore notation just

336
00:17:02,316 --> 00:17:04,246
to tie it all together
as one string,

337
00:17:04,586 --> 00:17:06,586
let's socks on feet equal zero.

338
00:17:06,846 --> 00:17:07,566
So this is easy.

339
00:17:07,566 --> 00:17:09,976
This is a variable, right,
just like X and Y and Z

340
00:17:09,976 --> 00:17:11,816
in high school were
variables that stored value,

341
00:17:11,816 --> 00:17:14,556
so is socks on feet a little
more interestingly named a

342
00:17:14,556 --> 00:17:17,196
variable that is in fact
consistent with reality.

343
00:17:17,196 --> 00:17:18,986
So we're initializing
our program here.

344
00:17:19,286 --> 00:17:21,716
So two, line two
induces what we'll call,

345
00:17:21,716 --> 00:17:23,966
and this too might be
self explanatory, a loop.

346
00:17:24,316 --> 00:17:27,036
So something that repeats
zero or more times.

347
00:17:27,036 --> 00:17:29,146
Now in this case I've
used the key word while

348
00:17:29,396 --> 00:17:32,706
which does imply doing something
while something is the case.

349
00:17:32,926 --> 00:17:35,546
We'll see terms like
for and do while

350
00:17:35,936 --> 00:17:39,226
and similar synonyms essentially
but for this we chose this

351
00:17:39,356 --> 00:17:43,826
so while socks on feet
exclamation point equals two.

352
00:17:44,366 --> 00:17:45,966
Now if you don't
know what that means,

353
00:17:46,326 --> 00:17:49,496
take a stab at what
it might mean.

354
00:17:49,716 --> 00:17:52,526
So not equal to,
so bang equal sign

355
00:17:52,526 --> 00:17:55,696
so computer scientist geeks
say bang not exclamation point

356
00:17:55,906 --> 00:17:58,046
so bang equals means
not equal to two.

357
00:17:58,046 --> 00:18:01,436
So we're going to do the
following while Josh presumably

358
00:18:01,436 --> 00:18:05,766
has a zero or one or you
know maybe even three socks

359
00:18:05,766 --> 00:18:06,786
on his feet cause we're going

360
00:18:06,786 --> 00:18:08,216
to do this while it's
not equal to two.

361
00:18:08,486 --> 00:18:11,106
So, here's where your
awkward role play comes in.

362
00:18:11,306 --> 00:18:14,096
Open the sock drawer, excellent.

363
00:18:14,346 --> 00:18:15,906
Look for sock.

364
00:18:16,986 --> 00:18:21,106
Okay if you find sock, so this
too is bold faced for a reason,

365
00:18:21,106 --> 00:18:22,466
this is a condition, a branch,

366
00:18:22,546 --> 00:18:25,336
something that happens
optionally if something is true

367
00:18:25,336 --> 00:18:27,656
or false so if you
find a sock, have you?

368
00:18:28,256 --> 00:18:31,266
Okay then and notice
the indentation here too

369
00:18:31,266 --> 00:18:33,526
so this is something
we'll use consistently.

370
00:18:33,526 --> 00:18:34,816
This is a matter of style.

371
00:18:34,816 --> 00:18:37,796
The computer often does not
care if you have spaces,

372
00:18:37,796 --> 00:18:40,586
tab characters, indentation,
no indentation and those

373
00:18:40,586 --> 00:18:43,516
of you have written HTML or
web pages before probably know

374
00:18:43,516 --> 00:18:45,566
that it might look pretty
to make everything look nice

375
00:18:45,566 --> 00:18:47,996
and hierarchical and
pretty printed as they say

376
00:18:48,126 --> 00:18:49,756
but the computer
certainly doesn't care.

377
00:18:49,976 --> 00:18:52,916
But here we're humans, we do
care so it's indented to imply

378
00:18:53,176 --> 00:18:57,156
that line 6 on down are
now encapsulated inside

379
00:18:57,156 --> 00:19:00,006
of what line 5 is
telling us to do.

380
00:19:00,136 --> 00:19:03,046
So if line 5 is true, if
find sock, which you did,

381
00:19:03,046 --> 00:19:04,936
then we proceed to
the indentation.

382
00:19:04,936 --> 00:19:09,846
Put on sock, excellent.

383
00:19:10,276 --> 00:19:12,826
Socks on feet plus plus,
this is shorthand notation

384
00:19:12,826 --> 00:19:15,106
and we'll see this in
C, JavaScript, PHP,

385
00:19:15,156 --> 00:19:18,366
plus plus as you might infer,
just means incremented by one.

386
00:19:18,366 --> 00:19:20,636
So socks on feet now equals one.

387
00:19:20,866 --> 00:19:21,836
Look for matching sock.

388
00:19:22,256 --> 00:19:23,796
And this we might
call a statement.

389
00:19:23,796 --> 00:19:25,696
Anytime it's just sort
of matter of fact here,

390
00:19:25,696 --> 00:19:26,646
we'll call that a statement.

391
00:19:26,646 --> 00:19:27,746
Okay you've looked.

392
00:19:27,946 --> 00:19:33,056
If you find a matching
sock then indentation,

393
00:19:33,056 --> 00:19:34,826
put on matching sock.

394
00:19:36,216 --> 00:19:37,336
So far so good.

395
00:19:37,516 --> 00:19:39,056
Socks on feet plus plus.

396
00:19:39,276 --> 00:19:42,736
Close sock drawer and now
notice I'm not going to proceed

397
00:19:42,736 --> 00:19:46,686
to do anything with line 13
cause line 13 is exclusive

398
00:19:46,686 --> 00:19:49,706
with respect to 9 so if
I did 9 I don't do 13.

399
00:19:49,936 --> 00:19:53,546
Similarly if I did
5, I don't do 16

400
00:19:53,736 --> 00:19:55,446
and now what do I go back up to?

401
00:19:55,446 --> 00:19:58,676
Well following the logic of
the indentation I now go back

402
00:19:58,706 --> 00:20:01,076
to line 2 and I check
this condition,

403
00:20:01,076 --> 00:20:05,746
while socks on feet not
equal to 2, what do I do now?

404
00:20:06,396 --> 00:20:08,846
[inaudible audience comment]
Yeah, I seem to move on.

405
00:20:08,846 --> 00:20:09,606
So what's next?

406
00:20:09,606 --> 00:20:13,686
Well line 18 which isn't even
present so we implicitly drop

407
00:20:13,686 --> 00:20:14,916
out of the program
and we're done.

408
00:20:15,266 --> 00:20:16,296
So that was actually very good.

409
00:20:16,296 --> 00:20:18,226
And now if you wouldn't
mind, let's restart this.

410
00:20:18,336 --> 00:20:22,416
Take off this socks if you would
and that will get awkward for me

411
00:20:22,456 --> 00:20:26,316
so I'm gonna take, say
this, this and this.

412
00:20:26,316 --> 00:20:27,216
Did we screw up?

413
00:20:27,216 --> 00:20:28,866
Is this program actually
correct?

414
00:20:29,026 --> 00:20:29,446
Well let's see.

415
00:20:29,446 --> 00:20:30,746
Let's socks on feet equal zero.

416
00:20:30,746 --> 00:20:32,946
Good, while socks on
feet not equal to 2.

417
00:20:32,946 --> 00:20:33,846
Good, that is true.

418
00:20:33,846 --> 00:20:36,636
Indentation, open sock
drawer, look for sock.

419
00:20:37,496 --> 00:20:40,996
If you find sock then,
[laughter] nice, put on sock.

420
00:20:46,936 --> 00:20:49,356
You will always remember
this moment in college.

421
00:20:50,076 --> 00:20:56,396
[laughter] Excellent,
socks on feet plus plus.

422
00:20:56,396 --> 00:20:58,466
Look for matching sock.

423
00:20:59,036 --> 00:21:03,146
Problem, if you find matching
sock then line 9 doesn't seem

424
00:21:03,146 --> 00:21:06,776
to apply so we go
to line 13 followed

425
00:21:06,776 --> 00:21:11,306
by 14 else remove
first sock from foot.

426
00:21:11,556 --> 00:21:15,366
Socks on feet, minus minus

427
00:21:15,706 --> 00:21:17,196
so now we're back
to zero presumably.

428
00:21:17,196 --> 00:21:22,336
What line do I go back to next?

429
00:21:22,336 --> 00:21:24,646
[inaudible audience comment]
So not 5, I heard but rather,

430
00:21:25,296 --> 00:21:28,946
so I'm back up at 2 while socks
on feet not equal to 2, okay,

431
00:21:29,776 --> 00:21:32,926
true so we proceed to open
sock drawer, look for sock,

432
00:21:33,106 --> 00:21:35,466
if you find sock
then put on sock.

433
00:21:36,186 --> 00:21:39,546
So at this point you're probably
getting a little bored, right?

434
00:21:39,696 --> 00:21:41,226
Because what's gonna
happen here?

435
00:21:41,846 --> 00:21:45,766
So this is one of these
things called an infinite loop

436
00:21:45,846 --> 00:21:47,926
or if you've never heard of that
expression, that's what it is

437
00:21:47,926 --> 00:21:50,066
and frankly you've probably
experienced this before,

438
00:21:50,066 --> 00:21:52,586
whether you have a Mac, a PC
or whatever, you've probably

439
00:21:52,586 --> 00:21:54,456
at least once in life
gotten really ticked off

440
00:21:54,456 --> 00:21:58,186
at your machine because it's
kind of just hanging and hanging

441
00:21:58,186 --> 00:22:00,786
and hanging or the stupid
hourglass is just spinning

442
00:22:00,786 --> 00:22:04,426
and spinning and spinning and
it never stops for some reason.

443
00:22:04,426 --> 00:22:05,166
Well what is that?

444
00:22:05,276 --> 00:22:07,256
Well technically it could
be any number of things

445
00:22:07,306 --> 00:22:11,126
but it's probably a mistake
that some programmer made,

446
00:22:11,366 --> 00:22:12,676
didn't anticipate the fact

447
00:22:12,676 --> 00:22:15,236
that maybe I haven't done
the laundry all that often

448
00:22:15,236 --> 00:22:17,676
and therefore I might not
have sufficiently many socks.

449
00:22:17,676 --> 00:22:20,326
Maybe I just don't have certain
socks anymore cause they got

450
00:22:20,326 --> 00:22:21,306
torn and I threw them out.

451
00:22:21,486 --> 00:22:24,006
These are what we'll call
corner cases and clearly bad,

452
00:22:24,356 --> 00:22:27,666
awkward things arise if you
don't handle these cases.

453
00:22:27,666 --> 00:22:30,356
So in those cases with your own
machines you probably rebooted

454
00:22:30,556 --> 00:22:32,686
or you force quit, you
pulled up the task manager,

455
00:22:32,686 --> 00:22:34,846
all these stupid real
world issues that boil

456
00:22:34,846 --> 00:22:37,416
down to stopping a
program that's caught

457
00:22:37,416 --> 00:22:38,216
in an infinite loop.

458
00:22:38,396 --> 00:22:41,876
Are there other bugs here
that we might trip over?

459
00:22:42,456 --> 00:22:42,606
[inaudible audience
comment] Sorry?

460
00:22:44,896 --> 00:22:49,576
[inaudible audience
comment] Yeah so I don't,

461
00:22:49,756 --> 00:22:52,156
so there's also some real
world problems here perhaps,

462
00:22:52,156 --> 00:22:54,406
especially if this is
again not Josh the human

463
00:22:54,406 --> 00:22:58,786
but Josh the robot where it
keeps saying open, what is,

464
00:22:58,866 --> 00:23:01,076
it said open sock
drawer, open sock drawer.

465
00:23:01,186 --> 00:23:02,486
Like, what does that even mean?

466
00:23:02,486 --> 00:23:04,576
I mean either the robot is
gonna start ripping the thing

467
00:23:04,576 --> 00:23:07,436
out of the wall or it's
just gonna get confused

468
00:23:07,436 --> 00:23:08,406
because it's ambiguous.

469
00:23:08,486 --> 00:23:12,616
And ambiguity is often a problem
too because what is going

470
00:23:12,666 --> 00:23:15,976
to happen in cases that are not
so explicitly specified by you.

471
00:23:16,206 --> 00:23:19,146
So why don't we put Josh
out of his awkwardness here.

472
00:23:19,346 --> 00:23:22,266
You get about 6 souvenirs today.

473
00:23:22,336 --> 00:23:23,076
They are clean.

474
00:23:23,076 --> 00:23:25,016
I can give you a
little shopping bag,

475
00:23:25,096 --> 00:23:26,546
if not the receipts if you want.

476
00:23:27,506 --> 00:23:29,706
But here we have some nice
little Harvard souvenirs

477
00:23:29,706 --> 00:23:31,156
and perhaps a quick round
of applause for Josh.

478
00:23:31,286 --> 00:23:36,796
Thank you very much.

479
00:23:36,796 --> 00:23:38,906
[applause] Alright so
this is what we did

480
00:23:38,906 --> 00:23:40,086
at the command line quickly

481
00:23:40,086 --> 00:23:42,226
and this is what we'll
do a week plus from now.

482
00:23:42,226 --> 00:23:44,086
This week we focus on
things more intuitive

483
00:23:44,356 --> 00:23:47,946
but this is a basic C
program, a canonical program

484
00:23:48,076 --> 00:23:49,776
and in fact it will get
much more interesting,

485
00:23:49,776 --> 00:23:50,946
much more powerful than this

486
00:23:51,166 --> 00:23:52,956
but what really happened
underneath the hood

487
00:23:52,956 --> 00:23:54,326
when I wrote this
little program?

488
00:23:54,566 --> 00:23:57,046
Well if that's my program
at top left and I ran

489
00:23:57,046 --> 00:23:59,596
that program called
GCC, the compiler,

490
00:23:59,596 --> 00:24:01,756
well what that compiler
did, it was written

491
00:24:01,756 --> 00:24:05,756
by some smart people a while
ago, it converts what kind

492
00:24:05,756 --> 00:24:08,326
of looks like English but is
definitely not English I'm used

493
00:24:08,326 --> 00:24:11,506
to writing in, it converts
it to zeros and ones

494
00:24:11,506 --> 00:24:13,256
that my particular
computer understands

495
00:24:13,316 --> 00:24:15,096
and this process is
called compiling.

496
00:24:15,356 --> 00:24:17,196
And this is partly
the reason why

497
00:24:17,406 --> 00:24:20,066
if you've ever bought a
program, maybe Microsoft Office,

498
00:24:20,316 --> 00:24:22,636
you either buy the shrink
wrapped box that says

499
00:24:22,636 --> 00:24:25,676
for Windows or you buy the
shrink wrapped box that says

500
00:24:25,926 --> 00:24:28,066
for Mac OS because
even companies

501
00:24:28,066 --> 00:24:29,626
like Microsoft very often have

502
00:24:29,796 --> 00:24:32,976
to essentially write
two versions of programs

503
00:24:32,976 --> 00:24:35,696
because a compiler, as I
hinted at with my own Mac,

504
00:24:36,006 --> 00:24:37,816
is kind of machine specific

505
00:24:37,816 --> 00:24:39,976
or at least it's
operating system specific.

506
00:24:40,236 --> 00:24:43,306
So unfortunately we don't
really live in a world that's

507
00:24:43,306 --> 00:24:46,186
as perfect enough whereby
Microsoft writes their program

508
00:24:46,186 --> 00:24:50,486
once and they run GCC on a
Mac, they run GCC on a PC.

509
00:24:50,486 --> 00:24:54,126
They copy the resulting A
dot out file or MS Word.

510
00:24:54,296 --> 00:24:55,956
Exe file to a CD and ship it.

511
00:24:55,956 --> 00:24:57,746
There's a lot more
to it than that.

512
00:24:58,086 --> 00:24:59,636
So one of the things
we'll talk about closer

513
00:24:59,636 --> 00:25:02,736
to terms end is the notion
of cross platform software.

514
00:25:02,736 --> 00:25:05,106
Java, one of the appeals
of Java, if I'm familiar,

515
00:25:05,296 --> 00:25:07,086
is that in theory you're
supposed to be able

516
00:25:07,086 --> 00:25:09,376
to write a Java program
once and then run it

517
00:25:09,376 --> 00:25:12,906
on any computer you want,
a Mac, a PC, a lynx box,

518
00:25:13,136 --> 00:25:14,626
even a cell phone these days.

519
00:25:14,876 --> 00:25:17,846
And you'll use a program like
this, the virtual terminal room

520
00:25:17,846 --> 00:25:20,236
to which I referred on Wednesday
for virtual office hours,

521
00:25:20,236 --> 00:25:21,316
if you decide to log on,

522
00:25:21,646 --> 00:25:24,246
that's actually you'll see
a program written in Java

523
00:25:24,516 --> 00:25:26,806
because that particular
company, called Illuminate,

524
00:25:27,026 --> 00:25:30,316
decided it's really expensive
and it's really uninteresting

525
00:25:30,316 --> 00:25:34,146
for us to have to write our
software multiple times just

526
00:25:34,286 --> 00:25:36,776
so that we can have Mac
customers, PC customers

527
00:25:36,776 --> 00:25:39,036
and so forth so they
went with Java.

528
00:25:39,036 --> 00:25:41,146
But there's a downside
even to that.

529
00:25:41,646 --> 00:25:45,036
So, what then are we
going to do this week?

530
00:25:45,196 --> 00:25:46,036
It is Scratch.

531
00:25:46,036 --> 00:25:47,986
And today what we'll do
is just lay the foundation

532
00:25:47,986 --> 00:25:50,546
for just these basic
programmatic constructs

533
00:25:50,656 --> 00:25:55,226
but by lectures and today we'll
be already begin our transition

534
00:25:55,226 --> 00:25:57,566
to C. Your first problem
set, which will be posted,

535
00:25:57,816 --> 00:26:00,166
as will be typically the
case, by 7:00 pm tonight

536
00:26:00,166 --> 00:26:01,206
on the courses website.

537
00:26:01,346 --> 00:26:03,506
If you're like me you're
welcome to dive in at 7:01.

538
00:26:03,976 --> 00:26:04,986
If you have more going

539
00:26:04,986 --> 00:26:07,856
on you can wait a few days cause
you have ample time for this one

540
00:26:08,036 --> 00:26:10,986
but they'll be posted on the
courses website at CS50.net

541
00:26:11,286 --> 00:26:14,426
but let me encourage you
to take note of this too.

542
00:26:14,666 --> 00:26:17,456
So especially for this first
problem set you'll find

543
00:26:17,696 --> 00:26:20,856
that it's rather fun and in
fact you'll probably get a kick

544
00:26:20,856 --> 00:26:23,366
out of sharing or making
your roommate take a look

545
00:26:23,366 --> 00:26:24,706
at what you've actually
been working on.

546
00:26:24,956 --> 00:26:27,386
But so that you don't
feel completely thrown

547
00:26:27,386 --> 00:26:29,256
into the lion's den,
so to speak,

548
00:26:29,256 --> 00:26:32,086
especially if you're feeling
a bit among those less

549
00:26:32,116 --> 00:26:34,836
comfortable, we will start
office hours this coming week.

550
00:26:34,836 --> 00:26:36,116
So no sections just yet.

551
00:26:36,256 --> 00:26:37,556
We will start the process

552
00:26:37,556 --> 00:26:39,426
of sectioning next
Wednesday onward,

553
00:26:39,426 --> 00:26:41,636
some more on that next
week but office hours,

554
00:26:41,636 --> 00:26:44,966
this list will grow as our
50 plus person team begins

555
00:26:44,966 --> 00:26:47,256
to figure out their own
schedules, we're gonna hang out.

556
00:26:47,256 --> 00:26:50,186
Next week on Monday, Tuesday,
Wednesday, Thursday evenings,

557
00:26:50,216 --> 00:26:52,826
7:00 pm onward in Cavet
[phonetic] Houses dining hall

558
00:26:53,016 --> 00:26:55,936
and in Quincy Houses dining hall
where you can bring your laptop,

559
00:26:55,936 --> 00:26:57,866
you can hang out for
brain break and a number

560
00:26:57,866 --> 00:27:00,006
of the teaching fellows and
CA's will be there to help you,

561
00:27:00,216 --> 00:27:04,576
to chat with you and just
generally tackle this as a team.

562
00:27:04,686 --> 00:27:06,326
So what are you going
to be tackling?

563
00:27:06,866 --> 00:27:08,866
Well this year it's Scratch.

564
00:27:08,916 --> 00:27:12,046
I went to a website
called scratch.mit.edu.

565
00:27:12,336 --> 00:27:14,196
It works for Mac's,
it works for PC's.

566
00:27:14,196 --> 00:27:17,176
I downloaded this program
and wala, this is what I get.

567
00:27:17,266 --> 00:27:18,576
But what is this interface?

568
00:27:18,666 --> 00:27:21,896
Well on the left hand side here
we have these puzzle pieces

569
00:27:21,896 --> 00:27:23,726
and they literally
look like puzzle pieces

570
00:27:23,726 --> 00:27:27,246
that in a moment I'll be able to
drag into this scripts area here

571
00:27:27,426 --> 00:27:29,296
and it's by dragging and
dropping these pieces

572
00:27:29,296 --> 00:27:31,856
that I will be programming,
so to speak.

573
00:27:32,046 --> 00:27:33,876
Well in the top right
we have what we're going

574
00:27:33,876 --> 00:27:34,936
to call the stage.

575
00:27:34,936 --> 00:27:37,786
This is where stuff happens,
as we've already seen.

576
00:27:38,066 --> 00:27:40,746
This little pat by
default, this is Scratch,

577
00:27:41,006 --> 00:27:42,186
sort of the mascot
they went with

578
00:27:42,186 --> 00:27:43,786
but we can change his
appearance at will.

579
00:27:44,016 --> 00:27:46,586
This is what we'll call a Sprite

580
00:27:46,586 --> 00:27:49,126
or really it's what we'll call
more technically a thread,

581
00:27:49,516 --> 00:27:51,846
a miniature program
that you can instruct

582
00:27:51,846 --> 00:27:54,366
to do things independently
of anything else.

583
00:27:54,366 --> 00:27:56,526
And at bottom right
here we'll see all

584
00:27:56,526 --> 00:27:59,736
of the many different Sprites
that I might possibly create

585
00:27:59,976 --> 00:28:00,886
so let me go and do this.

586
00:28:01,196 --> 00:28:03,976
So control, when
green flag clicked.

587
00:28:04,406 --> 00:28:07,026
So when I a moment ago
typed that cryptic sequence

588
00:28:07,026 --> 00:28:08,636
of instructions into main,

589
00:28:08,906 --> 00:28:10,646
main kind of has
some implied meaning,

590
00:28:10,646 --> 00:28:12,676
this is the main
guts of my program?

591
00:28:12,916 --> 00:28:14,566
Well this is sort of
the equivalent so those

592
00:28:14,566 --> 00:28:17,696
of you coming to the course
from like a Java background,

593
00:28:17,906 --> 00:28:19,526
it's like writing
your main method.

594
00:28:19,656 --> 00:28:23,006
So this block, just to be clear,
has this icon on it that says

595
00:28:23,066 --> 00:28:25,166
when green flag clicked
and that's why,

596
00:28:25,166 --> 00:28:26,636
if you've been paying
close attention,

597
00:28:26,846 --> 00:28:29,366
every time I start these
demos I click that green flag.

598
00:28:29,636 --> 00:28:30,466
So what does that mean?

599
00:28:30,466 --> 00:28:33,076
That means when we click
that green flag this cat,

600
00:28:33,446 --> 00:28:36,096
this Sprite, because
he's currently selected,

601
00:28:36,296 --> 00:28:39,756
is going to do whatever
the blocks tell him to do

602
00:28:39,756 --> 00:28:43,106
that are attached to this
particular starting point.

603
00:28:43,356 --> 00:28:44,256
So what do we want him to do?

604
00:28:44,256 --> 00:28:46,146
Well let's do something simple.

605
00:28:46,146 --> 00:28:48,276
I'm gonna click the little
pallet here for looks.

606
00:28:48,526 --> 00:28:52,216
So MIT has nicely
color coded things just

607
00:28:52,216 --> 00:28:54,876
so that all related blocks
are similarly colored.

608
00:28:54,876 --> 00:28:56,736
I'm gonna flip through
this and okay,

609
00:28:56,806 --> 00:28:58,436
this is interesting, say hello.

610
00:28:58,736 --> 00:29:01,436
So say hello for
2 seconds and just

611
00:29:01,716 --> 00:29:02,946
because it's a little
small that's what

612
00:29:02,946 --> 00:29:04,606
that puzzle piece looks
like and notice too,

613
00:29:04,606 --> 00:29:06,016
and this is the magic of it,

614
00:29:06,336 --> 00:29:08,256
you bring it close
enough, it snaps together.

615
00:29:08,256 --> 00:29:09,956
Right, it's a lot easier

616
00:29:09,956 --> 00:29:12,816
than remembering alright
I put the parenthesis

617
00:29:12,816 --> 00:29:14,636
and then the semicolon
and then hit enter.

618
00:29:14,976 --> 00:29:15,906
Well you just drag and drop

619
00:29:15,906 --> 00:29:17,596
and these things will
fit together logically

620
00:29:17,596 --> 00:29:18,466
if that makes since.

621
00:29:18,516 --> 00:29:22,156
Programmatically I'm gonna
click the green flag, wow,

622
00:29:22,536 --> 00:29:28,636
alright so I just wrote
what was this a moment ago.

623
00:29:28,636 --> 00:29:31,996
Not so bad but not that
interesting, in fairness.

624
00:29:32,236 --> 00:29:34,116
So let's do something a
little more interesting.

625
00:29:34,116 --> 00:29:37,136
So let me say something like
this, what is your name?

626
00:29:37,946 --> 00:29:41,166
So now I need a way of
actually getting input

627
00:29:41,366 --> 00:29:43,286
so how can I do this?

628
00:29:43,286 --> 00:29:45,806
Well under sensing here
there's this block,

629
00:29:46,056 --> 00:29:48,206
now ask what's your
name and weight?

630
00:29:48,206 --> 00:29:50,526
Well let me try and do this,
what's your name, oh wait,

631
00:29:50,526 --> 00:29:52,236
this is already a book, right?

632
00:29:52,276 --> 00:29:55,486
So I've kind of gotten ahead of
myself so let's do this, hello,

633
00:29:55,486 --> 00:29:58,616
we'll say that for 2 seconds,
then ask this and then wait.

634
00:29:58,616 --> 00:30:02,816
Well let's see, hello,
what's your name?

635
00:30:03,036 --> 00:30:04,106
Alright, David.

636
00:30:04,106 --> 00:30:07,516
[laughter] Kind of
stupid program, right?

637
00:30:07,516 --> 00:30:10,746
I've asked for my name but
I'm not doing anything yet.

638
00:30:10,746 --> 00:30:13,946
So let's do something a
little cute perhaps so looks,

639
00:30:13,946 --> 00:30:16,756
how about, hmm, so
there's the saved block,

640
00:30:16,756 --> 00:30:17,926
there's this think block.

641
00:30:17,926 --> 00:30:18,976
Let me plug this in.

642
00:30:19,196 --> 00:30:23,356
Now I don't want it to say,
well okay we'll let it say hmm

643
00:30:23,356 --> 00:30:25,406
but then I'm going
to be a little cute

644
00:30:25,856 --> 00:30:36,006
and say something like,
let's say doesn't ring a bell

645
00:30:36,616 --> 00:30:38,216
and actually hmm
is a little generic

646
00:30:38,216 --> 00:30:40,246
so let's do something even
more interesting still.

647
00:30:40,246 --> 00:30:42,436
Under sensing, notice that
there was this answer here

648
00:30:42,636 --> 00:30:44,036
and if I had actually
gotten some help

649
00:30:44,036 --> 00:30:46,586
about this block I wouldn't have
gotten a little cheat sheet.

650
00:30:46,586 --> 00:30:47,306
What does this do?

651
00:30:47,306 --> 00:30:49,546
So this programming environment,
it's actually very easy

652
00:30:49,546 --> 00:30:50,956
to bootstrap yourself cause all

653
00:30:50,956 --> 00:30:52,666
of the help is right
there for you.

654
00:30:52,876 --> 00:30:54,636
I'm actually gonna drag
this and then notice

655
00:30:54,636 --> 00:30:57,226
that the hmm doesn't have
to be me manually type.

656
00:30:57,226 --> 00:31:00,136
I can actually plug in this
variable so now I go ahead

657
00:31:00,136 --> 00:31:04,536
and say this, hello,
what's your name?

658
00:31:04,536 --> 00:31:08,406
Alright, David, hmm, and that's
a little cartoon bubble there,

659
00:31:09,126 --> 00:31:09,836
doesn't ring a bell.

660
00:31:09,836 --> 00:31:12,126
Alright so baby steps to be sure

661
00:31:12,126 --> 00:31:13,486
and still not all
that glamorous.

662
00:31:13,526 --> 00:31:14,866
Probably you're not
tapping your roommate

663
00:31:14,866 --> 00:31:15,836
on the shoulder just yet.

664
00:31:16,256 --> 00:31:18,236
So what more can
we do with this?

665
00:31:18,236 --> 00:31:21,816
Well let's see if we can't take
things progressively higher

666
00:31:21,816 --> 00:31:22,126
and higher.

667
00:31:22,126 --> 00:31:23,386
And what you'll see
is even though a lot

668
00:31:23,386 --> 00:31:25,726
of these initial demonstrations
are terribly simply,

669
00:31:25,726 --> 00:31:28,026
that's what programming
ultimately is about.

670
00:31:28,026 --> 00:31:30,936
It's taking some very basic
building blocks and building

671
00:31:31,076 --> 00:31:34,296
on top, on top, on top of
them, kind of combining them

672
00:31:34,296 --> 00:31:36,996
into things we'll start
calling functions and libraries

673
00:31:37,256 --> 00:31:37,806
so that you don't have

674
00:31:37,806 --> 00:31:40,286
to reinvent these uninteresting
wheels all the time

675
00:31:40,516 --> 00:31:43,606
until finally do you get much
more interesting software.

676
00:31:43,606 --> 00:31:46,626
Case in point, things like
Shuttleboy that we don't have

677
00:31:46,736 --> 00:31:49,346
to worry so much about
these minutia before long.

678
00:31:49,346 --> 00:31:50,696
So there are these
things called statements

679
00:31:50,696 --> 00:31:52,056
in programming and in Scratch.

680
00:31:52,366 --> 00:31:55,426
Anything that is a block that's
sort of imperative, say this,

681
00:31:55,506 --> 00:31:57,196
wait for this amount
of time, play sound,

682
00:31:57,426 --> 00:31:59,906
we'll call these statements and
these are just a few examples.

683
00:32:00,196 --> 00:32:02,016
So here are two programs.

684
00:32:02,286 --> 00:32:05,126
The left one we can
figure out intuitively

685
00:32:05,126 --> 00:32:06,346
so one green flag clicked,

686
00:32:06,636 --> 00:32:09,626
that example is gonna say
oh hi world, for one second,

687
00:32:09,656 --> 00:32:10,446
then it's gonna pause.

688
00:32:10,676 --> 00:32:12,806
It's gonna say it again,
then it's gonna pause.

689
00:32:12,916 --> 00:32:14,036
And then it's gonna
say it again,

690
00:32:14,536 --> 00:32:16,126
right, rather underwhelming.

691
00:32:16,126 --> 00:32:18,886
Let's go ahead and pull up the
third version of this though

692
00:32:18,886 --> 00:32:21,236
and all of these
examples you can download

693
00:32:21,236 --> 00:32:23,936
from the courses website, let
me go ahead and pull up hi3.sb,

694
00:32:23,936 --> 00:32:28,016
double clicking suffices,
that's the program.

695
00:32:28,016 --> 00:32:30,156
Notice there's really
not much complexity here.

696
00:32:30,156 --> 00:32:31,126
I still have just one cat

697
00:32:31,576 --> 00:32:33,526
but let me go ahead
and click play now.

698
00:32:37,846 --> 00:32:42,636
[noise] Okay still maybe not
roommate worthy but let's see

699
00:32:42,846 --> 00:32:44,036
where else we can go with this.

700
00:32:44,146 --> 00:32:46,346
Alright so there's
Boolean expressions

701
00:32:46,346 --> 00:32:47,786
in program languages.

702
00:32:47,786 --> 00:32:50,866
A Boolean expression,
named after a Mr. Bool,

703
00:32:51,066 --> 00:32:54,776
is an expression that in the end
is either true or it's false.

704
00:32:54,776 --> 00:32:56,786
It's either 1 or it's zero.

705
00:32:56,786 --> 00:32:57,986
It's yes. It's no.

706
00:32:57,986 --> 00:32:58,886
These two dichotomies.

707
00:32:58,886 --> 00:33:01,056
It doesn't matter what
label you slap on them

708
00:33:01,056 --> 00:33:02,436
but it's this notion
of true and false.

709
00:33:02,646 --> 00:33:05,336
So what kinds of questions
might return yes no answers,

710
00:33:05,336 --> 00:33:06,226
true false answers?

711
00:33:06,436 --> 00:33:08,996
Well is the cat touching
the mouse pointer?

712
00:33:09,256 --> 00:33:11,016
In other words is my
mouse hovering over him

713
00:33:11,016 --> 00:33:13,636
because if he is that could
be an interesting mode

714
00:33:13,636 --> 00:33:14,366
of interaction.

715
00:33:14,646 --> 00:33:15,896
Is the mouse button down?

716
00:33:16,186 --> 00:33:17,936
That too is kind of an
interesting question to ask

717
00:33:18,126 --> 00:33:20,646
because if so maybe I'll
do something interesting.

718
00:33:20,956 --> 00:33:22,666
Is something less
than something?

719
00:33:22,816 --> 00:33:27,156
Well sometimes you're supposed
to do numeric manipulations,

720
00:33:27,376 --> 00:33:28,716
even in a context like this.

721
00:33:28,716 --> 00:33:31,766
And then there's this,
this Boolean and operator.

722
00:33:31,766 --> 00:33:34,076
If any of you are
particularly savvy with Google

723
00:33:34,076 --> 00:33:37,546
or search engines, sometimes you
might realize that you can type,

724
00:33:37,786 --> 00:33:41,916
say, David and Harvard into
Google search box or something

725
00:33:41,916 --> 00:33:46,016
like that, to get results that
actually relate not to David

726
00:33:46,016 --> 00:33:48,416
or Harvard but both
of them, in fact.

727
00:33:48,596 --> 00:33:52,476
And so we can merge Boolean
expressions into singular units.

728
00:33:52,966 --> 00:33:54,196
Why is that interesting?

729
00:33:54,196 --> 00:33:57,446
Well if we can ask these yes
no questions we can do things

730
00:33:57,806 --> 00:33:59,376
or not conditionally.

731
00:33:59,376 --> 00:34:02,456
Just like Josh put on the
sock only if he found a match,

732
00:34:03,526 --> 00:34:05,196
can we similarly distinguish

733
00:34:05,196 --> 00:34:06,976
between different
states of our world here?

734
00:34:06,976 --> 00:34:08,936
So there's this if
block in Scratch,

735
00:34:08,936 --> 00:34:11,086
at the far left here
so if something.

736
00:34:11,086 --> 00:34:13,456
And notice there's this funky
triangular shape here too

737
00:34:13,456 --> 00:34:15,186
so you'll find that
there are similarities,

738
00:34:15,186 --> 00:34:17,506
not just with color in
this program but also

739
00:34:17,506 --> 00:34:18,476
in the shape of the block.

740
00:34:18,476 --> 00:34:20,876
So just as a visual
queue, what can you put

741
00:34:20,876 --> 00:34:21,966
into that place holder?

742
00:34:22,256 --> 00:34:23,746
But it's not just
interesting to say,

743
00:34:23,746 --> 00:34:26,616
if something's true do
this, sometimes you want it

744
00:34:26,616 --> 00:34:28,906
to be exclusive, do
this, else do that

745
00:34:29,106 --> 00:34:31,286
and so there's this
if else construct.

746
00:34:31,376 --> 00:34:33,436
Now sometimes you
might want to do this,

747
00:34:33,586 --> 00:34:34,836
that or the other thing.

748
00:34:35,296 --> 00:34:38,086
Well these things can nest so
even though it doesn't look

749
00:34:38,086 --> 00:34:41,856
like you can cram another block
inside that middle one, else,

750
00:34:41,856 --> 00:34:43,236
I mean it's only this big.

751
00:34:43,236 --> 00:34:44,876
How in the world do we
fit anything in there?

752
00:34:45,096 --> 00:34:45,946
Well what you'll find is

753
00:34:45,946 --> 00:34:48,036
that Scratch automatically
adapts itself.

754
00:34:48,036 --> 00:34:50,336
It just grows the size
of the block if you want

755
00:34:50,336 --> 00:34:52,006
to squeeze more blocks into it.

756
00:34:52,236 --> 00:34:54,866
So now we have the ability
to do things conditionally.

757
00:34:55,106 --> 00:34:56,286
Well why is that helpful?

758
00:34:56,286 --> 00:34:58,856
Well let's take a look
at iteration 4 of this.

759
00:34:59,066 --> 00:35:01,116
So I'm gonna go open hi4.sb.

760
00:35:01,116 --> 00:35:06,516
It's not a very interesting
program yet so let me hit play.

761
00:35:06,516 --> 00:35:10,006
[noise] I'm gonna hit it again.

762
00:35:12,046 --> 00:35:14,716
[noise] This is sort of
vacuous truth, right?

763
00:35:14,716 --> 00:35:18,006
This is not [background
noise] a very interesting use

764
00:35:18,676 --> 00:35:22,116
of a condition because one's
value is presumably never

765
00:35:22,116 --> 00:35:24,416
changing so let's hope I did
something a little smarter

766
00:35:24,416 --> 00:35:25,556
with version 5 here.

767
00:35:25,806 --> 00:35:27,686
This is version 5
of this same idea.

768
00:35:28,076 --> 00:35:32,766
Huh, so now the fact that we can
plug in values from elsewhere,

769
00:35:32,766 --> 00:35:34,266
this gets a little
more interesting.

770
00:35:34,266 --> 00:35:36,036
So this block here
let me zoom in,

771
00:35:36,366 --> 00:35:39,586
is what we'll call a sudo
random number generator.

772
00:35:39,876 --> 00:35:43,236
Now to the target audience of
Scratch, which is usually people

773
00:35:43,236 --> 00:35:45,506
about yay high so 10
year olds, 12 year olds,

774
00:35:45,876 --> 00:35:47,856
MIT teachers don't
whip out terms like,

775
00:35:48,136 --> 00:35:51,526
now use a sudo random number
generator to make your cat move

776
00:35:51,566 --> 00:35:53,806
but that's what we're
gonna call it, right?

777
00:35:53,986 --> 00:35:58,716
We're [laughter] in this case
going to pick a random number.

778
00:35:58,716 --> 00:36:01,606
How that gets picked is
actually an interesting question

779
00:36:01,606 --> 00:36:02,416
unto itself.

780
00:36:02,416 --> 00:36:04,996
And those of you who like
mathematics, like theory,

781
00:36:04,996 --> 00:36:08,246
I mean there's a whole world
of discovery and a whole world

782
00:36:08,246 --> 00:36:09,686
of research that
asks the question,

783
00:36:09,916 --> 00:36:13,576
how do you make machines, which
are by nature deterministic?

784
00:36:13,856 --> 00:36:15,566
They only do what you tell them.

785
00:36:15,876 --> 00:36:18,846
How do you make a machine like
that pick a random number?

786
00:36:19,156 --> 00:36:20,506
It would seem to be
the case that you have

787
00:36:20,546 --> 00:36:23,566
to tell the machine what number
to pick but that would sort

788
00:36:23,566 --> 00:36:25,906
of defeat the point of
having randomness at all.

789
00:36:26,106 --> 00:36:28,206
But in fact there are ways of
doing this or there are ways

790
00:36:28,206 --> 00:36:31,246
of simulating this and as an
aside you can kind of cheat.

791
00:36:31,486 --> 00:36:33,616
For instance, if your
computer has a microphone,

792
00:36:33,876 --> 00:36:35,256
well odds are you're
not going to be

793
00:36:35,256 --> 00:36:36,886
in a perfectly quiet
room all the time

794
00:36:36,886 --> 00:36:40,146
so maybe you get some
randomness, some random noise,

795
00:36:40,426 --> 00:36:41,896
by way of turning
on your microphone

796
00:36:41,896 --> 00:36:44,446
so there's some juicy
areas of discovery there.

797
00:36:44,626 --> 00:36:46,436
But for us, we're just
gonna use it mechanically.

798
00:36:46,436 --> 00:36:49,786
Pick a random number from 1
to 10, this has the effect,

799
00:36:50,496 --> 00:36:55,676
if all goes according to plan,
of the following, is that a bug?

800
00:36:55,676 --> 00:36:57,366
I click, nothing happens.

801
00:36:58,376 --> 00:37:01,496
It's kind of to be
expected I think, right?

802
00:37:01,496 --> 00:37:03,926
[noise] Okay that
time it worked.

803
00:37:03,976 --> 00:37:06,356
It didn't.

804
00:37:07,036 --> 00:37:07,336
[noise] Did.

805
00:37:08,346 --> 00:37:13,116
Didn't, [noise] did,
now as you may recall,

806
00:37:13,116 --> 00:37:15,396
if we do this an
infinite number of times,

807
00:37:15,606 --> 00:37:18,186
50 percent of them will in fact
meow, 50 percent of them won't.

808
00:37:18,266 --> 00:37:19,466
It's actually a good statistic

809
00:37:19,466 --> 00:37:21,646
so last year I kept
hitting the button

810
00:37:21,646 --> 00:37:23,666
and the damn thing kept
meowing, meowing, meowing

811
00:37:23,836 --> 00:37:26,446
but that actually speaks to the
fact that it is in fact random,

812
00:37:26,446 --> 00:37:27,496
at least in some part.

813
00:37:27,726 --> 00:37:28,896
So who cares?

814
00:37:28,896 --> 00:37:29,916
What can we do with this?

815
00:37:30,116 --> 00:37:31,876
Well let's see what
else we can build on.

816
00:37:32,016 --> 00:37:33,996
So there's a way of doing
things multiple times.

817
00:37:33,996 --> 00:37:37,126
Now in one of my first programs
I had the cat meow 3 times.

818
00:37:37,506 --> 00:37:41,476
It said play sound, meow, wait a
second, play it, wait a second,

819
00:37:41,566 --> 00:37:42,886
play it, wait a second.

820
00:37:43,126 --> 00:37:46,426
Well that's kind of a nice
capability if you can tell it

821
00:37:46,426 --> 00:37:47,956
to do something multiple times.

822
00:37:47,956 --> 00:37:50,156
Let me go ahead and
open up this thing here.

823
00:37:50,156 --> 00:37:52,386
And again this is what we
just glanced at at the slide.

824
00:37:53,596 --> 00:37:56,046
It's rather underwhelming
and in fact,

825
00:37:56,906 --> 00:37:58,536
it's perhaps more
interesting if we

826
00:37:58,536 --> 00:38:00,596
at least make it
slightly annoying

827
00:38:01,076 --> 00:38:02,836
by doing it this
way with the sound.

828
00:38:06,376 --> 00:38:08,046
[noise] And in fact I'll indulge

829
00:38:08,046 --> 00:38:10,876
in what you inevitably will find
a fun way of procrastinating,

830
00:38:10,876 --> 00:38:13,096
it turns out we don't
have to use just the meow.

831
00:38:13,096 --> 00:38:14,686
We can click things like import.

832
00:38:14,926 --> 00:38:16,396
You'll find that this
environment comes

833
00:38:16,396 --> 00:38:17,676
with all these various effects

834
00:38:17,956 --> 00:38:20,286
so maybe it would be more
fun to do something like.

835
00:38:20,496 --> 00:38:27,936
[noise and laughter]
Okay and there's the cat.

836
00:38:28,886 --> 00:38:30,206
There's other things too.

837
00:38:30,206 --> 00:38:34,036
I think if we go into
vocals we've got this one.

838
00:38:34,256 --> 00:38:38,966
[noise] So that's not that
bad in terms of annoying.

839
00:38:39,616 --> 00:38:42,776
[laughter] So let me do this
again, and please stop me,

840
00:38:42,946 --> 00:38:44,316
we can't go on this
tangent too long.

841
00:38:44,566 --> 00:38:46,096
So there's three tabs
up there and again,

842
00:38:46,096 --> 00:38:48,046
this is not interesting
stuff intellectually

843
00:38:48,046 --> 00:38:50,116
so we'll wave our hands at it
largely today but I'm going

844
00:38:50,116 --> 00:38:52,686
to go back to my scripts now and
now notice, to change the sound,

845
00:38:52,946 --> 00:38:55,126
alright so I'll do
this, this, this.

846
00:38:55,436 --> 00:38:58,436
But the point I wanted to
make is what do I do if I want

847
00:38:58,436 --> 00:39:01,116
to do not 3 times but 6 times?

848
00:39:01,316 --> 00:39:02,736
Well you can duplicate things.

849
00:39:03,116 --> 00:39:05,346
So actually let me go ahead
and duplicate it from here

850
00:39:05,346 --> 00:39:07,566
and I'm just right clicking
or control clicking.

851
00:39:07,566 --> 00:39:09,616
Let me get this stuff
out of the way there.

852
00:39:09,616 --> 00:39:16,056
And yep, so now I have it
going 1, 2, 3, 4, 5 times

853
00:39:16,056 --> 00:39:19,326
and actually let me
detach this, 6, 7 times.

854
00:39:19,326 --> 00:39:20,566
I'm gonna drop this over there

855
00:39:20,566 --> 00:39:22,946
and let's just see how
annoying this gets.

856
00:39:23,516 --> 00:39:30,546
[noise]

857
00:39:31,046 --> 00:39:37,176
Okay so the point is now not
to annoy but to demonstrate

858
00:39:37,176 --> 00:39:39,896
that this is not the smartest
approach to telling the computer

859
00:39:39,896 --> 00:39:41,266
to do something again and again.

860
00:39:41,436 --> 00:39:45,416
If you find yourself, whether
in C, Scratch, Java Script, PHP,

861
00:39:45,416 --> 00:39:47,496
just kind of copying and
pasting to get your program

862
00:39:47,496 --> 00:39:49,876
to do something multiple
times, multiple times,

863
00:39:49,876 --> 00:39:52,626
which may be compelling,
there's got to be another way

864
00:39:52,626 --> 00:39:53,466
and in fact there are.

865
00:39:53,466 --> 00:39:55,856
There are these constructs,
as we saw with Josh's example,

866
00:39:55,916 --> 00:39:59,236
of looping in Scratch and
in C and in Java Script

867
00:39:59,236 --> 00:40:00,756
and in a lot of modern
languages.

868
00:40:00,996 --> 00:40:03,866
These things are called
wild loops or for a loop.

869
00:40:04,066 --> 00:40:05,726
In Scratch they're
called forever loops

870
00:40:05,846 --> 00:40:07,906
or repeat some number of times.

871
00:40:08,136 --> 00:40:10,776
So already can you probably
imagine how you might

872
00:40:10,776 --> 00:40:11,646
integrate these.

873
00:40:11,826 --> 00:40:13,236
Well what about something
like this?

874
00:40:13,236 --> 00:40:15,346
If we wanted to create
that very simple effect

875
00:40:15,556 --> 00:40:19,956
of doing something ad nauseum,
if we pull up version 6 of this,

876
00:40:20,296 --> 00:40:23,356
notice that the blocks I've
chosen are one green flag click.

877
00:40:23,626 --> 00:40:25,166
I then grab that forever block.

878
00:40:25,406 --> 00:40:29,196
And to be clear, the forever
block is not normally that tall

879
00:40:29,196 --> 00:40:31,576
so notice the mere act
of dragging something

880
00:40:31,576 --> 00:40:34,736
like this inside the
forever loop makes it bigger.

881
00:40:34,736 --> 00:40:36,936
And then if I want to squeeze
this in it may trune [phonetic]

882
00:40:36,936 --> 00:40:38,956
but again everything
is similarly shaped.

883
00:40:39,106 --> 00:40:40,376
Now I'm gonna have
the same effect,

884
00:40:40,746 --> 00:40:41,746
slightly less annoying [noise]

885
00:40:42,936 --> 00:40:45,966
and now [background noise]
I've arguably implemented this

886
00:40:45,966 --> 00:40:47,706
with a much better design.

887
00:40:47,706 --> 00:40:51,276
There was no reason for the copy
paste and in fact if you sort

888
00:40:51,276 --> 00:40:54,336
of think about where
that bad habit leads you,

889
00:40:54,336 --> 00:40:56,016
if you're copying and
pasting constantly,

890
00:40:56,216 --> 00:40:58,386
imagine how much work it
would be for me to go back

891
00:40:58,726 --> 00:41:00,656
into the previous
version of that program

892
00:41:01,066 --> 00:41:03,546
and actually change something

893
00:41:03,546 --> 00:41:05,036
like the sound that's
being played.

894
00:41:05,036 --> 00:41:08,256
I mean my God I have to like
change it in 6 or 7 drop downs,

895
00:41:08,566 --> 00:41:11,376
here I just change
it in 1 place.

896
00:41:11,506 --> 00:41:13,586
So we already have this
ability to do things

897
00:41:13,586 --> 00:41:16,076
that are a little
better designed.

898
00:41:16,076 --> 00:41:17,656
Well what about this
thing at top right?

899
00:41:18,366 --> 00:41:20,796
So top right is saying
when green flag is clicked,

900
00:41:21,066 --> 00:41:22,216
do the following forever.

901
00:41:22,216 --> 00:41:22,976
Well what?

902
00:41:22,976 --> 00:41:25,376
If touching mouse
pointer, play sound meow

903
00:41:25,576 --> 00:41:27,816
and wait two seconds
and then repeat.

904
00:41:27,866 --> 00:41:28,886
Alright so let's open this.

905
00:41:28,946 --> 00:41:30,046
This is version 7.

906
00:41:30,046 --> 00:41:33,896
I'm gonna go ahead
and open this.

907
00:41:33,896 --> 00:41:35,436
If you wondered how I
was doing it before,

908
00:41:35,436 --> 00:41:37,296
at the top right
there's presentation mode

909
00:41:37,456 --> 00:41:39,246
where you can zoom in
and make it even bigger.

910
00:41:39,726 --> 00:41:40,746
So now I hit play.

911
00:41:41,526 --> 00:41:43,646
Nothing seems to be happening
but that's consistent

912
00:41:43,646 --> 00:41:47,006
with the previous slide because
I didn't use the save block,

913
00:41:47,006 --> 00:41:49,216
I didn't use the think
block but notice my mouse,

914
00:41:49,216 --> 00:41:50,926
if I move it closer and
closer to the cat [noise],

915
00:41:50,926 --> 00:41:56,536
it's kind of like petting
the cat, if you will.

916
00:41:57,306 --> 00:41:58,406
Clever? I thought so.

917
00:41:59,296 --> 00:42:02,616
Alright so kind of interesting.

918
00:42:02,616 --> 00:42:04,386
Let's take things
slightly higher.

919
00:42:04,386 --> 00:42:05,656
So that's version 7.

920
00:42:05,656 --> 00:42:07,676
Let me go to version 8 here.

921
00:42:09,096 --> 00:42:11,116
Version 8 is not
that much bigger

922
00:42:11,116 --> 00:42:12,926
but now notice, what's
gonna happen?

923
00:42:13,046 --> 00:42:14,676
You can kind of see
this one coming.

924
00:42:16,926 --> 00:42:17,146
[inaudible audience
comment] Alright

925
00:42:17,146 --> 00:42:19,326
so this was me thinking
this was really clever.

926
00:42:20,416 --> 00:42:22,366
Alright so let me go
ahead and full screen it,

927
00:42:22,616 --> 00:42:25,366
click the green flag and now
notice [background noise] he's

928
00:42:25,366 --> 00:42:28,316
just going about his business
meowing, okay that's all good.

929
00:42:28,596 --> 00:42:31,956
If I now try to touch him,
[noise] bad things happen

930
00:42:32,646 --> 00:42:35,286
and then he'll go back
ultimately to meowing.

931
00:42:35,286 --> 00:42:37,066
Okay so now we have this ability

932
00:42:37,066 --> 00:42:39,726
to do things conditionally
inside of loops.

933
00:42:39,926 --> 00:42:44,066
So already we seem to
have a fairly rich amount

934
00:42:44,066 --> 00:42:45,136
of expressiveness.

935
00:42:45,316 --> 00:42:48,296
We can kind of think about doing
something, like I did last night

936
00:42:48,326 --> 00:42:49,206
when putting that together.

937
00:42:49,206 --> 00:42:52,076
I wanted the cat to just meow,
meow, meow and then I wanted

938
00:42:52,076 --> 00:42:54,726
to do something optionally
so there you have loop.

939
00:42:54,726 --> 00:42:56,406
You have condition
sandwiched in between.

940
00:42:56,406 --> 00:42:57,666
Think about what
statements you want.

941
00:42:57,856 --> 00:42:59,136
And wala, we have that effect.

942
00:42:59,676 --> 00:43:00,956
Well what more can we do?

943
00:43:00,956 --> 00:43:02,016
Well we have variables.

944
00:43:02,016 --> 00:43:05,496
Now variables are useful,
certainly in the context of Josh

945
00:43:05,496 --> 00:43:07,016
where he had to maintain state.

946
00:43:07,016 --> 00:43:09,486
He had to remember how
many socks he had on.

947
00:43:09,736 --> 00:43:12,056
Otherwise he would just
keep putting on socks,

948
00:43:12,056 --> 00:43:13,626
putting on socks,
putting on socks.

949
00:43:13,896 --> 00:43:17,486
The use of that variable allowed
him to short circuit that loop.

950
00:43:17,686 --> 00:43:20,506
And terminate as soon as he
reached some happy place.

951
00:43:20,766 --> 00:43:22,586
Well what about these
two programs here?

952
00:43:22,886 --> 00:43:26,956
Count 1.sb is at left so when
green flag is clicked set a

953
00:43:27,136 --> 00:43:29,586
counter, that's apparently
a variable that I chose.

954
00:43:29,986 --> 00:43:32,286
Better named than X.
Better named than Y,

955
00:43:32,286 --> 00:43:34,606
which though you might have
used in algebra, really isn't

956
00:43:34,606 --> 00:43:35,466
that useful when you're trying

957
00:43:35,466 --> 00:43:37,116
to remember what
your own program does

958
00:43:37,116 --> 00:43:39,876
so again a good matter of
style is pick variable names

959
00:43:39,876 --> 00:43:41,496
that say something interesting.

960
00:43:41,796 --> 00:43:45,946
So say counter to zero, forever,
say counter for 2 seconds.

961
00:43:46,226 --> 00:43:48,666
Then wait a second,
change counter by 1.

962
00:43:48,836 --> 00:43:51,076
So this is Scratch's
approach to plus plus.

963
00:43:51,266 --> 00:43:56,186
Alright so in count 1.sb
we have the following,

964
00:43:56,606 --> 00:44:06,006
count 1 [silence] no sound but
I thought it was very clever too

965
00:44:06,006 --> 00:44:07,716
that the sheep was
doing the counting.

966
00:44:08,806 --> 00:44:10,396
[laughter] Ha, ha,
ha, see, geek humor.

967
00:44:10,896 --> 00:44:13,716
Alright so things are more
fun with sound so we'll kill

968
00:44:13,716 --> 00:44:16,476
that thing but it's going
to go on up at least

969
00:44:16,476 --> 00:44:18,236
until we hit some upper bounds.

970
00:44:18,266 --> 00:44:21,006
Can you anticipate maybe
if you're coming at this

971
00:44:21,006 --> 00:44:22,586
with a background
in some programming,

972
00:44:22,586 --> 00:44:28,576
what could still
happen that's bad here?

973
00:44:28,576 --> 00:44:28,856
[inaudible audience
comment] Sorry,

974
00:44:28,856 --> 00:44:30,636
might only be using 8 bits.

975
00:44:30,636 --> 00:44:33,366
So actually roll back mentally
to Wednesday when we talked

976
00:44:33,366 --> 00:44:34,656
about representing numbers

977
00:44:34,896 --> 00:44:37,866
and by default I kept using
8 bits to represent numbers.

978
00:44:38,006 --> 00:44:41,626
Well if you have 8 bits, each
one can be a zero or a 1,

979
00:44:41,896 --> 00:44:45,386
that means you have two possible
values times 2 here, 2, 2, 2, 2,

980
00:44:45,386 --> 00:44:47,166
2, so that's 2 to the eighth,

981
00:44:47,376 --> 00:44:52,466
right so that meant we had
256 possible values you can

982
00:44:52,466 --> 00:44:53,636
represent with 8 bits.

983
00:44:53,906 --> 00:44:57,396
The number zero then 1 then
2 then 3 then 4 then 5,

984
00:44:57,396 --> 00:44:59,166
all the way up to 255.

985
00:44:59,356 --> 00:45:01,566
And again, why 255, not 256?

986
00:45:01,706 --> 00:45:03,706
Computer scientists tend
to start counting at zero

987
00:45:03,706 --> 00:45:05,626
so we spend one of our
encodings that way.

988
00:45:05,906 --> 00:45:10,216
So a problem that might arise
here is MIT probably made a

989
00:45:10,216 --> 00:45:12,326
judgment call somewhere
underneath the hood

990
00:45:12,326 --> 00:45:15,156
when they implemented Scratch
itself where they decided,

991
00:45:15,156 --> 00:45:18,476
you know what, no 12 year old
is gonna count up to 4 billion.

992
00:45:18,476 --> 00:45:20,716
We don't need 32 bit integers.

993
00:45:20,906 --> 00:45:23,106
We don't need integers
that are represented

994
00:45:23,106 --> 00:45:27,806
by 32 individual bits, maybe
just 8 or maybe 16 but they had

995
00:45:27,806 --> 00:45:28,946
to make a judgment call.

996
00:45:29,116 --> 00:45:32,056
And just as, think back
to the Y2K problem,

997
00:45:32,056 --> 00:45:33,026
what was that problem?

998
00:45:33,236 --> 00:45:36,176
Well the world had decided,
in the 1970's give or take,

999
00:45:36,716 --> 00:45:39,036
you know this codes never going
to be used 30 years from now,

1000
00:45:39,036 --> 00:45:41,556
let's just use 2 digits
to represent the year

1001
00:45:41,696 --> 00:45:44,366
and then we all, you may
recall, bad things happened

1002
00:45:44,366 --> 00:45:46,326
or bad things were
predicted to happen.

1003
00:45:46,326 --> 00:45:47,816
Certainly expensive
things happen

1004
00:45:48,056 --> 00:45:49,456
when we roll back
from that mistake.

1005
00:45:49,456 --> 00:45:52,916
So same thing here, let's
not spend the time figuring

1006
00:45:52,916 --> 00:45:55,746
out just how high this sheep
can count but that too is going

1007
00:45:55,746 --> 00:45:59,636
to be an issue we trip over when
you start coding in C and in PHP

1008
00:45:59,636 --> 00:46:02,166
and in Java Script where
you have much more control

1009
00:46:02,166 --> 00:46:05,536
over the representation
of numbers and of data.

1010
00:46:05,806 --> 00:46:08,316
What about count 2.sb here?

1011
00:46:08,636 --> 00:46:12,076
So when green flag clicked,
set number to a random number,

1012
00:46:12,566 --> 00:46:14,806
say the number and then
if that number is less

1013
00:46:14,896 --> 00:46:16,296
than 6, play sheep.

1014
00:46:16,586 --> 00:46:19,926
So in English, in laymen's
terms, this thing is going

1015
00:46:19,926 --> 00:46:21,536
to something 50 percent
of the time

1016
00:46:21,776 --> 00:46:23,726
but at least now we're going
to have some introspection,

1017
00:46:23,726 --> 00:46:27,426
we're gonna see why it's
actually playing a sound or not

1018
00:46:27,656 --> 00:46:29,446
because it's going to tell
me what the number is,

1019
00:46:29,446 --> 00:46:31,846
whereas before I
simply had to guess.

1020
00:46:31,846 --> 00:46:36,206
So this is count 2, still
involving our little sheep here.

1021
00:46:36,206 --> 00:46:42,076
If I hit play 6, nothing happens
because 6 is not less than 6 .

1022
00:46:42,216 --> 00:46:46,596
Play 10, still nothing,
7, still nothing,

1023
00:46:47,596 --> 00:46:50,726
see this is where we
could be tossing a coin

1024
00:46:50,726 --> 00:46:52,516
and it might be coming
up heads that many times,

1025
00:46:52,516 --> 00:46:58,256
hopefully it will go [noise]
the other direction 50 percent

1026
00:46:58,256 --> 00:46:58,666
of the time.

1027
00:46:58,666 --> 00:46:59,916
So if do that an infinite number

1028
00:46:59,916 --> 00:47:02,966
of times hopefully the math
would in fact work out.

1029
00:47:03,146 --> 00:47:04,376
Alright so those are variables.

1030
00:47:04,766 --> 00:47:07,896
Well variables are useful
but only when you want

1031
00:47:07,896 --> 00:47:09,666
to store one piece
of information.

1032
00:47:10,046 --> 00:47:12,536
But you could just use
then multiple variables

1033
00:47:12,536 --> 00:47:14,496
to store multiple
pieces of information.

1034
00:47:14,756 --> 00:47:16,796
But if you think now
even in terms of Scratch

1035
00:47:17,036 --> 00:47:19,206
that each variable right
now is its own block.

1036
00:47:19,516 --> 00:47:24,506
Imagine how you might implement,
maybe a game like some kind

1037
00:47:24,506 --> 00:47:26,676
of RPG game where
you're a character,

1038
00:47:26,676 --> 00:47:28,656
ala Super Mario Brothers
and you have to go

1039
00:47:28,656 --> 00:47:29,796
around collecting things.

1040
00:47:29,796 --> 00:47:30,686
Any game like Zelda.

1041
00:47:30,756 --> 00:47:33,886
Any game where you
go, or even Halo.

1042
00:47:33,886 --> 00:47:36,116
Anytime you pick up
something and have to add it

1043
00:47:36,116 --> 00:47:39,286
to your inventory, it
would probably be a fairly

1044
00:47:39,286 --> 00:47:42,906
unmanageable program if you had
to have a separate variable,

1045
00:47:42,906 --> 00:47:46,606
a separately named variable
for every possible possession

1046
00:47:46,846 --> 00:47:50,006
that someone in a game
might need to carry around.

1047
00:47:50,006 --> 00:47:52,986
It would be a lot easier if you
could sort of have just a bucket

1048
00:47:53,156 --> 00:47:55,966
or a satchel, whatever the
analog is in the actual game

1049
00:47:56,146 --> 00:47:58,496
that can contain
zero or more items.

1050
00:47:58,926 --> 00:48:01,346
And that data structure,
as we'll start calling it

1051
00:48:01,346 --> 00:48:03,316
in programming, is
what's called an array,

1052
00:48:03,676 --> 00:48:07,686
sometimes called a vector,
also we'll see fancier versions

1053
00:48:07,686 --> 00:48:10,306
of this ultimately called
lists and trees but more

1054
00:48:10,306 --> 00:48:11,216
on that in the time to come.

1055
00:48:11,506 --> 00:48:14,796
So an array in Scratch
gives us this capability.

1056
00:48:14,796 --> 00:48:16,286
We give the array a name.

1057
00:48:16,966 --> 00:48:19,786
It's called a list in
the context of Scratch.

1058
00:48:19,876 --> 00:48:21,606
And then we can use
this block at top.

1059
00:48:21,736 --> 00:48:23,056
Add something to inventory.

1060
00:48:23,056 --> 00:48:25,146
Delete 1 from inventory.

1061
00:48:25,146 --> 00:48:29,056
Insert thing, a string, at
location 1 of inventory.

1062
00:48:29,366 --> 00:48:31,306
In other words, and we
won't spend too much time

1063
00:48:31,306 --> 00:48:33,776
on using these now because
you might not need them

1064
00:48:33,936 --> 00:48:35,856
but if you find ultimately
that you kind of need to keep

1065
00:48:35,856 --> 00:48:37,936
to keep an arbitrary amount
of information around,

1066
00:48:38,146 --> 00:48:40,216
well the data structure
you use to make that happen

1067
00:48:40,216 --> 00:48:42,626
in general is this
thing called an array.

1068
00:48:43,396 --> 00:48:45,086
Well let's see this for real.

1069
00:48:45,086 --> 00:48:46,646
So this is actually a program

1070
00:48:46,926 --> 00:48:49,166
that is a role playing
game of sorts.

1071
00:48:49,466 --> 00:48:50,596
It's called Fruit Craft,

1072
00:48:51,176 --> 00:48:53,726
a play on War Craft,
if you're familiar.

1073
00:48:53,956 --> 00:48:57,386
Let me go ahead and full screen
this one here, click play,

1074
00:48:57,386 --> 00:49:00,006
and we have a little
animation it seems.

1075
00:49:00,646 --> 00:49:07,376
Alright you can move
me with the arrow keys.

1076
00:49:07,756 --> 00:49:09,986
So just as our little
DDR game let me move,

1077
00:49:09,986 --> 00:49:13,696
notice and actually the goal
is to look for fruits and so

1078
00:49:13,696 --> 00:49:15,596
like an idiot I kind of
did this the first time.

1079
00:49:16,286 --> 00:49:18,836
Apparently this is a
picture [laughter] of fruit.

1080
00:49:20,346 --> 00:49:22,486
So there's clearly
some bugs if you will

1081
00:49:22,486 --> 00:49:25,746
or at least some disregard for
our 3-dimensional world here.

1082
00:49:26,066 --> 00:49:28,246
[laughter] So I'm
gonna go around

1083
00:49:28,246 --> 00:49:29,686
or over or through the table.

1084
00:49:29,686 --> 00:49:30,836
I'm gonna follow this arrow.

1085
00:49:30,836 --> 00:49:32,566
I'm just using my
arrow keys right now.

1086
00:49:32,956 --> 00:49:34,656
Alright so okay, there we go.

1087
00:49:34,656 --> 00:49:35,886
So now notice the top left.

1088
00:49:36,246 --> 00:49:39,926
Scratch is showing me what
is inside my list, aka array,

1089
00:49:40,216 --> 00:49:43,246
now I have cherries and
orange, this looks interesting.

1090
00:49:43,246 --> 00:49:44,856
Let me go here.

1091
00:49:44,856 --> 00:49:47,086
Oh okay so we have,
remember some sensing.

1092
00:49:47,086 --> 00:49:48,236
So remember just as I'm able

1093
00:49:48,236 --> 00:49:50,476
to sense is the Sprite
touching my mouse pointer,

1094
00:49:50,636 --> 00:49:53,486
I can also sense is a Sprite
touching another Sprite?

1095
00:49:53,616 --> 00:49:56,766
Is this person touching the
sign, if so do something.

1096
00:49:56,926 --> 00:49:59,116
And even though we might not
know all the blocks that we need

1097
00:49:59,116 --> 00:50:01,616
to make that happen, it's
these basic fundamentals,

1098
00:50:01,616 --> 00:50:05,366
right an if condition that says
if touching then say and then

1099
00:50:05,366 --> 00:50:06,766
when not touching don't say.

1100
00:50:06,766 --> 00:50:07,666
So hide that text.

1101
00:50:08,266 --> 00:50:11,526
Alright so let me go and pick
up these things, pineapple,

1102
00:50:11,526 --> 00:50:13,766
bananas, another apple,

1103
00:50:13,766 --> 00:50:16,166
hopefully this is the right
place, okay, interesting.

1104
00:50:17,386 --> 00:50:17,856
Great job.

1105
00:50:17,886 --> 00:50:21,786
You brought, cherries, orange,
bananas, pineapple, apple,

1106
00:50:21,976 --> 00:50:22,916
here's your fruit platter.

1107
00:50:24,056 --> 00:50:29,036
[laughter] Okay so simple
but again demonstrates

1108
00:50:29,166 --> 00:50:31,826
where you might, in a
fairly compelling context,

1109
00:50:31,926 --> 00:50:35,126
actually need some kind of
variable amount of storage.

1110
00:50:35,276 --> 00:50:37,596
Well let's make this
more sophisticated still.

1111
00:50:37,796 --> 00:50:40,386
When this is actually a topic
that you can't traditionally get

1112
00:50:40,486 --> 00:50:43,886
to in the first week of a course
let alone an entire semester

1113
00:50:43,886 --> 00:50:46,476
sometimes, this notion of
threading because a lot

1114
00:50:46,476 --> 00:50:50,166
of languages, whether it's C or
Java, really don't make it easy

1115
00:50:50,606 --> 00:50:53,336
of what we're about to do and
frankly what we've been doing,

1116
00:50:53,646 --> 00:50:56,606
which is having multiple
things happen at once.

1117
00:50:57,076 --> 00:50:59,686
So the program I wrote earlier
that was very underwhelming,

1118
00:50:59,686 --> 00:51:01,516
hello world, just
printing it to the screen?

1119
00:51:01,906 --> 00:51:03,426
That's kind of the
only thing it did.

1120
00:51:03,426 --> 00:51:06,356
I could add another print F
line, another print F line,

1121
00:51:06,356 --> 00:51:10,376
another print F line but those
are all gonna happen in series,

1122
00:51:10,426 --> 00:51:12,066
not in parallel at
the same time.

1123
00:51:12,066 --> 00:51:13,866
I'm gonna get one
output, one output,

1124
00:51:13,866 --> 00:51:17,826
one output so what's
nice about Scratch is

1125
00:51:17,826 --> 00:51:19,346
that you can have
the cat moving,

1126
00:51:19,346 --> 00:51:20,876
you can have a sheep
doing something,

1127
00:51:20,876 --> 00:51:23,206
you can have this
character interacting

1128
00:51:23,206 --> 00:51:24,486
with the guy behind the counter,

1129
00:51:24,626 --> 00:51:26,636
you have a multithreaded
environment.

1130
00:51:26,636 --> 00:51:27,706
Well what is a thread?

1131
00:51:27,846 --> 00:51:30,996
A thread is like a miniature
program that's running inside

1132
00:51:30,996 --> 00:51:33,046
the context of a larger program

1133
00:51:33,286 --> 00:51:36,366
and they can operate
what essentially looks

1134
00:51:36,366 --> 00:51:37,996
like simultaneously.

1135
00:51:38,196 --> 00:51:40,496
Now this is a bit of a white
lie because ultimately,

1136
00:51:40,496 --> 00:51:42,106
and we'll talk about this
towards the end of the course,

1137
00:51:42,596 --> 00:51:46,046
many computers have just one
CPU, central processing unit,

1138
00:51:46,046 --> 00:51:47,276
the brains of the computer.

1139
00:51:47,276 --> 00:51:49,886
When it says Intel inside
it means there's an Intel

1140
00:51:49,886 --> 00:51:50,936
CPU inside.

1141
00:51:51,196 --> 00:51:53,836
Well if the computer only
has one brain or one CPU,

1142
00:51:53,836 --> 00:51:58,796
the reality is that computer
can only do one thing at a time.

1143
00:51:59,246 --> 00:52:01,216
But the neat thing is
when you have a gigahertz,

1144
00:52:01,626 --> 00:52:02,906
so in other words a hertz,

1145
00:52:03,186 --> 00:52:05,346
something happening once
per second and you can think

1146
00:52:05,346 --> 00:52:07,986
of there being a clock
inside of your own CPU

1147
00:52:08,226 --> 00:52:11,026
so if you have a one gigahertz
CPU there's a little clock

1148
00:52:11,026 --> 00:52:13,986
or a little crystal that's
oscillating a billion times,

1149
00:52:14,086 --> 00:52:17,806
giga meaning billion, a
billion times per second.

1150
00:52:18,016 --> 00:52:20,166
Well my God, if you can
do something that fast,

1151
00:52:20,436 --> 00:52:23,176
clicking your fingers a
billion times per second,

1152
00:52:23,366 --> 00:52:25,886
surely we can dupe
humans into thinking

1153
00:52:26,116 --> 00:52:28,726
that two things are
happening simultaneously.

1154
00:52:28,986 --> 00:52:31,876
So if you've ever run AOL
instant messenger in a window

1155
00:52:32,096 --> 00:52:35,606
and also Microsoft Word and a
browser window and any number

1156
00:52:35,606 --> 00:52:37,856
of other programs,
you are multitasking.

1157
00:52:38,136 --> 00:52:40,086
But realize that you
don't have to quit one

1158
00:52:40,086 --> 00:52:41,666
of those programs
to load the other.

1159
00:52:41,666 --> 00:52:43,856
You're running them
all simultaneously.

1160
00:52:44,006 --> 00:52:45,286
But what Windows is doing,

1161
00:52:45,286 --> 00:52:48,216
what Mac OS is doing
is just giving everyone

1162
00:52:48,216 --> 00:52:51,446
of those programs only a
split second of computation

1163
00:52:51,446 --> 00:52:53,486
in the CPU, a split
second of attention

1164
00:52:53,696 --> 00:52:55,686
but because it does it
here, here, here, here,

1165
00:52:55,686 --> 00:52:58,046
here then here, here,
here, here, here where each

1166
00:52:58,046 --> 00:52:59,846
of these things has a
different program running

1167
00:52:59,846 --> 00:53:02,996
in your task bar, you the
human, being the slowest piece

1168
00:53:02,996 --> 00:53:05,316
of this puzzle, have
the illusion of oh,

1169
00:53:05,316 --> 00:53:07,356
I am running multiple
programs at once.

1170
00:53:07,596 --> 00:53:10,526
Those of you with iPhones might
be somewhat annoyed these days

1171
00:53:10,526 --> 00:53:13,156
that Apple doesn't support
background applications,

1172
00:53:13,426 --> 00:53:16,476
at least written by other
people besides Apple.

1173
00:53:16,476 --> 00:53:19,846
In other words you can't run
one program and then use another

1174
00:53:19,846 --> 00:53:21,866
at the same time, if
they're written by people

1175
00:53:21,866 --> 00:53:23,236
in the app store and that's

1176
00:53:23,236 --> 00:53:27,396
because Apple has essentially
said users may not run multiple

1177
00:53:27,396 --> 00:53:30,166
threads simultaneously
unless we, Apple,

1178
00:53:30,286 --> 00:53:32,006
implemented those
threads ourselves,

1179
00:53:32,006 --> 00:53:33,146
unless we wrote the code.

1180
00:53:33,386 --> 00:53:35,886
Now this is partly because the
battery life of this thing kind

1181
00:53:35,886 --> 00:53:39,326
of sucks right now and if you
allow people to write programs

1182
00:53:39,326 --> 00:53:40,356
that run in the background,

1183
00:53:41,006 --> 00:53:42,856
probably the concern
they have is

1184
00:53:42,856 --> 00:53:45,206
that you're gonna drain people's
batteries and most people

1185
00:53:45,206 --> 00:53:47,646
out there don't understand what
a thread is and they don't care

1186
00:53:47,646 --> 00:53:49,896
and they shouldn't have to
care but that's Apple's way,

1187
00:53:49,896 --> 00:53:52,586
probably, at least in part,
of dealing with that issue

1188
00:53:52,586 --> 00:53:55,086
of multithreading, being
expensive in terms of battery.

1189
00:53:55,356 --> 00:53:56,556
So here is a thread.

1190
00:53:56,776 --> 00:53:59,486
So move 1.sb tells us
to do the following,

1191
00:53:59,486 --> 00:54:03,126
when the green flag
is clicked, go to xy.

1192
00:54:03,486 --> 00:54:06,336
So it turns out this whole
time that stage can be modeled

1193
00:54:06,336 --> 00:54:08,506
and in fancy terms is
like a Cartesian plane

1194
00:54:08,506 --> 00:54:10,486
where you have X
coordinates and Y, right,

1195
00:54:10,486 --> 00:54:11,556
just like in mathematics.

1196
00:54:11,826 --> 00:54:14,816
So zero zero is pixel
location zero zero,

1197
00:54:14,816 --> 00:54:17,546
the little dot somewhere
on that stage points

1198
00:54:17,546 --> 00:54:20,266
in the direction 90 degrees
so that's either here, here,

1199
00:54:20,266 --> 00:54:22,416
here or here, we'll see
once we orient ourselves

1200
00:54:22,416 --> 00:54:24,286
and then forever
do the following.

1201
00:54:24,636 --> 00:54:27,156
If touching the edge,
play sound,

1202
00:54:27,186 --> 00:54:29,446
this could be interesting,
until done.

1203
00:54:29,586 --> 00:54:34,476
Turn 180 degrees, move 5 steps
and then do it again and again

1204
00:54:34,476 --> 00:54:37,476
because it's inside the forever
loop so this is move 1.sb.

1205
00:54:37,836 --> 00:54:44,496
So let me go down to this,
move 1.sb, I'll go ahead

1206
00:54:44,496 --> 00:54:46,976
and full screen it
again, hit play.

1207
00:54:47,516 --> 00:54:56,646
[noise]

1208
00:54:57,146 --> 00:54:59,286
[background noise] I
don't know if this is

1209
00:54:59,286 --> 00:55:03,036
like a pedagogical technique or
just a bad habit but it seems

1210
00:55:03,036 --> 00:55:06,016
if you right program
loud and annoying,

1211
00:55:06,016 --> 00:55:07,756
people remember them so be it.

1212
00:55:07,756 --> 00:55:10,026
So this thing, this
is the Sprite.

1213
00:55:10,236 --> 00:55:12,286
Notice I've changed
his costume, how?

1214
00:55:12,286 --> 00:55:14,646
Well I probably went up
here, I clicked costumes,

1215
00:55:14,646 --> 00:55:17,436
I fussed around with paint or
importing a JPEG or whatever,

1216
00:55:17,656 --> 00:55:19,356
some uninteresting
details intellectually

1217
00:55:19,356 --> 00:55:21,666
but you're nonetheless welcome
to play with and procrastinate

1218
00:55:21,666 --> 00:55:24,146
with for project zero
but I've made a duck

1219
00:55:24,146 --> 00:55:26,676
that orients himself 90
degrees and he starts moving

1220
00:55:26,916 --> 00:55:29,796
so this notion of walking is
really about moving some number

1221
00:55:29,796 --> 00:55:32,066
of steps, some number of
pixels and then notice

1222
00:55:32,066 --> 00:55:34,686
because I'm doing this again
and again and again forever,

1223
00:55:34,856 --> 00:55:37,136
I'm constantly checking,
am I touching the edge,

1224
00:55:37,136 --> 00:55:38,106
am I touching the edge?

1225
00:55:38,106 --> 00:55:42,306
If so, yell, then turn
yourself around and do it again.

1226
00:55:42,596 --> 00:55:44,596
So again, not a very
interactive program

1227
00:55:44,596 --> 00:55:46,776
but all these simple
puzzle pieces,

1228
00:55:46,776 --> 00:55:48,886
all these simple building
blocks we've been looking

1229
00:55:49,056 --> 00:55:53,016
at are really what comprised
Ann's DDR like game and Andrews,

1230
00:55:53,016 --> 00:55:54,246
Raining Men and some

1231
00:55:54,246 --> 00:55:56,446
of the other more sophisticated
programs that you'll write.

1232
00:55:56,666 --> 00:55:59,026
So here's one that
actually is multithreaded.

1233
00:55:59,026 --> 00:56:01,246
That was just one duck
doing his own thing.

1234
00:56:01,246 --> 00:56:04,946
Now we'll see a bird and a
cat, each of which is a Sprite

1235
00:56:05,176 --> 00:56:07,816
in Scratch speak and each
of which has its own script.

1236
00:56:08,096 --> 00:56:11,086
So notice in this program
move 2.sp, I'm actually going

1237
00:56:11,346 --> 00:56:15,166
to have 2 different
scripts, each associated

1238
00:56:15,166 --> 00:56:16,076
with a different Sprite.

1239
00:56:16,346 --> 00:56:19,296
So I've highlighted cat at the
moment, here is the cat script.

1240
00:56:19,426 --> 00:56:21,696
If I click bird, here
is the bird script

1241
00:56:21,696 --> 00:56:23,106
and they are clearly different.

1242
00:56:23,106 --> 00:56:24,446
Well how are they different?

1243
00:56:24,446 --> 00:56:25,756
Well let's look at
the cat first.

1244
00:56:26,116 --> 00:56:31,246
The cat is doing this, go to
negative 160, negative 160.

1245
00:56:31,246 --> 00:56:33,976
Why? Well again I was kind
of fussing around one day

1246
00:56:33,976 --> 00:56:36,626
and I decided let's have the
cat start there specifically,

1247
00:56:36,626 --> 00:56:37,846
trial and error in this case.

1248
00:56:38,216 --> 00:56:40,406
Point in direction,
well I wanted this demo

1249
00:56:40,406 --> 00:56:41,876
to be a little different
each time,

1250
00:56:41,876 --> 00:56:43,516
otherwise it's really
uninteresting

1251
00:56:43,716 --> 00:56:46,896
so pick a random degree
between 91 and 179,

1252
00:56:47,186 --> 00:56:50,036
then forever do the
following, if touching bird,

1253
00:56:50,286 --> 00:56:54,036
here's where it gets fun, play
sound, that particular sound,

1254
00:56:54,036 --> 00:56:56,086
line 4 and then stop the script.

1255
00:56:56,086 --> 00:56:59,216
In other words, game over is the
implication there, exit quit.

1256
00:56:59,676 --> 00:57:03,266
Otherwise, actually I
misspoke, it's not an else,

1257
00:57:03,536 --> 00:57:06,106
it's not an otherwise
but no matter what,

1258
00:57:06,106 --> 00:57:08,896
point yourself toward the
bird and move one step.

1259
00:57:09,146 --> 00:57:10,806
So in other words, if
you're touching the bird,

1260
00:57:11,266 --> 00:57:12,956
play the sound and
quit, game over.

1261
00:57:12,956 --> 00:57:15,476
No matter what though, proceed

1262
00:57:15,556 --> 00:57:18,196
to point toward the
bird and move one step.

1263
00:57:18,196 --> 00:57:20,276
And actually it is
implicitly an if else

1264
00:57:20,276 --> 00:57:22,686
because of the termination
so I retract my retraction.

1265
00:57:23,056 --> 00:57:24,766
So, let's take a
look at the bird.

1266
00:57:24,766 --> 00:57:25,666
What is the bird doing?

1267
00:57:25,666 --> 00:57:27,066
The bird's not doing
all that much.

1268
00:57:27,066 --> 00:57:29,876
The bird starts at a hard
coded place and I only did this

1269
00:57:29,876 --> 00:57:31,346
because it would
be really stupid

1270
00:57:31,346 --> 00:57:35,526
if the bird might sudo randomly
start on top of the cat and then

1271
00:57:35,526 --> 00:57:36,716
if the game has no point.

1272
00:57:36,976 --> 00:57:39,176
So I go to this location,
which I know is different

1273
00:57:39,176 --> 00:57:43,066
from the cat, point in 45 degree
direction and then forever.

1274
00:57:43,066 --> 00:57:44,076
Now notice what I did.

1275
00:57:44,076 --> 00:57:46,316
It turns out with Boolean
Expressions not only can you

1276
00:57:46,316 --> 00:57:49,006
check for true or false,
you can flip the meaning

1277
00:57:49,156 --> 00:57:51,876
so there's a block called
not which inverts meaning,

1278
00:57:51,876 --> 00:57:53,886
which is good because
I want to do forever,

1279
00:57:54,196 --> 00:57:58,486
the following forever if I'm not
touching the cat, move 3 steps

1280
00:57:58,486 --> 00:58:00,016
but if I'm on the edge, bounce.

1281
00:58:00,016 --> 00:58:03,436
So you'll also see in
Scratch, for user friendliness,

1282
00:58:03,796 --> 00:58:07,806
sometimes they merge the notion
of a condition and a statement

1283
00:58:07,806 --> 00:58:11,256
like this so if on edge
bounce, just happens

1284
00:58:11,256 --> 00:58:12,636
to be consolidated into 2.

1285
00:58:12,636 --> 00:58:13,356
So let's play.

1286
00:58:13,416 --> 00:58:15,076
I'll go ahead and
full screen it.

1287
00:58:15,496 --> 00:58:17,086
I'll go ahead and
click the green flag.

1288
00:58:17,766 --> 00:58:21,126
And notice this isn't random.

1289
00:58:21,126 --> 00:58:23,446
The cat is again
point toward bird.

1290
00:58:23,446 --> 00:58:26,546
The bird is moving around
somewhat randomly or [noise]

1291
00:58:26,546 --> 00:58:32,416
at least, alright so kind of sad
but kind of clever nonetheless,

1292
00:58:32,536 --> 00:58:33,996
at least programmatically.

1293
00:58:34,226 --> 00:58:36,476
So what more is there?

1294
00:58:36,476 --> 00:58:40,156
Well what about if we do
something like this interesting?

1295
00:58:40,156 --> 00:58:44,666
So this example, as we'll see,
not only uses multiple threads,

1296
00:58:44,666 --> 00:58:46,546
multiple scripts
happening in lock step

1297
00:58:46,746 --> 00:58:50,346
but I'm also using a variable to
maintain state and essentially

1298
00:58:50,346 --> 00:58:52,526
to communicate across
the two Sprites.

1299
00:58:52,526 --> 00:58:54,516
And this is useful if
you effectively want

1300
00:58:54,516 --> 00:58:55,866
to pass information
between the two.

1301
00:58:55,866 --> 00:58:59,596
So let me go to version
10 of my hi program,

1302
00:59:00,086 --> 00:59:03,776
that's not in this
folder, interesting.

1303
00:59:04,326 --> 00:59:10,776
Alright well rather than drag
our feet, well let's go ahead

1304
00:59:10,776 --> 00:59:14,366
and do, well you know what,
we're out of tape anyway.

1305
00:59:14,496 --> 00:59:16,816
Let's go ahead and take a 5
minute break, we'll change tapes

1306
00:59:16,816 --> 00:59:18,026
and then resume once
I've found this file.

1307
00:59:18,026 --> 00:59:26,716
[background noise] Alright so
version 10 has just gone missing

1308
00:59:26,716 --> 00:59:29,186
so fortunately we have
the source code here.

1309
00:59:29,476 --> 00:59:31,486
On the left hand side
we have a script,

1310
00:59:31,616 --> 00:59:33,976
a Sprite that just says
forever do the following,

1311
00:59:33,976 --> 00:59:36,596
if muted equals zero,
play sound sea lion,

1312
00:59:37,336 --> 00:59:40,746
think oh hi for 2 seconds,
then wait, then repeat

1313
00:59:40,746 --> 00:59:42,806
and then notice what the
right hand side is doing

1314
00:59:42,806 --> 00:59:44,366
and this is really the
only take away here.

1315
00:59:44,366 --> 00:59:46,926
Giving ourselves a mean of
communication between Sprites.

1316
00:59:47,186 --> 00:59:49,616
One the right hand side I have
this variable called muted.

1317
00:59:49,816 --> 00:59:51,146
I set muted to zero.

1318
00:59:51,246 --> 00:59:51,956
That is false.

1319
00:59:51,956 --> 00:59:53,886
It is not muted,
is the implication

1320
00:59:54,046 --> 00:59:55,346
and then forever
do the following.

1321
00:59:55,396 --> 00:59:58,296
If the key space pressed,
then check the following.

1322
00:59:58,296 --> 01:00:01,846
If it's muted, set muted to
1, else set muted to zero.

1323
01:00:02,066 --> 01:00:03,286
So the right hand side,

1324
01:00:03,656 --> 01:00:06,186
even though there's some new
blocks there, what is that doing

1325
01:00:06,186 --> 01:00:08,576
in laymen's terms, the
right hand side script?

1326
01:00:09,336 --> 01:00:10,546
It's just [inaudible
audience comment],

1327
01:00:10,786 --> 01:00:14,846
yeah it's just toggling the
state of this variable, right?

1328
01:00:14,846 --> 01:00:15,746
We have a forever loop

1329
01:00:15,746 --> 01:00:17,466
which just means forever
check the following.

1330
01:00:17,466 --> 01:00:18,426
Is the space bar pressed?

1331
01:00:18,426 --> 01:00:20,636
If so flip the value of muted.

1332
01:00:20,796 --> 01:00:22,906
So the goal of this
exercise here was again

1333
01:00:22,906 --> 01:00:24,256
to have a little
fun playing sound

1334
01:00:24,256 --> 01:00:26,416
but when I hit the space
bar it either mutes

1335
01:00:26,746 --> 01:00:29,926
or effectively unmutes,
what, unmutes the other guy

1336
01:00:30,066 --> 01:00:32,306
because the other guy,
because he's also happening

1337
01:00:32,306 --> 01:00:32,876
in lock step.

1338
01:00:32,876 --> 01:00:34,766
Whether he's a cat or a bird,
it really doesn't matter

1339
01:00:34,766 --> 01:00:37,696
in this case what the Sprite
is, who's using this script,

1340
01:00:37,966 --> 01:00:41,316
the left hand side guy is only
going to play the sound sea lion

1341
01:00:41,416 --> 01:00:44,376
if that value of muted is zero.

1342
01:00:44,576 --> 01:00:47,156
So in other words, if I hit
the space, I make him bark.

1343
01:00:47,386 --> 01:00:49,586
If I hit it again,
it makes him stop

1344
01:00:49,586 --> 01:00:51,296
and this just goes
on in perpetuity.

1345
01:00:51,526 --> 01:00:53,976
So it's not such a loss
if we have that one here.

1346
01:00:53,976 --> 01:00:55,816
This one's a little
more interesting anyway.

1347
01:00:56,146 --> 01:00:58,776
So I'll go ahead and beat
myself up here, if you will.

1348
01:00:59,036 --> 01:01:01,326
This is actually
borrowed from a version

1349
01:01:01,326 --> 01:01:02,606
of this program someone
else wrote,

1350
01:01:02,606 --> 01:01:04,176
it wasn't called
David at the time.

1351
01:01:04,176 --> 01:01:06,246
I took some liberties since I
didn't think it would be the

1352
01:01:06,246 --> 01:01:08,906
friendliest thing to have our
student punch him in the face

1353
01:01:09,286 --> 01:01:11,946
so I swapped in my own face
here and now think about it

1354
01:01:11,946 --> 01:01:13,866
for just a moment,
what's probably going

1355
01:01:13,866 --> 01:01:15,246
on underneath the hood here?

1356
01:01:15,576 --> 01:01:19,156
So I'm apparently moving back
and forth, as any boxer might

1357
01:01:19,456 --> 01:01:23,026
and if I use my arrow keys
here I can start punching

1358
01:01:23,366 --> 01:01:23,816
and missing.

1359
01:01:24,256 --> 01:01:27,426
[laughter] But there we
have here and actually

1360
01:01:27,426 --> 01:01:30,536
if we turn this feature
here on, notice too,

1361
01:01:31,116 --> 01:01:39,046
[music] notice my face is
clearly changing color.

1362
01:01:45,266 --> 01:01:48,166
[music and laughter]
So what's going on?

1363
01:01:48,166 --> 01:01:50,636
Well we don't have to even
dissect all the pieces cause

1364
01:01:50,636 --> 01:01:52,806
for the most part we
presented all the fundamentals.

1365
01:01:53,066 --> 01:01:55,576
There's more than just 1
script now, more than just 2.

1366
01:01:55,576 --> 01:01:58,246
There's actually 4 that
belong to the left glove.

1367
01:01:58,246 --> 01:02:00,976
It looks like there are as
many that belong to the right.

1368
01:02:01,236 --> 01:02:05,206
My own self has this slightly
larger one but it all boils

1369
01:02:05,206 --> 01:02:07,136
down if you start to
focus on the specifics,

1370
01:02:07,136 --> 01:02:09,576
and again all this code will
be online, it just boils

1371
01:02:09,576 --> 01:02:13,186
down to making the computer do
what you want it to do and each

1372
01:02:13,186 --> 01:02:15,926
of these things are Sprites
therefore they can detect

1373
01:02:15,926 --> 01:02:18,846
themselves, if touching
one Sprite, do this and so

1374
01:02:18,846 --> 01:02:20,676
with these very simple
building blocks,

1375
01:02:20,676 --> 01:02:22,616
almost all of which we've
rattled off already,

1376
01:02:22,616 --> 01:02:25,206
can you do things that are
increasingly compelling

1377
01:02:25,206 --> 01:02:28,176
and much more interesting
than just hello world.

1378
01:02:28,376 --> 01:02:32,126
Well this is one final
functionality that's provided

1379
01:02:32,126 --> 01:02:33,006
by Scratch.

1380
01:02:33,006 --> 01:02:35,966
It is not often found in at
least introductory courses

1381
01:02:36,116 --> 01:02:39,126
and this notion of events, we'll
actually see this in the context

1382
01:02:39,126 --> 01:02:41,566
of web programming later in
the semester with Java Script.

1383
01:02:41,826 --> 01:02:43,966
For C we won't really
play around with this idea

1384
01:02:43,966 --> 01:02:46,976
but this example here
simply has 2 Sprite's,

1385
01:02:46,976 --> 01:02:48,286
each of which has one script,

1386
01:02:48,496 --> 01:02:50,306
but these guys too
can communicate,

1387
01:02:50,436 --> 01:02:52,396
not by using a single variable,

1388
01:02:52,506 --> 01:02:54,696
so when I discussed a moment
ago the notion of muting,

1389
01:02:54,696 --> 01:02:57,486
that's what we'll call a global
variable, global in the sense

1390
01:02:57,486 --> 01:02:59,606
that anyone can use
or manipulate it.

1391
01:02:59,776 --> 01:03:01,936
We'll see that that's not
often the best practice

1392
01:03:02,216 --> 01:03:04,286
but with events can
you do the same thing.

1393
01:03:04,286 --> 01:03:06,806
You can broadcast an
event and the idea is

1394
01:03:06,806 --> 01:03:08,826
that any other Sprite
that wants to listen

1395
01:03:08,826 --> 01:03:11,086
for that event can
somehow respond to it.

1396
01:03:11,316 --> 01:03:15,666
So in this case here, Marko.sb
we have essentially 2 Sprite's

1397
01:03:15,666 --> 01:03:17,226
playing a game with each other.

1398
01:03:17,606 --> 01:03:21,406
If I go ahead and play this,
we'll see on the left hand side,

1399
01:03:21,656 --> 01:03:24,436
Marko is waiting forever
for me to do what?

1400
01:03:25,186 --> 01:03:26,456
Hit the space so I'm
gonna hit the space.

1401
01:03:26,846 --> 01:03:32,156
Marko and now the female says
polo so why did she say that?

1402
01:03:32,156 --> 01:03:33,556
Well let's take a look at her.

1403
01:03:33,556 --> 01:03:37,736
Oh, when I receive event,
say polo for 2 seconds.

1404
01:03:37,946 --> 01:03:40,466
So we have cross
thread communication,

1405
01:03:40,466 --> 01:03:43,506
inter-thread communication,
which is useful, you know yes

1406
01:03:43,576 --> 01:03:46,136
to make this simple example
happen but we offer it

1407
01:03:46,136 --> 01:03:49,066
for today's purposes as a
little tool for ones tool kit,

1408
01:03:49,106 --> 01:03:51,596
so to speak, that if
you need 2 Sprites

1409
01:03:51,596 --> 01:03:53,966
to somehow communicate you've
got a couple of options.

1410
01:03:53,966 --> 01:03:56,766
One, share a variable that
both of them check the value of

1411
01:03:56,766 --> 01:03:58,576
and maybe change
or you have events

1412
01:03:58,836 --> 01:04:00,646
which might be a little
cleaner than that,

1413
01:04:00,646 --> 01:04:02,116
if you want to send
something arbitrary.

1414
01:04:02,336 --> 01:04:05,156
Well it turns out that Scratch
also supports these things

1415
01:04:05,156 --> 01:04:08,596
and in problem said zero,
it already is coming

1416
01:04:08,596 --> 01:04:11,186
in 2 different editions,
standard and hacker.

1417
01:04:11,506 --> 01:04:13,786
So the standard edition is,
you'll see when it goes online

1418
01:04:13,786 --> 01:04:17,316
in PDF form tonight, invites
you to pretty much make anything

1419
01:04:17,316 --> 01:04:21,546
of interest to you so long
as it checks off a few boxes.

1420
01:04:21,546 --> 01:04:22,866
You need to use at
least a variable.

1421
01:04:22,866 --> 01:04:24,726
You need to have at least
a few different Sprites.

1422
01:04:24,726 --> 01:04:25,636
You need to use a sound.

1423
01:04:25,866 --> 01:04:28,786
So we lay some basic ground
rules just to kind of push you

1424
01:04:28,786 --> 01:04:30,326
to do more than just
scratch the surface.

1425
01:04:30,326 --> 01:04:32,916
But if you're really the
type who wants to get his

1426
01:04:32,916 --> 01:04:35,826
or her hands dirty very early
and maybe do have more of it,

1427
01:04:36,056 --> 01:04:38,716
a more comfortable background,
there's these things here.

1428
01:04:38,986 --> 01:04:41,616
So these are sensor boards
that we bought a whole bunch

1429
01:04:41,616 --> 01:04:44,416
of for students in the course
to use and they have buttons,

1430
01:04:44,476 --> 01:04:46,646
they have little sliders
that move left to right.

1431
01:04:46,876 --> 01:04:49,376
They have little inputs that
you can connect alligator clips

1432
01:04:49,476 --> 01:04:51,216
to so that you can
check resistance,

1433
01:04:51,486 --> 01:04:54,396
whether through some physical
medium like yourself, left hand

1434
01:04:54,396 --> 01:04:57,236
and right hand, water,
metal, anything like that.

1435
01:04:57,456 --> 01:04:58,496
There's a light sensor.

1436
01:04:58,496 --> 01:04:59,736
There's an audio sensor.

1437
01:04:59,846 --> 01:05:03,356
And so these things too, very
simple ideas, taking sounds,

1438
01:05:03,476 --> 01:05:05,576
do something with it, you
can actually detect it

1439
01:05:05,576 --> 01:05:07,186
programmatically
within the program.

1440
01:05:07,436 --> 01:05:10,676
So this one is actually a simple
example of precisely this,

1441
01:05:10,756 --> 01:05:12,816
singer.sb, let me go ahead

1442
01:05:12,816 --> 01:05:15,056
and open this one here,
I'll full screen him.

1443
01:05:15,336 --> 01:05:20,076
He doesn't do all that
much and here we go,

1444
01:05:20,076 --> 01:05:23,006
[noise] he responds to sounds.

1445
01:05:23,546 --> 01:05:25,656
Now he also responds to
this and now it just looks

1446
01:05:25,656 --> 01:05:27,706
like I'm talking so it's
not terribly sensitive

1447
01:05:27,706 --> 01:05:33,336
to different types of speech but
we're clearly still responding

1448
01:05:33,336 --> 01:05:35,286
to something a little
binary in nature.

1449
01:05:35,286 --> 01:05:36,776
I wonder how long we
could give lecture like,

1450
01:05:36,776 --> 01:05:39,146
and see how distracting that is.

1451
01:05:39,456 --> 01:05:42,416
[laughter] Alright so
this one, let me go ahead

1452
01:05:42,416 --> 01:05:43,606
and full screen this one.

1453
01:05:44,516 --> 01:05:49,586
[noise]

1454
01:05:50,086 --> 01:05:52,716
We need to raise the
house lights again.

1455
01:05:53,936 --> 01:05:57,736
[background music] Can
we pull them down again?

1456
01:05:59,566 --> 01:06:01,376
So it is in fact a light sensor

1457
01:06:01,376 --> 01:06:04,276
and the game gets a little
freaky when the lights go down.

1458
01:06:04,276 --> 01:06:06,006
That was actually
unintentional the first time.

1459
01:06:06,006 --> 01:06:07,446
Normally I use my
hand but taking

1460
01:06:07,446 --> 01:06:10,166
down Sanders Theatre lights
also works as well there.

1461
01:06:10,436 --> 01:06:11,856
Well we have one other here

1462
01:06:11,856 --> 01:06:15,176
so this one was particularly
sophisticated but again speaks

1463
01:06:15,276 --> 01:06:18,266
to the relative complexity
you can create

1464
01:06:18,266 --> 01:06:21,026
with what is ultimately
quite simple building blocks.

1465
01:06:21,376 --> 01:06:24,486
This is one by David
Woo, now a junior.

1466
01:06:24,486 --> 01:06:26,056
I'm gonna click green flag here.

1467
01:06:27,766 --> 01:06:29,886
[background music] This one
uses the little sensor bar

1468
01:06:30,516 --> 01:06:34,566
to move this guy back and
forth and the goal is I think

1469
01:06:34,596 --> 01:06:37,196
to get the bananas and
not get hit by these,

1470
01:06:37,196 --> 01:06:40,416
the monkey's thumb counter has
just gone up which means we're

1471
01:06:40,416 --> 01:06:43,626
like 174 milliseconds
or whatever, I'm stunned

1472
01:06:43,626 --> 01:06:45,806
and he's not responding
now, notice his eyes.

1473
01:06:45,866 --> 01:06:46,906
So that's bad.

1474
01:06:48,626 --> 01:06:54,016
[noise and laughter] So that
actually use the slider back

1475
01:06:54,016 --> 01:06:55,976
and forth but this one
actually gets some notion

1476
01:06:55,976 --> 01:06:57,326
of some friction coefficient.

1477
01:06:57,546 --> 01:06:59,856
So it's not just going left
and right in response to me.

1478
01:06:59,856 --> 01:07:02,786
He kind of skids out on what is
ultimately supposed to be ice.

1479
01:07:02,786 --> 01:07:04,726
So again, very simple
primitives, right?

1480
01:07:04,726 --> 01:07:08,006
Light on, light off, sound
on, sound off, slider left,

1481
01:07:08,006 --> 01:07:10,426
slider right but even Scratch
can interface with this.

1482
01:07:10,456 --> 01:07:15,346
So realize that for problems
set zero the hacker edition is

1483
01:07:15,346 --> 01:07:20,656
by nature meant to be very, very
accessible and we have over 100

1484
01:07:20,656 --> 01:07:22,736
or 200 of these things out in
the hall that you'll be able

1485
01:07:22,736 --> 01:07:25,516
to collect today and at
any office hours next week

1486
01:07:25,866 --> 01:07:28,466
for problem set zero.

1487
01:07:28,746 --> 01:07:30,566
So this is this program
now that I promised.

1488
01:07:30,566 --> 01:07:32,536
This is the first thing
that I wrote and this is one

1489
01:07:32,536 --> 01:07:35,046
of those either sad or
really impressive stories

1490
01:07:35,046 --> 01:07:38,086
of late Friday night 8
hours straight playing this.

1491
01:07:38,426 --> 01:07:41,056
You'll hear in a moment
that I chose a song

1492
01:07:41,056 --> 01:07:43,696
that is perhaps the most
annoying song in the world

1493
01:07:43,696 --> 01:07:47,516
to play ad nauseum for 8 hours
while you debug your own program

1494
01:07:47,766 --> 01:07:49,906
but this is a little
something called Oscar time

1495
01:07:50,366 --> 01:07:52,066
and its source code
too will be distributed

1496
01:07:52,066 --> 01:07:54,396
on the courses website just
so that you can see a lot

1497
01:07:54,396 --> 01:07:56,246
of different interactions
going on.

1498
01:07:56,526 --> 01:07:58,846
So I stole this actually
from an old piece

1499
01:07:58,846 --> 01:08:00,226
of software from Mac OS.

1500
01:08:00,226 --> 01:08:03,626
So back when I was a kid growing
up with like a Macintosh SC,

1501
01:08:03,796 --> 01:08:08,776
there was this plug in called
Oscar and you could install it

1502
01:08:08,776 --> 01:08:11,706
and then anytime you dragged
trash to your trash can,

1503
01:08:11,966 --> 01:08:13,996
Oscar would pop out,
sing a little song,

1504
01:08:13,996 --> 01:08:14,986
and then go back in, right?

1505
01:08:14,986 --> 01:08:17,536
[laughter] Completely amusing
for like the first few weeks,

1506
01:08:17,536 --> 01:08:20,346
first few months and then
you realize this is just,

1507
01:08:20,636 --> 01:08:22,276
you get, the slippery slope.

1508
01:08:22,636 --> 01:08:25,236
So let me go ahead
and play this,

1509
01:08:25,326 --> 01:08:28,096
perhaps not in its entirety
less you never again be able

1510
01:08:28,096 --> 01:08:31,296
to watch Sesame Street but
as the instructions say,

1511
01:08:31,296 --> 01:08:34,156
drag as much falling trash as
you can to Oscar's trashcan.

1512
01:08:35,286 --> 01:08:38,446
[background music] So notice
this game too is going

1513
01:08:38,446 --> 01:08:40,106
to interact with my keyboard.

1514
01:08:40,136 --> 01:08:49,126
This is trash, click, drag and
I won't go too far but notice

1515
01:08:49,126 --> 01:08:52,846
if I pull it into the trash
quickly, the trash reappears

1516
01:08:52,846 --> 01:08:55,346
and now notice it's appearing
in a different place each time

1517
01:08:55,676 --> 01:09:00,426
so even this game is using
some sudo randomness.

1518
01:09:00,426 --> 01:09:03,996
And what we'll see is that
this gets progressively faster

1519
01:09:03,996 --> 01:09:06,996
and progressively faster,
much like the DDR game before.

1520
01:09:07,696 --> 01:09:12,866
So I thought a fun note,
perhaps to conclude on, is this,

1521
01:09:13,166 --> 01:09:15,526
throughout today
we've been dragging

1522
01:09:15,526 --> 01:09:18,186
and dropping puzzle pieces,
it's a very intuitive language,

1523
01:09:18,186 --> 01:09:20,286
problem set zero will task
you with precisely that.

1524
01:09:20,286 --> 01:09:23,456
Implementing the program
that most amuses you.

1525
01:09:24,336 --> 01:09:26,186
We spoke of cell
phones earlier today,

1526
01:09:26,186 --> 01:09:28,456
albeit in the context
of iPhones.

1527
01:09:28,456 --> 01:09:30,576
Yes it is the case, if you
read this morning's Crimson,

1528
01:09:30,576 --> 01:09:33,876
that Google very kindly donated
some 20 telephones to CS50

1529
01:09:33,916 --> 01:09:38,746
and CS1 so that we could empower
some of you, if you so desire

1530
01:09:38,746 --> 01:09:41,576
at terms end, to implement any
final project that you want

1531
01:09:41,576 --> 01:09:43,526
but also one if you so choose,

1532
01:09:43,796 --> 01:09:47,086
on what's called an android
cell phone, marketed by T-Mobile

1533
01:09:47,086 --> 01:09:48,436
and some other folks
throughout the world.

1534
01:09:48,676 --> 01:09:49,746
Well we read this article.

1535
01:09:49,746 --> 01:09:51,406
The Crimson has always
been very good to us

1536
01:09:51,406 --> 01:09:52,716
in recent years, press wise.

1537
01:09:52,756 --> 01:09:54,646
We were quite flattered
that they took an interest

1538
01:09:54,646 --> 01:09:57,486
in this project but we
were also quite amused.

1539
01:09:57,486 --> 01:09:59,766
And within minutes of
this article going online,

1540
01:09:59,926 --> 01:10:03,526
on the front page of today's
Crimson, I got this note here,

1541
01:10:03,526 --> 01:10:05,426
whose names and such
have been redacted.

1542
01:10:11,436 --> 01:10:15,396
[laughter] So let's be clear,
this is the email I got.

1543
01:10:16,886 --> 01:10:18,586
So then I got a little
curious, right?

1544
01:10:18,646 --> 01:10:21,756
So I'm like okay, interesting,
there's an article here

1545
01:10:21,756 --> 01:10:24,666
about CS50 so thecrimson.com.

1546
01:10:25,466 --> 01:10:28,596
Okay, yep we are in fact on
the front page there as well

1547
01:10:28,596 --> 01:10:29,656
as in the print version.

1548
01:10:30,366 --> 01:10:37,896
So what you hopefully realize
today, paragraph 2, look for it.

1549
01:10:38,716 --> 01:10:40,596
[laughter] So bless
their hearts, right?

1550
01:10:42,506 --> 01:10:45,026
This is in fact drag
and drop programming

1551
01:10:45,026 --> 01:10:47,766
that you will be focusing
on for problem set zero

1552
01:10:47,766 --> 01:10:51,306
but I did think, in fairness
cause again we're big fans

1553
01:10:51,306 --> 01:10:53,256
and frankly a lot of
our team members are

1554
01:10:53,256 --> 01:10:54,826
on the Crimson so it's all good.

1555
01:10:55,006 --> 01:10:57,636
I did think that I could
at least make this so.

1556
01:10:57,686 --> 01:11:00,806
So instead of Oscar Time which
drops this trash, I went ahead

1557
01:11:00,806 --> 01:11:01,956
and decided we could do this.

1558
01:11:01,956 --> 01:11:03,456
You know so I'm gonna
go to the trash here,

1559
01:11:03,736 --> 01:11:05,256
some little on the
fly programming.

1560
01:11:05,256 --> 01:11:05,506
You know what?

1561
01:11:05,506 --> 01:11:07,046
I need a different costume here.

1562
01:11:07,266 --> 01:11:08,466
I'm gonna click import.

1563
01:11:08,706 --> 01:11:10,596
I kind of prepared
in advance here.

1564
01:11:10,596 --> 01:11:12,156
I'm going to go to
my desktop folder.

1565
01:11:12,156 --> 01:11:15,876
I'm going to go into my
zero folder where I hid it.

1566
01:11:15,876 --> 01:11:17,546
I've got a little costume here.

1567
01:11:19,096 --> 01:11:22,446
[laughter] So if you will,
let me go to this thing here.

1568
01:11:22,446 --> 01:11:23,926
I'm gonna go ahead
and click this button

1569
01:11:23,926 --> 01:11:25,606
to shrink him down a little bit.

1570
01:11:25,976 --> 01:11:28,596
You have the ability to
rotate Sprites as you'll see.

1571
01:11:28,596 --> 01:11:31,246
I'm gonna tell it not to
rotate anymore and now

1572
01:11:31,246 --> 01:11:34,216
if we could crank the volume
up just a little bit we have

1573
01:11:34,276 --> 01:11:37,966
for you, in conclusion today,
drag and drop programming.

1574
01:11:38,516 --> 01:11:45,786
[laughter and music]

1575
01:11:46,286 --> 01:11:46,976
We'll see you on Wednesday.

1576
01:11:47,516 --> 01:12:21,890
[applause and music]