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[ Silence ]

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[ Noise ]

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[ Music ]

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>> This-- This is CS50.

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Welcome back or welcome
for the first time

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to the end of week zero.

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So today's opening act is
courtesy of Nicole Bowman,

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a student self-described
last year is among those less

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comfortable when she
implemented that visualization

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in a programming
language called "Scratch"

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into which we will
dive as a group today.

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But recall couple of
invitations from Wednesday.

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So this year is a
representative puzzle

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for the first ever
CS50 puzzle day

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to take place tomorrow at noon.

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For those who only glimpse this
earlier who happen struggling

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with this sort of puzzle,
I'll offer the following clue

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if you focus on the
very top line.

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You might think to
yourself 1, 2, 3, 4,

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we there yet, 5, and so forth.

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The other lines are in
quite as straightforward

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but if you actually
fill in those blanks,

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you'll come out with some
word and that's represented

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over the types of puzzles you'll
be challenged to solve tomorrow.

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So if of interest and you
have not RSVP just yet,

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do go to cs50.net/register
to sign

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up for the Facebook
sponsored CS50 puzzle day.

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It will take place
at noon to 3 p.m.

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in a building called Northwest
Science which is right

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across the street, just
off the street from here.

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You'll be greeted with pizza
as well as seven puzzles,

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either in printed or
electronic form and once you do,

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you and your team of friends of
2 or 3 or size 4 will be able

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to go often to some corner,

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opt into some conference
room outside and figure

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out the seven-letter
clue that will lead you

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to some prize on campus.

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Among the prizes will be a Wii
with the Mario cart bundle,

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raffled off to any possible
participant not just those

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who win some 50 dollar, 25
dollar gift certificates

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as well some mice
[phonetic] as well

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as some other giveaways
during that.

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And our friends from
Facebook will be in attendance

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if you would like to see what
life is like their real world.

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Also with of interest,

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dinner tonight you are
all cordially invited.

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Do go to cs50.net/rsvp.

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If you did not so already and
you would like to join us,

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the plan is to meet at 6:30
tonight at Maxwell-Dworkin

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which is also just
up the street.

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If these names are
unfamiliar to you,

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just go to the course's
home page, cs50.net,

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and we have direct
links to the addresses

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so that you can find
your way on campus.

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So now those of you who are
joining us for the first

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and even those who are here on
Wednesday, allow me to excerpt

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from the syllabus something
that is particularly important

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to me and to the course.

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So there's this excerpt
there, what ultimately matters

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in this course is not so much
where you end up relative

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to your classmates,
but where you end up--

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but where you in week 12,
at the end of the course,

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end up relative to
yourself in week 0.

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There's no predetermined
distribution of grades,

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there's no curve per se.

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We very much do grade
students in this course

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in an individual basic
whether doing a pass-fail

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which certainly encouraged
per the pink pass-fail

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of change slips if you
need them at any point

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or if you take it
for a letter grade.

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And more over as we
hit it on Wednesday,

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we do take into account students
[inaudible] backgrounds.

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So once it comes time to section

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and sectioning will
start next week,

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you needn't do anything
just yet,

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we'll announce it next week.

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We'll have three different
tracks of sections.

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The track of section for
those less comfortable,

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those more comfortable, and
those somewhere in between.

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And when it comes
time to section,

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you kinda know what
bucket you're in.

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If you're in the less
comfortable group,

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odds are that's a label you'll
immediately gravitate toward.

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And similarly if you're
particularly advance,

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you're eager to those
hacker editions,

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you're probably among
those come comfortable.

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And if you're kinda
of on the fence,

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somewhere in between is a--

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is a large portion
of the class as well.

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So realize that the course
tries to keep everyone

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on track not only for
those without experience

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but also keeping interested

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and challenged those
with more experience.

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In terms of the support
structure that awakes--

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that awaits, realize that the
first problems sets PDF was post

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into the course's
websites this morning

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and it's due next Thursday.

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But toward that end,
there's this walkthrough,

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every week for every problem

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that we'll have a code
walkthrough so to speak,

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where about one of the
teaching fellows will walk you

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through the process of
diving into that problem set.

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They're not mandatory.

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They are simply there,
so that if you're one

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of those students whether
in this course or in general

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who picks up a P
set and is often--

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I'm not really sure where to
begin or how to tackle it, well,

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that's exactly what these video
tape walkthroughs will be.

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We'll walk you through
the spec itself.

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We'll propose ideas,
things to think about.

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So realize that is
the support structure

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that would be taking place

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and almost every Sunday whenever
there's a P set of the week,

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at 7 p.m. also

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in Maxwell-Dworkin whose
address can be found

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on the course's website
under problem sets.

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Office hours too will
commence this year coinciding

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with Brain Break in four-- in
three different dining halls,

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simply check out the course's
website for the schedule thereof

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and you will see and you
we will work out the kings

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of [inaudible], our brand new
fancy online system iPad based

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systems [inaudible] coming
up on Monday in [inaudible],

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Tuesday in Leverett,
and then Wednesday

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and Thursday Lowell House.

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So keep an eye on the
course's website for details.

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So rather than just tell you
a little bit about 50 in terms

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of pictures, what we thought
we do is actually draw

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on some voices of CS50
past from both former TFs,

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former CAs as well
as former students.

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So if we could dim
the light again,

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allow me to convey some
messages from your predecessors.

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[ Noise ]

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[ Beep Sound ]

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>> My name is Jason, and
I took CS50 in the fall.

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I'm a freshman.

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And I was sort of looking

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for something I never
studied before.

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So-- And I knew it's gonna be
fun but also a lot of work.

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And I found all of
those and more in CS50.

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So I had a fantastic experience

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and I'm pretty sure
it changed my life

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and changed what I
was interested in.

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So I'm here to encourage other
people to take CS50 as well.

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So hopefully they can as
wonderful experience as I did.

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>> So tell us how you got
into the CS concentration.

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>> I took CS in high school
and I really liked it,

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and I real like it
all at Harvard,

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so I'm happy with it, I guess.

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>> Very cool!

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What do you think about
CS as a potential minor,

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do you think it's a good
option for lot people?

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What would you sell
some around that way?

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>> I think it's really useful
because CS is really liberating,

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once you know how do just like
a small script you can make

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that [inaudible] people have to
do or you can do, I don't know,

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minor things just make your life
easier, which is really nice.

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So [inaudible] is good too.

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>> [Inaudible], thank
you so much.

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There you have it,
CS empowering.

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>> So this is my final project
that I developed last year.

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Actually, was up in space for a
majority the time taking a lot

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of these photos which was
the probably the biggest time

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consuming aspect to this.

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But it's pretty cool.

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Apple got it lot of their ideas
from me, so did Google Earth.

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It's really, really unique,
you know, can find your house,

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even if you live in the middle
of the ocean like over here.

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Good place.

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Also if you like wanna play--

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let's play some chess,
you can play chess.

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It's a great little
development too.

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I learned a lot and thanks

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to CS50 you can create
things just like this.

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[ Laughter ]

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>> Oh, definitely,
CS50 was a lot fun.

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I remember when I was
looking at the courses

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like before I came here.

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Like I knew it's gonna my
math course and, you know, oh,

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a lot of them I pick

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out of having trying CS50 I did
[inaudible] I was gonna take,

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but after the first day
of shopping, you know,

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and then I entered,
okay, this is for me.

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>> [Inaudible] about CS50
at 4 a.m. in the morning.

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>> I like it.

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It's-- It really has a sense of
creativity 'cause there's a lot

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of creativity flowing
around everywhere.

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[ Laughter ]

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>> Everybody just seems like
really excited and it's already

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like 4:30 and everyone is awake
and working on their projects.

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It's pretty cool.

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[ Laughter ]

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>> Like hackaton.

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[ Simultaneous Talking ]

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>> We had so much fun.

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>> We were raging in the corner.

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Yeah.

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>> Music.

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>> Eating all the candy.

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>> Eating-- They're
eating lots of candy,

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that was disgusting,
that was fun.

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>> Yeah

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>> So.

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>> What do you think of
the CS50 [inaudible]?

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>> There're a lot
of people here.

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I think it's amazing
that there're

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so many really great
projects that are going.

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But it's definitely a
challenging semester but I think

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at the end-- at the
end you can say

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like I've gained some
very tangible skills.

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So definitely use the future.

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>> I've really thought
that has been one

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of the most practical courses
that I've taken at Harvard

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and also lot of fun and I've
really enjoyed learning how

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to use the technology.

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I'm really interested
in medicine

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as you can probably
tell from this.

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So I think I'm gonna go like
to-- go and be a doctor.

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But I think that being able
to work with programming

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and websites and computers
is such a useful skill

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and everybody should
really know it.

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>> Harvard President Faust,
we just wanna ask you,

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what you think of
the [inaudible]?

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>> Well, it was really fun.

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First I was struck by how much
energy there is in this room

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and everyone so excited
about their projects

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and everyone else's projects.

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But I was so struck by the
ingenuity of the projects I saw.

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Lost and found seems
to be a shared theme

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that obviously [inaudible]
university why it's lost

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and found.

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So I-- and then I just saw
an application which is

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about debating across Harvard

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where people can debate issues
online and I thought there're

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so many questions
that it would be great

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to just get people's
responses on.

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So I though about
what a contribution

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at the community that could be.

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And then I just saw a lot of
other very ingenious websites

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about where to find libraries
and different functions

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in libraries and a
variety of things.

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So, what a great event.

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[ Noise ]

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[ Applause ]

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>> So, even though the CS50
fair each year is the climax

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00:11:58,976 --> 00:12:01,146
of the course and the
terminus of the course,

246
00:12:01,146 --> 00:12:03,516
many students do go
on with their projects

247
00:12:03,516 --> 00:12:07,626
or with variants thereof to
create their own startups,

248
00:12:07,626 --> 00:12:10,506
to incubate their ideas with
friends particularly now

249
00:12:10,506 --> 00:12:12,086
that Harvard has its own J term.

250
00:12:12,086 --> 00:12:14,306
And so those of you who
are particularly interested

251
00:12:14,306 --> 00:12:16,786
in pursuing some startup
with your own or externally

252
00:12:17,026 --> 00:12:20,206
or just generally interest
in on entrepreneurship,

253
00:12:20,416 --> 00:12:23,046
know that any number of
resources exist that you'll come

254
00:12:23,046 --> 00:12:26,086
across particularly toward terms
and Hack Harvard for instance.

255
00:12:26,086 --> 00:12:28,386
It's a relatively new
student group on which some

256
00:12:28,386 --> 00:12:30,926
of our own teaching fellows
and CAs now participate

257
00:12:31,076 --> 00:12:33,416
and this will be an opportunity
and J term to come back

258
00:12:33,416 --> 00:12:36,246
to campus early and immerse
yourself with the team

259
00:12:36,246 --> 00:12:38,226
of your friends and
work on some project.

260
00:12:38,226 --> 00:12:41,666
Maybe even your CS50 project
and take it to the next level,

261
00:12:41,666 --> 00:12:45,696
[inaudible] wonderful support
structure surrounded 24/7

262
00:12:45,696 --> 00:12:47,576
by peers doing something
similar.

263
00:12:47,816 --> 00:12:50,476
Know that the i3, the Harvard
Collage Innovation Challenge

264
00:12:50,476 --> 00:12:50,916
will come up.

265
00:12:50,916 --> 00:12:53,606
And this is an opportunity to
actually get potentially funding

266
00:12:53,606 --> 00:12:56,076
for some idea that you might
have and so we'll be sure

267
00:12:56,076 --> 00:12:58,476
to announce the application
deadline for an opportunity

268
00:12:58,476 --> 00:13:00,636
like that when it arises.

269
00:13:00,876 --> 00:13:04,136
Know too that Harvard and all
of its schools is in a process

270
00:13:04,136 --> 00:13:06,026
in building out a little
something like this

271
00:13:06,026 --> 00:13:09,576
across the river at HPS to
be called the innovation lab

272
00:13:10,076 --> 00:13:12,596
for students and faculty and
staff from across Harvard.

273
00:13:12,826 --> 00:13:15,036
This is meant to become
a center of innovation,

274
00:13:15,036 --> 00:13:17,926
a place where you can go
to work with work spaces,

275
00:13:17,926 --> 00:13:20,316
for invited guest,
for food and such.

276
00:13:20,586 --> 00:13:21,986
And so this will be debuting

277
00:13:21,986 --> 00:13:23,946
in a few months once
construction is complete

278
00:13:23,946 --> 00:13:26,276
and the idea is to really
bring people and ideas

279
00:13:26,276 --> 00:13:29,576
from across campus particularly
across schools to work

280
00:13:29,576 --> 00:13:31,156
on all things innovative.

281
00:13:31,326 --> 00:13:33,756
In particular if you're
interested in J term

282
00:13:33,756 --> 00:13:37,116
in joining some of those
folks out in Silicon Valley

283
00:13:37,116 --> 00:13:38,916
on a little field
trip over J term,

284
00:13:38,916 --> 00:13:40,956
know that there'll be an
application opportunity

285
00:13:41,186 --> 00:13:44,396
to just a couple of weeks
to go with some 35 peers

286
00:13:44,696 --> 00:13:45,866
out to Silicon Valley.

287
00:13:45,866 --> 00:13:49,466
Meet with startup, see startups,
talk to VCs, venture capitalist,

288
00:13:49,736 --> 00:13:53,366
and talk with folks who live in
breath this particular world.

289
00:13:53,366 --> 00:13:55,796
And also closer to home,
know that in a short term,

290
00:13:55,796 --> 00:13:58,146
there are mailing lists
and plenty of your peers,

291
00:13:58,326 --> 00:14:01,226
an upper class man who
discuss and explore these kinds

292
00:14:01,226 --> 00:14:04,016
of things, the whipping through
these URLs fairly quickly.

293
00:14:04,016 --> 00:14:06,146
The slides are online so
you can grab this later.

294
00:14:06,146 --> 00:14:09,016
But in email list called the
Harvard College Entrepreneurship

295
00:14:09,056 --> 00:14:12,766
Form as well as the Harvard
College Venture Partners exist

296
00:14:12,896 --> 00:14:13,486
as well.

297
00:14:13,846 --> 00:14:16,536
So in terms of CS50 today,
we do have these pink forms

298
00:14:16,536 --> 00:14:19,226
which are only necessary if you
decide to switch grading status

299
00:14:19,226 --> 00:14:20,556
between now and the
5th and Monday.

300
00:14:20,806 --> 00:14:23,586
We also have copies of the
same handouts from Wednesday,

301
00:14:23,586 --> 00:14:26,236
one of which is the syllabus,
the other of which is this,

302
00:14:26,236 --> 00:14:28,836
the unofficial CS50 guide to
computer science at Harvard.

303
00:14:29,106 --> 00:14:30,906
The goal here is not to
turn everyone in this room

304
00:14:30,906 --> 00:14:32,906
into a computer science
concentrate or a minor,

305
00:14:33,106 --> 00:14:35,056
but to give you a sense
of where else you can go.

306
00:14:35,056 --> 00:14:36,156
And particular the last--

307
00:14:36,266 --> 00:14:38,706
very last back page of
this is a cheat sheet

308
00:14:38,706 --> 00:14:40,946
of all the various
directions in which you can go

309
00:14:41,166 --> 00:14:42,836
after just this course alone.

310
00:14:43,006 --> 00:14:44,776
And this brochure here
is what was handed

311
00:14:44,776 --> 00:14:48,426
to all 2,000 attendees
of the CS50 fair in 2010

312
00:14:48,586 --> 00:14:50,666
from which you saw
excerpts just a moment ago.

313
00:14:50,916 --> 00:14:53,276
And these we handout so
that you can get a sense

314
00:14:53,316 --> 00:14:57,176
in your own hands of the
500 some odd projects

315
00:14:57,176 --> 00:14:58,896
that your predecessors
implemented along

316
00:14:58,896 --> 00:15:00,436
with their names,
the descriptions

317
00:15:00,746 --> 00:15:02,926
and additional details as well

318
00:15:03,196 --> 00:15:05,036
as to what those
folks were like.

319
00:15:05,076 --> 00:15:09,476
So do explore that if this
is of interest to you.

320
00:15:09,816 --> 00:15:13,386
Now even now today, we're gonna
dive into one of the core skills

321
00:15:13,386 --> 00:15:15,276
of computer science,
namely programming.

322
00:15:15,276 --> 00:15:18,426
Bear in mind one that it's just
tool the computer scientists

323
00:15:18,426 --> 00:15:20,746
tend to use and folks from
other domains tend to use

324
00:15:20,896 --> 00:15:24,046
to solve problems on
interest to themselves.

325
00:15:24,046 --> 00:15:26,596
It is not programming
a goal into itself,

326
00:15:26,596 --> 00:15:29,606
but really a tool you can
use expedite data analysis

327
00:15:29,606 --> 00:15:32,236
as we heard one of our
student-- our own student say,

328
00:15:32,506 --> 00:15:34,676
to expedite just solving
some real world problem

329
00:15:34,676 --> 00:15:37,376
that you wanna do more
efficiently or mechanically

330
00:15:37,376 --> 00:15:39,096
than you could do, say by hand.

331
00:15:39,376 --> 00:15:42,166
But toward that end
of actually learning

332
00:15:42,166 --> 00:15:44,046
to program it's perhaps useful

333
00:15:44,046 --> 00:15:47,576
to understand why these same
ideas now increasingly imply

334
00:15:47,686 --> 00:15:48,546
in the real world?

335
00:15:48,546 --> 00:15:50,656
So a lot of you, many
of you will not go off

336
00:15:50,656 --> 00:15:53,216
to take computer science
minor or concentration

337
00:15:53,216 --> 00:15:55,346
and maybe not even a second
course in computer science,

338
00:15:55,576 --> 00:15:57,916
but you're gonna go off at some
point and some number of years

339
00:15:57,916 --> 00:16:00,736
to the real world, to business,
to politics and the like,

340
00:16:00,736 --> 00:16:02,216
and presumably making decisions.

341
00:16:02,426 --> 00:16:04,786
And if you watch the news,
particularly the daily show,

342
00:16:05,016 --> 00:16:06,766
you'll know that there's
a whole lot of adults

343
00:16:06,766 --> 00:16:09,286
out there making really
bad uninformed decisions

344
00:16:09,286 --> 00:16:11,896
particularly when it comes to
technology, whether they're

345
00:16:11,896 --> 00:16:14,226
in positions of power
or just consumers

346
00:16:14,226 --> 00:16:15,536
who are using technologies

347
00:16:15,536 --> 00:16:17,626
without even understanding
how they work,

348
00:16:17,816 --> 00:16:20,606
or how they might threaten
their own privacy and security.

349
00:16:20,836 --> 00:16:22,926
One of themes we'll
discuss term's end

350
00:16:22,926 --> 00:16:24,806
when you introduce
web programming

351
00:16:25,036 --> 00:16:28,166
and how you actually program
dynamic websites and tools

352
00:16:28,166 --> 00:16:30,876
for the internet, we'll
discuss issues of security,

353
00:16:31,086 --> 00:16:33,016
but just to tease you
now, I though I'd share

354
00:16:33,016 --> 00:16:35,566
with you an example of
something that literally crapped

355
00:16:35,566 --> 00:16:40,556
up the day before our one of our
lecture's last fall on inter--

356
00:16:40,556 --> 00:16:42,436
on the web programming
in the internet.

357
00:16:42,686 --> 00:16:44,506
There's this tool with
which from the whisperers,

358
00:16:44,506 --> 00:16:46,536
some of you are already
clearly familiar with already.

359
00:16:46,536 --> 00:16:48,446
If not, you're about
to be educated.

360
00:16:48,666 --> 00:16:53,036
So number months ago, a
security researcher presented

361
00:16:53,036 --> 00:16:56,386
to the world a challenge that
we as a society really need

362
00:16:56,386 --> 00:16:58,976
to fix some flaws that
have existed in the web,

363
00:16:59,126 --> 00:17:02,176
in that protocol called "HTTP,"
which you've seen in front

364
00:17:02,176 --> 00:17:05,056
of URLs now for years that
have existed for ages.

365
00:17:05,516 --> 00:17:06,916
Really since the
beginning of a lot

366
00:17:06,916 --> 00:17:08,146
of these technologies' adaption

367
00:17:08,146 --> 00:17:10,796
but no one really
took it seriously

368
00:17:10,796 --> 00:17:14,646
because it required exploiting
weaknesses in these technologies

369
00:17:14,646 --> 00:17:19,536
and hacking certain things so to
speak took more savvy than, say,

370
00:17:19,536 --> 00:17:21,006
the typical population
might have.

371
00:17:21,006 --> 00:17:24,746
So this fellow wrote a plug
in, an extension for firefox,

372
00:17:24,746 --> 00:17:26,436
the browser with which
you're probably familiar,

373
00:17:26,696 --> 00:17:30,936
that makes it super easy to just
log in to the Facebook account,

374
00:17:30,936 --> 00:17:33,206
the GMail account, the Twitter
account, the Yahoo account,

375
00:17:33,206 --> 00:17:35,506
any number of big name
accounts if you're

376
00:17:35,506 --> 00:17:37,886
within wireless range
of these people.

377
00:17:37,886 --> 00:17:38,656
And when you run this tool,

378
00:17:38,656 --> 00:17:39,996
it looks a little
something like this.

379
00:17:40,066 --> 00:17:41,326
This might be a browser window.

380
00:17:41,596 --> 00:17:43,056
A typical Firefox window

381
00:17:43,056 --> 00:17:45,156
and with this freely
available tool installed,

382
00:17:45,416 --> 00:17:49,786
you can see a button at top left
that just says start capturing.

383
00:17:50,066 --> 00:17:53,426
And so long as you're on
a wireless Wi-Fi laptop

384
00:17:53,426 --> 00:17:55,816
and you're surrounded by other
people whether it's on campus

385
00:17:55,816 --> 00:17:57,516
or in Starbucks or
in an airport,

386
00:17:57,676 --> 00:17:58,946
there's all of these bits.

387
00:17:58,946 --> 00:18:01,176
All of this 0s and 1s
going through the air

388
00:18:01,486 --> 00:18:03,706
that pretty much all
of our laptops are

389
00:18:03,706 --> 00:18:05,416
on the honor system
not to listen

390
00:18:05,416 --> 00:18:09,136
to unless they're destined
from my own computer's address.

391
00:18:09,446 --> 00:18:11,546
Well, this tool simply
turns itself into--

392
00:18:11,786 --> 00:18:13,506
which definitely called
"promiscuous mode"

393
00:18:13,576 --> 00:18:16,076
where it listens to all
of these 0s and 1s flowing

394
00:18:16,076 --> 00:18:19,616
out the room whether or not
they're belonging to me.

395
00:18:19,756 --> 00:18:22,026
And what you see with
this particular tool,

396
00:18:22,026 --> 00:18:23,656
and the challenge
this fellow laid

397
00:18:23,656 --> 00:18:26,326
down is once you start
running this, you see everyone

398
00:18:26,326 --> 00:18:29,386
in the room or the wireless
vicinity who's actually using

399
00:18:29,386 --> 00:18:31,316
Facebook, or Google,
or the like.

400
00:18:31,636 --> 00:18:34,736
And with the simple double click
on any one of these names bam,

401
00:18:34,736 --> 00:18:37,616
in that browser window would
be Ian Gallagher's [phonetic]

402
00:18:37,616 --> 00:18:40,806
Facebook account or Eric's
Google account or the like.

403
00:18:41,146 --> 00:18:43,976
And the reason behind
this derives from these--

404
00:18:44,056 --> 00:18:45,766
the use of websites of cookies.

405
00:18:45,826 --> 00:18:47,556
Something you're probably
generally familiar with.

406
00:18:47,556 --> 00:18:50,196
And we'll talk about in more
detail towards term's end.

407
00:18:50,456 --> 00:18:53,326
But these cookies are not
often kept particularly secure.

408
00:18:53,486 --> 00:18:57,706
A lot of websites do not have
URLs that start with https,

409
00:18:58,186 --> 00:18:59,586
where the S denotes secure.

410
00:18:59,826 --> 00:19:02,266
And even those that do, only
use it some of the time,

411
00:19:02,266 --> 00:19:03,776
Facebook itself included.

412
00:19:03,776 --> 00:19:05,716
In fact there was
just a few months ago,

413
00:19:05,906 --> 00:19:09,506
that Facebook finally started
offering the option site-wide

414
00:19:09,766 --> 00:19:12,716
to use HTTPS to ward
off this threat.

415
00:19:12,886 --> 00:19:15,936
But as recently as last fall,
there was a good several months

416
00:19:16,226 --> 00:19:19,116
where your best friend or your
roommate might very well been

417
00:19:19,176 --> 00:19:21,916
seeing everything you were doing
inside of you Facebook account

418
00:19:21,916 --> 00:19:24,136
or the like because
it is incredibly easy

419
00:19:24,136 --> 00:19:26,016
to steal cookies.

420
00:19:26,216 --> 00:19:28,336
And that's a topic we'll
discuss towards term's end.

421
00:19:28,336 --> 00:19:30,126
And we'll also discuss defenses

422
00:19:30,126 --> 00:19:31,606
against these kinds
of mechanisms.

423
00:19:31,866 --> 00:19:34,806
But if you're sort of wide
eyed at this moment like,

424
00:19:34,806 --> 00:19:37,906
my God it's that easy or
it's this possible to do.

425
00:19:37,906 --> 00:19:40,086
Well one of the goals
of this course is again,

426
00:19:40,086 --> 00:19:42,906
not just about computer science
and not just about programming

427
00:19:43,146 --> 00:19:46,166
but to empower you to then
make educated decisions

428
00:19:46,166 --> 00:19:49,476
and maybe you don't care that
anyone around you could log

429
00:19:49,476 --> 00:19:51,666
in to your Facebook profile
'cause you don't really use it

430
00:19:51,666 --> 00:19:54,366
anyway, but at least
that's a conscious decision

431
00:19:54,366 --> 00:19:55,816
and not just an accidental one

432
00:19:56,026 --> 00:19:58,976
because you don't understand
how these technologies work.

433
00:19:59,096 --> 00:20:01,906
>> Another exploit
that came to light

434
00:20:01,906 --> 00:20:04,506
or another security
implication that came

435
00:20:04,506 --> 00:20:07,936
to light a few months back too,
related to Apple and the iPhone.

436
00:20:07,936 --> 00:20:09,436
So Apple, if you
actually read the

437
00:20:09,436 --> 00:20:12,976
like 23-page document you have
to agree to just to use iTunes

438
00:20:12,976 --> 00:20:15,156
and listen to music,
let alone use an iPhone,

439
00:20:15,406 --> 00:20:16,936
it actually does say
somewhere in there

440
00:20:16,936 --> 00:20:18,886
that we actually log
everywhere you're going.

441
00:20:19,176 --> 00:20:22,456
And we store all of this
information in iTunes

442
00:20:22,456 --> 00:20:24,916
and we even upload this
information to our servers.

443
00:20:25,156 --> 00:20:27,386
Now as an aside, there're
actually a lot of benefits

444
00:20:27,386 --> 00:20:29,846
to all 500 of us
walking around the world

445
00:20:30,076 --> 00:20:32,106
with GPS transponders
in our pockets.

446
00:20:32,106 --> 00:20:33,766
If we have iPhones
or androids or any

447
00:20:33,766 --> 00:20:35,216
of these moderns smartphones

448
00:20:35,506 --> 00:20:39,416
because then we can provide
useful services to us

449
00:20:39,416 --> 00:20:41,996
like what's near by me, Google
Maps, these kinds of things.

450
00:20:41,996 --> 00:20:43,796
So there's a compelling
user case for this.

451
00:20:44,146 --> 00:20:46,286
But what happens in this
case a few months back,

452
00:20:46,516 --> 00:20:49,376
was apple accidentally
or unknowingly

453
00:20:49,376 --> 00:20:53,426
or consciously was not
encrypting the GPS coordinates

454
00:20:53,426 --> 00:20:56,586
of everywhere you and in
turn your phone had been.

455
00:20:56,726 --> 00:20:58,246
And so this is minor
worry [inaudible] 'cause

456
00:20:58,246 --> 00:21:00,016
if anyone has physical access

457
00:21:00,016 --> 00:21:03,256
to your computer whether it's a
roommate or spouse or children

458
00:21:03,256 --> 00:21:05,126
or parents, well, they could--

459
00:21:05,476 --> 00:21:07,506
they could certainly
just start poking around.

460
00:21:07,506 --> 00:21:08,746
They could run this free tool

461
00:21:08,746 --> 00:21:11,156
which is again it was
another proof of concept

462
00:21:11,206 --> 00:21:13,766
to demonstrate just how easy
it is to do these things

463
00:21:14,036 --> 00:21:16,246
and how scary it is scary it
is if you don't understand.

464
00:21:16,546 --> 00:21:19,366
Well, this will plot on the map
absolutely everywhere you have

465
00:21:19,406 --> 00:21:21,496
been because this
file was on encrypted.

466
00:21:21,496 --> 00:21:23,926
And so I actually did this
on my phone out of curiosity.

467
00:21:23,926 --> 00:21:26,556
I ran this free program on
my own installation of iTunes

468
00:21:26,776 --> 00:21:28,866
and what it generates
for you is this nice map.

469
00:21:29,286 --> 00:21:32,016
And these big blue dots
represents proportional

470
00:21:32,016 --> 00:21:35,696
to their size where I spent
over that past year or two most

471
00:21:35,696 --> 00:21:37,596
of my time and you can
see I spent a lot of time

472
00:21:37,596 --> 00:21:39,456
in the Northeast around
Cambridge and such.

473
00:21:39,696 --> 00:21:42,376
I apparently went to Texas
and somewhere in Pennsylvania

474
00:21:42,636 --> 00:21:43,726
as well as California.

475
00:21:44,016 --> 00:21:47,696
The tiny little red dot at top
left there, anyone had a hunch

476
00:21:47,696 --> 00:21:50,756
as to why that one is so
small relative to the others?

477
00:21:52,376 --> 00:21:54,956
So was actually just a
layover in an airplane.

478
00:21:54,956 --> 00:21:57,636
When my phone actually got
reception once we had a layover,

479
00:21:57,636 --> 00:22:00,026
I got out in the terminal,
viola, Apple knows where I am,

480
00:22:00,026 --> 00:22:02,496
get back in the plane and they
see me again ones I hit Texas

481
00:22:02,756 --> 00:22:04,206
or the East Coast.

482
00:22:04,206 --> 00:22:05,816
Well, if you zoom in
on this it's one thing

483
00:22:05,816 --> 00:22:06,976
that kinda see these
big blotches,

484
00:22:07,336 --> 00:22:09,966
most people in the world could
guess I have spend a lot of time

485
00:22:09,966 --> 00:22:11,846
in Cambridge but if
you really dive in,

486
00:22:11,846 --> 00:22:13,596
you just see how
granular this is.

487
00:22:13,756 --> 00:22:15,476
So this is the greater
Boston area

488
00:22:15,476 --> 00:22:18,816
and the dark splotch does indeed
kind of overlap with Boston

489
00:22:18,816 --> 00:22:21,486
and Cambridge and I apparently
spent sometime in the suburbs

490
00:22:21,486 --> 00:22:22,486
but not all that much.

491
00:22:22,706 --> 00:22:24,826
But you can see now the
concentration of my time.

492
00:22:25,116 --> 00:22:28,356
I also happened to do a lot of
work for a startup in New York

493
00:22:28,356 --> 00:22:30,326
and so I take the
train and truck back

494
00:22:30,326 --> 00:22:31,706
and forth along the East Coast,

495
00:22:31,876 --> 00:22:33,586
and sure enough,
Apple new that too.

496
00:22:34,206 --> 00:22:37,756
So at top right there,
that's Boston up there,

497
00:22:37,876 --> 00:22:39,826
at the bottom left is Manhattan,

498
00:22:39,826 --> 00:22:41,676
in the middle there is
Stanford Connecticut

499
00:22:41,676 --> 00:22:44,456
where Amtrak lets off and we
typically pick up a rental car.

500
00:22:44,646 --> 00:22:46,306
And you can see that I've
occasionally vacationed

501
00:22:46,306 --> 00:22:49,056
on Cape Cod which is that
little hook shape thing going

502
00:22:49,056 --> 00:22:50,756
out into the Atlantic Ocean.

503
00:22:50,756 --> 00:22:53,076
And so, it was cool
on the one hand,

504
00:22:53,076 --> 00:22:55,456
it was also a little worrisome,
when I had no recollection

505
00:22:55,456 --> 00:22:58,166
of being in Pennsylvania and
apparently my phone had been.

506
00:22:58,166 --> 00:22:59,976
And then finally, I figured it

507
00:22:59,976 --> 00:23:03,736
out that I was just visiting
some university folks there.

508
00:23:04,056 --> 00:23:06,906
So this sort of stuff,
these sorts of risks are all

509
00:23:06,906 --> 00:23:09,366
around us, and so by semester's
end, not only will you know how

510
00:23:09,366 --> 00:23:13,366
to do things as those images
and videos of your predecessors

511
00:23:13,366 --> 00:23:15,536
to pick, you'll also
understand these things.

512
00:23:15,536 --> 00:23:18,446
So that even 50 is in fact the
terminal course for you in CS,

513
00:23:18,866 --> 00:23:20,546
you're better equipped
in the real world just

514
00:23:20,546 --> 00:23:22,896
to make smart decisions when
it comes to your own phone,

515
00:23:23,146 --> 00:23:26,386
let alone policy and business
decisions moving forward.

516
00:23:26,926 --> 00:23:31,286
So, with that said, I thought
we would come back to this issue

517
00:23:31,536 --> 00:23:33,406
of the program we
wrote on Wednesday

518
00:23:33,406 --> 00:23:34,976
that didn't really
turn out so well.

519
00:23:34,976 --> 00:23:37,336
There were clearly some
opportunities for improvements.

520
00:23:37,336 --> 00:23:40,706
And recall that the program that
we wrote collectively as a class

521
00:23:41,196 --> 00:23:45,006
or as the volunteer gave us his
form was this five-step program.

522
00:23:45,306 --> 00:23:48,156
And odds are, if you kind
of reflected on the steps,

523
00:23:48,156 --> 00:23:49,656
you yourself wrote on Wednesday,

524
00:23:49,836 --> 00:23:51,826
well yours was probably
relatively

525
00:23:51,826 --> 00:23:54,836
as short somewhat cursory
maybe even incomplete.

526
00:23:54,966 --> 00:23:57,716
And granted if we gave you
more than 60 or 120 seconds,

527
00:23:57,716 --> 00:23:59,656
maybe you'd have come up
with something a little more

528
00:23:59,756 --> 00:24:00,326
fleshed out.

529
00:24:00,766 --> 00:24:02,876
But the reality is
there're probably a lot

530
00:24:02,876 --> 00:24:04,536
of things you didn't
in fact consider.

531
00:24:04,536 --> 00:24:06,206
And they're representative
of the type--

532
00:24:06,316 --> 00:24:08,186
the types of things
you need to consider

533
00:24:08,416 --> 00:24:10,776
when commanding a
computer to do something.

534
00:24:10,776 --> 00:24:12,196
So for instance, and if Rob,

535
00:24:12,196 --> 00:24:13,576
you could join me up
here for a moment.

536
00:24:13,866 --> 00:24:18,456
What's was one of the most
obvious flaws in step one?

537
00:24:18,456 --> 00:24:20,436
Take two slices bread
from the loaf,

538
00:24:20,606 --> 00:24:22,506
what is that clearly
assuming as Rob

539
00:24:22,556 --> 00:24:25,396
so professionally
demonstrated yesterday?

540
00:24:26,596 --> 00:24:28,206
>> It's a loaf with
sliced bread.

541
00:24:28,276 --> 00:24:30,026
>> So it's a loaf with
sliced bread, right?

542
00:24:30,026 --> 00:24:32,686
So with this-- okay, so here
is the loaf and we were--

543
00:24:32,786 --> 00:24:34,186
we told you're allowed
to assume.

544
00:24:34,186 --> 00:24:36,116
You have a jar of each
of these things, a knife,

545
00:24:36,116 --> 00:24:39,116
and a loaf of bread, take two
slices of bread from the loaf.

546
00:24:39,696 --> 00:24:40,696
You know, what is-- I mean,

547
00:24:40,696 --> 00:24:42,506
as you saw as from
our volunteers ripped

548
00:24:42,506 --> 00:24:45,796
into to this thing because what
does it mean to take two slices

549
00:24:45,796 --> 00:24:46,926
from this loaf, right?

550
00:24:46,926 --> 00:24:47,506
So what would be better?

551
00:24:47,506 --> 00:24:49,576
What would be a marginal
improvement if we wanna go back

552
00:24:49,986 --> 00:24:53,286
and actually improve this
with a better step one?

553
00:24:54,506 --> 00:24:56,036
Okay, so open the package.

554
00:24:56,226 --> 00:24:59,506
Well, let me just play kind
of-- okay so open the package.

555
00:25:00,006 --> 00:25:02,626
So now we do this and I
turn the little twisty thing

556
00:25:02,626 --> 00:25:04,876
and we can assume certain things
otherwise this will very quickly

557
00:25:04,876 --> 00:25:06,436
devolve into complete tedium.

558
00:25:06,436 --> 00:25:09,166
So we know we can assume that
I know how to open the bag.

559
00:25:09,376 --> 00:25:11,936
But now we've kind of written
slightly flawed program, right.

560
00:25:11,936 --> 00:25:15,036
What if I make my
sandwich and I then decide

561
00:25:15,036 --> 00:25:17,156
to make another sandwich,
what am I now assuming?

562
00:25:17,156 --> 00:25:18,006
[ Inaudible Remarks ]

563
00:25:18,006 --> 00:25:19,426
>> It's already open, right.

564
00:25:19,426 --> 00:25:22,256
And I better say close the
bag at the end of the program

565
00:25:22,256 --> 00:25:25,506
or maybe I should say something
like, "If bag is closed,

566
00:25:25,746 --> 00:25:27,196
then opened bag, right."

567
00:25:27,196 --> 00:25:29,246
I wanna actually make
an intelligent decision

568
00:25:29,436 --> 00:25:31,786
that takes us input, the
state of my world to the state

569
00:25:31,786 --> 00:25:35,406
of the loaf of bread and then
decide if in only if it's closed

570
00:25:35,636 --> 00:25:36,986
to open the bag, alright.

571
00:25:36,986 --> 00:25:39,416
So notice what's Rob is doing
here is gonna be a little

572
00:25:39,416 --> 00:25:40,596
something called pseudocode.

573
00:25:40,596 --> 00:25:41,806
And it will get us
into the habit

574
00:25:41,806 --> 00:25:44,576
of thinking more
algorithmically, more carefully,

575
00:25:44,576 --> 00:25:47,016
more procedurally which
for today's purpose is can

576
00:25:47,016 --> 00:25:47,666
be synonymous.

577
00:25:47,976 --> 00:25:50,586
So he's doing this little
thing of indentation just

578
00:25:50,586 --> 00:25:53,596
to make clear that you only
open the bag if the thing that's

579
00:25:53,596 --> 00:25:57,176
above it and slightly to
the left is actually true,

580
00:25:57,176 --> 00:25:58,846
or is actually the
case in reality.

581
00:25:59,016 --> 00:26:01,816
Alright, so now if bag is
close, I opened it, otherwise,

582
00:26:01,816 --> 00:26:02,936
it's presumably just open.

583
00:26:02,936 --> 00:26:03,536
Now, what do I do?

584
00:26:05,096 --> 00:26:09,906
Let's start making the
sandwich more properly.

585
00:26:10,216 --> 00:26:10,636
Yeah?

586
00:26:10,636 --> 00:26:10,936
[ Inaudible Remark ]

587
00:26:10,936 --> 00:26:11,236
>> Okay.

588
00:26:11,236 --> 00:26:12,246
[ Inaudible Remark ]

589
00:26:12,246 --> 00:26:14,656
>> Okay, good.

590
00:26:14,716 --> 00:26:17,386
So if loaf is sliced
then take two slices.

591
00:26:17,676 --> 00:26:19,606
And again, just to be
a little nitpicky here.

592
00:26:19,726 --> 00:26:21,996
You know, there's nothing saying
I should be taking the bread

593
00:26:21,996 --> 00:26:23,366
from the open end of the bag.

594
00:26:23,366 --> 00:26:25,296
So there's another
opportunity for improvement.

595
00:26:25,536 --> 00:26:26,666
But I can kinda push back

596
00:26:26,666 --> 00:26:29,706
on this one other else
slice bread into--

597
00:26:29,706 --> 00:26:31,536
slice loaf into slices, right.

598
00:26:31,816 --> 00:26:33,956
Like we kinda need to do
something before that, right?

599
00:26:35,026 --> 00:26:37,886
Suppose that it is not sliced,
what do I obviously need

600
00:26:37,886 --> 00:26:40,476
to do first before
slicing this thing?

601
00:26:40,556 --> 00:26:41,836
So remove loaf from bag.

602
00:26:41,836 --> 00:26:43,016
So we might do that.

603
00:26:43,196 --> 00:26:45,296
But now, let's-- because this
is pretty sliced, let's go back

604
00:26:45,296 --> 00:26:47,536
to the other condition and let's
start to record both of these.

605
00:26:47,536 --> 00:26:51,696
So if loaf is already sliced,
remove two slices from bag,

606
00:26:51,886 --> 00:26:55,156
from open end, it is
fairly precise, else.

607
00:26:55,436 --> 00:26:57,166
So now let's introduce
a fork in the road.

608
00:26:57,166 --> 00:26:59,706
Be previously we did something
if something was true,

609
00:26:59,706 --> 00:27:01,256
but there was really
no alternative.

610
00:27:01,256 --> 00:27:03,316
This time it kinda makes
sense to have an alternative.

611
00:27:03,446 --> 00:27:06,636
So if loaf is already
sliced, then remove two slices

612
00:27:06,636 --> 00:27:11,516
from the open bag, else,
what do I wanna do?

613
00:27:11,516 --> 00:27:11,766
[ Inaudible Remark ]

614
00:27:11,766 --> 00:27:12,036
>> Sorry.

615
00:27:12,036 --> 00:27:12,926
[ Inaudible Remark ]

616
00:27:12,926 --> 00:27:15,816
>> Okay, so get a knife at
some points and still, else,

617
00:27:15,816 --> 00:27:17,296
if it isn't sliced, what needs

618
00:27:17,326 --> 00:27:19,686
to happen first before I
start slicing this bread?

619
00:27:20,466 --> 00:27:20,676
Yeah.

620
00:27:20,676 --> 00:27:21,136
[ Inaudible Remarks ]

621
00:27:21,136 --> 00:27:22,836
>> Remove bread from bag.

622
00:27:22,916 --> 00:27:25,186
So I would actually slide
all of the bread out

623
00:27:25,186 --> 00:27:26,946
and then I could
probably proceed

624
00:27:26,946 --> 00:27:27,986
to use this part of the knife.

625
00:27:28,056 --> 00:27:30,706
So now we're getting somewhere.

626
00:27:30,706 --> 00:27:34,106
So we've introduced what we'll
call conditions, or branches,

627
00:27:34,106 --> 00:27:37,496
or decision points
really in the real world.

628
00:27:37,496 --> 00:27:39,366
So now what we-- what
might we do here?

629
00:27:39,366 --> 00:27:40,466
At this point in the story,

630
00:27:40,736 --> 00:27:42,826
we have removed potentially
the bread from the bag

631
00:27:42,826 --> 00:27:46,616
and let's just put one more
statement also indented cuts two

632
00:27:46,616 --> 00:27:48,486
slices of bread from loaf.

633
00:27:48,486 --> 00:27:49,956
Now here too, we
could be nitpicky,

634
00:27:49,956 --> 00:27:51,496
I could cut a slice
that's this big

635
00:27:51,496 --> 00:27:53,386
and be completely
ridiculous unless you start

636
00:27:53,386 --> 00:27:54,896
to make some more
forward progress here.

637
00:27:55,116 --> 00:27:57,316
But notice what Rob
has done in pseudocode,

638
00:27:57,576 --> 00:28:00,536
both of these lines
removed and cut or indented

639
00:28:00,766 --> 00:28:03,596
and this suggests that both of
those things should now happen

640
00:28:03,746 --> 00:28:07,866
if the decision that we just
made if it isn't sliced is true.

641
00:28:07,956 --> 00:28:10,716
So again, an example
of logical flow here.

642
00:28:10,886 --> 00:28:12,446
Well let's try to
wrap this one up now.

643
00:28:12,446 --> 00:28:17,846
So if we have at this point
two slices of bread in hand,

644
00:28:17,976 --> 00:28:19,816
now what we-- now
what should we do?

645
00:28:21,056 --> 00:28:21,166
Yeah.

646
00:28:22,026 --> 00:28:23,026
>> Put them on the table.

647
00:28:23,136 --> 00:28:23,936
>> Put them on the table.

648
00:28:24,116 --> 00:28:28,596
Okay. It could be
better, but it's alright.

649
00:28:28,676 --> 00:28:31,776
So, you know, separately
or some such adverb.

650
00:28:32,336 --> 00:28:33,606
What's next?

651
00:28:33,606 --> 00:28:34,526
[ Inaudible Remarks ]

652
00:28:34,526 --> 00:28:35,536
>> Okay, open-- right.

653
00:28:35,676 --> 00:28:39,146
And so this was the key point
missing yesterday, right?

654
00:28:39,146 --> 00:28:40,686
Open the jar of peanut butter.

655
00:28:40,686 --> 00:28:42,196
That sort of something
we took for granted.

656
00:28:42,196 --> 00:28:43,866
If this is the first
time using it,

657
00:28:43,866 --> 00:28:46,256
here's another possible
corner case so to speak.

658
00:28:46,256 --> 00:28:48,846
And this is a piece of jargon
we'll often use in programming.

659
00:28:49,086 --> 00:28:51,356
Corner case is sort of a
situation that, you know,

660
00:28:51,356 --> 00:28:52,496
make sense that it might happen

661
00:28:52,496 --> 00:28:53,786
but you didn't really
think about it.

662
00:28:53,946 --> 00:28:55,446
And even though it
might only happen one

663
00:28:55,446 --> 00:28:58,486
out of a hundred times, that
one time it does happen, right,

664
00:28:58,486 --> 00:29:00,146
you're not gonna be able
to get any peanut butter

665
00:29:00,146 --> 00:29:03,256
out of it unless you have
another condition or branch.

666
00:29:03,506 --> 00:29:05,656
So let's go ahead and say,
open the jar of peanut butter;

667
00:29:05,936 --> 00:29:08,936
if sealed, remove seal
or something like that.

668
00:29:08,936 --> 00:29:11,606
And so that gets me to the point
of conditionally doing this.

669
00:29:12,126 --> 00:29:14,416
So now I have the opened peanut
butter, I have my two slices

670
00:29:14,416 --> 00:29:16,506
of bread separately what's next?

671
00:29:16,506 --> 00:29:17,836
[ Inaudible Remark ]

672
00:29:17,836 --> 00:29:19,056
>> Pick up knife?

673
00:29:19,556 --> 00:29:22,776
[ Inaudible Remark ]

674
00:29:23,276 --> 00:29:24,326
>> Or put it-- sorry?

675
00:29:24,326 --> 00:29:26,046
[ Inaudible Remark ]

676
00:29:26,046 --> 00:29:29,336
>> Put knife in peanut butter?

677
00:29:29,336 --> 00:29:29,536
[ Inaudible Remark ]

678
00:29:29,536 --> 00:29:30,716
>> Scoop out peanut butter?

679
00:29:30,716 --> 00:29:33,076
Right, this is very
quickly gonna devolved

680
00:29:33,076 --> 00:29:34,676
into yesterday, right.

681
00:29:34,676 --> 00:29:37,026
Scoop out-- At least we got
this part yesterday, right?

682
00:29:37,026 --> 00:29:39,006
Scoop out two table
spoons of peanut butter.

683
00:29:39,006 --> 00:29:40,186
So let's go ahead and do that.

684
00:29:40,186 --> 00:29:45,556
So, scoop out two tea spoons
of peanut butter and then?

685
00:29:45,556 --> 00:29:45,836
[ Inaudible Remark ]

686
00:29:45,836 --> 00:29:47,206
>> Spread it on piece of bread.

687
00:29:47,206 --> 00:29:50,096
But again, here too, you
know this kind of reasonable.

688
00:29:50,166 --> 00:29:51,866
Stupid but correct.

689
00:29:53,296 --> 00:29:56,546
Or at least consistent with what
you told me to do, so here too.

690
00:29:56,776 --> 00:29:59,166
So again, we don't need to turn
this into the complete drama

691
00:29:59,166 --> 00:29:59,976
that we had yesterday.

692
00:30:00,286 --> 00:30:03,806
>> But just notice all of the
various opportunities there are

693
00:30:03,806 --> 00:30:06,716
to make mistakes, all of
these possible corner cases,

694
00:30:06,896 --> 00:30:08,526
and so one of the things
that's a little tougher,

695
00:30:08,526 --> 00:30:10,536
a lot of people with
programming initially is

696
00:30:10,536 --> 00:30:12,456
that you just kinda want
this computer to behave

697
00:30:12,456 --> 00:30:15,936
as through it's a human just
responding and interpreting

698
00:30:15,936 --> 00:30:17,666
with their best human
judgment, your command.

699
00:30:17,896 --> 00:30:20,126
But the reality is that's not
gonna happen and we're gonna see

700
00:30:20,126 --> 00:30:22,636
that today with our
own programs.

701
00:30:22,636 --> 00:30:24,486
So thanks very much to Rob.

702
00:30:25,076 --> 00:30:25,756
Very well done.

703
00:30:26,516 --> 00:30:30,886
[ Applause ]

704
00:30:31,386 --> 00:30:32,966
>> Let's see if we
can now translate.

705
00:30:32,966 --> 00:30:34,116
Well, again, we'll
call pseudocode.

706
00:30:34,116 --> 00:30:36,546
And to be clear pseudocode is
something that akin to English.

707
00:30:36,546 --> 00:30:38,136
It's not a programing language.

708
00:30:38,136 --> 00:30:39,196
It's not really English.

709
00:30:39,336 --> 00:30:41,546
It's some amalgam of the two
and there's no one's standard.

710
00:30:41,546 --> 00:30:44,656
You just kinda go with your
instincts and write out words

711
00:30:44,656 --> 00:30:46,096
that kind of communicate
your idea.

712
00:30:46,096 --> 00:30:48,286
But when you're actually
commanding a computer,

713
00:30:48,446 --> 00:30:49,866
you're actually gonna
write something like this

714
00:30:49,866 --> 00:30:52,546
and we saw this in a slightly
different screen on Wednesday.

715
00:30:52,546 --> 00:30:54,946
It was a little color coded
but this is perhaps one

716
00:30:54,946 --> 00:30:58,516
of the simplest programs you can
write that a computer can run

717
00:30:58,516 --> 00:31:01,686
in this language called
C. So this is the language

718
00:31:01,686 --> 00:31:02,546
in which we'll spend a lot

719
00:31:02,546 --> 00:31:06,366
of our time transitioning
ultimately the PHP and HTML

720
00:31:06,496 --> 00:31:08,756
and CSS and SQL and
various markup

721
00:31:08,756 --> 00:31:10,876
and programming languages,
but for now we have this.

722
00:31:10,876 --> 00:31:13,536
And there's gonna be a lot
of cryptic details here.

723
00:31:13,536 --> 00:31:15,426
These include the
angle brackets,

724
00:31:15,466 --> 00:31:17,146
the parenthesis,
the curly braces.

725
00:31:17,416 --> 00:31:19,006
This is one of the
challenges early

726
00:31:19,006 --> 00:31:20,326
on for those less comfortable

727
00:31:20,326 --> 00:31:22,146
and that there's all
this crazy syntax.

728
00:31:22,216 --> 00:31:24,756
And the frustrating thing
about computers is that even

729
00:31:24,756 --> 00:31:27,876
if you do something stupid and,
you know, not even unreasonable

730
00:31:28,106 --> 00:31:30,666
like leave off the
semicolon 'cause you forgot

731
00:31:30,666 --> 00:31:32,786
or you hit a colon or some
other character instead,

732
00:31:32,786 --> 00:31:36,336
the whole thing is gonna break
whether it's 5 or 6 lines

733
00:31:36,336 --> 00:31:38,956
or 5 million lines,
a single flaw

734
00:31:38,956 --> 00:31:41,346
like that can derail
an entire program.

735
00:31:41,596 --> 00:31:43,916
And know now that these are
the kinds of silly things

736
00:31:43,916 --> 00:31:46,966
that you do tend to trip
up-- trip over early on.

737
00:31:46,966 --> 00:31:48,476
But these are the kinds
of things that you start

738
00:31:48,476 --> 00:31:50,326
to spot spontaneously.

739
00:31:50,326 --> 00:31:52,856
And so you'll find over the
next 12 weeks that your--

740
00:31:53,056 --> 00:31:55,826
the types of mistakes that
you make become increasingly

741
00:31:55,826 --> 00:31:57,646
sophisticated and
that's a good thing

742
00:31:57,906 --> 00:32:00,536
and they become increasingly
interesting to solve

743
00:32:00,536 --> 00:32:02,566
as you begin to speak
this language

744
00:32:02,566 --> 00:32:03,916
like you would of
foreign language.

745
00:32:04,136 --> 00:32:06,976
Unfortunately computers don't
really speak this language.

746
00:32:06,976 --> 00:32:10,766
This thing C is what we'll
call a high level language

747
00:32:11,006 --> 00:32:13,806
because it's pretty close
to a human's conversation.

748
00:32:13,986 --> 00:32:16,586
Alright, this is still English
word, a few new things like int.

749
00:32:16,646 --> 00:32:17,786
I'm not really sure
what that is.

750
00:32:18,076 --> 00:32:21,106
Integer it turns out, but it's
still readable by a human.

751
00:32:21,106 --> 00:32:22,316
And this wasn't always the case.

752
00:32:22,316 --> 00:32:24,796
If you heard about things
like punch cards in the like,

753
00:32:24,886 --> 00:32:28,806
programming was not always
so relatively human friendly

754
00:32:29,066 --> 00:32:31,686
because at the end of the day
computers still only understand

755
00:32:31,916 --> 00:32:32,666
things like this.

756
00:32:33,216 --> 00:32:35,316
So you might know
just in generally

757
00:32:35,316 --> 00:32:38,816
that computer somehow operate
in 0s and 1s and that's useful

758
00:32:38,816 --> 00:32:42,706
because 0s and 1s, very nicely
map to electrical things, right?

759
00:32:42,706 --> 00:32:44,466
Right now in sanders,
the lights are on.

760
00:32:44,686 --> 00:32:47,866
We could say that this huge
theater is actually storing the

761
00:32:47,866 --> 00:32:48,666
number 1.

762
00:32:48,956 --> 00:32:51,166
If we killed all of the
lights it might be storing the

763
00:32:51,166 --> 00:32:51,996
number 0.

764
00:32:52,176 --> 00:32:56,746
And this is why we have such a
simple system, a simple alphabet

765
00:32:56,746 --> 00:32:58,646
for computers because
they're electrical.

766
00:32:58,646 --> 00:32:59,776
You plug them into the wall.

767
00:32:59,776 --> 00:33:00,816
They operate on batteries.

768
00:33:01,016 --> 00:33:04,526
You have disability to either
have electricity flowing or not

769
00:33:04,836 --> 00:33:06,556
and that gives you
these two binary.

770
00:33:06,556 --> 00:33:08,156
"Bi" meaning two states

771
00:33:08,346 --> 00:33:11,496
and so you can represent
data, things in this way.

772
00:33:11,496 --> 00:33:14,716
Now these are literally the
0s and 1s that are created.

773
00:33:14,906 --> 00:33:16,566
When you start with
the program like this,

774
00:33:17,056 --> 00:33:19,626
you run it to a little
tool called a compiler,

775
00:33:19,896 --> 00:33:22,616
you get out 0s and 1s
like this and these 0s

776
00:33:22,616 --> 00:33:24,216
and 1s are just stored
in the file.

777
00:33:24,216 --> 00:33:28,216
Akin to any sort of MP3
or Microsoft Word file.

778
00:33:28,436 --> 00:33:30,706
They're just stored in
some file on a computer.

779
00:33:30,706 --> 00:33:31,826
And when you double click it

780
00:33:31,826 --> 00:33:34,366
or run some text-based
commands these 0s

781
00:33:34,366 --> 00:33:38,346
and 1s will be fed usually
8 at a time, 32 at a time,

782
00:33:38,346 --> 00:33:40,636
64 at a time into that
thing called the CPU

783
00:33:40,636 --> 00:33:42,566
which you probably
generally know is the brains

784
00:33:42,566 --> 00:33:45,546
of your computer and that
CPU then does something.

785
00:33:45,546 --> 00:33:49,586
And in this case it might print
to the screen, "Oh, hi, world."

786
00:33:49,916 --> 00:33:52,156
Well now how do you actually
get from all of these 0s

787
00:33:52,156 --> 00:33:53,506
and 1s to useful data?

788
00:33:53,996 --> 00:33:55,996
Well, it's actually
relatively straightforward.

789
00:33:56,186 --> 00:33:58,856
So I'm just gonna open up again
a simple little text document

790
00:33:58,886 --> 00:34:02,946
here just so we have something
to type with and it turns

791
00:34:02,946 --> 00:34:07,466
out that if I want to represent
the number we humans know as 0

792
00:34:07,676 --> 00:34:09,996
in binary, otherwise
known as base 2.

793
00:34:10,176 --> 00:34:13,936
Well, the world has decided
that 0 for a computer means 0

794
00:34:13,936 --> 00:34:15,486
to a human, so pretty simple.

795
00:34:15,716 --> 00:34:17,336
Well, if we wanna
do the number 1.

796
00:34:17,336 --> 00:34:18,836
It's actually pretty simple.

797
00:34:19,036 --> 00:34:21,336
Unfortunately, now we're
out of digits, right?

798
00:34:21,336 --> 00:34:25,606
The alphabet is only so
expressive so now using only 0s

799
00:34:25,606 --> 00:34:26,996
and 1s we have to
somehow be able

800
00:34:26,996 --> 00:34:30,156
to encode what we humans know
is the number 2, otherwise,

801
00:34:30,156 --> 00:34:32,136
this is gonna be the most
useless computer ever

802
00:34:32,306 --> 00:34:34,186
if the best it can
do is count up to 1.

803
00:34:34,476 --> 00:34:35,536
Surely we can do better.

804
00:34:35,886 --> 00:34:37,286
So what can we do?

805
00:34:37,556 --> 00:34:39,316
Well, much like in
the real world

806
00:34:39,316 --> 00:34:42,506
when you need another
digit, you add 9 plus 2.

807
00:34:42,506 --> 00:34:45,076
Well 9 plus 2, what do
you do when you kind

808
00:34:45,076 --> 00:34:45,956
of do that in grade school?

809
00:34:45,956 --> 00:34:48,356
You put a-- You carry
the 1, right?

810
00:34:48,356 --> 00:34:49,466
So you get another column.

811
00:34:49,736 --> 00:34:51,056
We can do the same thing here.

812
00:34:51,256 --> 00:34:54,206
So to represent the number
2, why don't we do something

813
00:34:54,206 --> 00:34:57,416
like 1-0, and the number 3, 1-1.

814
00:34:57,596 --> 00:34:59,906
And now just to make
things lined up prettily,

815
00:35:00,126 --> 00:35:03,016
let's just assume that every
number in the world uses 2 bits

816
00:35:03,016 --> 00:35:05,126
and a bit is just a 0 or a 1.

817
00:35:05,186 --> 00:35:06,376
Binary digit, bit.

818
00:35:06,826 --> 00:35:10,406
So now using 0s and 1s I can
represent what we humans think

819
00:35:10,406 --> 00:35:13,606
of as 0 and 1 and 2 and 3,

820
00:35:13,646 --> 00:35:15,686
and take a guess what's
the number 4 gonna be

821
00:35:15,686 --> 00:35:16,436
represented with?

822
00:35:17,596 --> 00:35:18,946
Yeah, 1-0-0.

823
00:35:19,126 --> 00:35:20,856
And now I can go back
and clean this up.

824
00:35:21,166 --> 00:35:23,976
To much like in the real
world, you can have as many 0s

825
00:35:23,976 --> 00:35:25,216
to the left of some number.

826
00:35:25,216 --> 00:35:28,366
It makes no material difference,
same thing with a computer here.

827
00:35:28,596 --> 00:35:32,076
But in reality computers do
use some fixed number of bits.

828
00:35:32,476 --> 00:35:35,266
So anytime you store a number
or a letter in a computer,

829
00:35:35,436 --> 00:35:39,846
it's almost always using, say,
8 bits at a time or 16 or 32

830
00:35:39,846 --> 00:35:42,806
or 64, generally,
some multiple of 2.

831
00:35:42,806 --> 00:35:44,136
There conventions
that we'll see.

832
00:35:44,406 --> 00:35:46,756
But this is precisely
the same idea.

833
00:35:47,236 --> 00:35:52,136
Now how if you were given a
number like let's say 1-1-1.

834
00:35:52,136 --> 00:35:56,796
How do you get from 1-1-1 to the
decimal, the base 10 equivalent,

835
00:35:56,796 --> 00:35:58,906
just the real world
expression of that number?

836
00:35:59,186 --> 00:36:01,936
Well, think of it how we
do this in the real world.

837
00:36:02,096 --> 00:36:04,516
If I just gonna-- let me just
move my cursor over here.

838
00:36:04,796 --> 00:36:08,596
If you have the number like
123 in the real world, well,

839
00:36:08,596 --> 00:36:12,306
how do this series
of digits, 1, 2, 3,

840
00:36:12,556 --> 00:36:15,666
translate into conceptually
what we know as 123?

841
00:36:15,876 --> 00:36:16,826
Well, again, if you think back

842
00:36:16,826 --> 00:36:18,916
to grade school this
is ones column

843
00:36:19,206 --> 00:36:23,856
and this is the tens column and
this is the hundreds column,

844
00:36:24,076 --> 00:36:26,136
and now let me just space
this out so that they kind

845
00:36:26,136 --> 00:36:27,356
of not line up nicely.

846
00:36:27,586 --> 00:36:29,026
So how do you get from 1, 2,

847
00:36:29,086 --> 00:36:32,136
3 to what we conceptually
think of as 123?

848
00:36:32,406 --> 00:36:36,186
Well, in the ones column we
have 3 so that's 1 times 3.

849
00:36:36,556 --> 00:36:39,186
In the tens column we have
a 2, so that's 10 times 2.

850
00:36:39,186 --> 00:36:42,276
In one hundreds column we have
a 1 so that's 100 times 1.

851
00:36:42,506 --> 00:36:45,196
Add all those up,
100 plus 20 plus 3.

852
00:36:45,366 --> 00:36:48,756
Viola! You get this
notion of 123.

853
00:36:49,086 --> 00:36:51,016
So now let's go back
to this challenge.

854
00:36:51,016 --> 00:36:53,846
We kind of figured this out
just by working our way up.

855
00:36:53,846 --> 00:36:55,296
Let's now work our
way backwards.

856
00:36:55,296 --> 00:36:58,746
If we start with 1-1-1, how
does the binary system work?

857
00:36:58,746 --> 00:37:01,686
Well in the binary system,
the system that computers use,

858
00:37:01,896 --> 00:37:04,636
we actually start with,
thankfully, the ones column.

859
00:37:05,066 --> 00:37:06,926
But it's not the tens
column that comes next,

860
00:37:07,026 --> 00:37:10,706
it's the twos column and after
that it's the fours column.

861
00:37:11,076 --> 00:37:15,496
And so whereas in the real
world of base 10 and decimal,

862
00:37:15,496 --> 00:37:18,916
the columns-- the columns
represent powers of 10,

863
00:37:19,176 --> 00:37:21,426
in the binary world,
binary again meaning two,

864
00:37:21,426 --> 00:37:22,816
it's just powers of 2.

865
00:37:22,916 --> 00:37:23,736
It's as simple as that.

866
00:37:23,736 --> 00:37:26,126
So you have the ones column,
the twos columns, the fours,

867
00:37:26,126 --> 00:37:29,356
the eight, sixteen, thirty
two, sixty four, and so forth.

868
00:37:29,356 --> 00:37:31,326
And so now we have the
ability to express numbers

869
00:37:31,366 --> 00:37:32,336
in just the different way.

870
00:37:32,336 --> 00:37:36,776
But the advantage is we don't
need 0 and 1 and 2 and 3 and 4

871
00:37:36,776 --> 00:37:40,236
and 5 and say, all of these-- we
don't need 10 different digits.

872
00:37:40,466 --> 00:37:43,176
All we need is to turn the
power on or turn it off.

873
00:37:43,176 --> 00:37:46,006
And by doing that rapidly
in sequence, we can build

874
00:37:46,006 --> 00:37:48,456
up some representation
in electricity

875
00:37:48,936 --> 00:37:51,216
with numbers like this.

876
00:37:51,636 --> 00:37:54,336
So with that said, if I have
a 1 in the fours column,

877
00:37:54,336 --> 00:37:56,796
a 1 in the twos column,
and the 1

878
00:37:56,886 --> 00:37:59,016
in the ones column what
does that now give me?

879
00:37:59,016 --> 00:37:59,676
[ Inaudible Remarks ]

880
00:37:59,676 --> 00:38:00,656
>> 7, right?

881
00:38:00,706 --> 00:38:04,126
1, 4 plus 1, 2 plus
1, 1, that gives me 7.

882
00:38:04,216 --> 00:38:06,576
And we can work our
way up from there.

883
00:38:06,816 --> 00:38:08,786
So just to round this out,

884
00:38:09,416 --> 00:38:13,676
the reason that we often hear
the term not so much bits

885
00:38:13,676 --> 00:38:17,176
in the real world but bytes,
megabytes, gigabytes, terabytes

886
00:38:17,176 --> 00:38:19,526
and so forth is 'cause our
single bit is actually pretty

887
00:38:19,526 --> 00:38:20,926
useless certainly these days

888
00:38:21,106 --> 00:38:22,946
and so somewhere along
the way the world decided

889
00:38:22,946 --> 00:38:26,676
that it's more useful to express
units of measure in terms

890
00:38:26,676 --> 00:38:28,356
of eight of these
things so8 bits,

891
00:38:28,356 --> 00:38:31,886
eight 0s and/or 1s is
what we call a byte.

892
00:38:32,356 --> 00:38:36,306
So if you have some crazy
short like MP3 or music file

893
00:38:36,306 --> 00:38:38,556
on your computer that
was just a single byte,

894
00:38:39,046 --> 00:38:40,026
obviously, unrealistic.

895
00:38:40,026 --> 00:38:41,206
They're usually millions
of bytes.

896
00:38:41,476 --> 00:38:43,036
That would just mean
there're 8 bits

897
00:38:43,036 --> 00:38:46,606
to compose whatever song you're
very briefly listening to.

898
00:38:46,606 --> 00:38:49,846
And if it's 3 megabytes, that
means you have 3 million bytes

899
00:38:50,196 --> 00:38:52,056
or really 24 million bits

900
00:38:52,056 --> 00:38:54,256
if you multiply the
number of bytes by 8.

901
00:38:54,356 --> 00:38:56,406
So when we talk about
a bytes worth

902
00:38:56,406 --> 00:38:58,556
of bits we have not
just the ones, the twos,

903
00:38:58,556 --> 00:39:00,216
the fours column,
we have all the way

904
00:39:00,216 --> 00:39:03,456
up to the one hundred
twenty eights column there.

905
00:39:03,456 --> 00:39:06,166
So now, a little something silly

906
00:39:06,166 --> 00:39:07,746
like this might make
some more sense.

907
00:39:08,516 --> 00:39:13,181
[ Noise ]

908
00:39:13,681 --> 00:39:18,346
[ Laughter ]

909
00:39:18,846 --> 00:39:20,126
>> We got through it at least 1.

910
00:39:20,166 --> 00:39:20,826
Excellent!

911
00:39:21,646 --> 00:39:24,176
So if it's not clicking,
come back to it later

912
00:39:24,176 --> 00:39:27,546
but now perhaps even more comic
books even will make sense.

913
00:39:27,636 --> 00:39:29,746
But numbers alone are not
all that useful, right?

914
00:39:29,746 --> 00:39:32,086
The last-- You might use
numbers in a computer

915
00:39:32,296 --> 00:39:34,696
but more likely these days
you're sending instant messages

916
00:39:34,696 --> 00:39:37,196
or downloading photos or
reading emails in the like.

917
00:39:37,456 --> 00:39:39,716
So the next step up from
just numbers is probably

918
00:39:39,716 --> 00:39:40,906
something alphabetical.

919
00:39:40,906 --> 00:39:44,196
The ability to express yourself
as a human in English or Spanish

920
00:39:44,196 --> 00:39:46,116
or whatever language,
you need to be able

921
00:39:46,116 --> 00:39:49,116
to represent other symbols
using though at the end

922
00:39:49,116 --> 00:39:50,246
of the day electricity.

923
00:39:50,246 --> 00:39:52,546
That's on or off, 0s or 1s.

924
00:39:52,686 --> 00:39:55,246
Well, thankfully the world
years ago came up with a system

925
00:39:55,376 --> 00:39:57,156
and this chart actually
looks more overwhelming

926
00:39:57,156 --> 00:39:57,816
that it needs to.

927
00:39:57,816 --> 00:39:59,646
It's just because there's
a whole lot of information.

928
00:40:00,046 --> 00:40:03,396
>> Notice that you have the
same two columns repeated again

929
00:40:03,396 --> 00:40:03,866
and again.

930
00:40:03,866 --> 00:40:06,516
Code and Char for
character, code, char,

931
00:40:06,516 --> 00:40:07,786
code, char, code, char.

932
00:40:08,046 --> 00:40:10,876
So the way to read this
is that at the top left,

933
00:40:11,156 --> 00:40:14,436
it says the number 32 and
that's a decimal number 32,

934
00:40:14,776 --> 00:40:17,056
and that means that
the space bar character

935
00:40:17,056 --> 00:40:20,166
on the keyboard is
implemented inside of a computer

936
00:40:20,336 --> 00:40:23,106
by just storing the number 32.

937
00:40:23,166 --> 00:40:24,576
How do you store the number 32?

938
00:40:24,576 --> 00:40:26,276
Well you use some
sequence of bits.

939
00:40:26,416 --> 00:40:28,876
Now how do we get to this
sequence of bits of 32?

940
00:40:28,876 --> 00:40:31,296
Well, let me just again
come to a place holder here.

941
00:40:31,296 --> 00:40:34,396
If I've got the ones column,
the twos, the fours, the eights,

942
00:40:34,396 --> 00:40:37,226
the sixteens, the thirty-twos,
I can kind of figure this out.

943
00:40:37,256 --> 00:40:40,616
This is the ones column, this
is the twos column, the fours,

944
00:40:40,616 --> 00:40:43,636
eights, sixteen,
thirty-two, wahla!

945
00:40:43,786 --> 00:40:46,786
If you wanna represent that
space bar character in something

946
00:40:46,786 --> 00:40:48,986
like an e-mail or a
Microsoft Word document,

947
00:40:49,196 --> 00:40:51,546
all the computer
stores is the bits 10000

948
00:40:51,546 --> 00:40:56,836
and actually it probably
uses at least a byte's worth.

949
00:40:56,836 --> 00:40:59,116
So this is one, two,
three, four, five, six,

950
00:40:59,386 --> 00:41:01,196
so probably what
it's really storing

951
00:41:01,196 --> 00:41:03,556
for the spacebar character
is eight total bits

952
00:41:03,596 --> 00:41:05,346
but the first two are zeros.

953
00:41:05,726 --> 00:41:06,636
Now, the spacebar is kind

954
00:41:06,666 --> 00:41:08,566
of underwhelming 'cause you
can't really even see it

955
00:41:08,566 --> 00:41:10,946
but if you look back at
this chart and fast-forward

956
00:41:10,946 --> 00:41:14,056
to roughly the middle of
the chart, what is the code,

957
00:41:14,056 --> 00:41:16,736
the decimal number for
the letter capital A?

958
00:41:16,736 --> 00:41:21,696
Yeah, it's apparently 65 and
if you scroll down to the rest

959
00:41:21,696 --> 00:41:24,986
of the chart, the lower case
A is represented with what?

960
00:41:26,206 --> 00:41:27,266
97, right?

961
00:41:27,266 --> 00:41:28,706
So let's do the 65.

962
00:41:28,736 --> 00:41:31,436
Well, 65 is also pretty easy
'cause things line up nicely.

963
00:41:31,656 --> 00:41:34,146
So this is the spacebar
character which we'll represent

964
00:41:34,146 --> 00:41:36,776
with this, even though that's a
different character altogether.

965
00:41:37,016 --> 00:41:41,256
The capital A then is gonna have
again eight bits typically, one,

966
00:41:41,256 --> 00:41:42,676
two, three, four,
five, six, seven,

967
00:41:42,676 --> 00:41:45,136
eight but this time
I need the number 65.

968
00:41:45,136 --> 00:41:48,626
So I have the ones column, the
twos, the fours, the eights,

969
00:41:48,896 --> 00:41:50,446
sixteens, thrity-twos.

970
00:41:50,446 --> 00:41:53,926
Aha! 64, that's helpful
but I need something else.

971
00:41:54,516 --> 00:41:56,146
This now needs to become a one.

972
00:41:56,406 --> 00:41:57,016
And so that's it.

973
00:41:57,016 --> 00:41:58,956
When you're hitting the
letter A on your keyboard

974
00:41:58,956 --> 00:42:01,686
to do something completely
simple like send and IM

975
00:42:01,686 --> 00:42:04,776
or send an e-mail, what's really
being stored in that e-mail

976
00:42:04,776 --> 00:42:06,806
by your computer or
in turn Gmail or the

977
00:42:06,806 --> 00:42:12,056
like is some sequence of bits,
0100001 and what you seen

978
00:42:12,056 --> 00:42:16,206
on the screen is
the letter A. So,

979
00:42:16,556 --> 00:42:19,486
how do these things actually
get stored in reality?

980
00:42:19,746 --> 00:42:22,386
Let's see if we can translate
this idea of electricity

981
00:42:22,386 --> 00:42:23,936
to something that's
a little more visual.

982
00:42:23,936 --> 00:42:26,506
Let me go ahead and open
up a little demo here,

983
00:42:26,506 --> 00:42:29,166
and we can dim the lights
just a bit if you don't mind,

984
00:42:29,626 --> 00:42:32,846
for this look inside of what's
been inside your computer all

985
00:42:32,846 --> 00:42:33,416
these years.

986
00:42:33,416 --> 00:42:34,476
[ Background Music ]

987
00:42:34,476 --> 00:42:36,996
>> The hard drive is
where your PC stores most

988
00:42:36,996 --> 00:42:38,116
of its permanent data.

989
00:42:38,706 --> 00:42:41,706
To do that, the data
travels from RAM along

990
00:42:41,706 --> 00:42:44,306
with software signals that
tell the hard drive how

991
00:42:44,306 --> 00:42:45,426
to store that data.

992
00:42:46,086 --> 00:42:48,616
The hard drive circuits
translate those signals

993
00:42:48,616 --> 00:42:50,226
into voltage fluctuations.

994
00:42:50,956 --> 00:42:53,706
These in turn control the
hard drive's moving parts,

995
00:42:54,296 --> 00:42:57,156
some of the few moving parts
left in the modern computer.

996
00:42:57,926 --> 00:42:59,586
Some of the signals
control a motor

997
00:42:59,586 --> 00:43:01,446
which spins metal
coated platters.

998
00:43:02,166 --> 00:43:04,666
Your data is actually
stored on these platters.

999
00:43:05,486 --> 00:43:08,536
Other signals move the
read/write heads to read

1000
00:43:08,536 --> 00:43:09,996
or write data on the platters.

1001
00:43:10,746 --> 00:43:12,576
This machinery is so precise

1002
00:43:13,066 --> 00:43:15,716
that a human hair couldn't
even pass between the heads

1003
00:43:15,716 --> 00:43:19,726
and spinning platters, yet it
all works at terrific speeds.

1004
00:43:20,556 --> 00:43:23,136
>> So if you've even wondered
why dropping a laptop is

1005
00:43:23,166 --> 00:43:24,976
generally a bad idea,
it's because one

1006
00:43:24,976 --> 00:43:29,206
of the few remaining mechanical
parts inside of a laptop is one,

1007
00:43:29,206 --> 00:43:31,216
the hard drive and two, the fan.

1008
00:43:31,216 --> 00:43:33,566
And that fan, is actually the
cheap, easily replaceable one.

1009
00:43:33,566 --> 00:43:35,746
This thing is a bad break
because obviously all

1010
00:43:35,746 --> 00:43:36,636
of your data is on it.

1011
00:43:36,916 --> 00:43:38,816
But we're also getting
to the point as in a side

1012
00:43:38,846 --> 00:43:41,956
where hard drives that spin like
this are increasingly passe.

1013
00:43:41,956 --> 00:43:46,986
In fact, a lot of you with Macs
or relatively new PCs have SSD,

1014
00:43:47,066 --> 00:43:49,146
solid state drives, and
what's nice there is

1015
00:43:49,146 --> 00:43:51,476
that they're still electrical
but they're indeed solid

1016
00:43:51,476 --> 00:43:53,236
and that there are
no such moving parts,

1017
00:43:53,406 --> 00:43:56,576
which means you're much
safer in tossing your laptop

1018
00:43:56,576 --> 00:43:58,426
across the room to let
it land on you bed.

1019
00:43:58,426 --> 00:44:00,836
The only thing you really risk
breaking then is maybe the

1020
00:44:00,836 --> 00:44:03,386
screen or the fan, but
at least your data,

1021
00:44:03,386 --> 00:44:05,866
with much higher probability
these days is going

1022
00:44:05,866 --> 00:44:06,296
to be in tact.

1023
00:44:06,696 --> 00:44:08,916
Though perhaps don't
try that at home.

1024
00:44:09,226 --> 00:44:11,346
Let's dive in just a little
deeper to now translate

1025
00:44:11,346 --> 00:44:15,626
that high level visual to the
actual bits that we promised.

1026
00:44:15,626 --> 00:44:15,693
[ Background Music ]

1027
00:44:15,693 --> 00:44:17,696
>> Let's look at what we
just saw in slow motion.

1028
00:44:18,396 --> 00:44:20,766
When a great pulse of
electricity is sent

1029
00:44:20,766 --> 00:44:23,906
to the read/write head it
flips on a tiny electromagnet

1030
00:44:23,906 --> 00:44:24,976
for a fraction of a second.

1031
00:44:25,126 --> 00:44:29,116
The magnet creates a field which
changes the polarity of a tiny,

1032
00:44:29,116 --> 00:44:31,236
tiny portion of the
metal particles

1033
00:44:31,286 --> 00:44:32,436
which coat each platter surface.

1034
00:44:32,436 --> 00:44:36,036
A pattern series of these
tiny charged batteries

1035
00:44:36,036 --> 00:44:39,516
on the disk represents
a single bit of data

1036
00:44:39,516 --> 00:44:43,486
in the binary number
system used by computers.

1037
00:44:43,516 --> 00:44:44,386
Now, if the current
is sent one way

1038
00:44:44,416 --> 00:44:45,526
through the read/write
head the area is polarized

1039
00:44:45,556 --> 00:44:45,976
in one direction.

1040
00:44:46,046 --> 00:44:48,636
If the current is sent in
the opposite direction,

1041
00:44:48,636 --> 00:44:50,726
the polarization is reversed.

1042
00:44:50,726 --> 00:44:53,386
How do you get data
off the hard disk?

1043
00:44:53,386 --> 00:44:54,406
Just reverse the process.

1044
00:44:54,946 --> 00:44:56,746
So as the particles on the
disk that get the current

1045
00:44:56,746 --> 00:45:00,146
in the read/write head
moving put together millions

1046
00:45:00,146 --> 00:45:01,976
of these magnetized segments
and you've got a file.

1047
00:45:02,876 --> 00:45:06,186
Now, the pieces of a
single may be scattered all

1048
00:45:06,186 --> 00:45:08,576
over the drive's platters,
kind of like the mess

1049
00:45:08,636 --> 00:45:09,776
of papers on your desk.

1050
00:45:10,836 --> 00:45:14,136
So a special extra file keeps
track of where everything is.

1051
00:45:14,276 --> 00:45:15,936
Don't you wish you had
something like that?

1052
00:45:17,126 --> 00:45:19,926
>> So to summarize, even though
we began the notion of bits

1053
00:45:19,926 --> 00:45:22,686
with electricity, you can't
always store things electrically

1054
00:45:22,686 --> 00:45:24,356
if there's no power
and certainly most

1055
00:45:24,356 --> 00:45:27,336
of us unplug our laptops or
desktops or just lose power once

1056
00:45:27,336 --> 00:45:29,566
in a while and then it would
be nice if we not lose all

1057
00:45:29,566 --> 00:45:30,646
of these zeros and ones.

1058
00:45:30,866 --> 00:45:32,866
And so, thus was
born magnetic storage

1059
00:45:32,866 --> 00:45:34,196
which again per the inside

1060
00:45:34,196 --> 00:45:37,126
of this hard drive stores
tiny little magnetic particles

1061
00:45:37,156 --> 00:45:39,296
that have some poles,
north pole, south pole,

1062
00:45:39,466 --> 00:45:41,196
and the idea is that
you take that particle

1063
00:45:41,196 --> 00:45:42,586
and if you wanna
represent a zero,

1064
00:45:42,796 --> 00:45:44,116
you sort of orient it this way

1065
00:45:44,286 --> 00:45:46,716
or if you instead you
wanna represent in a one,

1066
00:45:46,716 --> 00:45:50,046
you flip it around so that
it's south-north instead

1067
00:45:50,046 --> 00:45:50,806
of north-south.

1068
00:45:50,806 --> 00:45:53,666
And then-- there are yet other
ways to represent these bits

1069
00:45:53,796 --> 00:45:57,476
but for now at this point in
the story, we can now just take

1070
00:45:57,476 --> 00:46:00,136
for granted that there
exists some technology

1071
00:46:00,136 --> 00:46:03,396
or some technologies with which
to represent zeros and ones

1072
00:46:03,436 --> 00:46:06,936
and to covert high level
things like C and as we're

1073
00:46:06,936 --> 00:46:09,586
about to see Scratch
into the zeros and ones

1074
00:46:09,856 --> 00:46:11,236
that computers understand.

1075
00:46:11,476 --> 00:46:15,276
So the program with which we
began today was called Scratch.

1076
00:46:15,546 --> 00:46:17,646
This is a graphical
programming language developed

1077
00:46:17,646 --> 00:46:19,596
by MIT's media lab
down the road the

1078
00:46:19,736 --> 00:46:23,196
and Nicole's program here is
the result of her dragging

1079
00:46:23,196 --> 00:46:27,076
and dropping puzzle pieces
into this middle palette here,

1080
00:46:27,076 --> 00:46:30,676
the so-called scripts area,
such that anytime she wanted

1081
00:46:30,676 --> 00:46:33,596
to command one of these little
characters to do something,

1082
00:46:33,856 --> 00:46:37,366
she had to tell it very
precisely to move up,

1083
00:46:37,366 --> 00:46:39,046
to move down, to move
left, to move right,

1084
00:46:39,266 --> 00:46:42,796
maybe to conditionally move in
some direction, if too close

1085
00:46:43,136 --> 00:46:47,226
to the policeman, then move to
the left else move to the right.

1086
00:46:47,226 --> 00:46:49,896
So you can implement
interactions between all

1087
00:46:49,896 --> 00:46:51,856
of these various
characters on the screen

1088
00:46:52,076 --> 00:46:54,186
and you can have
sounds and costumes

1089
00:46:54,186 --> 00:46:55,186
and you can really have fun

1090
00:46:55,186 --> 00:46:57,916
with it once you
understand the basics.

1091
00:46:57,916 --> 00:46:58,806
So let me go ahead and do this.

1092
00:46:58,806 --> 00:47:01,056
Let me go ahead and just
create a new file with this.

1093
00:47:01,346 --> 00:47:03,736
This author is freely available
and in problem set one,

1094
00:47:03,736 --> 00:47:05,976
both standard and hacker edition
where you'll be challenged

1095
00:47:05,976 --> 00:47:07,226
to use this for the first time.

1096
00:47:07,466 --> 00:47:08,746
Let's just get a sense of this

1097
00:47:08,746 --> 00:47:11,636
and how we can now express
ourselves using this very

1098
00:47:11,776 --> 00:47:14,666
simple, very low-barrier
language, but you'll see

1099
00:47:14,666 --> 00:47:17,816
in just a minute with some more
visual demos just how fancy your

1100
00:47:17,816 --> 00:47:18,966
programs can become.

1101
00:47:19,266 --> 00:47:22,426
So at top right here, we'll
have what we'll call the stage.

1102
00:47:22,426 --> 00:47:24,806
So this is just a rectangular
area in which Scratch,

1103
00:47:25,136 --> 00:47:28,266
the default sprite who looks
like this cat here lives.

1104
00:47:28,306 --> 00:47:31,216
And the stage is where things
can move up, down, left, right,

1105
00:47:31,276 --> 00:47:32,816
spin around, and so forth.

1106
00:47:33,076 --> 00:47:36,746
On the middle of the screen, we
have the so-called scripts area

1107
00:47:37,006 --> 00:47:39,976
as this tab name suggests and
this is where we're gonna drag

1108
00:47:39,976 --> 00:47:42,846
and drop these puzzle pieces
and hopefully they'll interlock

1109
00:47:42,846 --> 00:47:45,836
in a nice graphical way that
informs me, the programmer,

1110
00:47:45,836 --> 00:47:48,806
that indeed this is okay to
connect them in this way.

1111
00:47:49,016 --> 00:47:52,486
The end result might not be what
I intend but at least if I want

1112
00:47:52,486 --> 00:47:55,406
to do something conditionally
just like Rob indented

1113
00:47:55,406 --> 00:47:57,996
with the spacebar when using
a little text editor here,

1114
00:47:58,156 --> 00:48:00,906
we're gonna see puzzle pieces
that look graphically like this,

1115
00:48:01,216 --> 00:48:03,486
that's an if condition,
and the--

1116
00:48:03,486 --> 00:48:05,466
it has just enough
space on the inside

1117
00:48:05,466 --> 00:48:07,276
so that you can fit another
puzzle piece in there

1118
00:48:07,486 --> 00:48:10,226
and it only interlocks
if it's appropriate in--

1119
00:48:10,606 --> 00:48:12,756
to actually put those
two pieces together.

1120
00:48:12,966 --> 00:48:14,126
Well, what are all these pieces?

1121
00:48:14,156 --> 00:48:19,016
Well, the third and final
feature of the program is the--

1122
00:48:19,016 --> 00:48:22,296
the palette here of all of
your various puzzle pieces

1123
00:48:22,296 --> 00:48:25,016
and they're nicely
color-coded into categories

1124
00:48:25,016 --> 00:48:29,016
like motion-related puzzle
pieces, looks, sounds, controls,

1125
00:48:29,016 --> 00:48:31,876
sensing and so forth, and
so initially you'll see

1126
00:48:31,876 --> 00:48:34,486
that you just don't really know
where to look but you kinda know

1127
00:48:34,486 --> 00:48:36,726
as you'll probably find in
the problem set, what you want

1128
00:48:36,726 --> 00:48:38,206
to achieve and as in the side,

1129
00:48:38,206 --> 00:48:40,056
the problem set is
actually quite open-ended.

1130
00:48:40,056 --> 00:48:43,286
As you'll see, it's going to
be make something of interest

1131
00:48:43,286 --> 00:48:46,226
to you with Scratch, and so
you'll come up with an idea

1132
00:48:46,226 --> 00:48:49,456
and you'll then have to figure
how now to express that.

1133
00:48:49,506 --> 00:48:51,346
So let's express the
simplest of programs.

1134
00:48:51,346 --> 00:48:53,016
I'm gonna click the
control palette,

1135
00:48:53,236 --> 00:48:55,366
which is where I have
puzzle pieces with which

1136
00:48:55,366 --> 00:48:58,006
to really get things
started and I'm gonna drag

1137
00:48:58,006 --> 00:49:01,476
and drop the very first
one when green flag clicks.

1138
00:49:01,906 --> 00:49:04,856
Now this shape, notice, has
a little curved edge on top

1139
00:49:04,856 --> 00:49:06,686
and that just kind of
implies to you graphically

1140
00:49:06,686 --> 00:49:09,476
that you can't put anything on
top but the little indentation

1141
00:49:09,476 --> 00:49:11,536
on the bottom is much like
a real world puzzle piece.

1142
00:49:11,586 --> 00:49:12,766
Something can lock into that.

1143
00:49:13,166 --> 00:49:15,276
Well what do I want
Scratch to perhaps do?

1144
00:49:15,596 --> 00:49:18,846
Let me go to the looks category
and there's a whole bunch

1145
00:49:18,846 --> 00:49:21,676
of stuff here and I don't really
care about all the intricacies

1146
00:49:21,676 --> 00:49:23,536
but I'm gonna go ahead
and drag this one, say.

1147
00:49:23,536 --> 00:49:26,256
And now notice graphically
if I move it close to it,

1148
00:49:26,256 --> 00:49:27,876
it kind of wants
to snap together.

1149
00:49:28,086 --> 00:49:30,986
So if I let go of my mouse,
it indeed snaps together

1150
00:49:30,986 --> 00:49:34,726
and I can make it say exactly
what that other program might--

1151
00:49:34,806 --> 00:49:37,246
might say, "Oh, hi world!"

1152
00:49:37,246 --> 00:49:40,176
And now if I zoom out,
nothing's happened just

1153
00:49:40,176 --> 00:49:43,036
yet because the way
to start the program,

1154
00:49:43,036 --> 00:49:46,516
to double click its icons so to
speak is, as you might infer,

1155
00:49:46,516 --> 00:49:47,896
by clicking this green flag.

1156
00:49:47,996 --> 00:49:50,616
So if I click the green
flag, the program runs,

1157
00:49:50,816 --> 00:49:53,286
which means all those puzzle
pieces start executing.

1158
00:49:53,286 --> 00:49:56,396
They start running top to bottom
and as soon as it hits the one

1159
00:49:56,686 --> 00:49:58,376
that it says, say
"Oh, hi world",

1160
00:49:58,766 --> 00:50:00,906
that's indeed what Scratch does.

1161
00:50:01,426 --> 00:50:02,896
>> Well, what is
this an example of?

1162
00:50:02,936 --> 00:50:04,916
Well, we can start
tossing some jargon

1163
00:50:04,916 --> 00:50:07,496
on this that's actually
going to come back

1164
00:50:07,496 --> 00:50:10,276
in every single future
language we explore.

1165
00:50:10,486 --> 00:50:12,926
So, this here is a
self-contained program

1166
00:50:12,926 --> 00:50:14,126
that we ourselves just wrote.

1167
00:50:14,296 --> 00:50:16,216
But generally, anything
that's purple

1168
00:50:16,216 --> 00:50:17,906
in Scratch is what
we'll call a statement.

1169
00:50:17,986 --> 00:50:20,056
It just tells the
Scratch program

1170
00:50:20,236 --> 00:50:22,366
or the Scratch character
to do something.

1171
00:50:22,596 --> 00:50:24,286
You might have "Wait
one second".

1172
00:50:24,426 --> 00:50:26,496
This would be another statement
that does what it says.

1173
00:50:26,736 --> 00:50:28,836
You might have-- and then
now it's getting interesting,

1174
00:50:28,836 --> 00:50:29,726
"Play sound meow."

1175
00:50:30,286 --> 00:50:31,786
So now we have the means

1176
00:50:31,786 --> 00:50:33,346
of expressing things
audio [inaudible]

1177
00:50:33,786 --> 00:50:35,636
like Nicole did with
her program.

1178
00:50:35,806 --> 00:50:37,726
And if we actually now
wanna integrate these

1179
00:50:37,726 --> 00:50:39,816
into our programs, let's
see what we might do.

1180
00:50:39,856 --> 00:50:42,016
I'm gonna go ahead
and now open up rather

1181
00:50:42,016 --> 00:50:43,966
than writing all these
programs from Scratch.

1182
00:50:44,306 --> 00:50:46,906
Another such example, all
of which are posted online

1183
00:50:46,906 --> 00:50:49,326
so that can play
yourself, and you can see

1184
00:50:49,326 --> 00:50:51,076
and infer pretty
straight forwardly even

1185
00:50:51,076 --> 00:50:53,586
if you've never programmed
a computer before what this

1186
00:50:53,586 --> 00:50:54,196
thing's gonna do.

1187
00:50:54,576 --> 00:50:56,226
It's apparently gonna
say, "Oh, hi world,"

1188
00:50:56,226 --> 00:50:57,746
for a second and
it's gonna wait.

1189
00:50:57,746 --> 00:50:59,456
It's gonna say it
again, it's gonna wait,

1190
00:50:59,456 --> 00:51:01,236
it's gonna say it
again, alright?

1191
00:51:01,236 --> 00:51:02,316
So let's see what happens.

1192
00:51:02,356 --> 00:51:03,526
Let's go up to the top right.

1193
00:51:03,746 --> 00:51:05,906
As in the side, you can also
full screen these things

1194
00:51:05,906 --> 00:51:07,596
by clicking a certain icon.

1195
00:51:07,926 --> 00:51:09,576
So now if I wanna
play this full screen,

1196
00:51:10,216 --> 00:51:14,176
we have a program that's
three times as good

1197
00:51:14,176 --> 00:51:16,356
but just as underwhelming.

1198
00:51:16,356 --> 00:51:19,286
So let's see if we can make
this a little more interesting.

1199
00:51:19,286 --> 00:51:20,536
Let me go ahead instead and open

1200
00:51:20,536 --> 00:51:22,086
up version three
of this program.

1201
00:51:23,436 --> 00:51:25,996
And now, it's gonna be a little
more interesting 'cause I'm

1202
00:51:25,996 --> 00:51:27,846
gonna make use of that
other type of statement.

1203
00:51:28,516 --> 00:51:30,846
[ Cat Meowing ]

1204
00:51:31,346 --> 00:51:33,676
[ Laughter ]

1205
00:51:34,176 --> 00:51:36,556
>> Alright, so now,
how did this happen?

1206
00:51:36,786 --> 00:51:38,606
Well, we just used a
different statement.

1207
00:51:38,606 --> 00:51:40,946
Instead of doing the say,
which was a visual one,

1208
00:51:40,946 --> 00:51:44,306
we use the audio one, play sound
meow, and it just starts to flex

1209
00:51:44,306 --> 00:51:45,406
around with the various menus,

1210
00:51:45,406 --> 00:51:47,656
you can import other
sounds as in the side.

1211
00:51:47,926 --> 00:51:51,396
You can go here, you can
say import, we can then go

1212
00:51:51,396 --> 00:51:57,366
into things like animal and
we can-- we hear crickets,

1213
00:51:57,626 --> 00:52:03,846
we can hear [noise] oh,
I got to click on it.

1214
00:52:03,846 --> 00:52:04,456
[Noise] And so forth.

1215
00:52:04,586 --> 00:52:13,936
That has no academic
bearing onto that.

1216
00:52:14,396 --> 00:52:16,916
So, let's see if we
now can take things

1217
00:52:16,956 --> 00:52:18,916
to the next level 'cause
this is not the most exciting

1218
00:52:18,916 --> 00:52:19,346
of programs.

1219
00:52:19,346 --> 00:52:21,196
Yes, we're commanding the
computer to do something

1220
00:52:21,196 --> 00:52:23,976
but I'm not yet all that
excited, so I need a way

1221
00:52:23,976 --> 00:52:26,436
to express more interesting
forms of logic.

1222
00:52:26,776 --> 00:52:29,716
So it turns out once upon a time
there was a man named Mr. Boole,

1223
00:52:29,716 --> 00:52:31,506
whose name is now
attached to this notion,

1224
00:52:31,676 --> 00:52:34,446
a Boolean expression in
programming, not just Scratch,

1225
00:52:34,446 --> 00:52:37,216
really any programming
language out there is something

1226
00:52:37,216 --> 00:52:40,616
that is either true
or false, yes or no,

1227
00:52:40,616 --> 00:52:42,746
on or off, one or zero.

1228
00:52:42,936 --> 00:52:45,706
So long as you have some kind of
binary answer, you can express

1229
00:52:45,706 --> 00:52:47,566
that as a Boolean expression.

1230
00:52:47,786 --> 00:52:51,266
Now in Scratch, Boolean
expressions have this diagonal

1231
00:52:51,266 --> 00:52:53,556
shape here and inside of that,

1232
00:52:53,556 --> 00:52:56,246
in this case is touching
mouse pointer question mark.

1233
00:52:56,456 --> 00:52:58,946
So a Boolean expression is
indeed some kind of question

1234
00:52:58,946 --> 00:53:01,696
that you can ask and it's
either going to evaluate

1235
00:53:01,926 --> 00:53:04,326
to true or false, yes or no.

1236
00:53:04,326 --> 00:53:07,006
So in this case, you can ask
Scratch to do something only

1237
00:53:07,006 --> 00:53:08,936
if it's touching
the mouse pointer.

1238
00:53:08,936 --> 00:53:11,516
That is the little arrow that
I move around with my mouse.

1239
00:53:11,746 --> 00:53:15,186
You might also do something
like this, is the mouse down?

1240
00:53:15,456 --> 00:53:17,976
You might also say is some
number on the left less

1241
00:53:18,096 --> 00:53:19,276
than some number on the right?

1242
00:53:19,516 --> 00:53:21,586
You might instead say is it--

1243
00:53:21,836 --> 00:53:25,156
the case that the thing
on the left and the thing

1244
00:53:25,156 --> 00:53:26,966
on the right are both true.

1245
00:53:26,966 --> 00:53:28,796
So you can check two
conditions at once.

1246
00:53:28,796 --> 00:53:31,586
Am I touching some sprite and
am I also touching the mouse,

1247
00:53:31,816 --> 00:53:33,226
and together can you then build

1248
00:53:33,226 --> 00:53:35,206
up even more interesting
expressions.

1249
00:53:35,476 --> 00:53:37,986
And there are not-- this is
not an unfamiliar concept.

1250
00:53:37,986 --> 00:53:40,666
If you use for instance
CS50's encore shopping tool

1251
00:53:40,796 --> 00:53:43,486
or really any, frankly,
forum on the web,

1252
00:53:43,716 --> 00:53:45,406
you've often encountered
check boxes

1253
00:53:45,406 --> 00:53:47,006
and radio buttons and so forth.

1254
00:53:47,126 --> 00:53:48,836
Well, how are we
implementing something

1255
00:53:48,836 --> 00:53:50,746
like these filters
one Harvard courses?

1256
00:53:51,016 --> 00:53:53,036
Well, we're essentially
implementing this notion

1257
00:53:53,246 --> 00:53:54,386
of a Boolean expression.

1258
00:53:54,646 --> 00:53:58,916
If you check the box offered
this year, that is only going

1259
00:53:58,916 --> 00:54:02,256
to return true for courses
that are indeed offered

1260
00:54:02,256 --> 00:54:05,206
in fall 2011 or spring 2012.

1261
00:54:05,416 --> 00:54:07,006
So we too are embodying
this idea

1262
00:54:07,006 --> 00:54:09,276
of a Boolean expression all
throughout the source code

1263
00:54:09,276 --> 00:54:10,546
that implements Harvard courses

1264
00:54:10,826 --> 00:54:14,126
and asking ourselves before
we show any search results,

1265
00:54:14,386 --> 00:54:17,296
yes or no, true or
false, one or zero.

1266
00:54:17,296 --> 00:54:18,226
And it's the same idea.

1267
00:54:18,226 --> 00:54:19,556
It's not expressed graphically.

1268
00:54:19,556 --> 00:54:21,196
It's expressed in a
language called PHP,

1269
00:54:21,196 --> 00:54:23,606
which we ourselves will
use to our term's end.

1270
00:54:23,876 --> 00:54:24,996
The ideas are the same.

1271
00:54:25,186 --> 00:54:26,596
Now, Boolean expressions
are alone--

1272
00:54:27,036 --> 00:54:28,756
are only yes or no questions.

1273
00:54:28,976 --> 00:54:31,636
You still need to act on them
and go off in this direction

1274
00:54:31,636 --> 00:54:33,996
or this way, when you
encounter some fork in the road.

1275
00:54:34,216 --> 00:54:36,286
So Scratch has puzzle
pieces that looks like this

1276
00:54:36,286 --> 00:54:39,476
as I depicted earlier where if
something is true then you wanna

1277
00:54:39,476 --> 00:54:41,426
do what's inside of this block.

1278
00:54:41,636 --> 00:54:43,006
But you can be fancier still

1279
00:54:43,056 --> 00:54:45,356
if something is true
else you can do this,

1280
00:54:45,356 --> 00:54:47,286
and Rob did that in our example

1281
00:54:47,496 --> 00:54:50,396
of the slightly refined PBK
example and you can actually

1282
00:54:50,396 --> 00:54:51,706
in Scratch stack these things.

1283
00:54:51,976 --> 00:54:53,996
So even though the
space between the if

1284
00:54:53,996 --> 00:54:55,736
and the else there is
actually pretty small

1285
00:54:55,936 --> 00:54:58,256
such that there's really no
puzzle pieces that fit in there.

1286
00:54:58,436 --> 00:55:01,716
As we'll see in a moment as you
drag puzzle pieces toward things

1287
00:55:01,716 --> 00:55:04,416
like this they will also
grow in a similar way

1288
00:55:04,656 --> 00:55:06,436
to fit additional puzzle pieces.

1289
00:55:06,436 --> 00:55:07,856
So we can even do
things like this.

1290
00:55:08,296 --> 00:55:12,856
If else if else, and now you can
have a three-way intersection,

1291
00:55:13,036 --> 00:55:15,346
a three-way decision
points in your program.

1292
00:55:15,606 --> 00:55:17,526
So let's now see what
this allows us to do.

1293
00:55:17,526 --> 00:55:19,706
Let me go back to
my examples of high.

1294
00:55:20,156 --> 00:55:21,936
Let me open the fourth
such iteration,

1295
00:55:22,356 --> 00:55:24,726
and I can do something
pretty stupid but at least

1296
00:55:24,726 --> 00:55:28,736
to demonstrate that this works
if 1 is less than 2, play meow.

1297
00:55:28,936 --> 00:55:29,636
Meow! Works.

1298
00:55:30,876 --> 00:55:32,656
Meow! Works.

1299
00:55:34,006 --> 00:55:38,656
Meow! Okay, obviously,
it's gonna keep working.

1300
00:55:38,966 --> 00:55:41,196
So we can do more
interesting conditions.

1301
00:55:41,196 --> 00:55:44,466
So let's open a fifth iteration
of this, and now you see

1302
00:55:44,466 --> 00:55:46,386
that the program can get
more interesting especially

1303
00:55:46,386 --> 00:55:49,066
if the program you might start
creating in your head is a game.

1304
00:55:49,066 --> 00:55:52,126
You probably don't want the game
to be completely deterministic.

1305
00:55:52,206 --> 00:55:54,116
Every time you play
it, the bad guys are

1306
00:55:54,156 --> 00:55:56,826
in the exact same place, makes
it pretty easy to win, right?

1307
00:55:56,826 --> 00:55:59,726
Most any popular game has some
kind of randomness built in

1308
00:55:59,726 --> 00:56:02,526
and computers can allow us
what's called pseudorandomness.

1309
00:56:02,596 --> 00:56:04,426
They can come up with
[inaudible] looks random

1310
00:56:04,426 --> 00:56:06,686
but it actually isn't but
more on that in the future.

1311
00:56:06,866 --> 00:56:09,536
Scratch too can do something
like pick a random number from 1

1312
00:56:09,536 --> 00:56:13,246
to 10 and you can then ask the
questions if it's less than 6,

1313
00:56:13,246 --> 00:56:16,356
well, in other words, with 50
percent probability, go ahead

1314
00:56:16,356 --> 00:56:17,616
and play the sound meow.

1315
00:56:17,866 --> 00:56:18,706
So let's try this now.

1316
00:56:18,706 --> 00:56:22,356
I'll click the green flag,
nothing, click it again.

1317
00:56:23,576 --> 00:56:28,806
Meow! Yes, again,
nothing, nothing, nothing.

1318
00:56:29,476 --> 00:56:30,506
Meow! There we go.

1319
00:56:30,666 --> 00:56:32,416
If we do this in
infinite number of times,

1320
00:56:32,586 --> 00:56:34,896
50 percent of them
will indeed be on

1321
00:56:35,056 --> 00:56:36,996
and the other 50
percents will be off.

1322
00:56:37,236 --> 00:56:40,106
Now this too is making
for very short programs.

1323
00:56:40,106 --> 00:56:42,556
And so in most any program we
use today, there are some notion

1324
00:56:42,556 --> 00:56:44,476
of looping, doing
something again

1325
00:56:44,476 --> 00:56:45,956
and again and again or forever.

1326
00:56:46,116 --> 00:56:48,346
Even the simplest thing on
your computer like the clock

1327
00:56:48,376 --> 00:56:49,446
that tells you what time it is,

1328
00:56:49,706 --> 00:56:51,986
that's probably a program
implemented with some kind

1329
00:56:51,986 --> 00:56:55,096
of loop, an infinite loop
at that because you want it

1330
00:56:55,096 --> 00:56:58,496
to do something, keep
updating the seconds every time

1331
00:56:58,616 --> 00:57:01,336
on every strike of the clock.

1332
00:57:01,336 --> 00:57:03,606
So you might wanna have
loops that repeat infinitely,

1333
00:57:03,816 --> 00:57:07,276
some loops that repeat finitely
but in short you wanna be able

1334
00:57:07,276 --> 00:57:08,876
to do something again and again.

1335
00:57:09,136 --> 00:57:12,046
And now our programs again
can get progressively neither.

1336
00:57:12,296 --> 00:57:16,036
So in variant 6 here, notice
I don't have to drag and drop

1337
00:57:16,036 --> 00:57:18,776
at [inaudible] all these puzzle
pieces saying again and again

1338
00:57:18,826 --> 00:57:20,326
and again and again,
do something.

1339
00:57:20,616 --> 00:57:22,636
I can express it much more
succinctly with a loop.

1340
00:57:22,966 --> 00:57:25,256
And so here, I might play
this particular program.

1341
00:57:30,066 --> 00:57:34,436
[Cat sound] This too will
get annoying frankly quickly.

1342
00:57:35,636 --> 00:57:37,446
Your roommates will
be thrilled this week.

1343
00:57:38,046 --> 00:57:39,286
But let's open something

1344
00:57:39,346 --> 00:57:41,606
that does a little
something more dynamic.

1345
00:57:41,656 --> 00:57:43,406
This is version 7 of this.

1346
00:57:43,806 --> 00:57:45,076
So this is kind of interesting.

1347
00:57:45,076 --> 00:57:47,056
And now again, the programs
are getting bigger and bigger.

1348
00:57:47,156 --> 00:57:49,326
And to be clear, this is
exactly where Nicole started.

1349
00:57:49,326 --> 00:57:50,766
You'll see that we're
only gonna equip you

1350
00:57:50,766 --> 00:57:53,296
with the basic concepts
and fundamental skills here

1351
00:57:53,486 --> 00:57:55,716
and then it will be up
to you to run with it.

1352
00:57:55,926 --> 00:57:58,636
So when green flag is clicked,
do the following forever.

1353
00:57:58,906 --> 00:58:02,346
If I'm ever touching the mouse
pointer, that is the cat,

1354
00:58:02,706 --> 00:58:04,886
play sound meow then
wait 2 seconds.

1355
00:58:05,106 --> 00:58:06,246
So let's see how this works.

1356
00:58:06,246 --> 00:58:08,516
Let me hit play,
nothing happens.

1357
00:58:08,516 --> 00:58:10,586
Now, notice the puzzle
pieces do highlight in white

1358
00:58:10,586 --> 00:58:11,536
when they're actually running

1359
00:58:11,536 --> 00:58:13,076
so you can see your
program running.

1360
00:58:13,326 --> 00:58:17,886
Now if I move my mouse
toward the cat, [cat sound].

1361
00:58:18,276 --> 00:58:20,106
So it's kind of like
petting the cat.

1362
00:58:20,456 --> 00:58:22,896
So that might be how
we implement that.

1363
00:58:23,096 --> 00:58:26,296
Now we might go to
say version 8 of this

1364
00:58:26,296 --> 00:58:28,656
and let's see how
this cat behaves.

1365
00:58:28,686 --> 00:58:30,076
So this is sort of
the opposite kind

1366
00:58:30,076 --> 00:58:35,936
of cat whereby [background
cat sound] he meows and meow.

1367
00:58:35,936 --> 00:58:37,316
So again there's
some loop in there.

1368
00:58:37,316 --> 00:58:39,066
This is not a cat
you wanna touch.

1369
00:58:39,206 --> 00:58:42,106
[Growl] And then he does that.

1370
00:58:42,106 --> 00:58:43,696
Well, how does that
actually get implemented?

1371
00:58:43,696 --> 00:58:45,936
Well we just need another
fork in the road, right?

1372
00:58:45,936 --> 00:58:49,396
If touching mouse pointer, then
play some lion sound, else,

1373
00:58:49,556 --> 00:58:52,526
play meow again and
again and again.

1374
00:58:53,026 --> 00:58:55,636
Well, another very common
constructing programming is this

1375
00:58:55,636 --> 00:58:58,396
notion of a variable, something
that you take for granted

1376
00:58:58,396 --> 00:59:01,836
in algebra but in algebra, X and
Y and Z are not very interesting

1377
00:59:01,836 --> 00:59:03,816
or expressive but they
still store values.

1378
00:59:03,816 --> 00:59:06,876
In programming, you wanna store
more interesting things not just

1379
00:59:06,876 --> 00:59:09,206
numbers but maybe
sentences or user input,

1380
00:59:09,306 --> 00:59:10,466
photos, they've uploaded.

1381
00:59:10,686 --> 00:59:13,266
And so in Scratch, you can
define what are called variables

1382
00:59:13,496 --> 00:59:17,366
not using just boring names like
X, Y and Z but socks on feet

1383
00:59:17,456 --> 00:59:21,376
or the slices of bread
left and so forth.

1384
00:59:21,476 --> 00:59:23,706
And you can check those
values in your program.

1385
00:59:23,926 --> 00:59:27,726
So now we'll transition to a
much more dynamic program also

1386
00:59:27,726 --> 00:59:28,896
in by a former student.

1387
00:59:29,246 --> 00:59:31,176
This one called Fruit-Craft RPG.

1388
00:59:31,176 --> 00:59:35,486
And in this Scratch program,
which again written by a student

1389
00:59:35,486 --> 00:59:36,976
after just a couple of days,

1390
00:59:37,166 --> 00:59:38,556
runs a little something
like this.

1391
00:59:38,556 --> 00:59:40,696
At top left, you see
a graphical depiction

1392
00:59:40,856 --> 00:59:43,886
and this is just something he
drag and drop into his program

1393
00:59:43,886 --> 00:59:46,676
and it shows you the
contents of these variables.

1394
00:59:47,166 --> 00:59:48,666
Now notice that with my mouse,

1395
00:59:48,926 --> 00:59:50,606
I can now have things
that are interactive.

1396
00:59:50,916 --> 00:59:53,176
I can walk around this
guy's home and table.

1397
00:59:53,476 --> 00:59:55,736
I can follow the
arrow out the door.

1398
00:59:55,916 --> 00:59:58,096
And now think about how
this might be implemented.

1399
00:59:58,416 --> 01:00:01,436
It's many more script, it's
many more puzzle pieces

1400
01:00:01,436 --> 01:00:01,976
but what's it doing?

1401
01:00:02,046 --> 01:00:04,666
>> There's probably a
loop that says forever.

1402
01:00:04,876 --> 01:00:07,916
If down arrow pressed,
then what does it do?

1403
01:00:08,006 --> 01:00:09,916
Well, there's another
puzzle piece that says move

1404
01:00:09,916 --> 01:00:12,716
and you say 10 pixels,
10 dots on the screen.

1405
01:00:12,906 --> 01:00:14,536
And if you have another
"if" condition that says,

1406
01:00:14,536 --> 01:00:16,856
"If up arrow pressed,
go up or left

1407
01:00:17,126 --> 01:00:18,876
or right," the exact same ideas.

1408
01:00:19,186 --> 01:00:20,796
So now notice I'm
outside my house.

1409
01:00:20,796 --> 01:00:22,046
I can go over to this orange.

1410
01:00:22,486 --> 01:00:23,996
And now notice at top left,

1411
01:00:24,256 --> 01:00:26,846
we see what's generally
called an array.

1412
01:00:26,846 --> 01:00:29,276
An array is the simplest of this
thing called a data structure,

1413
01:00:29,276 --> 01:00:30,436
a word I use on Wednesday.

1414
01:00:30,436 --> 01:00:32,616
But a data structure is
generally like a bucket

1415
01:00:32,816 --> 01:00:35,476
where you can store not just
one but any number of things,

1416
01:00:35,516 --> 01:00:36,986
in this case some fruit.

1417
01:00:36,986 --> 01:00:37,676
Let's go get this.

1418
01:00:38,516 --> 01:00:39,266
Let's go get this

1419
01:00:39,266 --> 01:00:40,606
on the question marks
we're [inaudible].

1420
01:00:40,946 --> 01:00:42,116
So remember that you can check

1421
01:00:42,116 --> 01:00:43,976
if you're touching
something else on the screen,

1422
01:00:43,976 --> 01:00:45,466
and sure enough, if
I go close enough,

1423
01:00:45,666 --> 01:00:46,846
there's an "if' condition
that says,

1424
01:00:46,846 --> 01:00:49,806
"If sprite touching
question mark sign,

1425
01:00:50,106 --> 01:00:52,426
apparently it's gonna
then say, got some fruit,

1426
01:00:52,426 --> 01:00:53,286
bring it to the fruit place."

1427
01:00:53,286 --> 01:00:54,726
So there are some
built-in instructions.

1428
01:00:55,026 --> 01:00:56,176
Well, let's go get
the rest of this.

1429
01:00:56,176 --> 01:00:58,506
And notice my inventory
at top left change

1430
01:00:58,856 --> 01:01:00,776
and now I'm gonna
enter the fruit place.

1431
01:01:00,906 --> 01:01:04,986
And now notice here, if touching
door change the whole scenery.

1432
01:01:04,986 --> 01:01:07,616
And so the costume of the
stage has just changed.

1433
01:01:07,846 --> 01:01:11,326
Now let's see if this guy has
got any final words for us here

1434
01:01:11,326 --> 01:01:14,436
and then we'll introduce
some actual sound.

1435
01:01:15,006 --> 01:01:19,176
Now that over there was the
grand prize of this game.

1436
01:01:19,776 --> 01:01:23,136
So we need just a couple more
tricks to express ourselves

1437
01:01:23,136 --> 01:01:25,096
and implement more
sophisticated things.

1438
01:01:25,096 --> 01:01:27,766
So this notion of an array in
Scratch looks a little something

1439
01:01:27,766 --> 01:01:29,556
like this where you can
add something like a word

1440
01:01:29,556 --> 01:01:31,946
like orange or apple
to a variable

1441
01:01:31,946 --> 01:01:33,426
in this case called "inventory."

1442
01:01:33,666 --> 01:01:36,896
But you can also do multiple
things at once, right?

1443
01:01:36,896 --> 01:01:40,126
Thus far the only example we've
seen involving multiple sprites

1444
01:01:40,126 --> 01:01:41,976
on this screen was
this RPG game.

1445
01:01:42,186 --> 01:01:44,706
But here, we see more
generally the notion of threads.

1446
01:01:44,746 --> 01:01:47,726
A computer program these days
can do multiple things at once

1447
01:01:47,726 --> 01:01:48,946
and a slight white lie.

1448
01:01:49,156 --> 01:01:51,656
It can create the illusion of
doing multiple things at once.

1449
01:01:51,656 --> 01:01:53,866
It's just so much faster
than you, the human are,

1450
01:01:54,036 --> 01:01:55,846
you don't really notice that
it's doing a little bit of this,

1451
01:01:55,846 --> 01:01:58,036
a little bit of this, a little
bit of this and repeating

1452
01:01:58,036 --> 01:01:59,266
so fast that you don't realize

1453
01:01:59,306 --> 01:02:01,646
that actually it's not all
happening at the same time.

1454
01:02:01,866 --> 01:02:04,476
But in Scratch we can implement
this idea of multiple threads,

1455
01:02:04,656 --> 01:02:07,576
multiple scripts or
multiple sprites executing

1456
01:02:07,576 --> 01:02:10,126
at the very same time rather
than one after the other.

1457
01:02:10,126 --> 01:02:13,256
And what this allows us to do
is write a program like this

1458
01:02:13,256 --> 01:02:16,396
where two different
sprites somehow interact.

1459
01:02:16,866 --> 01:02:19,956
So now notice one last piece of
the UI here, at bottom right,

1460
01:02:20,556 --> 01:02:22,576
you see icons for each

1461
01:02:22,636 --> 01:02:24,946
of the multiple sprites
in your program.

1462
01:02:24,946 --> 01:02:27,406
If you don't want just a
cat, you can have a bird,

1463
01:02:27,406 --> 01:02:29,226
you can have a piece of
fruit, you can have a house,

1464
01:02:29,306 --> 01:02:31,806
any number of sprites and then
you just add a graphic to them

1465
01:02:31,806 --> 01:02:34,736
and they look more interesting
than just a simple orange cat.

1466
01:02:35,256 --> 01:02:38,366
So now notice I have the cat
selected so these scripts

1467
01:02:38,366 --> 01:02:41,476
at top left are all of
these scripts that belong

1468
01:02:41,476 --> 01:02:45,296
to that sprite, to that
cat and that script,

1469
01:02:45,366 --> 01:02:47,956
those scripts will only
control that particular cat.

1470
01:02:48,686 --> 01:02:50,806
Well now, if we click
over to the bird here,

1471
01:02:50,986 --> 01:02:53,446
the bird has his own
set of puzzle pieces

1472
01:02:53,796 --> 01:02:56,536
and now let's see what
this game is gonna do.

1473
01:02:56,756 --> 01:02:57,876
One green flag clicked,

1474
01:02:57,936 --> 01:03:01,916
apparently the bird is gonna
go to negative 150, 150.

1475
01:03:02,086 --> 01:03:05,226
So it turns out that stage, it's
like a graphic in like Photoshop

1476
01:03:05,226 --> 01:03:07,056
or some program where you
have all these little dots

1477
01:03:07,056 --> 01:03:08,466
and you just figure out by trial

1478
01:03:08,466 --> 01:03:11,046
and error what numbers represent
what position on the screen.

1479
01:03:11,376 --> 01:03:14,176
Point in direction 45
degrees so at a certain angle.

1480
01:03:14,276 --> 01:03:15,196
And then forever,

1481
01:03:15,196 --> 01:03:17,776
if not touching the
cat, do the following.

1482
01:03:17,776 --> 01:03:19,786
Move three steps and if
you're on the edge, bounce.

1483
01:03:20,486 --> 01:03:22,466
Now the cat is gonna
something a little different

1484
01:03:22,516 --> 01:03:24,546
so let's play this in parallel.

1485
01:03:25,516 --> 01:03:30,666
[ Noise ]

1486
01:03:31,166 --> 01:03:32,666
>> Right? So think about
what the cat is doing.

1487
01:03:32,756 --> 01:03:36,386
If touching bird--
[growl], okay,

1488
01:03:36,526 --> 01:03:38,176
not so good for the
bird in this case.

1489
01:03:38,376 --> 01:03:39,696
But there's now interaction

1490
01:03:39,696 --> 01:03:41,416
that you can see 'cause
what was the cat doing?

1491
01:03:41,416 --> 01:03:43,286
Well, let's go back to
his scripts and zoom in.

1492
01:03:43,536 --> 01:03:45,866
He too goes to some
specific X, Y coordinate.

1493
01:03:46,116 --> 01:03:47,936
But then, point in
direction random.

1494
01:03:48,056 --> 01:03:49,936
So we decided, let's
at least have him point

1495
01:03:49,986 --> 01:03:52,116
into some direction so that
every time we do this demo,

1496
01:03:52,306 --> 01:03:53,916
it's not the same
pan demonstration.

1497
01:03:53,916 --> 01:03:55,506
There's a little
something variant.

1498
01:03:55,786 --> 01:03:57,526
And then forever
the cat does this.

1499
01:03:57,526 --> 01:04:01,256
If touching bird play the
lion sound then stop, else,

1500
01:04:01,326 --> 01:04:02,746
and this time the
"else" is implicit

1501
01:04:02,746 --> 01:04:04,676
which is perfectly fine
too if you have an--

1502
01:04:04,746 --> 01:04:06,126
or rather it's not implicit,

1503
01:04:06,426 --> 01:04:08,346
rather the "if" condition
is gonna execute

1504
01:04:08,516 --> 01:04:11,546
but the bottom two blocks will
always execute point towards

1505
01:04:11,546 --> 01:04:12,966
bird, move one step.

1506
01:04:13,376 --> 01:04:15,626
So we can actually make the
cat much better at this game

1507
01:04:15,816 --> 01:04:18,276
if we make the cat move
say 10 steps at a time

1508
01:04:18,276 --> 01:04:19,546
by just changing that number.

1509
01:04:19,946 --> 01:04:21,896
And now we go back
here and hit play.

1510
01:04:23,176 --> 01:04:25,586
[Growl] Okay, happens all
that much more quickly.

1511
01:04:26,156 --> 01:04:27,566
So there's one last
feature here.

1512
01:04:27,716 --> 01:04:29,916
And this in programming in
general, not just Scratch,

1513
01:04:29,916 --> 01:04:34,386
it's called "event," whereby you
can two sprites, 2 characters

1514
01:04:34,386 --> 01:04:36,996
on the screen actually
somehow intercommunicate

1515
01:04:37,126 --> 01:04:39,956
and we can implement this
age old game of Marco Polo

1516
01:04:39,956 --> 01:04:43,206
that you might have played in
a pool whereby two humans need

1517
01:04:43,206 --> 01:04:44,426
to somehow communicate.

1518
01:04:44,616 --> 01:04:47,876
So here we have the boy, Marco,
and he's highlighted down here.

1519
01:04:48,036 --> 01:04:49,596
And so at top left, we see

1520
01:04:49,596 --> 01:04:51,146
that Marco is gonna
behave as follows.

1521
01:04:51,386 --> 01:04:55,416
Forever if the space key is
pressed, he's going to say Marco

1522
01:04:55,416 --> 01:04:58,496
for 2 seconds and then he's
gonna broadcast an event.

1523
01:04:59,076 --> 01:05:02,096
Thus far, we've seen ways
where two sprites can interact

1524
01:05:02,096 --> 01:05:03,486
by checking where they are,

1525
01:05:03,746 --> 01:05:06,766
which we haven't seen one sprite
send another sprite a message.

1526
01:05:07,036 --> 01:05:09,966
When we say, say Marco, that's
just for our human benefit,

1527
01:05:09,966 --> 01:05:12,056
to see some little cartoon
bubble on the screen.

1528
01:05:12,246 --> 01:05:14,486
If we want the boy
to communicate

1529
01:05:14,486 --> 01:05:18,336
to the girl programmatically
by sending some kind of 0

1530
01:05:18,336 --> 01:05:21,376
and 1 message, we have to do
what's called broadcast an event

1531
01:05:21,826 --> 01:05:25,886
which is just a message that now
other sprites can listen for.

1532
01:05:25,886 --> 01:05:26,966
So let's look at the girl.

1533
01:05:27,196 --> 01:05:30,086
The girl is actually
defined quite simply not

1534
01:05:30,086 --> 01:05:31,206
with the green flag trick

1535
01:05:31,476 --> 01:05:34,366
but with this other starting
point when I receive event.

1536
01:05:34,696 --> 01:05:37,476
So she's listening for Marco
to send her this message,

1537
01:05:37,536 --> 01:05:38,856
this event called "event."

1538
01:05:39,236 --> 01:05:43,446
And when she receives it, she
is gonna say Polo for 2 seconds.

1539
01:05:43,876 --> 01:05:45,436
So let's go ahead and
see this in action.

1540
01:05:45,786 --> 01:05:47,576
Nothing happens at
first but remember,

1541
01:05:47,606 --> 01:05:49,706
once I hit the space
bar, I see this

1542
01:05:49,706 --> 01:05:53,316
and now the girl
replies in turn.

1543
01:05:53,316 --> 01:05:54,306
So now we have the means

1544
01:05:54,526 --> 01:05:57,046
of conveying two different
messages to people.

1545
01:05:57,456 --> 01:05:59,076
Well, these are just
building blocks,

1546
01:05:59,076 --> 01:06:01,026
how can we take things
up to the next level?

1547
01:06:01,296 --> 01:06:05,036
Well, let me go ahead and we
could call one brave volunteer

1548
01:06:05,036 --> 01:06:08,456
to the stage and show
you yet another example.

1549
01:06:08,946 --> 01:06:11,936
Only us in the front are
playing along today I see.

1550
01:06:12,206 --> 01:06:15,796
Okay. How 'bout, let's say you,

1551
01:06:15,796 --> 01:06:17,326
right in front, easier
to get to.

1552
01:06:17,326 --> 01:06:17,716
Come on up.

1553
01:06:17,716 --> 01:06:18,076
What's your name?

1554
01:06:18,526 --> 01:06:18,796
>> Trevor [phonetic].

1555
01:06:19,056 --> 01:06:19,406
>> Trevor.

1556
01:06:19,406 --> 01:06:20,196
Trevor, come on up.

1557
01:06:20,196 --> 01:06:21,806
You have to be comfortable
appearing on camera

1558
01:06:22,496 --> 01:06:24,406
and also performing
poorly on a game.

1559
01:06:25,856 --> 01:06:28,876
So here we have a little
something also made

1560
01:06:28,876 --> 01:06:30,136
by one of our own alumni.

1561
01:06:30,136 --> 01:06:34,256
Come on up and I give you in a
moments the culmination of some

1562
01:06:34,256 --> 01:06:37,806
of these fundamentals called
Scratch, Scratch revolution.

1563
01:06:38,596 --> 01:06:40,216
[Laughter] And if Barry
[phonetic] we could turn

1564
01:06:40,216 --> 01:06:41,466
up the sound a bit more.

1565
01:06:42,346 --> 01:06:42,676
>> Oh, great.

1566
01:06:42,676 --> 01:06:45,376
>> Here we have-- let me
set the stage verbally.

1567
01:06:45,736 --> 01:06:47,486
Notice we got a whole
bunch of variables

1568
01:06:47,586 --> 01:06:49,896
that this former student
dragged into the stage

1569
01:06:49,896 --> 01:06:51,846
so that you could see
the variable score,

1570
01:06:51,936 --> 01:06:54,926
the awesome score, the cool
score, the good, the boo score.

1571
01:06:55,206 --> 01:06:56,866
Then there are these
arrows and each

1572
01:06:56,866 --> 01:06:58,016
of these arrows is a sprite.

1573
01:06:58,016 --> 01:07:00,116
And notice he used
some randomness

1574
01:07:00,116 --> 01:07:02,006
to have these things appear
in different locations.

1575
01:07:02,266 --> 01:07:03,736
Here is just a very
poor man's way

1576
01:07:03,766 --> 01:07:05,216
of implementing instructions.

1577
01:07:05,216 --> 01:07:07,316
You just kind of do it with
the built-in text editor

1578
01:07:07,316 --> 01:07:07,906
in the program.

1579
01:07:08,176 --> 01:07:10,056
Press Spacebar to begin
then you're gonna--

1580
01:07:10,156 --> 01:07:11,956
Trevor is gonna hit
the arrow keys in time

1581
01:07:11,956 --> 01:07:13,246
with arrows to score points.

1582
01:07:13,246 --> 01:07:15,856
Watch out if your score gets
negative, bad things are going

1583
01:07:15,856 --> 01:07:18,426
to happen, little
like DDR, if familiar.

1584
01:07:18,806 --> 01:07:20,666
So if our volumes
indeed cranked up,

1585
01:07:21,096 --> 01:07:22,586
we'll see how well
you can do here.

1586
01:07:22,586 --> 01:07:23,756
So you got the instructions
in mind?

1587
01:07:24,026 --> 01:07:24,446
>> I guess so.

1588
01:07:24,446 --> 01:07:26,766
>> Okay, arrow keys are
right here, spacebar begins.

1589
01:07:27,516 --> 01:08:06,516
[ Music ]

1590
01:08:07,016 --> 01:08:07,083
[ Background Music ]

1591
01:08:07,486 --> 01:08:11,366
>> Now notice, it's gonna
be some kind of timer

1592
01:08:11,366 --> 01:08:13,356
that realizes it's
time to raise the bar.

1593
01:08:14,516 --> 01:08:17,266
[ Music ]

1594
01:08:17,766 --> 01:08:20,516
[ Laughter ]

1595
01:08:21,016 --> 01:08:21,083
[ Background Music ]

1596
01:08:21,236 --> 01:08:23,046
>> So a big hand for
Trevor if we could here.

1597
01:08:23,046 --> 01:08:25,956
[Background applause]
Very well done.

1598
01:08:29,006 --> 01:08:30,466
So unless we have
the culmination

1599
01:08:30,466 --> 01:08:32,106
of all these various
puzzle pieces--

1600
01:08:32,106 --> 01:08:33,626
but it doesn't even stop there.

1601
01:08:33,626 --> 01:08:36,486
In fact, for the hacker
edition of this week's problem

1602
01:08:36,536 --> 01:08:38,586
that you'll be challenged not
only to implement something

1603
01:08:38,586 --> 01:08:39,336
of interest to you

1604
01:08:39,576 --> 01:08:41,526
but to integrate a little
something like this.

1605
01:08:41,606 --> 01:08:44,726
A sensor board which is just
this little plastic board that's

1606
01:08:44,726 --> 01:08:48,226
got a whole bunch of metallic
traces, wires connected to it

1607
01:08:48,366 --> 01:08:49,826
and it takes a whole
bunch of inputs.

1608
01:08:49,866 --> 01:08:51,926
There's this little slider
which is like a joystick.

1609
01:08:52,206 --> 01:08:53,846
There's this little connector
here which allows you

1610
01:08:53,846 --> 01:08:55,846
to connect this to
your Mac or PC.

1611
01:08:56,066 --> 01:08:57,466
There's a light sensor.

1612
01:08:57,466 --> 01:08:59,506
There are resistance sensors.

1613
01:08:59,506 --> 01:09:01,416
There is a sound
and light sensors.

1614
01:09:01,416 --> 01:09:03,036
There're all sorts
of sensors built

1615
01:09:03,036 --> 01:09:05,076
into this otherwise very
primitive looking device.

1616
01:09:05,336 --> 01:09:07,766
And so if you think now about
some of the toys you might have

1617
01:09:07,766 --> 01:09:10,376
or electronic devices, what's
generally inside of them

1618
01:09:10,376 --> 01:09:13,426
if they respond to inputs
like touch and sound and feel,

1619
01:09:13,616 --> 01:09:15,346
are these basic building blocks.

1620
01:09:15,346 --> 01:09:17,086
So for the hacker of the
addition of the problem

1621
01:09:17,086 --> 01:09:19,306
that you'll be challenged
to connect up a cable

1622
01:09:19,306 --> 01:09:22,056
to your own computer and not
just take input from the user

1623
01:09:22,056 --> 01:09:26,296
at the keyboard but rather from
this actual physical device.

1624
01:09:26,526 --> 01:09:31,926
Well, we thought we would share
just one last example here also

1625
01:09:31,926 --> 01:09:33,336
by one of our former students.

1626
01:09:33,336 --> 01:09:37,076
This was to a song that you'll
see takes advantage of loops

1627
01:09:37,256 --> 01:09:40,196
and conditions and
sounds and animation

1628
01:09:40,526 --> 01:09:43,276
to give us this final song.

1629
01:09:44,516 --> 01:10:20,766
[ Music ]

1630
01:10:21,266 --> 01:10:57,516
[ Movie Clip ]

1631
01:10:58,016 --> 01:10:58,083
[ Background Music ]

1632
01:10:58,336 --> 01:10:59,386
>> That's it for today.

1633
01:10:59,886 --> 01:11:01,616
This is CS50 Problem Set
[inaudible] on the website.

1634
01:11:02,296 --> 01:11:04,136
We will see you next week.

1635
01:11:05,516 --> 01:11:11,693
[ Music ]

1636
01:11:12,193 --> 01:11:18,370
[ Applause ]