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[Introduction]

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DAVID MALAN: This is CS50,
Harvard University's introduction

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to the intellectual
enterprises of computer science

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and the art of programming.

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My name is David Malan,
and if you are among those

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in the room who are thinking, why
am I in a class of computer science,

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realize that I too felt
that exact same way.

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In fact, my freshman
year, I didn't quite

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get up the nerve to take this class
or computer science more generally,

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and that was largely because
I was intimidated by it.

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I was a little nervous.

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It felt well out of my comfort zone.

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And I really didn't know at the end
of the day what it actually was.

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But realize if you, too, are
feeling a little bit of that,

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or even if you're among
those more comfortable who

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have dabbled in computer
science or programming,

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realize that there's so many blanks
that we can fill in along the way

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so that ultimately, at the
end of the semester, everyone

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will feel themselves on the same page.

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And until then, rest assured that 68%
of the people sitting to your left

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and to your right and
behind and in front

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have never taken a CS course
before, which may very well be

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the demographic into which you fit.

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But realize, too, that with
such an amazing support

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structure with so many office hours
and sections and materials and beyond,

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realize that what's ultimately
important in this course

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is not so much where you end
up relative to your classmates

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in week 10, our final week, but
where you end up relative to yourself

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in week zero.

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And indeed, that is where we now are.

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And as it turns out, computer
scientists start counting at zero.

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And so over the next 11
weeks, we will take you

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from being among those
less comfortable or perhaps

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somewhere in between
less comfortable and more

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to feeling much more comfortable
and confident and capable than that.

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But to get there, we need to understand
what computer science really is.

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And this was something I didn't
understand until I set foot in a room

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like this.

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And I dare say we can distill computer
science into just this picture.

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Computer science is
about problem solving.

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And I know that high school
courses typically do kind of paint

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a misleading picture
that it's only about

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and it's entirely about programming
and people with their heads

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down in the computer lab working
fairly anti-socially on code,

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but the reality is it's all about
solving problems, and very often,

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solving problems collaboratively
either in person or by leveraging code,

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programs that others
have written in the past.

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And what does it mean
to solve a problem?

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Well, you need inputs.

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So there's a problem
you're trying to solve.

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That is the input.

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And you want output.

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You want the solution to that problem.

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And the sort of secret
sauce of computer science

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is going to be everything
in this proverbial black box

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in the middle over the
next several weeks,

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where you begin to understand
exactly what you can do with that.

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But in order to start solving
problems, we kind of just

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need to decide as a group how we're
going to represent these problems

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and what might a problem be.

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Well, in this room, there's
a whole bunch of people.

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If we wanted to take attendance or
count the number of people in this room,

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I might need to start keeping
track of how many people I see.

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But how do I represent the
number of people I see?

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Well, I can do it sort of old school
and I can just take out a piece of chalk

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or whatnot and say, all right.

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I see 1, 2, 3, 4, 5.

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I can do little stylistic
conventions like that

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to save space and remind myself.

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6, 7, 8, 9, 10, and so forth.

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Or I can, of course, just
do that on my own hand.

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So 1, 2, 3, 4, 5, and so forth.

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But obviously, how high can
I count on just one hand?

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So 5 you would think, but that's
just because we haven't really

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thought hard enough about this problem.

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It turns out that with just these five
fingers, let alone these five more,

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I can actually count rather higher
because after all, the system

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I'm using of hashmarks
on the board or just

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now with my fingers is just kind of
keeping my fingers down and putting

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them up to represent ones, really.

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But what if I actually took into
account the order of my fingers

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and sort of permuted them, so to speak,
so that it's really patterns of fingers

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that represent the number
of people in the room,

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and not just the mere presence
of a finger going up or down.

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In other words, this can remain zero.

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This could still be one.

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But what if two is not
just this, the obvious?

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But what if it's just this?

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So raising just one, my second finger.

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What if, then, three is this?

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So we have 0, 1, 2, 3.

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That's going to lead us to
four somewhat offensively.

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But if we begin to jump
ahead to five, I might now

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permute this finger and this finger up.

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And if I want to now represent
six, I could do this.

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And now seven.

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In other words, I've expressed so
many more patterns on my hand already

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and if we keep doing this,
I think I can actually

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represent painfully perhaps like 32
different patterns, and therefore

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32 different people, on my hands alone.

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Or 31 people if I
start counting at zero.

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So what is that--
what's the relationship

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and how did we even get here?

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Well, it turns out that
computers are kind of simplistic,

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much like our hands here.

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At the end of the day, your
computer is plugged into the wall

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or it's got a battery, so it either
has or it does not have electricity.

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At the end of the day, that
is the physical resource

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that drives these things and our
phones and all of technology today.

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So if there is either electricity
or not, that kind of maps nicely

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to no finger or yes finger.

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And indeed, computers, as you probably
know, only speak what language?

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What alphabet, so to speak?

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Yeah.

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Binary.

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Bi meaning two.

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And indeed, that refers to the
fact that in binary in computers,

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you only have two digits--

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zero and one.

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We humans, of course, have
0, 1, 2, 3, 4, 5, 6, 7, 8, 9,

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and then we can combine
those to count even higher.

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But computers only have
0, 1, and then that's it.

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Because at the end of
the day, there's actually

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a direct mapping between power being off
and it being a zero or power being on

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and it being one, or some electrons
or whatever flowing from your battery

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or from the wall.

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So this is why computers
tend to speak only binary,

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because at the end of the day,
it just maps really cleanly

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to what it is that's powering
them in the first place.

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But how is this actually useful?

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If computers only have zeros and
ones, how can they do anything useful?

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Well, think about our human world, where
you might have this pattern of symbols.

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This is decimal, dec meaning 10
because you have 0 through 9.

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And this is, of course, 123.

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But why?

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If you haven't thought about
this in quite some time,

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this is really just a pattern of three
symbols, one and two and three shapes,

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or glyphs, on the screen.

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But we humans, ever since grade
school, have started ascribing meaning

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to each of these numbers, right?

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If you think back, this is the ones
column, this is the tens column,

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this is the hundreds column, and
so forth, and we could keep going.

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And so why does this pattern--
one, two, three-- mean 123?

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Well, it's because all
of us sort of intuitively

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nowadays are just quickly in our
head doing 100 times 1 plus 10 times

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2 plus 1 times 3, which of course
gives us 100 plus 20 plus three,

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and then the number we
know mathematically as 123.

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But we're all doing this so quickly, you
don't really think about this anymore.

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Well, computers work
fundamentally the same way.

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They don't have as many digits--

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0 through 9-- as we do.

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They only have zeros and ones.

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And so if they were to
store values, you're

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only going to see zeros
and ones on the screen,

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but those zeros and ones
just mean different things.

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Instead of having a ones
place, tens, a hundreds,

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they're going to have a ones
place, a twos place, a fours place,

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and then eights and 16 and beyond.

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Now, why?

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Well, one and 10 and 100, turns
out those are powers of 10.

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10 to the 0 is technically 1.

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10 to the 1 is just 10.

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10 to the 2 is 100.

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And that's why you have ones, tens
hundreds, thousands, and so forth.

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Computers are apparently
using powers of 2.

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Not surprising.

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Binary-- two.

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So if you only have ones, twos,
and fours as your placeholders,

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if a computer were
storing these digits--

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0, 0, 0-- that computer is presumably
storing what number so far as we

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humans understand it?

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Well, that's how a
computer would store zero.

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If a computer is storing
literally 0, 0, 0, just

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like in our human world, that
also is 0, but that's technically

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because it's 4 times 0 plus 2 times 0
plus 1 times zero, which is obviously

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zero.

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Meanwhile, if a computer is
actually storing not just,

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say, 0, 0, 0, but instead is
storing this value in binary,

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what does that map to in decimal?

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So that's one.

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And now, why, if we change this 0 and
1 to this value here, is this two?

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Well, mathematically, for
the exact same reasons.

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And so earlier, I had five fingers, but
if you consider just my first three,

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when I did this holding up one
finger, I was representing two.

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And if I want to represent three,
recall that I put up the second finger.

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And so the reason that
could nicely represent three

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is because all I was doing with my
human hand was counting in binary.

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And I could keep counting
more and more and more.

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And so if I have five fingers or
five bits, bit meaning binary digits,

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I could count up, it turns
out, if we do the math,

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as high as 31 by starting to zero.

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It's going to be hard to
physically do that, but we could.

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So why is this useful?

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At the end of the day,
a computer, therefore,

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can represent any number of values from
0 to 1 to 2 to 3 to some number much,

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much, much higher than that.

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All it needs is enough
bits, enough zeros and ones.

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Well, what are those bits?

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Well, all of us have these days in our
phones sources of light, for instance.

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So I could actually say that
this physical device right now--

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might be a little hard to tell--

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it does have a flashlight and it's
technically off at the moment.

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But if I turn this flashlight on,
thereby using some of the electricity,

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now, I'm storing a one.

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And so the phone is on.

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Now, it's off.

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Now, it's on.

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And if I see--

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can I borrow someone's phone real quick?

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May I?

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OK.

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And flashlight.

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How do I turn on the flashlight?

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Oh.

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Shake it.

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That's OK.

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OK.

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Thank you.

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Oh.

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Thank you.

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OK.

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So this is great.

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Now, I can count higher.

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Now, this represents the number what if
I have two light bulbs or two switches

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on at the moment?

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Yeah.

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Three.

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Because I have a one,
I have a one, and I

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have two, which of course is
going to end up equaling three.

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And if I pick up a third phone
somehow, I could count even higher.

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Technically, if I had
three light bulbs on--

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one, one, one-- what
would that value be?

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Seven.

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Because it's a four plus a
two plus a one, and so forth.

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Thank you so much for the spontaneity.

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So why does this not lead
to limitations for us?

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I can count in decimal
as high as I want.

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I can now count in binary as high as
I want, so long as I have enough bits.

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But how do I actually
represent other information?

240
00:10:52,310 --> 00:10:57,250
Well, if I want to represent something
like a letter, how do I get there?

241
00:10:57,250 --> 00:11:01,540
If computers only have electricity
in them and they use binary to count,

242
00:11:01,540 --> 00:11:04,300
and yet somehow they're much more
useful than just doing math--

243
00:11:04,300 --> 00:11:08,050
they can have text messages and e-mails
and websites and videos and more--

244
00:11:08,050 --> 00:11:11,140
how do we get from zeros
and ones to letters?

245
00:11:11,140 --> 00:11:12,400
Well, we-- yeah.

246
00:11:15,120 --> 00:11:15,620
Sorry.

247
00:11:15,620 --> 00:11:18,140
A little louder.

248
00:11:18,140 --> 00:11:18,650
Yeah.

249
00:11:18,650 --> 00:11:20,630
We just need to kind of
relate the numbers to letters.

250
00:11:20,630 --> 00:11:23,510
In other words, all the people in this
room just need to decide at some point

251
00:11:23,510 --> 00:11:24,410
that, you know what?

252
00:11:24,410 --> 00:11:27,200
If we want to represent something
like the capital letter A,

253
00:11:27,200 --> 00:11:29,180
we just need to decide
on a pattern of bits,

254
00:11:29,180 --> 00:11:31,730
a pattern of fingers,
that's going to represent A.

255
00:11:31,730 --> 00:11:34,670
And it turns out humans
years ago just unilaterally

256
00:11:34,670 --> 00:11:39,840
decided 65 shall be the decimal number
that represents capital letter A.

257
00:11:39,840 --> 00:11:43,700
And you might guess capital B is
represented by what decimal number?

258
00:11:43,700 --> 00:11:44,420
66.

259
00:11:44,420 --> 00:11:46,500
And then C is 67 and so forth.

260
00:11:46,500 --> 00:11:50,630
And there's a mapping of like
128 or even 256 possible values

261
00:11:50,630 --> 00:11:52,880
for all the keys you might
see on a typical keyboard

262
00:11:52,880 --> 00:11:54,680
in order to represent letters.

263
00:11:54,680 --> 00:11:58,260
Now, how does a computer distinguish,
though, numbers from letters?

264
00:11:58,260 --> 00:11:59,680
Well, just depends on the context.

265
00:11:59,680 --> 00:12:01,880
If you're using like a
calculator program on your Mac

266
00:12:01,880 --> 00:12:05,600
or PC or iPhone or Android,
well, the computer, the device,

267
00:12:05,600 --> 00:12:09,050
is just going to know contextually,
let me interpret this pattern of zeros

268
00:12:09,050 --> 00:12:11,330
and ones as actual numbers to do math.

269
00:12:11,330 --> 00:12:14,880
But if you're using the SMS app
or the messages app on your phone,

270
00:12:14,880 --> 00:12:17,060
you're going to actually
be in the context of text,

271
00:12:17,060 --> 00:12:20,380
and so your phone is going to interpret
that same pattern of zeros and ones,

272
00:12:20,380 --> 00:12:23,780
or light bulbs being off, or, at
the end of the day, transistors,

273
00:12:23,780 --> 00:12:27,450
tiny pieces of hardware and
computers that are either on or off--

274
00:12:27,450 --> 00:12:30,470
it's going to interpret those patterns
as just representing a letter.

275
00:12:30,470 --> 00:12:34,220
If you're in the context of a text
messaging application or Microsoft Word

276
00:12:34,220 --> 00:12:38,180
or Google Docs or the like, it
completely depends on context.

277
00:12:38,180 --> 00:12:41,780
The system we humans came up with just
called ASCII, American Standard Code

278
00:12:41,780 --> 00:12:43,100
for Information Interchange.

279
00:12:43,100 --> 00:12:46,760
The name isn't interesting, but the
fact that we all agreed years ago

280
00:12:46,760 --> 00:12:50,210
that 65 is A and so forth
is what's important.

281
00:12:50,210 --> 00:12:52,800
And so for instance, if we
look at this mapping here

282
00:12:52,800 --> 00:12:56,340
of just the first few
letters, what does this mean?

283
00:12:56,340 --> 00:13:00,920
If I were to now get a text message
and I had the ability somehow

284
00:13:00,920 --> 00:13:04,190
to look underneath the hood, so to
speak, at the pattern of zeros and ones

285
00:13:04,190 --> 00:13:06,890
that someone had just
texted me, and that pattern,

286
00:13:06,890 --> 00:13:12,230
if I convert it to decimal, technically
said, let's say, 72 and 73--

287
00:13:12,230 --> 00:13:13,970
so I get a whole bunch
of zeros and ones.

288
00:13:13,970 --> 00:13:17,480
I do some math and I realize,
OK, I just received 72 and 73,

289
00:13:17,480 --> 00:13:20,690
but this is texting, and so it's not
just numbers my friend is sending me.

290
00:13:20,690 --> 00:13:21,690
It's a message.

291
00:13:21,690 --> 00:13:27,840
What message did my friend likely send
me if he or she sent 72 and then 73?

292
00:13:27,840 --> 00:13:28,370
Yeah.

293
00:13:28,370 --> 00:13:28,860
Hi.

294
00:13:28,860 --> 00:13:31,280
H-I. Because if you skim
ahead at the right there,

295
00:13:31,280 --> 00:13:36,720
that just happens to be in ASCII the
mapping between 72 and 73, H and I.

296
00:13:36,720 --> 00:13:40,240
If technically, the message
had a third byte, if you will.

297
00:13:40,240 --> 00:13:43,090
A byte, if you've ever
wondered, is just eight bits.

298
00:13:43,090 --> 00:13:45,260
It's convenient to talk not
in terms of single bits,

299
00:13:45,260 --> 00:13:48,440
where you can't count very high,
but with a byte, or eight bits,

300
00:13:48,440 --> 00:13:49,700
you can count higher.

301
00:13:49,700 --> 00:13:53,600
And so it turns out if I received a
third byte, another sequence of eight

302
00:13:53,600 --> 00:13:55,520
zeros and ones together--

303
00:13:55,520 --> 00:13:56,150
33.

304
00:13:56,150 --> 00:13:59,490
How would we know what
this message now is?

305
00:13:59,490 --> 00:13:59,990
Yeah.

306
00:13:59,990 --> 00:14:01,640
So it turns out you
would not know this other

307
00:14:01,640 --> 00:14:04,430
than by guessing or Googling or
just coming in with this knowledge.

308
00:14:04,430 --> 00:14:05,360
This is now "HI!"

309
00:14:05,360 --> 00:14:09,020
with an exclamation point because 33
just so happens, if you look it up,

310
00:14:09,020 --> 00:14:12,180
to map to an exclamation point, as well.

311
00:14:12,180 --> 00:14:14,360
Now, if we actually looked
at the binary of this,

312
00:14:14,360 --> 00:14:16,610
you would actually see this
pattern of zeros and ones.

313
00:14:16,610 --> 00:14:18,920
This is how you represent 72 in binary.

314
00:14:18,920 --> 00:14:20,690
This is you represent 73.

315
00:14:20,690 --> 00:14:22,820
And this is how you represent 33.

316
00:14:22,820 --> 00:14:26,810
And notice I've only used one,
two, three, four, five, six bits,

317
00:14:26,810 --> 00:14:30,200
even though I technically tend
to receive things in units of 8,

318
00:14:30,200 --> 00:14:31,220
units of bytes.

319
00:14:31,220 --> 00:14:37,770
But why did I not bother writing
another 00 here and another 0 here?

320
00:14:37,770 --> 00:14:40,660
Does it matter when you
write these things out?

321
00:14:40,660 --> 00:14:41,160
No.

322
00:14:41,160 --> 00:14:41,670
Not really.

323
00:14:41,670 --> 00:14:46,010
Like in English, in our human world,
if you were to write one, two, three,

324
00:14:46,010 --> 00:14:47,280
that's 123.

325
00:14:47,280 --> 00:14:50,850
If you were to write 0,
1, 2, 3, that's still 123.

326
00:14:50,850 --> 00:14:53,670
So even though we tend to
get them in clusters of 8,

327
00:14:53,670 --> 00:14:56,660
we don't necessarily need to write
those when just talking about them.

328
00:14:56,660 --> 00:14:57,540
So what have we done?

329
00:14:57,540 --> 00:15:00,160
Well, let me introduce a fancier
word now known as abstraction.

330
00:15:00,160 --> 00:15:03,480
Abstraction is just a term generally
used in computer science and we'll soon

331
00:15:03,480 --> 00:15:07,530
see in programming for taking some
low level-- like literally low level--

332
00:15:07,530 --> 00:15:11,700
implementation details, like
minutiae even, and understanding them

333
00:15:11,700 --> 00:15:15,450
at some point, but then deciding this
is not a useful level conceptually

334
00:15:15,450 --> 00:15:16,750
to think about problems.

335
00:15:16,750 --> 00:15:20,280
I really don't want to solve problems in
this world of thinking in 0's and 1's.

336
00:15:20,280 --> 00:15:23,910
I'd much rather think about things
minimally in decimal, or better yet,

337
00:15:23,910 --> 00:15:26,640
in the context of letters if I'm
actually receiving text, or even

338
00:15:26,640 --> 00:15:28,110
some other representation.

339
00:15:28,110 --> 00:15:32,040
So abstraction is about taking
fairly low level details

340
00:15:32,040 --> 00:15:35,980
and just simplifying them so that we
can have a more useful conversation

341
00:15:35,980 --> 00:15:39,300
and never again worry about where
the electricity is coming from.

342
00:15:39,300 --> 00:15:42,730
We can just stipulate my computer
can represent zeros and ones.

343
00:15:42,730 --> 00:15:44,230
Therefore, it can represent numbers.

344
00:15:44,230 --> 00:15:47,010
Therefore, it can also
represent ASCII or letters.

345
00:15:47,010 --> 00:15:50,850
And we can kind of move on and start
solving more interesting problems.

346
00:15:50,850 --> 00:15:56,700
But it would seem that we can't solve
all problems because on my keyboard

347
00:15:56,700 --> 00:16:00,300
here, this American keyboard here,
there's a whole bunch of symbols,

348
00:16:00,300 --> 00:16:03,810
like a 100 or 200 maybe in total if we
actually hit shift and option and all

349
00:16:03,810 --> 00:16:04,560
of that.

350
00:16:04,560 --> 00:16:07,170
But what you don't see are
some pretty common characters.

351
00:16:07,170 --> 00:16:09,010
Especially in a very
international audience,

352
00:16:09,010 --> 00:16:12,490
what can I apparently not
even type on this keyboard?

353
00:16:12,490 --> 00:16:13,410
What kinds of symbols?

354
00:16:13,410 --> 00:16:15,000
Yeah.

355
00:16:15,000 --> 00:16:16,190
Anything with an accent.

356
00:16:16,190 --> 00:16:18,240
If you have accents over
vowels or other letters.

357
00:16:18,240 --> 00:16:19,740
What else?

358
00:16:19,740 --> 00:16:20,810
I'm sorry.

359
00:16:20,810 --> 00:16:22,560
Umlauts or other
characters above letters.

360
00:16:22,560 --> 00:16:23,060
Yeah.

361
00:16:25,200 --> 00:16:26,170
Like pound symbol?

362
00:16:26,170 --> 00:16:26,670
Oh.

363
00:16:26,670 --> 00:16:27,710
Like the UK pound symbol.

364
00:16:27,710 --> 00:16:28,210
Sure.

365
00:16:28,210 --> 00:16:29,340
And other countries, too.

366
00:16:29,340 --> 00:16:30,790
Any number of Asian languages.

367
00:16:30,790 --> 00:16:33,410
There's so many symbols that are
not depicted on this keyboard,

368
00:16:33,410 --> 00:16:36,630
and yet somehow, all of us with
international keyboards or phones

369
00:16:36,630 --> 00:16:38,410
can surely express themselves.

370
00:16:38,410 --> 00:16:42,180
But that's because phones and computers
these days don't just use ASCII.

371
00:16:42,180 --> 00:16:45,030
ASCII literally used
just eight bits total.

372
00:16:45,030 --> 00:16:47,290
Technically seven, but then,
ultimately, really eight.

373
00:16:47,290 --> 00:16:49,380
And with eight bits, if
you actually do the math--

374
00:16:49,380 --> 00:16:51,750
if you have eight bits
or eight fingers, you

375
00:16:51,750 --> 00:16:56,130
can only permute them in 256
total possible ways, which

376
00:16:56,130 --> 00:17:01,620
is to say that you can only represent
256 characters using ASCII with numbers

377
00:17:01,620 --> 00:17:03,930
underneath the hood, and
that's not enough to represent

378
00:17:03,930 --> 00:17:06,220
so many different symbols
like those enumerated here.

379
00:17:06,220 --> 00:17:08,850
You can't represent any of the
accents that you can nonetheless

380
00:17:08,850 --> 00:17:11,400
type on your Macs and
PCs and you certainly

381
00:17:11,400 --> 00:17:13,740
can't type these things,
which are very much in vogue.

382
00:17:13,740 --> 00:17:17,520
Which even though they're pictures,
they're actually just characters.

383
00:17:17,520 --> 00:17:22,020
Because it turns out some years ago, the
world decided eight bits is not enough.

384
00:17:22,020 --> 00:17:25,020
Let's start using something called
Unicode, where you actually use

385
00:17:25,020 --> 00:17:28,200
one or two or three or even four bytes.

386
00:17:28,200 --> 00:17:33,450
So eight bits or 16 bits, 24 bits, or
even 32 bits to represent characters.

387
00:17:33,450 --> 00:17:35,940
And now, we have the ability
to represent thousands or even

388
00:17:35,940 --> 00:17:37,440
millions of characters.

389
00:17:37,440 --> 00:17:42,540
And frankly, daresay, the result
of that huge amount of availability

390
00:17:42,540 --> 00:17:45,340
is partly why there are so many
of these things these days.

391
00:17:45,340 --> 00:17:48,540
And they just keep making more because
there's just so many different numbers

392
00:17:48,540 --> 00:17:49,320
available to us.

393
00:17:49,320 --> 00:17:52,950
So Unicode is often a specific
version of it called UTF-8,

394
00:17:52,950 --> 00:17:54,210
which we'll see before long.

395
00:17:54,210 --> 00:17:56,060
But let me ask this question here.

396
00:17:56,060 --> 00:18:01,710
This is a crying face with
joy, I think this is called.

397
00:18:01,710 --> 00:18:03,830
So it turns out,
according to Apple or iOS,

398
00:18:03,830 --> 00:18:06,420
this is the most popular emoji
that at least iPhone people

399
00:18:06,420 --> 00:18:07,710
are sending to each other.

400
00:18:07,710 --> 00:18:10,710
So when you're receiving this, though,
if we can really take the fun out

401
00:18:10,710 --> 00:18:14,860
of this, what pattern of bits are you
actually receiving from your friend?

402
00:18:14,860 --> 00:18:17,580
He or she is clearly trying
to express some emotion,

403
00:18:17,580 --> 00:18:24,030
but really, what your friend is sending
you-- the decimal number 128,514.

404
00:18:24,030 --> 00:18:26,670
Or really, if you looked at
the zeros and ones coming

405
00:18:26,670 --> 00:18:29,220
to you over the internet
or airwaves, you're

406
00:18:29,220 --> 00:18:33,540
getting this pattern of zeros and
ones, which is hardly joyful or hardly

407
00:18:33,540 --> 00:18:36,060
descriptive, but all
your phone or computer

408
00:18:36,060 --> 00:18:38,040
are doing is seeing
this pattern of bits,

409
00:18:38,040 --> 00:18:40,530
looking it up in like a little
cheat sheet, and saying, oh.

410
00:18:40,530 --> 00:18:44,220
Whenever I see this pattern of bits
in the context of text like texting,

411
00:18:44,220 --> 00:18:47,490
I should actually display
it as that picture.

412
00:18:47,490 --> 00:18:50,550
Now, that picture has a lot
of yellow and other colors

413
00:18:50,550 --> 00:18:53,050
in it, but how do we even get there?

414
00:18:53,050 --> 00:18:55,410
Well, it turns out that this
same pattern of numbers--

415
00:18:55,410 --> 00:19:00,040
72, 73, 33-- which just to be
sure, a moment ago, meant what?

416
00:19:00,040 --> 00:19:00,540
Hi.

417
00:19:00,540 --> 00:19:04,650
In the context of a textual program like
Microsoft Word, Google Docs, texting,

418
00:19:04,650 --> 00:19:05,940
this means hi.

419
00:19:05,940 --> 00:19:09,900
But what if you saw this
same pattern of bytes--

420
00:19:09,900 --> 00:19:11,820
and again, we could
draw the zeros and ones,

421
00:19:11,820 --> 00:19:13,820
but it's not interesting
anymore, so we're going

422
00:19:13,820 --> 00:19:15,540
to abstract away at the decimal level.

423
00:19:15,540 --> 00:19:17,450
If you got this same
pattern of zeros and ones

424
00:19:17,450 --> 00:19:21,060
or numbers in the context of
like Photoshop or a browser

425
00:19:21,060 --> 00:19:24,900
or some kind of photo program,
well, it might make more sense

426
00:19:24,900 --> 00:19:28,660
to interpret it not as text, but
as imagery, some kind of colors.

427
00:19:28,660 --> 00:19:31,120
Well, it turns out there's this
other system in the world--

428
00:19:31,120 --> 00:19:32,820
you might have seen
this acronym before--

429
00:19:32,820 --> 00:19:35,560
called RGB-- red, green, blue.

430
00:19:35,560 --> 00:19:38,980
And this is just a way of
humans having standardized years

431
00:19:38,980 --> 00:19:39,940
ago that you know what?

432
00:19:39,940 --> 00:19:42,910
If we want to represent a dot on
someone's screen, otherwise known

433
00:19:42,910 --> 00:19:44,950
as a pixel a tiny little
square on the screen

434
00:19:44,950 --> 00:19:47,880
of your phone, your laptop,
or even TV these days,

435
00:19:47,880 --> 00:19:49,900
we're going to use three bytes--

436
00:19:49,900 --> 00:19:54,010
one byte to specify how much red
should be in that specific pixel,

437
00:19:54,010 --> 00:19:57,940
one more byte to specify how much green
should be combined with red to form

438
00:19:57,940 --> 00:20:00,400
that pixel, and then
one more byte, a third,

439
00:20:00,400 --> 00:20:03,550
to represent how much blue to
combine with those other two colors

440
00:20:03,550 --> 00:20:05,620
to make a new color all together.

441
00:20:05,620 --> 00:20:09,230
So it's kind of like combining
paints, except in this case,

442
00:20:09,230 --> 00:20:13,810
it's more really waves of light in
order to get a specific color using just

443
00:20:13,810 --> 00:20:16,510
red, green, and blue as your palette.

444
00:20:16,510 --> 00:20:20,180
And so if we were to see this
red, green, blue pattern and say,

445
00:20:20,180 --> 00:20:20,770
you know what?

446
00:20:20,770 --> 00:20:25,780
Give me 72 red, 73 of
green, and 33 of blue.

447
00:20:25,780 --> 00:20:30,490
If the total possible range that I
alluded to earlier is like 0 to 256,

448
00:20:30,490 --> 00:20:35,770
or technically 0 to 255 if you start
counting in computer science from zero,

449
00:20:35,770 --> 00:20:38,590
this is like a medium amount
of red, medium amount of green,

450
00:20:38,590 --> 00:20:42,140
and just a little bit of blue,
if the range goes from 0 to 255.

451
00:20:42,140 --> 00:20:44,100
So if you combine these
three things together

452
00:20:44,100 --> 00:20:46,790
and you want to know
what color you get--

453
00:20:46,790 --> 00:20:47,770
yeah.

454
00:20:47,770 --> 00:20:50,840
So it's kind of a light
yellow that looks like this.

455
00:20:50,840 --> 00:20:54,360
So if a computer is storing a
single dot on the screen that

456
00:20:54,360 --> 00:20:57,720
happens to be in yellow, what
the computer's actually storing

457
00:20:57,720 --> 00:21:00,640
is not this dot physically,
but a pattern of three

458
00:21:00,640 --> 00:21:03,600
bytes-- how much red, how much green,
how much blue should the computer

459
00:21:03,600 --> 00:21:05,560
display at this particular point.

460
00:21:05,560 --> 00:21:10,230
So if we look at this crying face of
joy and we kind of enhance or zoom in

461
00:21:10,230 --> 00:21:14,320
on it here, you can actually see
it start to pixelate, so to speak,

462
00:21:14,320 --> 00:21:15,660
where you start to see the dots.

463
00:21:15,660 --> 00:21:17,970
If I punch in a little
more, now you can really

464
00:21:17,970 --> 00:21:20,170
start to see the dots on the screen.

465
00:21:20,170 --> 00:21:22,230
And if I go an even
further, you can actually

466
00:21:22,230 --> 00:21:26,250
see the tiny little squares that
compose this image, most of which

467
00:21:26,250 --> 00:21:29,430
at the zoom level are yellow,
but a bunch of which are black,

468
00:21:29,430 --> 00:21:33,310
a bunch of which are light
black or dark yellow.

469
00:21:33,310 --> 00:21:35,790
And that's what composes
this image ultimately.

470
00:21:35,790 --> 00:21:39,990
So this is to say if you count up
all of the pixels on the screen

471
00:21:39,990 --> 00:21:43,260
and then multiply it by one,
two, three bytes, that's

472
00:21:43,260 --> 00:21:46,950
how many bytes or kilobytes or
megabytes, if you've heard those terms,

473
00:21:46,950 --> 00:21:51,670
are going to be stored on your
computer just to represent an image.

474
00:21:51,670 --> 00:21:56,130
So we've gone from electricity to down
here, so to speak, to zeros and ones,

475
00:21:56,130 --> 00:21:58,050
to decimal, now to colors.

476
00:21:58,050 --> 00:22:00,300
Well, with colors, you can get images.

477
00:22:00,300 --> 00:22:01,540
What comes after images?

478
00:22:01,540 --> 00:22:05,280
Well, we've all watched videos or
movies certainly digitally these days.

479
00:22:05,280 --> 00:22:07,530
Well, what is a movie or a video file?

480
00:22:07,530 --> 00:22:09,220
How might that be implemented?

481
00:22:11,770 --> 00:22:13,380
Say it a little louder.

482
00:22:13,380 --> 00:22:13,880
Yeah.

483
00:22:13,880 --> 00:22:15,120
It's a collection of images.

484
00:22:15,120 --> 00:22:16,910
If you've ever heard
of frames per second--

485
00:22:16,910 --> 00:22:20,750
like movies tend to be 24 frames
per second or 30 frames per second--

486
00:22:20,750 --> 00:22:23,510
that just means that a
typical movie, every second

487
00:22:23,510 --> 00:22:26,810
is showing you 24 or
30 images per second

488
00:22:26,810 --> 00:22:29,240
and they're just flying by
so quickly that you actually

489
00:22:29,240 --> 00:22:31,970
don't notice you're just watching
a sequence of static images.

490
00:22:31,970 --> 00:22:35,270
It's like as a kid, if you ever had one
of those paper flip books where there's

491
00:22:35,270 --> 00:22:38,060
tons of drawings in them, and
as you flip through the pages,

492
00:22:38,060 --> 00:22:40,770
you see things moving, but that's
just because your eyes are just

493
00:22:40,770 --> 00:22:45,230
seeing little snapshots ever so quickly
of something moving on the paper.

494
00:22:45,230 --> 00:22:47,010
That's all a video file actually is.

495
00:22:47,010 --> 00:22:49,850
So if you have an iPhone and you've
ever played with these animojis,

496
00:22:49,850 --> 00:22:53,180
so to speak, well, all those
are are little video files

497
00:22:53,180 --> 00:22:55,700
composed of lots and
lots and lots of images

498
00:22:55,700 --> 00:23:00,060
that you have saved on your
phone or texted to someone else.

499
00:23:00,060 --> 00:23:03,150
And if we just think, now, we're at
the point of video, but that's OK.

500
00:23:03,150 --> 00:23:04,700
Videos are just bunches of images.

501
00:23:04,700 --> 00:23:06,380
Images are just bunches of colors.

502
00:23:06,380 --> 00:23:07,940
Colors are just patterns of bits.

503
00:23:07,940 --> 00:23:09,650
And bits, at the end
of the day, are just

504
00:23:09,650 --> 00:23:12,800
the result of electricity coming
into my machine or transistors

505
00:23:12,800 --> 00:23:14,570
turning switches on and off.

506
00:23:14,570 --> 00:23:18,170
Like we've all of a sudden to hold
this entire story, but none of us

507
00:23:18,170 --> 00:23:21,710
ever is going to need to really think
about binary in the context of videos

508
00:23:21,710 --> 00:23:25,400
because a video is just an abstraction
on top of bunches of images,

509
00:23:25,400 --> 00:23:29,430
and images are just an abstraction on
top bunches of pixels, and so forth.

510
00:23:29,430 --> 00:23:31,550
So we can keep painting
this hierarchy that

511
00:23:31,550 --> 00:23:34,010
just allows us to talk about
things at a more useful level,

512
00:23:34,010 --> 00:23:35,930
and the reason we had
this conversation is

513
00:23:35,930 --> 00:23:38,480
just because we needed
a way to represent

514
00:23:38,480 --> 00:23:41,440
inputs and outputs to problems.

515
00:23:41,440 --> 00:23:46,400
Let me pause there for just a second
to see if there's any questions.

516
00:23:46,400 --> 00:23:48,830
Anything at all?

517
00:23:48,830 --> 00:23:49,460
No?

518
00:23:49,460 --> 00:23:49,960
All right.

519
00:23:49,960 --> 00:23:52,480
So what's inside this black box?

520
00:23:52,480 --> 00:23:56,080
Well, it turns out this is where
the really interesting work starts

521
00:23:56,080 --> 00:23:58,100
to happen and the thought
starts to come in.

522
00:23:58,100 --> 00:24:01,810
This is the proverbial algorithms,
step by step instructions

523
00:24:01,810 --> 00:24:03,160
for solving some problem.

524
00:24:03,160 --> 00:24:05,410
And some of you might have
solved this problem before,

525
00:24:05,410 --> 00:24:07,480
either digitally or
textually, but of course,

526
00:24:07,480 --> 00:24:09,770
if you have contacts in
your phone these days

527
00:24:09,770 --> 00:24:11,650
and you've got bunches
of friends and family,

528
00:24:11,650 --> 00:24:14,040
odds are they're alphabetized
by first name or last name.

529
00:24:14,040 --> 00:24:16,540
And you have auto complete these
days, but it really is just

530
00:24:16,540 --> 00:24:18,300
a long list of names and numbers.

531
00:24:18,300 --> 00:24:20,890
That's not all that different
from yesteryear's implementation

532
00:24:20,890 --> 00:24:24,640
of the same problem, which was
this device here, a phonebook.

533
00:24:24,640 --> 00:24:28,120
Now, this phonebook might have a
friend of ours in it, say Mike Smith,

534
00:24:28,120 --> 00:24:30,490
whose last name starts with
S. And I could, of course,

535
00:24:30,490 --> 00:24:32,800
if trying to find Mike
Smith, start by looking

536
00:24:32,800 --> 00:24:37,780
at the first page, the second page,
the third page, the fourth page,

537
00:24:37,780 --> 00:24:39,790
and eventually, just
hopefully find Mike Smith.

538
00:24:39,790 --> 00:24:42,640
Indeed, is this algorithm,
this step by step process,

539
00:24:42,640 --> 00:24:45,710
correct for finding
someone like Mike Smith?

540
00:24:45,710 --> 00:24:46,210
Yeah.

541
00:24:46,210 --> 00:24:46,940
It's correct.

542
00:24:46,940 --> 00:24:48,970
It's stupid and slow
perhaps because it's

543
00:24:48,970 --> 00:24:51,430
going to take forever in
a phone book of this size,

544
00:24:51,430 --> 00:24:54,390
but it is correct because if Mike's
in here, I will, in fact, find him.

545
00:24:54,390 --> 00:24:55,520
But I could do this better.

546
00:24:55,520 --> 00:24:57,140
I could do it sort of two at a time.

547
00:24:57,140 --> 00:25:00,880
So two, four, six, eight,
10-- or imperfectly--

548
00:25:00,880 --> 00:25:02,560
10, 12, 14.

549
00:25:02,560 --> 00:25:04,450
Is that faster?

550
00:25:04,450 --> 00:25:06,640
Obviously, it's going twice as fast.

551
00:25:06,640 --> 00:25:08,470
Is it correct?

552
00:25:08,470 --> 00:25:08,970
No.

553
00:25:08,970 --> 00:25:10,640
Why is it not correct?

554
00:25:10,640 --> 00:25:11,140
Yeah.

555
00:25:11,140 --> 00:25:12,260
I might miss him, right?

556
00:25:12,260 --> 00:25:15,560
Mike just accidentally might eventually
get sandwiched between two pages

557
00:25:15,560 --> 00:25:18,380
and I have the unlucky
experience of just missing him.

558
00:25:18,380 --> 00:25:19,430
Now, is it fixable?

559
00:25:19,430 --> 00:25:20,030
Yeah.

560
00:25:20,030 --> 00:25:24,440
I can probably, once I hit like SN
or the T section, for instance--

561
00:25:24,440 --> 00:25:26,690
I can just say, obviously,
I've gone too far for Mike.

562
00:25:26,690 --> 00:25:28,830
Let me just double back
one or just a few pages.

563
00:25:28,830 --> 00:25:30,030
So it is fixable.

564
00:25:30,030 --> 00:25:33,380
And so long as I've saved time by
flying through this twice as fast,

565
00:25:33,380 --> 00:25:37,310
can I at least afford to spend a
few more steps at the very end just

566
00:25:37,310 --> 00:25:38,420
to find Mike Smith?

567
00:25:38,420 --> 00:25:39,920
But none of us are going to do that.

568
00:25:39,920 --> 00:25:42,440
And our Apple devices and
Android devices certainly

569
00:25:42,440 --> 00:25:44,330
don't do that for efficiency today.

570
00:25:44,330 --> 00:25:48,210
Odds are most of us are going to do what
to find someone in any book like this?

571
00:25:48,210 --> 00:25:48,710
Yeah.

572
00:25:48,710 --> 00:25:51,740
Open to roughly the middle or
maybe bias ourselves toward the end

573
00:25:51,740 --> 00:25:53,090
because S is after the middle.

574
00:25:53,090 --> 00:25:55,340
But you know, I'm in the
middle of the phonebook here.

575
00:25:55,340 --> 00:25:58,730
And now, if I know that Mike is in
the S's and therefore over here,

576
00:25:58,730 --> 00:26:01,260
where do I know he's not?

577
00:26:01,260 --> 00:26:03,200
He's not in the beginning
and I can literally

578
00:26:03,200 --> 00:26:07,670
tear a problem like this in half,
throw figuratively and literally half

579
00:26:07,670 --> 00:26:11,540
of the problem away, and be left
with fundamentally the same problem,

580
00:26:11,540 --> 00:26:12,470
but it's half as big.

581
00:26:12,470 --> 00:26:13,670
I went from like-- whatever--

582
00:26:13,670 --> 00:26:17,300
1,000 pages to 500 pages and I
can now repeat this algorithm.

583
00:26:17,300 --> 00:26:18,160
I look down.

584
00:26:18,160 --> 00:26:19,120
I'm a little too far.

585
00:26:19,120 --> 00:26:20,450
I'm in the T section now.

586
00:26:20,450 --> 00:26:20,990
OK.

587
00:26:20,990 --> 00:26:25,010
I can again tear the problem
in half, throw that half away,

588
00:26:25,010 --> 00:26:29,180
taking a 500 page byte out, a 250
page byte out, now leaving myself

589
00:26:29,180 --> 00:26:31,040
with just 250 pages more.

590
00:26:31,040 --> 00:26:32,720
And notice just how quickly I got here.

591
00:26:32,720 --> 00:26:39,260
The first two algorithms got me from
1,000 to 999 to 998, or 1,000 to 998

592
00:26:39,260 --> 00:26:40,910
to 996.

593
00:26:40,910 --> 00:26:43,760
But here, I went from
1,000 to 500 to 250.

594
00:26:43,760 --> 00:26:45,660
Feels like we're making up time here.

595
00:26:45,660 --> 00:26:47,540
And indeed, if I keep
repeating this process,

596
00:26:47,540 --> 00:26:50,390
hopefully, I'll be left with
just one page of the book

597
00:26:50,390 --> 00:26:55,100
that Mike is either on or not,
at which point, I will call him.

598
00:26:55,100 --> 00:26:58,850
And so that's an algorithm that honestly
leverages probably all the intuition we

599
00:26:58,850 --> 00:27:01,130
have and a lot of what
programming is going to be,

600
00:27:01,130 --> 00:27:04,480
is thinking about a problem like this,
figuring out how to divide and conquer

601
00:27:04,480 --> 00:27:06,500
it, and then expressing
yourself in a way

602
00:27:06,500 --> 00:27:09,030
that the computer can then
solve that problem for you.

603
00:27:09,030 --> 00:27:12,530
And just to paint a picture of how
much better this algorithm is, well,

604
00:27:12,530 --> 00:27:15,890
if this is just a very abstract
chart where we have on the vertical,

605
00:27:15,890 --> 00:27:18,530
or y-axis, how much time it
takes to solve a problem,

606
00:27:18,530 --> 00:27:21,260
and on the horizontal axis
how big the problem is--

607
00:27:21,260 --> 00:27:24,330
so the farther out you go this
way, the more pages in the problem,

608
00:27:24,330 --> 00:27:25,670
the more pages in the phonebook.

609
00:27:25,670 --> 00:27:28,070
And the higher you go up
here, the more seconds or page

610
00:27:28,070 --> 00:27:29,270
turns it's going to take.

611
00:27:29,270 --> 00:27:32,720
That first algorithm is
just like a linear slope,

612
00:27:32,720 --> 00:27:35,660
so to speak, because for every
additional page in the book,

613
00:27:35,660 --> 00:27:37,190
it might take me one more second.

614
00:27:37,190 --> 00:27:38,670
Right, up.

615
00:27:38,670 --> 00:27:41,700
It's just a one for one
relationship with pages.

616
00:27:41,700 --> 00:27:43,820
The second algorithm,
if I plot it, where

617
00:27:43,820 --> 00:27:45,530
I'm flying through
twice as fast, what is

618
00:27:45,530 --> 00:27:47,070
that line going to look like instead?

619
00:27:51,430 --> 00:27:51,930
Yeah.

620
00:27:51,930 --> 00:27:53,550
It's going to look lower than this one.

621
00:27:53,550 --> 00:27:56,550
It's still going to be a straight
line because now, there's a two to one

622
00:27:56,550 --> 00:27:59,580
relationship, but if you've got a
phone book that's got this many pages,

623
00:27:59,580 --> 00:28:02,910
and in the first algorithm,
it took this long, here,

624
00:28:02,910 --> 00:28:07,410
well, in the second algorithm, it will
take half as many steps, plus or minus

625
00:28:07,410 --> 00:28:09,840
or two if you need to actually
double back a little bit.

626
00:28:09,840 --> 00:28:12,660
But that third algorithm is
what we'll call logarithmic.

627
00:28:12,660 --> 00:28:15,030
If n is the number of
pages in the phone book,

628
00:28:15,030 --> 00:28:16,890
the first algorithm,
in the very worst case,

629
00:28:16,890 --> 00:28:18,640
might take all n pages
to find Mike Smith.

630
00:28:18,640 --> 00:28:21,470
The second algorithm is going to
take half as many steps because I'm

631
00:28:21,470 --> 00:28:22,920
flying through it two at a time.

632
00:28:22,920 --> 00:28:26,250
But the third algorithm is going
to look and feel like this.

633
00:28:26,250 --> 00:28:30,720
It's going to be curved and ever so
slowly rising and rising and rising,

634
00:28:30,720 --> 00:28:33,950
the implication of which is if
Verizon or the phone company

635
00:28:33,950 --> 00:28:35,790
doubles the number of
pages in the phonebook

636
00:28:35,790 --> 00:28:39,090
next year because Cambridge and
Somerville merged together in the phone

637
00:28:39,090 --> 00:28:41,230
book and we now have 2,000 pages.

638
00:28:41,230 --> 00:28:45,150
Well, how many more steps
does my third algorithm take?

639
00:28:45,150 --> 00:28:45,690
Just one.

640
00:28:45,690 --> 00:28:48,300
Because I can take a 1,000
page bite out of the problem

641
00:28:48,300 --> 00:28:52,140
with that clever algorithm, whereas
my first two algorithms would take it

642
00:28:52,140 --> 00:28:54,150
one or just two pages at a time.

643
00:28:54,150 --> 00:28:58,740
So that is to say we have to hugely
increase the size of this problem

644
00:28:58,740 --> 00:29:03,450
just for the number of seconds or page
turns to appreciably actually increase.

645
00:29:03,450 --> 00:29:05,490
And so as we start to
learn about programming,

646
00:29:05,490 --> 00:29:08,670
it's, again, going to be leveraging
of this intuition in order

647
00:29:08,670 --> 00:29:13,260
to actually solve problems and
code more effectively than we

648
00:29:13,260 --> 00:29:17,820
might without that intuition alone.

649
00:29:17,820 --> 00:29:19,560
So let's formalize this now.

650
00:29:19,560 --> 00:29:22,920
So that was kind of a very intuitive way
of dividing and conquering a problem.

651
00:29:22,920 --> 00:29:24,750
Just kind of made sense
to go in the middle,

652
00:29:24,750 --> 00:29:26,940
tear it, then go to the
other half or the other half

653
00:29:26,940 --> 00:29:29,010
and tear it again, and so forth.

654
00:29:29,010 --> 00:29:33,990
But a computer, even as cool as Alexa
and Google Home and all of this are,

655
00:29:33,990 --> 00:29:36,120
you can't really just talk
to them as another human

656
00:29:36,120 --> 00:29:37,800
and have them execute things correctly.

657
00:29:37,800 --> 00:29:41,400
I struggle just to get Siri
to set a timer on my phone.

658
00:29:41,400 --> 00:29:43,950
So we're not quite there yet,
so we're still at the age

659
00:29:43,950 --> 00:29:46,830
where we have to be ever
so precise with computers,

660
00:29:46,830 --> 00:29:50,430
voice activated or otherwise, and
so thus enter pseudocode for now.

661
00:29:50,430 --> 00:29:52,560
Pseudocode has no formal definition.

662
00:29:52,560 --> 00:29:57,000
This is just a way of saying use English
like syntax or any spoken language

663
00:29:57,000 --> 00:30:00,210
and just express yourself
succinctly and correctly

664
00:30:00,210 --> 00:30:03,090
so that a computer or a
robot or even another person

665
00:30:03,090 --> 00:30:05,640
can understand what it
is you're trying to say.

666
00:30:05,640 --> 00:30:08,340
So here, I propose, is
an algorithm written

667
00:30:08,340 --> 00:30:11,650
in pseudocode, English like syntax,
that just gets my point across.

668
00:30:11,650 --> 00:30:13,530
And I could write this
in any number of ways.

669
00:30:13,530 --> 00:30:16,680
I've numbered the steps from zero on
up, just for the sake of discussion,

670
00:30:16,680 --> 00:30:18,900
but this would seem to
capture what I did there.

671
00:30:18,900 --> 00:30:20,220
Pick up the phone book.

672
00:30:20,220 --> 00:30:21,840
Open to the middle of the phone.

673
00:30:21,840 --> 00:30:22,710
Look at the names.

674
00:30:22,710 --> 00:30:25,260
If Smith is among the names, call Mike.

675
00:30:25,260 --> 00:30:27,660
Else, if Mike Smith is
earlier in the book,

676
00:30:27,660 --> 00:30:31,050
go to the left, specifically the
middle of the left half of the book,

677
00:30:31,050 --> 00:30:32,910
and then go back to step two.

678
00:30:32,910 --> 00:30:35,650
Because indeed, I was just doing
the same thing again and again,

679
00:30:35,650 --> 00:30:38,350
and the reason I wasn't doing it
forever was because every time I

680
00:30:38,350 --> 00:30:42,060
repeated myself by opening and
tearing, I was shrinking the problem.

681
00:30:42,060 --> 00:30:44,850
And I can only shrink a problem
of some fixed finite size

682
00:30:44,850 --> 00:30:47,220
so many times until I
get just one page, and so

683
00:30:47,220 --> 00:30:51,060
if I continue this logic looking to the
right or to the left or just quitting,

684
00:30:51,060 --> 00:30:53,460
if I don't find Mike at
all on the last page,

685
00:30:53,460 --> 00:30:56,700
this would seem to capture
more precisely that code.

686
00:30:56,700 --> 00:30:59,640
Well, let's actually excerpt
from this now a few concepts

687
00:30:59,640 --> 00:31:01,980
and then start to apply
them to actual code.

688
00:31:01,980 --> 00:31:05,730
Highlighted in yellow here, I dare
say, are all of the verbs or actions.

689
00:31:05,730 --> 00:31:08,550
These are the functions, as we're
going to start calling them,

690
00:31:08,550 --> 00:31:09,330
in this algorithm.

691
00:31:09,330 --> 00:31:12,480
A function is just a specific
step, a specific action

692
00:31:12,480 --> 00:31:15,600
you take in order to do something.

693
00:31:15,600 --> 00:31:19,800
And so in yellow here-- pick up,
open to, look at, call, open, quit

694
00:31:19,800 --> 00:31:20,940
are all actions or verbs.

695
00:31:20,940 --> 00:31:23,400
Are henceforth, we'll
call them functions.

696
00:31:23,400 --> 00:31:27,450
Meanwhile, highlighted in yellow
here-- if, else if, else if, else.

697
00:31:27,450 --> 00:31:29,320
These are kind of
starting to ask questions.

698
00:31:29,320 --> 00:31:32,620
What might these be called
if you have some familiarity?

699
00:31:32,620 --> 00:31:33,120
Yes.

700
00:31:33,120 --> 00:31:35,490
Turns out many programming
languages, if you've seen any before,

701
00:31:35,490 --> 00:31:36,650
would call these conditions.

702
00:31:36,650 --> 00:31:39,060
They're branches, or
proverbial forks in the road.

703
00:31:39,060 --> 00:31:40,840
If this is true, go this way.

704
00:31:40,840 --> 00:31:44,160
Else, maybe go this other way, or
perhaps a third or fourth direction

705
00:31:44,160 --> 00:31:45,030
altogether.

706
00:31:45,030 --> 00:31:48,630
Meanwhile, if we actually look
at these highlighted phrases--

707
00:31:48,630 --> 00:31:51,420
if Smith is among names or
if Smith is earlier in book

708
00:31:51,420 --> 00:31:52,710
or Smith is later in book--

709
00:31:52,710 --> 00:31:56,440
these are the specific questions we're
asking in order to make that decision.

710
00:31:56,440 --> 00:31:59,130
These are known as Boolean
expressions, named after a gentleman

711
00:31:59,130 --> 00:32:01,170
by the last name of
Boole some years ago.

712
00:32:01,170 --> 00:32:03,120
And so a Boolean expression
is just a question

713
00:32:03,120 --> 00:32:07,340
that has a yes or no answer, a
true false answer, a one zero

714
00:32:07,340 --> 00:32:08,710
answer, if you will.

715
00:32:08,710 --> 00:32:11,340
And that's a nice mapping to what
computers are really good at.

716
00:32:11,340 --> 00:32:13,420
So within conditions, you
have Boolean expressions

717
00:32:13,420 --> 00:32:16,680
to decide which fork in the
road you want to go down.

718
00:32:16,680 --> 00:32:18,510
And then lastly,
highlighted in yellow here

719
00:32:18,510 --> 00:32:21,210
is go back to step 2
in a couple of places.

720
00:32:21,210 --> 00:32:23,940
This is inducing some
kind of cycle or loop

721
00:32:23,940 --> 00:32:27,750
that's telling the computer to do
something again and again and again.

722
00:32:27,750 --> 00:32:31,620
So in short, we have these building
blocks already conceptually.

723
00:32:31,620 --> 00:32:34,260
And it turns out, we can
now start to translate these

724
00:32:34,260 --> 00:32:36,220
to an actual programming language.

725
00:32:36,220 --> 00:32:38,310
The first of the languages
we'll introduce in CS50

726
00:32:38,310 --> 00:32:39,830
is something called Scratch.

727
00:32:39,830 --> 00:32:41,670
Turns out this is not
a text based language,

728
00:32:41,670 --> 00:32:44,130
like in my English pseudocode
there, but it's graphical

729
00:32:44,130 --> 00:32:46,680
and things look like puzzle
pieces that you can drag and drop

730
00:32:46,680 --> 00:32:48,870
and they interconnect if it
makes logical sense to do so.

731
00:32:48,870 --> 00:32:51,390
And in fact, some of you might have
played this back in the day as kids

732
00:32:51,390 --> 00:32:54,030
or even more recently because
it's actually targeted typically

733
00:32:54,030 --> 00:32:55,910
at students in like
after school programs who

734
00:32:55,910 --> 00:33:00,170
just want to learn more methodical, more
algorithmic, or computational thinking.

735
00:33:00,170 --> 00:33:02,930
And we're going to use it to
explore not only these building

736
00:33:02,930 --> 00:33:04,940
blocks, but a few others, as well.

737
00:33:04,940 --> 00:33:08,600
It turns out in the other languages
we'll explore in CS50 and beyond,

738
00:33:08,600 --> 00:33:12,560
are languages like C that we'll actually
transition to as quickly as next week,

739
00:33:12,560 --> 00:33:16,580
to then translate what we do this
week in Scratch to next week in C.

740
00:33:16,580 --> 00:33:18,590
And in languages like
Python and JavaScript

741
00:33:18,590 --> 00:33:21,650
and SQL, which we'll also explore,
do we have other capabilities--

742
00:33:21,650 --> 00:33:24,020
the ability to store data
in variables, so to speak,

743
00:33:24,020 --> 00:33:27,830
to use threads, which means get the
computer to do multiple things at once,

744
00:33:27,830 --> 00:33:30,920
events, to mean listen for
things happening, like a click

745
00:33:30,920 --> 00:33:33,920
on the page or a human typing
or even saying something.

746
00:33:33,920 --> 00:33:36,050
We'll be able to do all of
the things that you take

747
00:33:36,050 --> 00:33:38,180
for granted in your very own phones.

748
00:33:38,180 --> 00:33:41,040
And we'll do this first
by way of this guy.

749
00:33:41,040 --> 00:33:44,060
So this is Scratch, the default cat
that comes with this programming

750
00:33:44,060 --> 00:33:45,890
language from MIT's media lab.

751
00:33:45,890 --> 00:33:48,990
And via Scratch can we start
programming him to move up,

752
00:33:48,990 --> 00:33:52,550
down, left, right, say something,
utter something, and other commands

753
00:33:52,550 --> 00:33:53,580
all together.

754
00:33:53,580 --> 00:33:56,720
In fact, let me go ahead
and switch contexts here

755
00:33:56,720 --> 00:33:59,200
to show you the very first
thing I ever wrote in Scratch.

756
00:33:59,200 --> 00:34:01,370
It was back in the day when
I was in graduate school

757
00:34:01,370 --> 00:34:03,720
and Scratch had just
been invented by MIT.

758
00:34:03,720 --> 00:34:05,540
Let me go ahead and open this.

759
00:34:05,540 --> 00:34:08,710
And I called it Oscar Time.

760
00:34:08,710 --> 00:34:11,630
And if we could perhaps have a
volunteer come on up for just a moment.

761
00:34:11,630 --> 00:34:14,180
You have to be comfortable being
on stage and on the internet.

762
00:34:14,180 --> 00:34:15,590
How about here in the white shirt?

763
00:34:15,590 --> 00:34:16,550
I saw your hand first.

764
00:34:16,550 --> 00:34:18,020
Come on down.

765
00:34:18,020 --> 00:34:19,460
So this is Oscar Time.

766
00:34:19,460 --> 00:34:21,740
It's implemented in a
language called Scratch.

767
00:34:21,740 --> 00:34:25,850
And at the end of the day, all that
is underneath the hood of this program

768
00:34:25,850 --> 00:34:29,190
is functions and loops and conditions
and a few other of these concepts.

769
00:34:29,190 --> 00:34:29,690
Hi.

770
00:34:29,690 --> 00:34:30,080
What's your name?

771
00:34:30,080 --> 00:34:30,870
AVIVA: Aviva.

772
00:34:30,870 --> 00:34:31,280
DAVID MALAN: Aviva.

773
00:34:31,280 --> 00:34:31,710
David.

774
00:34:31,710 --> 00:34:32,410
Nice to meet you.

775
00:34:32,410 --> 00:34:33,980
Come on over here.

776
00:34:33,980 --> 00:34:37,070
And in just a moment, I'm
going to go ahead and click

777
00:34:37,070 --> 00:34:39,230
the green flag at the top
left hand corner, which

778
00:34:39,230 --> 00:34:40,460
is going to play this game.

779
00:34:40,460 --> 00:34:42,330
And we'll see on the
screen the instructions.

780
00:34:42,330 --> 00:34:45,810
[MUSIC PLAYING]

781
00:34:45,810 --> 00:34:50,300
OSCAR: (SINGING) Oh, I love trash.

782
00:34:50,300 --> 00:34:54,500
Anything dirty or dingy or dusty.

783
00:34:54,500 --> 00:34:58,670
Anything ragged or rotten or rusty.

784
00:34:58,670 --> 00:35:02,710
Yes, I love trash.

785
00:35:02,710 --> 00:35:05,300
If you really want to see
something trashy, look at this.

786
00:35:05,300 --> 00:35:09,400
I have here a sneaker
that's tattered and worn.

787
00:35:09,400 --> 00:35:13,340
It's all full of holes
and the laces are torn.

788
00:35:13,340 --> 00:35:17,710
A gift from my mother
the day I was born.

789
00:35:17,710 --> 00:35:21,270
I love it because it's trash.

790
00:35:21,270 --> 00:35:25,650
Oh, I love trash.

791
00:35:25,650 --> 00:35:29,880
Anything dirty or dingy or dusty.

792
00:35:29,880 --> 00:35:34,190
Anything ragged or rotten or rusty.

793
00:35:34,190 --> 00:35:37,290
Yes, I love trash.

794
00:35:37,290 --> 00:35:40,610
Here's some more rotten stuff.

795
00:35:40,610 --> 00:35:43,370
I have here some
newspaper 13 months old.

796
00:35:43,370 --> 00:35:44,330
DAVID MALAN: All right.

797
00:35:44,330 --> 00:35:46,950
Everybody, give a round of applause
for Aviva for coming on up.

798
00:35:46,950 --> 00:35:47,750
Thank you.

799
00:35:47,750 --> 00:35:48,250
Here.

800
00:35:48,250 --> 00:35:48,750
Aviva.

801
00:35:48,750 --> 00:35:51,230
[APPLAUSE]

802
00:35:51,230 --> 00:35:53,180
A little CS50 stress ball.

803
00:35:53,180 --> 00:35:56,150
So suffice it to say, if
you're tired of this song,

804
00:35:56,150 --> 00:35:59,900
consider how tired I was eight hours
later while debugging and building

805
00:35:59,900 --> 00:36:00,590
this program.

806
00:36:00,590 --> 00:36:02,090
But consider what it is we just saw.

807
00:36:02,090 --> 00:36:05,610
It's this interactive game and stuff
is animated and music is playing.

808
00:36:05,610 --> 00:36:09,560
But if you focus on decomposing,
so to speak, this program into just

809
00:36:09,560 --> 00:36:13,520
basic building blocks, this is just kind
of a big abstraction over some lower

810
00:36:13,520 --> 00:36:15,380
level pieces of functionality.

811
00:36:15,380 --> 00:36:16,580
Like this trash can here.

812
00:36:16,580 --> 00:36:18,830
At the moment, it's just a
picture, and on occasion,

813
00:36:18,830 --> 00:36:21,580
as soon as Aviva dropped something
into the trash, the lid came up

814
00:36:21,580 --> 00:36:24,680
and Oscar came out, he said
something, and then he went back down.

815
00:36:24,680 --> 00:36:26,720
But that animation is super simplistic.

816
00:36:26,720 --> 00:36:31,490
It was just a sequence of 1, 2, 3,
or so images displaying and then

817
00:36:31,490 --> 00:36:33,920
going back down to create
the illusion of animation.

818
00:36:33,920 --> 00:36:36,290
Meanwhile, every time
Oscar said something,

819
00:36:36,290 --> 00:36:39,320
that was keeping track of her
score in what's called a variable.

820
00:36:39,320 --> 00:36:42,900
In algebra, you have x and y and z, but
in programming, you have the same idea,

821
00:36:42,900 --> 00:36:45,530
but it's generally more useful
to call them more descriptively,

822
00:36:45,530 --> 00:36:46,370
like your score.

823
00:36:46,370 --> 00:36:48,620
And so there's probably a
variable in this game called

824
00:36:48,620 --> 00:36:51,290
score that was just keeping
track of how many times

825
00:36:51,290 --> 00:36:53,240
Aviva had dropped
something into the trash.

826
00:36:53,240 --> 00:36:57,650
Meanwhile, the trash itself and the
shoe and the newspaper-- and even more

827
00:36:57,650 --> 00:37:00,470
things happen eventually--
were falling from the sky

828
00:37:00,470 --> 00:37:03,680
at sort of random locations, and
that's because I programmed the game

829
00:37:03,680 --> 00:37:06,230
to sort of start the trash
here or over here, just

830
00:37:06,230 --> 00:37:08,950
to make it a little more
challenging as the game picked up.

831
00:37:08,950 --> 00:37:11,540
And in fact, things start falling
faster and faster over time,

832
00:37:11,540 --> 00:37:14,550
like a typical game, getting
more and more difficult.

833
00:37:14,550 --> 00:37:16,830
So how do we get to something like that?

834
00:37:16,830 --> 00:37:19,130
Well, let me go ahead and
open up Scratch itself

835
00:37:19,130 --> 00:37:21,240
and introduce the environment.

836
00:37:21,240 --> 00:37:24,200
So in Scratch, you essentially
have three general areas.

837
00:37:24,200 --> 00:37:26,660
And it's web based, and so you
can do this on any computer.

838
00:37:26,660 --> 00:37:29,510
And in the left hand side here,
you have those puzzle pieces

839
00:37:29,510 --> 00:37:30,920
to which I referred earlier.

840
00:37:30,920 --> 00:37:36,140
These puzzle pieces are all mapping
to functions or loops or conditions

841
00:37:36,140 --> 00:37:38,140
or variables, things that
we saw before, and I'm

842
00:37:38,140 --> 00:37:41,180
going to able to drag and drop
them into the middle in order

843
00:37:41,180 --> 00:37:43,040
to interconnect them
and write my program,

844
00:37:43,040 --> 00:37:44,430
which we'll do in just a moment.

845
00:37:44,430 --> 00:37:48,330
Meanwhile, Scratch lives in this stage,
this world, where he can move up,

846
00:37:48,330 --> 00:37:49,250
down, left, right.

847
00:37:49,250 --> 00:37:50,870
You can change what Scratch looks like.

848
00:37:50,870 --> 00:37:53,300
You can add other characters,
otherwise known as sprites,

849
00:37:53,300 --> 00:37:55,140
in order to have multiple
things happening at once.

850
00:37:55,140 --> 00:37:56,680
And of course, you can fullscreen it.

851
00:37:56,680 --> 00:38:00,410
And so the Oscar Time game a moment ago
was actually a whole bunch of sprites.

852
00:38:00,410 --> 00:38:02,250
Oscar's trash can was one.

853
00:38:02,250 --> 00:38:04,640
Each piece of trash was another sprite.

854
00:38:04,640 --> 00:38:07,050
The newspaper was a
sprite, and so forth.

855
00:38:07,050 --> 00:38:11,250
So each of them were separate programs
running in parallel at the same time.

856
00:38:11,250 --> 00:38:13,110
So let's actually make him do something.

857
00:38:13,110 --> 00:38:16,770
It turns out that if I
jump down to, say, events,

858
00:38:16,770 --> 00:38:18,770
I'm going to see one of
the most powerful blocks

859
00:38:18,770 --> 00:38:22,000
from the get go, which is
this when green flag clicked.

860
00:38:22,000 --> 00:38:24,000
That's indeed how I started
the game with Aviva,

861
00:38:24,000 --> 00:38:27,260
by clicking just above
Scratch's world this green flag.

862
00:38:27,260 --> 00:38:29,030
And if I wanted to
stop it, as I did, you

863
00:38:29,030 --> 00:38:31,130
can click the red stop sign to say stop.

864
00:38:31,130 --> 00:38:34,730
Meanwhile, the green
flag, I can constantly

865
00:38:34,730 --> 00:38:37,220
listen for by dragging and
dropping this puzzle piece.

866
00:38:37,220 --> 00:38:39,710
When the green flag is
clicked, what do I want to do?

867
00:38:39,710 --> 00:38:41,470
Well, let me go up to looks.

868
00:38:41,470 --> 00:38:43,140
And these are just different categories.

869
00:38:43,140 --> 00:38:45,180
And we can scroll through all
the different colorful blocks,

870
00:38:45,180 --> 00:38:47,240
but they pretty much
just do what they say.

871
00:38:47,240 --> 00:38:50,370
I'm going to go under looks, where I
know there to be a block that's called

872
00:38:50,370 --> 00:38:53,990
say, and I'm going to go ahead and type
the most canonical computer science

873
00:38:53,990 --> 00:38:55,500
thing-- hello world--

874
00:38:55,500 --> 00:38:56,160
in this box.

875
00:38:56,160 --> 00:38:58,550
So notice that functions
themselves can actually

876
00:38:58,550 --> 00:39:01,610
take inputs and the input
to this function, say,

877
00:39:01,610 --> 00:39:03,080
is going to be hello world.

878
00:39:03,080 --> 00:39:06,530
If I now go over to the
green flag and click it--

879
00:39:06,530 --> 00:39:07,530
hello world.

880
00:39:07,530 --> 00:39:08,030
All right.

881
00:39:08,030 --> 00:39:11,000
So not all that difficult.
Not all that interesting.

882
00:39:11,000 --> 00:39:14,810
But it actually got the job done, and
so my program is indeed just this.

883
00:39:14,810 --> 00:39:17,060
Well, how might I make this
a little more interesting?

884
00:39:17,060 --> 00:39:19,730
Just saying, hello world all the
time isn't all that compelling.

885
00:39:19,730 --> 00:39:20,620
Well, you know what?

886
00:39:20,620 --> 00:39:21,170
Let me think.

887
00:39:21,170 --> 00:39:22,550
Let me undo this.

888
00:39:22,550 --> 00:39:24,380
Let me scroll down to sensing.

889
00:39:24,380 --> 00:39:25,880
And notice this.

890
00:39:25,880 --> 00:39:28,490
Functions can also
take input from a human

891
00:39:28,490 --> 00:39:32,270
and functions can hand you back a
value, a so-called return value.

892
00:39:32,270 --> 00:39:35,090
So this block here, ask
something-- by default, it says,

893
00:39:35,090 --> 00:39:36,500
what's your name and weight--

894
00:39:36,500 --> 00:39:39,350
is another function built into
Scratch that allows me to do this.

895
00:39:39,350 --> 00:39:41,420
So I'm going to go
ahead and drag this here

896
00:39:41,420 --> 00:39:43,950
and I'm going to let it
say, what's your name?

897
00:39:43,950 --> 00:39:46,990
Notice now that below this
block is a special block,

898
00:39:46,990 --> 00:39:48,650
whatever it is the block returns.

899
00:39:48,650 --> 00:39:51,560
So answer is whatever the
human is going to type in.

900
00:39:51,560 --> 00:39:56,060
And if I want to now say what the human
typed in, let me go again to looks.

901
00:39:56,060 --> 00:39:57,060
Go to say.

902
00:39:57,060 --> 00:39:59,150
And notice that these
blocks are kind of magnetic.

903
00:39:59,150 --> 00:40:00,340
They want to snap together.

904
00:40:00,340 --> 00:40:02,130
So I'm going to go
ahead and let go there.

905
00:40:02,130 --> 00:40:05,160
And if I go back to
sensing and grab answer,

906
00:40:05,160 --> 00:40:07,730
notice that even though it's
not quite the same size,

907
00:40:07,730 --> 00:40:09,740
it's going to grow to
fill, and now, I can

908
00:40:09,740 --> 00:40:12,890
have my program ask the user
what his or her name is and then

909
00:40:12,890 --> 00:40:14,930
say whatever that answer is.

910
00:40:14,930 --> 00:40:18,020
So let me go ahead and
stop and click play again.

911
00:40:18,020 --> 00:40:21,020
Notice it's asking me for my name,
so let me go ahead and type in David.

912
00:40:21,020 --> 00:40:22,020
Enter.

913
00:40:22,020 --> 00:40:22,520
OK.

914
00:40:22,520 --> 00:40:24,770
It's a little weird
way to greet someone.

915
00:40:24,770 --> 00:40:25,790
David.

916
00:40:25,790 --> 00:40:28,490
So it'd be nice to clean that up a bit.

917
00:40:28,490 --> 00:40:29,330
So you know what?

918
00:40:29,330 --> 00:40:31,380
I know this only from
having poked around before.

919
00:40:31,380 --> 00:40:33,230
Not all of this is
obvious at first glance.

920
00:40:33,230 --> 00:40:36,320
But it turns out that under
operators, the category,

921
00:40:36,320 --> 00:40:39,070
there's this thing here--
join apple and banana.

922
00:40:39,070 --> 00:40:40,320
Which are just default values.

923
00:40:40,320 --> 00:40:41,460
You can change them.

924
00:40:41,460 --> 00:40:42,710
Because what do I want to do?

925
00:40:42,710 --> 00:40:47,720
I want to say hello, David, or whoever,
so I kind of want to say hello, comma,

926
00:40:47,720 --> 00:40:49,710
and then, David-- whatever
the human typed in.

927
00:40:49,710 --> 00:40:51,090
And that's what join lets you do.

928
00:40:51,090 --> 00:40:54,880
It lets you join or concatenate
two phrases that are somehow

929
00:40:54,880 --> 00:40:56,140
provided by you or the user.

930
00:40:56,140 --> 00:40:58,690
So let me pull this out, the answer.

931
00:40:58,690 --> 00:41:00,750
Let me go ahead and grab the join block.

932
00:41:00,750 --> 00:41:02,650
Notice it, too, is
going to grow to fill.

933
00:41:02,650 --> 00:41:09,010
Let me go ahead and say, hello, comma,
space, and now, drag answer into there.

934
00:41:09,010 --> 00:41:11,770
And notice this nesting.

935
00:41:11,770 --> 00:41:12,520
Just like in math.

936
00:41:12,520 --> 00:41:14,360
This nesting of functions.

937
00:41:14,360 --> 00:41:18,700
I can first join hello and answer
by taking those two things as input

938
00:41:18,700 --> 00:41:20,710
and then pass them to
say as another input

939
00:41:20,710 --> 00:41:22,480
because these things are layered on top.

940
00:41:22,480 --> 00:41:26,420
And so now, if I stop this and
play it again and say, David--

941
00:41:26,420 --> 00:41:27,250
hello, David.

942
00:41:27,250 --> 00:41:30,700
Now, we have the makings of a more
interesting interactive program

943
00:41:30,700 --> 00:41:32,290
that isn't just hardcoded.

944
00:41:32,290 --> 00:41:38,080
Of course, it's not nearly as audible as
something like Oscar Time a moment ago.

945
00:41:38,080 --> 00:41:39,380
So let me go ahead and do this.

946
00:41:39,380 --> 00:41:43,660
Let me start over altogether and
treat Scratch like the cat he is

947
00:41:43,660 --> 00:41:45,570
and just start the sound called meow.

948
00:41:45,570 --> 00:41:48,130
So it turns out there's a
category of blocks called sound,

949
00:41:48,130 --> 00:41:50,740
and within sound, there is
play some default sounds.

950
00:41:50,740 --> 00:41:52,400
So start sound meow.

951
00:41:52,400 --> 00:41:54,070
And now, things will get a little cuter.

952
00:41:54,070 --> 00:41:55,480
[MEOW]

953
00:41:55,980 --> 00:41:56,940
Aw.

954
00:41:56,940 --> 00:41:58,010
And now, again.

955
00:41:58,010 --> 00:41:59,010
[MEOW]

956
00:41:59,510 --> 00:42:03,200
And I can kind of simulate a cat
by [MEOW] standing here for a while

957
00:42:03,200 --> 00:42:04,610
and keeping to click this button.

958
00:42:04,610 --> 00:42:05,360
But you know what?

959
00:42:05,360 --> 00:42:07,890
Let me make him meow few times
because that's more realistic.

960
00:42:07,890 --> 00:42:09,580
So let me grab a second
one and a third one.

961
00:42:09,580 --> 00:42:11,570
And you can get this
infinite supply of blocks.

962
00:42:11,570 --> 00:42:12,230
Let me hit play.

963
00:42:12,230 --> 00:42:13,620
[MEOW]

964
00:42:16,490 --> 00:42:17,320
Seems like a bug.

965
00:42:17,320 --> 00:42:17,990
Let's try again.

966
00:42:17,990 --> 00:42:18,780
Play.

967
00:42:18,780 --> 00:42:20,140
[MEOW]

968
00:42:21,050 --> 00:42:23,570
This is my first bug, or mistake.

969
00:42:23,570 --> 00:42:24,710
This looks correct.

970
00:42:24,710 --> 00:42:28,700
It says when green flag clicked,
start sound meow, start sound meow,

971
00:42:28,700 --> 00:42:30,020
start sound meow.

972
00:42:30,020 --> 00:42:33,490
Why am I only hearing one meow?

973
00:42:33,490 --> 00:42:34,980
Yeah.

974
00:42:34,980 --> 00:42:37,960
They're kind of at the same time
or so close to the same time

975
00:42:37,960 --> 00:42:41,050
that the sounds are kind of tripping
over each other and just overlapping,

976
00:42:41,050 --> 00:42:41,550
right?

977
00:42:41,550 --> 00:42:43,720
The block literally
says, start sound meow.

978
00:42:43,720 --> 00:42:45,120
But computers are really fast.

979
00:42:45,120 --> 00:42:48,030
If you've heard of the expression
gigahertz, that's a unit of measure.

980
00:42:48,030 --> 00:42:52,300
And if your computer has a one gigahertz
CPU, central processing unit, or brain,

981
00:42:52,300 --> 00:42:55,660
that means it can literally do
like a billion things per second.

982
00:42:55,660 --> 00:42:59,200
It can certainly start
three sounds super fast.

983
00:42:59,200 --> 00:43:02,200
And if they're effectively all
happening one after the other

984
00:43:02,200 --> 00:43:05,290
before the sound even finishes,
you're just hearing one net effect.

985
00:43:05,290 --> 00:43:06,680
So how can we fix this?

986
00:43:06,680 --> 00:43:09,880
Well, I can actually go and
fix this with this block here--

987
00:43:09,880 --> 00:43:12,190
play sound meow until done.

988
00:43:12,190 --> 00:43:15,550
Play sound meow until done.

989
00:43:15,550 --> 00:43:16,240
And now.

990
00:43:16,240 --> 00:43:18,510
[MEOWING]

991
00:43:18,510 --> 00:43:19,010
OK.

992
00:43:19,010 --> 00:43:20,890
It's a little unhappy,
this particular cat,

993
00:43:20,890 --> 00:43:22,730
but at least it's now more correct.

994
00:43:22,730 --> 00:43:25,420
And, as it turns out,
if I go to control--

995
00:43:25,420 --> 00:43:26,100
you know what?

996
00:43:26,100 --> 00:43:28,770
There's this block here--
wait some number of seconds.

997
00:43:28,770 --> 00:43:30,940
I can go ahead and insert this here.

998
00:43:30,940 --> 00:43:32,560
Let me do another one here.

999
00:43:32,560 --> 00:43:33,510
And now, hit play.

1000
00:43:33,510 --> 00:43:36,910
[MEOWING]

1001
00:43:38,730 --> 00:43:40,170
You know, it's not bad.

1002
00:43:40,170 --> 00:43:43,230
It now sounds a little more realistic.

1003
00:43:43,230 --> 00:43:45,510
But honestly, if I keep doing
this-- or you know what?

1004
00:43:45,510 --> 00:43:47,840
You can actually right click
or control click on blocks,

1005
00:43:47,840 --> 00:43:51,020
duplicate them, and just copy and
paste even more if you want them.

1006
00:43:51,020 --> 00:43:55,440
So if I were to do this, now,
it's just going to go six times.

1007
00:43:55,440 --> 00:43:57,390
And then I could copy it
again and go 12 times.

1008
00:43:57,390 --> 00:43:59,140
But there's got to be
a better way, right?

1009
00:43:59,140 --> 00:44:00,480
This is now bad programming.

1010
00:44:00,480 --> 00:44:01,770
This is bad design.

1011
00:44:01,770 --> 00:44:04,620
Because I'm literally copying
and pasting, albeit, graphically.

1012
00:44:04,620 --> 00:44:07,170
But we've already seen a
building block with which

1013
00:44:07,170 --> 00:44:09,390
we can design this program better.

1014
00:44:09,390 --> 00:44:11,190
It's correct, but it's
not well designed.

1015
00:44:11,190 --> 00:44:15,820
What would the building block be that
I need to make this a little cleaner?

1016
00:44:15,820 --> 00:44:16,320
OK.

1017
00:44:16,320 --> 00:44:16,860
A four loop.

1018
00:44:16,860 --> 00:44:18,150
Doesn't quite exist in Scratch.

1019
00:44:18,150 --> 00:44:21,300
But a loop fundamentally
does something cyclically.

1020
00:44:21,300 --> 00:44:23,980
And indeed, if I go under
control and start poking around,

1021
00:44:23,980 --> 00:44:26,910
you'll notice that there's a few
blocks that might apply here.

1022
00:44:26,910 --> 00:44:29,010
There's the repeat block
some number of times

1023
00:44:29,010 --> 00:44:32,400
or the forever block, both of
which sound like loops, or cycles.

1024
00:44:32,400 --> 00:44:35,460
So sure enough, let me go ahead
here and I can throw away blocks

1025
00:44:35,460 --> 00:44:37,110
by just dragging them to the left.

1026
00:44:37,110 --> 00:44:40,080
Let me pull this out for a second.

1027
00:44:40,080 --> 00:44:44,460
And then just say forever play this
sound, and then wait one second.

1028
00:44:44,460 --> 00:44:47,070
So now, my program looks like this.

1029
00:44:47,070 --> 00:44:50,360
[MEOWING]

1030
00:44:53,770 --> 00:44:56,770
You know, we'll never know if it's
technically correct because it's just

1031
00:44:56,770 --> 00:45:00,130
going to go, we think, forever,
but it looks like this is correct.

1032
00:45:00,130 --> 00:45:02,320
And it was a lot less
code and it's a lot easier

1033
00:45:02,320 --> 00:45:05,260
to maintain because if I want
him to kind of get sleepy,

1034
00:45:05,260 --> 00:45:09,150
I can then maybe say
two seconds instead.

1035
00:45:09,150 --> 00:45:10,560
[MEOW]

1036
00:45:11,060 --> 00:45:13,730
You know, and we can adjust
this on the fly as we go.

1037
00:45:13,730 --> 00:45:15,710
But let's start to combine
some of these ideas

1038
00:45:15,710 --> 00:45:19,460
now and change what it is
the ultimate effect is.

1039
00:45:19,460 --> 00:45:22,310
Let me go ahead and open an
example I made in advance.

1040
00:45:22,310 --> 00:45:24,260
This one's called Count Zero.

1041
00:45:24,260 --> 00:45:27,510
And we'll put this on the website later
so that you can play with if you like.

1042
00:45:27,510 --> 00:45:29,630
And this is kind of the
opposite of counting sheep.

1043
00:45:29,630 --> 00:45:32,180
Rather than me or the person
sleeping counting sheep,

1044
00:45:32,180 --> 00:45:34,440
this sheep will count itself.

1045
00:45:34,440 --> 00:45:37,130
So let me go ahead and just play.

1046
00:45:37,130 --> 00:45:41,720
And adorably, he seems to
just be counting 1, 2, 3.

1047
00:45:41,720 --> 00:45:42,800
But why is that?

1048
00:45:42,800 --> 00:45:44,420
He's just going to count forever.

1049
00:45:44,420 --> 00:45:47,330
But let's look at the blocks with
which he is counting forever.

1050
00:45:47,330 --> 00:45:49,420
When green flag clicked, set counter.

1051
00:45:49,420 --> 00:45:51,720
Turns out this orange box is
what we called a variable.

1052
00:45:51,720 --> 00:45:53,570
So in algebra, it would
be like x or y or z.

1053
00:45:53,570 --> 00:45:54,780
Those are not descriptive.

1054
00:45:54,780 --> 00:45:58,490
I called this one counter instead, but
I could have called it x or y or z.

1055
00:45:58,490 --> 00:46:03,080
And then I forever say the counter
for one second, then wait one second,

1056
00:46:03,080 --> 00:46:04,910
and then change the
counter by one, which

1057
00:46:04,910 --> 00:46:06,410
technically means just increment it.

1058
00:46:06,410 --> 00:46:07,430
Add 1 to it.

1059
00:46:07,430 --> 00:46:10,190
And the sheep is just going to
therefore count up and up and up.

1060
00:46:10,190 --> 00:46:11,900
Now, this is a little
tedious, but that's

1061
00:46:11,900 --> 00:46:15,120
kind of the point of counting
sheep, of course, to fall asleep.

1062
00:46:15,120 --> 00:46:18,960
But what if the sheep actually kind
of liked counting a little faster?

1063
00:46:18,960 --> 00:46:21,500
Well, let me go under operators here.

1064
00:46:21,500 --> 00:46:24,860
Multiplication sounds like it
could get us places quicker.

1065
00:46:24,860 --> 00:46:27,260
And let me go ahead and go to variables.

1066
00:46:27,260 --> 00:46:29,830
And instead of changing
the counter by one,

1067
00:46:29,830 --> 00:46:32,290
let me go ahead and just keep
setting it to something else.

1068
00:46:32,290 --> 00:46:33,530
So let me drag and drop this.

1069
00:46:33,530 --> 00:46:38,360
Set the counter equal to something
times something, specifically

1070
00:46:38,360 --> 00:46:44,810
the counter times two, thereby
doubling, doubling, doubling, doubling.

1071
00:46:44,810 --> 00:46:47,250
That would seem to grow,
so to speak, a lot faster.

1072
00:46:47,250 --> 00:46:49,400
Let's see.

1073
00:46:49,400 --> 00:46:54,210
1, 2, 4.

1074
00:46:54,210 --> 00:46:56,780
So he's counting faster, but
it's still kind of tedious.

1075
00:46:56,780 --> 00:46:58,640
What if we instead do this?

1076
00:46:58,640 --> 00:47:02,660
Let's stop waiting and let's
go ahead and, with the looks,

1077
00:47:02,660 --> 00:47:06,960
not say counter for one second,
but let's just quickly say counter.

1078
00:47:06,960 --> 00:47:08,300
So I'm going to say the counter.

1079
00:47:08,300 --> 00:47:09,290
Whoops.

1080
00:47:09,290 --> 00:47:13,460
I'm going to say the counter and then
I'm going to set it to itself times 2.

1081
00:47:13,460 --> 00:47:14,560
So here's where we're at.

1082
00:47:14,560 --> 00:47:18,950
Initialize, or set the counter to 1
initially, say it, then double it,

1083
00:47:18,950 --> 00:47:21,930
then double it, then double
it, saying it along the way.

1084
00:47:21,930 --> 00:47:23,690
So here we go.

1085
00:47:23,690 --> 00:47:26,720
That's impressive.

1086
00:47:26,720 --> 00:47:32,320
So now the sheep has counted
up to 10 to the 60th so far.

1087
00:47:32,320 --> 00:47:34,160
10 to the 100th.

1088
00:47:34,160 --> 00:47:34,660
OK.

1089
00:47:34,660 --> 00:47:39,370
Now, it doesn't even fit in the
speech bubble, but he's still going.

1090
00:47:39,370 --> 00:47:41,380
How high can he go?

1091
00:47:41,380 --> 00:47:45,970
What's the biggest number you
can count to in a computer?

1092
00:47:45,970 --> 00:47:49,410
Anyone want to guess?

1093
00:47:49,410 --> 00:47:51,400
Could be here a while.

1094
00:47:51,400 --> 00:47:54,130
10 to the 270th now.

1095
00:47:54,130 --> 00:47:57,530
How high can you count, or rather--

1096
00:47:57,530 --> 00:47:58,030
OK.

1097
00:47:58,030 --> 00:48:01,010
So we gave up and just
called it infinity.

1098
00:48:01,010 --> 00:48:03,340
So it turns out infinity
does have a precise value--

1099
00:48:03,340 --> 00:48:05,680
10 to the 250th or so.

1100
00:48:05,680 --> 00:48:07,150
But what happens here?

1101
00:48:07,150 --> 00:48:09,490
Well, because computers,
at the end of the day,

1102
00:48:09,490 --> 00:48:12,700
are just storing information digitally--
but that information digitally has

1103
00:48:12,700 --> 00:48:15,640
to be physically stored using
electricity, using these lower level

1104
00:48:15,640 --> 00:48:16,990
switches called transistors.

1105
00:48:16,990 --> 00:48:20,200
At the end of the day, my phone, my
laptop, whatever device in question

1106
00:48:20,200 --> 00:48:22,510
only has a finite
amount of those things.

1107
00:48:22,510 --> 00:48:24,380
I only have a finite number of fingers.

1108
00:48:24,380 --> 00:48:26,800
Using unary, my old
school hashmark approach,

1109
00:48:26,800 --> 00:48:28,360
I can count to five on this hand.

1110
00:48:28,360 --> 00:48:32,380
Using binary, I claimed I
could count to 31 on this hand.

1111
00:48:32,380 --> 00:48:33,610
But it's still finite.

1112
00:48:33,610 --> 00:48:37,270
I cannot count to infinity on this
hand because I only have five fingers.

1113
00:48:37,270 --> 00:48:40,570
Similarly does a computer only
have so many transistors or so

1114
00:48:40,570 --> 00:48:43,240
many bytes or bits of
memory, and at some point,

1115
00:48:43,240 --> 00:48:45,790
the programmer has to
think about what is he

1116
00:48:45,790 --> 00:48:48,580
or she going to do when the user
wants to count so high that you

1117
00:48:48,580 --> 00:48:51,080
can't physically fit it anymore.

1118
00:48:51,080 --> 00:48:54,310
You have to give up like this
and say something semi accurately

1119
00:48:54,310 --> 00:48:57,980
or you have to handle that
issue in some other way.

1120
00:48:57,980 --> 00:49:01,180
And we'll see when we get to C that
how you handle this problem is not

1121
00:49:01,180 --> 00:49:03,730
necessarily straightforward,
and indeed a lot of software

1122
00:49:03,730 --> 00:49:05,610
out there does not handle this problem.

1123
00:49:05,610 --> 00:49:07,660
And odds are, all of us
have programs that if you

1124
00:49:07,660 --> 00:49:10,180
type big enough words or big
enough numbers into them,

1125
00:49:10,180 --> 00:49:12,970
they might very well
break or crash or freeze

1126
00:49:12,970 --> 00:49:16,600
because the humans, unlike MIT, did
not anticipate that that might actually

1127
00:49:16,600 --> 00:49:17,890
happen and handle it.

1128
00:49:17,890 --> 00:49:19,310
Well, let me go ahead and do this.

1129
00:49:19,310 --> 00:49:22,780
Let me open up this program and
see if we can't read the code now.

1130
00:49:22,780 --> 00:49:28,330
This is called Pet Zero and this is
a program that simulates petting.

1131
00:49:28,330 --> 00:49:31,630
So if I click play and
don't touch the keyboard,

1132
00:49:31,630 --> 00:49:35,350
nothing seems to be happening, but if
I now move my cursor over to the cat--

1133
00:49:35,350 --> 00:49:36,800
[MEOW]

1134
00:49:36,800 --> 00:49:37,770
Aw.

1135
00:49:37,770 --> 00:49:38,590
It's kind of cute.

1136
00:49:38,590 --> 00:49:39,280
[MEOW]

1137
00:49:39,280 --> 00:49:42,790
Right now, it's more only meowing
on demand when you pet the cat.

1138
00:49:42,790 --> 00:49:43,480
Why?

1139
00:49:43,480 --> 00:49:45,640
Well, notice I've added
some other building blocks.

1140
00:49:45,640 --> 00:49:48,130
We haven't used this one
before, but it intuitively

1141
00:49:48,130 --> 00:49:49,630
probably makes pretty clear sense.

1142
00:49:49,630 --> 00:49:52,660
When the green flag is clicked,
forever do the following.

1143
00:49:52,660 --> 00:49:56,070
If the cat is touching the mouse
pointer-- this thing in blue

1144
00:49:56,070 --> 00:49:58,030
is what we called earlier
a Boolean expression.

1145
00:49:58,030 --> 00:50:00,760
It has a yes/no, a
true/false, a one/zero answer.

1146
00:50:00,760 --> 00:50:03,560
And touching mouse pointer is one
of the options in the little drop

1147
00:50:03,560 --> 00:50:05,150
down here if you tinker with it.

1148
00:50:05,150 --> 00:50:08,290
So if the cat is touching the
mouse pointer, then and only then,

1149
00:50:08,290 --> 00:50:10,450
play sound meow until done.

1150
00:50:10,450 --> 00:50:14,200
So we've combined now functions
with loops with a condition.

1151
00:50:14,200 --> 00:50:16,330
But why the loop?

1152
00:50:16,330 --> 00:50:19,390
The cat's only meowing
once when I pet him.

1153
00:50:19,390 --> 00:50:21,580
Why am I doing anything forever here?

1154
00:50:24,490 --> 00:50:25,530
Someone-- yeah.

1155
00:50:28,600 --> 00:50:29,100
Yeah.

1156
00:50:29,100 --> 00:50:32,910
I might want to pet it again, so I
want the program to anticipate that.

1157
00:50:32,910 --> 00:50:36,480
And honestly, if I omitted this
forever block and my program

1158
00:50:36,480 --> 00:50:39,130
instead looked just like this--

1159
00:50:39,130 --> 00:50:40,890
so let me get rid of that and this--

1160
00:50:40,890 --> 00:50:45,780
and then I clicked play,
and now, I hover over him,

1161
00:50:45,780 --> 00:50:47,550
why is it not working even once?

1162
00:50:51,540 --> 00:50:52,090
Say it again.

1163
00:50:56,700 --> 00:50:57,200
Yeah.

1164
00:50:57,200 --> 00:51:00,740
So I mean, at this point,
if I can summarize,

1165
00:51:00,740 --> 00:51:03,080
the computer is so damn
fast that this already

1166
00:51:03,080 --> 00:51:07,190
happened by the time I moved my cursor
over to the cat, and at the moment

1167
00:51:07,190 --> 00:51:10,220
I clicked play, I was
not touching the cat.

1168
00:51:10,220 --> 00:51:12,780
Those blocks executed, so
to speak, top to bottom.

1169
00:51:12,780 --> 00:51:13,970
That's it for the program.

1170
00:51:13,970 --> 00:51:16,970
So by the time I move the cursor
over to the cat, the program is over.

1171
00:51:16,970 --> 00:51:17,760
It's not listening.

1172
00:51:17,760 --> 00:51:21,960
And so forever, this way I can actually
listen in perpetuity for something

1173
00:51:21,960 --> 00:51:23,430
to actually happen.

1174
00:51:23,430 --> 00:51:26,000
What if I want to do something
not just if something is true,

1175
00:51:26,000 --> 00:51:27,300
but handle two cases?

1176
00:51:27,300 --> 00:51:28,340
If or else.

1177
00:51:28,340 --> 00:51:31,130
Well, let me go ahead
and open up Pet One.

1178
00:51:31,130 --> 00:51:32,870
And this is another example.

1179
00:51:32,870 --> 00:51:37,310
And could someone perhaps describe,
after reading this code, what

1180
00:51:37,310 --> 00:51:38,810
this program is going to do instead?

1181
00:51:42,290 --> 00:51:42,790
Yeah.

1182
00:51:53,790 --> 00:51:54,300
Exactly.

1183
00:51:54,300 --> 00:51:55,750
And let me summarize more verbally.

1184
00:51:55,750 --> 00:51:59,590
So if this time, you're touching
the cat, it's going to roar instead.

1185
00:51:59,590 --> 00:52:01,950
Else, it's just going to meow sweetly.

1186
00:52:01,950 --> 00:52:05,830
So this time, it is meowing
perpetually once every second,

1187
00:52:05,830 --> 00:52:08,850
but if you touch this particular
cat, it doesn't like it.

1188
00:52:08,850 --> 00:52:09,660
So play.

1189
00:52:09,660 --> 00:52:10,570
[MEOW]

1190
00:52:10,570 --> 00:52:11,070
Meow.

1191
00:52:11,070 --> 00:52:12,310
[MEOW]

1192
00:52:12,310 --> 00:52:13,130
Meow.

1193
00:52:13,130 --> 00:52:14,350
[MEOW]

1194
00:52:14,350 --> 00:52:15,360
And now.

1195
00:52:15,360 --> 00:52:16,320
[ROAR]

1196
00:52:16,320 --> 00:52:17,640
Don't touch the cat.

1197
00:52:17,640 --> 00:52:20,160
So now, we might interact
in two different ways

1198
00:52:20,160 --> 00:52:22,970
by having two different
roads that you can go down.

1199
00:52:22,970 --> 00:52:25,560
Well, let's actually make something
a little more interactive.

1200
00:52:25,560 --> 00:52:27,730
Let me go ahead and
open another example.

1201
00:52:27,730 --> 00:52:29,850
This one's called Bounce
Zero because now, we

1202
00:52:29,850 --> 00:52:33,690
can start to see some design
elements from what Oscar Time was.

1203
00:52:33,690 --> 00:52:36,360
Like this now it's getting
a little interesting.

1204
00:52:36,360 --> 00:52:38,350
What is actually going on here?

1205
00:52:38,350 --> 00:52:40,470
So let me zoom in on the blocks here.

1206
00:52:40,470 --> 00:52:43,980
This block is just saying
forever move 10 steps, which

1207
00:52:43,980 --> 00:52:45,360
is another block we haven't seen.

1208
00:52:45,360 --> 00:52:46,920
But 10 steps is like 10 pixels.

1209
00:52:46,920 --> 00:52:48,930
So move 10 pixels on the screen.

1210
00:52:48,930 --> 00:52:52,330
But if you're touching the edge,
then turn around 180 degrees.

1211
00:52:52,330 --> 00:52:54,200
And you can see exactly that happening.

1212
00:52:54,200 --> 00:52:57,540
Scratch is turning around 180
degrees and this rotation style just

1213
00:52:57,540 --> 00:52:58,320
means double back.

1214
00:52:58,320 --> 00:53:00,810
Don't like loop around 180 degrees.

1215
00:53:00,810 --> 00:53:02,020
So that's kind of cool.

1216
00:53:02,020 --> 00:53:05,100
But this is not how humans or cats walk.

1217
00:53:05,100 --> 00:53:10,340
Like what is obviously
unnatural about this?

1218
00:53:10,340 --> 00:53:10,840
Yeah.

1219
00:53:10,840 --> 00:53:12,840
I mean, I mean, I can't
even simulate it, right?

1220
00:53:12,840 --> 00:53:15,580
Like his feet are in static
position, yet sliding back and forth

1221
00:53:15,580 --> 00:53:16,540
on the screen.

1222
00:53:16,540 --> 00:53:18,160
And yet, that is not what walking is.

1223
00:53:18,160 --> 00:53:20,950
Like walking presumably has
some kind of movement and what?

1224
00:53:20,950 --> 00:53:22,870
Well, we could just kind
of simulate it like--

1225
00:53:22,870 --> 00:53:24,190
OK, I could just walk--

1226
00:53:24,190 --> 00:53:28,420
walking and you can imagine taking like
really quick photographs of my legs

1227
00:53:28,420 --> 00:53:31,450
or the cat's leg moving and
then just deciding this photo

1228
00:53:31,450 --> 00:53:33,400
will be representative of one step.

1229
00:53:33,400 --> 00:53:35,530
This photo will be
representative another.

1230
00:53:35,530 --> 00:53:37,810
And you know, with just
two of those steps,

1231
00:53:37,810 --> 00:53:41,320
I'd wager we could actually do
a pretty good job of simulating

1232
00:53:41,320 --> 00:53:42,620
what walking looks like.

1233
00:53:42,620 --> 00:53:46,210
In fact, if I go back to where we
began, this picture of Scratch,

1234
00:53:46,210 --> 00:53:51,040
what if I just move his legs
ever so slightly, then go back,

1235
00:53:51,040 --> 00:53:52,280
then go forward?

1236
00:53:52,280 --> 00:53:56,050
And even just in my PDF, I can
simulate animation by hitting up arrow,

1237
00:53:56,050 --> 00:53:58,670
down arrow, up arrow, down
arrow because it kind of

1238
00:53:58,670 --> 00:54:01,720
looks like he's walking now, when
really, your human eyes are just seeing

1239
00:54:01,720 --> 00:54:04,250
two different pictures again and again.

1240
00:54:04,250 --> 00:54:05,340
So how can I do this?

1241
00:54:05,340 --> 00:54:08,350
Well, if I go back to
Scratch, he's still walking.

1242
00:54:08,350 --> 00:54:15,500
Let me go ahead and open up Bounce One,
the second version of this, and now,

1243
00:54:15,500 --> 00:54:16,300
do this.

1244
00:54:16,300 --> 00:54:18,630
OK.

1245
00:54:18,630 --> 00:54:19,570
So how did I add this?

1246
00:54:19,570 --> 00:54:21,870
There's a little purple block
that we haven't seen yet,

1247
00:54:21,870 --> 00:54:23,740
but if you poke around
the categories, you'll

1248
00:54:23,740 --> 00:54:26,070
see other blocks like this
next costume that just keeps

1249
00:54:26,070 --> 00:54:28,230
changing the costume that he's wearing.

1250
00:54:28,230 --> 00:54:31,860
It turns out Scratch exists as a
picture and his default picture

1251
00:54:31,860 --> 00:54:35,760
is him not moving, but if I go up
here to top left and click costumes,

1252
00:54:35,760 --> 00:54:38,220
you can actually see that
here's his one costume.

1253
00:54:38,220 --> 00:54:39,960
Here's his second costume.

1254
00:54:39,960 --> 00:54:43,140
And so that purple block that
says next costume, because it's

1255
00:54:43,140 --> 00:54:47,460
in the forever loop, it just keeps
doing next, next, next, next, next, just

1256
00:54:47,460 --> 00:54:49,240
showing one costume or the other.

1257
00:54:49,240 --> 00:54:52,210
They're clearly mimicking walking.

1258
00:54:52,210 --> 00:54:53,640
Now, this is not very unnatural.

1259
00:54:53,640 --> 00:54:59,040
Why don't we slow him down to, say, five
steps at a time and have him go again?

1260
00:54:59,040 --> 00:55:00,660
Now, this is still going pretty fast.

1261
00:55:00,660 --> 00:55:03,240
Let me go ahead and say--

1262
00:55:03,240 --> 00:55:05,010
we could have control.

1263
00:55:05,010 --> 00:55:11,100
We could have him wait a second
after moving very dramatically.

1264
00:55:11,100 --> 00:55:12,550
We could probably speed this up.

1265
00:55:12,550 --> 00:55:15,540
So let's wait 1/10 of a second, 0.1.

1266
00:55:15,540 --> 00:55:18,650
Or maybe let's do 0.01,
1/100 of a second.

1267
00:55:18,650 --> 00:55:20,400
Now, it's getting a
little more realistic.

1268
00:55:20,400 --> 00:55:21,810
But this is what animation is.

1269
00:55:21,810 --> 00:55:25,510
If you've ever watched a cartoon or
a movie based on pictures like this,

1270
00:55:25,510 --> 00:55:28,530
you're just tinkering with some
of these parameters, these inputs,

1271
00:55:28,530 --> 00:55:30,780
in order to produce this
output by understanding

1272
00:55:30,780 --> 00:55:34,260
what the fundamental representation
of these things is, which in this case

1273
00:55:34,260 --> 00:55:37,200
are just pictures, again
and again and again in order

1274
00:55:37,200 --> 00:55:38,700
to create that animation.

1275
00:55:38,700 --> 00:55:40,400
But what about interactivity?

1276
00:55:40,400 --> 00:55:41,490
Let me do this one myself.

1277
00:55:41,490 --> 00:55:45,960
Let me go ahead and get rid of
this, go back to events, and say,

1278
00:55:45,960 --> 00:55:47,670
when green flag clicked.

1279
00:55:47,670 --> 00:55:50,250
Then, let me go ahead
and grab a forever block

1280
00:55:50,250 --> 00:55:52,230
so that this keeps
going again and again.

1281
00:55:52,230 --> 00:55:54,150
And then, let me go
ahead to go to motion.

1282
00:55:54,150 --> 00:55:57,270
It turns out that under motion,
there's this block we haven't seen--

1283
00:55:57,270 --> 00:55:59,340
point towards the mouse pointer.

1284
00:55:59,340 --> 00:56:01,650
And let me go ahead
and pull this in here.

1285
00:56:01,650 --> 00:56:06,480
And then, let me have it move just
like one step at a time, instead of 10.

1286
00:56:06,480 --> 00:56:09,560
What is this going to do?

1287
00:56:09,560 --> 00:56:12,490
What's this program do?

1288
00:56:12,490 --> 00:56:12,990
Yeah.

1289
00:56:12,990 --> 00:56:14,370
Say it again.

1290
00:56:14,370 --> 00:56:15,280
Follow the mouse.

1291
00:56:15,280 --> 00:56:15,780
Yeah.

1292
00:56:15,780 --> 00:56:20,580
This is kind of like a way of
taking your cat for a walk.

1293
00:56:20,580 --> 00:56:24,930
Perhaps not quite the animal we
intended, but he'll follow the cursor.

1294
00:56:24,930 --> 00:56:27,170
And I can actually speed
this up a little bit.

1295
00:56:27,170 --> 00:56:28,780
So let's have him move 10 steps.

1296
00:56:28,780 --> 00:56:29,280
OK.

1297
00:56:29,280 --> 00:56:32,020
Now, there we go.

1298
00:56:32,020 --> 00:56:34,960
So now, he's moving up and down,
and so now, it's interactive.

1299
00:56:34,960 --> 00:56:38,650
So you might recall that when we were
playing Oscar Time earlier and picking

1300
00:56:38,650 --> 00:56:39,150
up--

1301
00:56:39,150 --> 00:56:40,530
OK.

1302
00:56:40,530 --> 00:56:41,110
Don't do that.

1303
00:56:41,110 --> 00:56:42,870
See, that's a bug.

1304
00:56:42,870 --> 00:56:43,710
He's just confused.

1305
00:56:43,710 --> 00:56:45,660
He's constantly moving toward
it, but you're already--

1306
00:56:45,660 --> 00:56:45,940
OK.

1307
00:56:45,940 --> 00:56:46,930
So we're going to stop.

1308
00:56:46,930 --> 00:56:47,430
OK.

1309
00:56:47,430 --> 00:56:50,610
So now, he's following, but
that's how we might now create,

1310
00:56:50,610 --> 00:56:52,980
for instance, the ability to
move those pieces of trash

1311
00:56:52,980 --> 00:56:54,950
around and have them
follow the mouse cursor.

1312
00:56:54,950 --> 00:56:58,070
If you think back to Oscar Time, every
time you picked up a piece of trash,

1313
00:56:58,070 --> 00:57:01,320
he'd follow the cursor because there
was a forever loop and a block like this

1314
00:57:01,320 --> 00:57:03,180
pointing toward the mouse pointer.

1315
00:57:03,180 --> 00:57:05,760
Well, now, let's
integrate multiple ideas

1316
00:57:05,760 --> 00:57:07,590
and actually have multiple scripts.

1317
00:57:07,590 --> 00:57:12,240
I proposed earlier that programs
can actually have multiple threads.

1318
00:57:12,240 --> 00:57:15,780
A thread is just a fancy way of saying,
in our context, multiple scripts.

1319
00:57:15,780 --> 00:57:21,120
Multiple scripts in one program that
are happening essentially in parallel.

1320
00:57:21,120 --> 00:57:24,570
A computer can effectively do multiple
things at a time thanks to threading,

1321
00:57:24,570 --> 00:57:26,010
and more on that down the road.

1322
00:57:26,010 --> 00:57:27,720
So these are more
involved, but let's see

1323
00:57:27,720 --> 00:57:30,120
if we can-- let's understand
first what this program does.

1324
00:57:30,120 --> 00:57:31,320
Let me go ahead and hit play.

1325
00:57:31,320 --> 00:57:33,950
And this one tends to be a little loud.

1326
00:57:33,950 --> 00:57:37,180
[SEA LION BARKING]

1327
00:57:38,120 --> 00:57:42,800
So the sea lion is just
barking endlessly, annoyingly.

1328
00:57:42,800 --> 00:57:48,650
So by reading the code, how
can I stop him from barking?

1329
00:57:48,650 --> 00:57:49,890
Hit the spacebar.

1330
00:57:49,890 --> 00:57:50,390
All right.

1331
00:57:50,390 --> 00:57:52,770
So hit the spacebar.

1332
00:57:52,770 --> 00:57:53,750
OK.

1333
00:57:53,750 --> 00:57:55,700
I could just stop the
program, obviously,

1334
00:57:55,700 --> 00:57:57,800
but this program is still
running, technically.

1335
00:57:57,800 --> 00:57:59,270
But why did that work?

1336
00:57:59,270 --> 00:58:02,300
Well, notice this on the left
hand side is the first script.

1337
00:58:02,300 --> 00:58:06,370
When the green flag is clicked, set this
variable that I called muted to false.

1338
00:58:06,370 --> 00:58:08,960
Could have called it x or y or
z or counter, but none of those

1339
00:58:08,960 --> 00:58:10,620
really make sense, so I called it muted.

1340
00:58:10,620 --> 00:58:13,400
And I set it equal to false,
which is, again, a Boolean value.

1341
00:58:13,400 --> 00:58:16,170
True or false just mean yes or no.

1342
00:58:16,170 --> 00:58:19,920
Forever, if the keyspace
is pressed, then do this.

1343
00:58:19,920 --> 00:58:24,200
If muted is currently false,
then change muted to true.

1344
00:58:24,200 --> 00:58:26,670
Else, change muted to false.

1345
00:58:26,670 --> 00:58:29,510
So if muted is false, change it to true.

1346
00:58:29,510 --> 00:58:31,840
If muted is true, change it to false.

1347
00:58:31,840 --> 00:58:35,910
Any time the human hits the
spacebar, update that variable.

1348
00:58:35,910 --> 00:58:39,500
Now, if we look at the other script,
which is also driving the sea lion,

1349
00:58:39,500 --> 00:58:40,530
what is he doing?

1350
00:58:40,530 --> 00:58:43,580
Forever, if muted is false.

1351
00:58:43,580 --> 00:58:44,750
So if he's not muted.

1352
00:58:44,750 --> 00:58:46,820
If muted is false means not muted.

1353
00:58:46,820 --> 00:58:50,360
Start the sound sea lion and then
think hi, hi, hi for two seconds,

1354
00:58:50,360 --> 00:58:52,160
and then wait for one more second.

1355
00:58:52,160 --> 00:58:53,900
And then just repeat, repeat, repeat.

1356
00:58:53,900 --> 00:58:56,750
But if I change with the
spacebar muted to true,

1357
00:58:56,750 --> 00:59:00,230
he's going to say if muted
equals false, that's not so.

1358
00:59:00,230 --> 00:59:01,950
I'm not going to play a sound this time.

1359
00:59:01,950 --> 00:59:05,360
And so now, we have the ability to
integrate multiple scripts together

1360
00:59:05,360 --> 00:59:08,390
in order to achieve a
more interactive result.

1361
00:59:08,390 --> 00:59:09,670
And what about this?

1362
00:59:09,670 --> 00:59:15,560
Back when I was a kid, might have
played in the summers Marco Polo.

1363
00:59:15,560 --> 00:59:16,650
Super simple game.

1364
00:59:16,650 --> 00:59:18,650
We played it in the pool,
for some reason, where

1365
00:59:18,650 --> 00:59:20,840
one person in the pool
very safely is blindfolded,

1366
00:59:20,840 --> 00:59:22,550
and then he or she yells Marco.

1367
00:59:22,550 --> 00:59:25,550
And then, everyone around him
or her is supposed to yell polo.

1368
00:59:25,550 --> 00:59:27,260
And then, the person
who's blindfolded is

1369
00:59:27,260 --> 00:59:29,540
supposed to go chase the other
kids in the pool and tag them,

1370
00:59:29,540 --> 00:59:30,650
and then they become it.

1371
00:59:30,650 --> 00:59:33,020
But in other words, it's this
like signaling mechanism.

1372
00:59:33,020 --> 00:59:35,750
Someone yells, Marco and
everyone else responds

1373
00:59:35,750 --> 00:59:38,180
to that broadcast of the word Marco.

1374
00:59:38,180 --> 00:59:40,910
Well, it turns out we can simulate
this with these two puppets.

1375
00:59:40,910 --> 00:59:43,550
This guy here-- notice that I've
highlighted the orange puppet

1376
00:59:43,550 --> 00:59:45,350
because there's a second
blue puppet there.

1377
00:59:45,350 --> 00:59:46,220
Separate sprites.

1378
00:59:46,220 --> 00:59:49,490
And these are just photographs
we uploaded to the game.

1379
00:59:49,490 --> 00:59:53,780
Forever, if the key space is pressed,
so if the spacebar is pressed,

1380
00:59:53,780 --> 00:59:57,290
say Marco for two seconds
and then broadcast an event.

1381
00:59:57,290 --> 01:00:02,350
Meanwhile, the blue puppet
here has a super simple block,

1382
01:00:02,350 --> 01:00:04,770
but it's fundamentally different
from the ones we've seen.

1383
01:00:04,770 --> 01:00:07,580
He's not starting when
the green flag is clicked.

1384
01:00:07,580 --> 01:00:10,730
He is starting only when
he receives an event.

1385
01:00:10,730 --> 01:00:13,470
So it turns out that sprites
and Scratch can't hear or see

1386
01:00:13,470 --> 01:00:15,680
what the other one is saying
in those speech bubbles.

1387
01:00:15,680 --> 01:00:17,420
You have to use a
fancier technique, which

1388
01:00:17,420 --> 01:00:19,430
is the special block
called broadcast, which

1389
01:00:19,430 --> 01:00:23,390
is like passing a note digitally
from one sprite to another

1390
01:00:23,390 --> 01:00:26,210
that the other one can read
or receive, so to speak.

1391
01:00:26,210 --> 01:00:28,430
So only when he receives
this event, so to speak,

1392
01:00:28,430 --> 01:00:30,800
does he say polo for two seconds.

1393
01:00:30,800 --> 01:00:34,350
And again, the orange puppet
sends that secret message

1394
01:00:34,350 --> 01:00:35,810
just using this other puzzle piece.

1395
01:00:35,810 --> 01:00:40,530
Broadcast an event, like passing a note
that the human doesn't actually see.

1396
01:00:40,530 --> 01:00:45,410
So if I now hit the green flag and
hit the spacebar, orange yells Marco.

1397
01:00:45,410 --> 01:00:48,380
Blue guy yells polo in response.

1398
01:00:48,380 --> 01:00:50,270
But those aren't timed together.

1399
01:00:50,270 --> 01:00:53,690
Rather, the blue guy is hearing
with the orange one has said,

1400
01:00:53,690 --> 01:00:58,740
thereby allowing multiple sprites
to actually intercommunicate.

1401
01:00:58,740 --> 01:00:59,780
So how did we get here?

1402
01:00:59,780 --> 01:01:02,930
Well, recall that we had all of
these building blocks a moment ago.

1403
01:01:02,930 --> 01:01:05,270
First, we started out with
just functions and conditions

1404
01:01:05,270 --> 01:01:06,710
and Boolean expressions and loops.

1405
01:01:06,710 --> 01:01:09,040
We've now added to that the
ability to store information

1406
01:01:09,040 --> 01:01:12,530
in variables and threads to do multiple
things at once, and then, if you do

1407
01:01:12,530 --> 01:01:14,840
have multiple things
happening, events, where

1408
01:01:14,840 --> 01:01:17,180
they can enter communicate somehow.

1409
01:01:17,180 --> 01:01:18,530
Yet another building block.

1410
01:01:18,530 --> 01:01:21,620
So if we now take a
step back and consider

1411
01:01:21,620 --> 01:01:26,360
how we can make functions of
our own, we have the final piece

1412
01:01:26,360 --> 01:01:27,890
of the puzzle, so to speak.

1413
01:01:27,890 --> 01:01:29,090
Let me go ahead and do this.

1414
01:01:29,090 --> 01:01:33,500
Let me go ahead and create a simple
program with, when green flag clipped,

1415
01:01:33,500 --> 01:01:35,570
that simply simulates
coughing for a cat.

1416
01:01:35,570 --> 01:01:40,760
So this cat is going to say not
hello, but cough for one second.

1417
01:01:40,760 --> 01:01:44,540
And then he's going to go
ahead and wait for one second.

1418
01:01:44,540 --> 01:01:48,200
And then I'm going to go ahead
and copy paste, as I did before--

1419
01:01:48,200 --> 01:01:52,040
this is one of those do as I say, not as
I do-- to implement this program here,

1420
01:01:52,040 --> 01:01:54,170
where he coughs three times.

1421
01:01:54,170 --> 01:01:57,020
We already know, though, from
earlier that this is not good design.

1422
01:01:57,020 --> 01:01:59,430
Why?

1423
01:01:59,430 --> 01:02:00,520
You're repeating yourself.

1424
01:02:00,520 --> 01:02:02,080
Don't repeat yourself.

1425
01:02:02,080 --> 01:02:03,570
DRY is an acronym, actually.

1426
01:02:03,570 --> 01:02:05,320
Don't repeat yourself
because you're doing

1427
01:02:05,320 --> 01:02:09,340
three times as many times something
that you only really need to do once.

1428
01:02:09,340 --> 01:02:12,680
The solution before, of course,
was just use a loop of some sort.

1429
01:02:12,680 --> 01:02:14,230
So let me actually take that out.

1430
01:02:14,230 --> 01:02:18,130
Let me use a repeat
block, change 10 to three,

1431
01:02:18,130 --> 01:02:19,810
and then just use two of these blocks.

1432
01:02:19,810 --> 01:02:22,150
And notice already, the program
is so much more compact.

1433
01:02:22,150 --> 01:02:27,040
And now, if I want to change the three
to a 30 or to a 10 or any number,

1434
01:02:27,040 --> 01:02:28,550
I just change one simple value.

1435
01:02:28,550 --> 01:02:31,840
I don't have to rewrite or
copy paste or delete things.

1436
01:02:31,840 --> 01:02:33,820
I can update the program
much more readily,

1437
01:02:33,820 --> 01:02:39,370
and now, the same thing is going to
happen with just cough, cough, cough.

1438
01:02:39,370 --> 01:02:43,010
But it turns out that it would be nice
to henceforth abstract away from this,

1439
01:02:43,010 --> 01:02:43,510
right?

1440
01:02:43,510 --> 01:02:46,960
I just want any program I
write to know how to cough.

1441
01:02:46,960 --> 01:02:49,210
And coughing is really just
saying something, perhaps

1442
01:02:49,210 --> 01:02:50,540
some number of times.

1443
01:02:50,540 --> 01:02:53,110
But it turns out we can
abstract this away in code.

1444
01:02:53,110 --> 01:02:55,930
Let me go down to my
blocks here and this allows

1445
01:02:55,930 --> 01:02:57,710
me to click this button-- make a block.

1446
01:02:57,710 --> 01:02:59,440
It allows me to make my own function.

1447
01:02:59,440 --> 01:03:01,120
I get this dialog window here.

1448
01:03:01,120 --> 01:03:03,850
And I'm just going to
call this block cough.

1449
01:03:03,850 --> 01:03:05,870
I'm going to go ahead and click OK.

1450
01:03:05,870 --> 01:03:09,420
And now, I have this new pink block that
itself can have blocks underneath it.

1451
01:03:09,420 --> 01:03:10,840
And you know what I'm going to do?

1452
01:03:10,840 --> 01:03:12,410
I'm going to go ahead and do this.

1453
01:03:12,410 --> 01:03:16,570
I'm going to go ahead and
say cough under there.

1454
01:03:16,570 --> 01:03:20,260
And now, notice on the left, I now
have access to this new pink piece.

1455
01:03:20,260 --> 01:03:22,220
I can now put this in here.

1456
01:03:22,220 --> 01:03:26,470
So now, notice even though, yes,
this is how coughing is implemented

1457
01:03:26,470 --> 01:03:30,320
on the left hand side here, next
time, when I write a program,

1458
01:03:30,320 --> 01:03:31,960
I just want to call cough.

1459
01:03:31,960 --> 01:03:34,630
And I don't care about those lower
level implementation details.

1460
01:03:34,630 --> 01:03:36,460
I don't care about the
party or any of that.

1461
01:03:36,460 --> 01:03:38,230
I just want this to be an abstraction.

1462
01:03:38,230 --> 01:03:40,510
But I could do better
than this wouldn't it

1463
01:03:40,510 --> 01:03:43,730
be nice if instead of just
repeating cough three times,

1464
01:03:43,730 --> 01:03:45,670
what if I made that a feature of cough?

1465
01:03:45,670 --> 01:03:46,760
So let me do this.

1466
01:03:46,760 --> 01:03:51,010
I can go ahead and right click on
this pink piece and I can edit it.

1467
01:03:51,010 --> 01:03:52,990
That brings up that
same window from before.

1468
01:03:52,990 --> 01:03:53,710
And notice this.

1469
01:03:53,710 --> 01:03:55,070
Add an input.

1470
01:03:55,070 --> 01:03:58,210
So when I make a custom block, I can
actually make pretty fancy blocks

1471
01:03:58,210 --> 01:04:01,570
just like the ones MIT
gives us with the software,

1472
01:04:01,570 --> 01:04:03,700
and now, I can type in something like n.

1473
01:04:03,700 --> 01:04:07,060
And if I add a label just to make it
more descriptive, I can just say times.

1474
01:04:07,060 --> 01:04:09,040
So now, I've made a
special custom puzzle

1475
01:04:09,040 --> 01:04:12,880
piece that says cough some number
of times, where n for number

1476
01:04:12,880 --> 01:04:16,340
is just the go to variable
that programmers tend to use.

1477
01:04:16,340 --> 01:04:24,260
So now, I can actually move this
repeat block into cough itself,

1478
01:04:24,260 --> 01:04:26,290
but rather than hard
code 3, notice this.

1479
01:04:26,290 --> 01:04:31,360
I can steal that variable and now say
cough this many times by repeating

1480
01:04:31,360 --> 01:04:34,630
saying this again and again and again.

1481
01:04:34,630 --> 01:04:39,880
And now, when I cough in my actual
program, I just type in three here.

1482
01:04:39,880 --> 01:04:42,350
So I have this beautiful
abstraction now, so to speak.

1483
01:04:42,350 --> 01:04:45,490
Cough this many times and I
and no one else in the world

1484
01:04:45,490 --> 01:04:48,730
never again needs to care about what
it means to cough because we've already

1485
01:04:48,730 --> 01:04:50,330
implemented that before.

1486
01:04:50,330 --> 01:04:52,810
And so just as MIT has given
us so much functionality

1487
01:04:52,810 --> 01:04:54,940
that we ourselves don't
have to think about, so can

1488
01:04:54,940 --> 01:04:57,560
I now make functionality that
I don't have to think about.

1489
01:04:57,560 --> 01:05:00,340
And as we progress to higher level
languages like C and JavaScript

1490
01:05:00,340 --> 01:05:02,800
and Python, we're going to
continue this process, sometimes

1491
01:05:02,800 --> 01:05:06,460
solving problems ourselves by
making our own custom puzzle pieces,

1492
01:05:06,460 --> 01:05:08,620
but very often using
things called libraries,

1493
01:05:08,620 --> 01:05:13,060
code that other humans wrote before us
that's just useful to get the job done,

1494
01:05:13,060 --> 01:05:16,970
just as Scratch has done
here in part for us.

1495
01:05:16,970 --> 01:05:19,360
Let me go ahead, then,
and bring all of this

1496
01:05:19,360 --> 01:05:23,750
together by opening
this other example here.

1497
01:05:23,750 --> 01:05:27,470
Let me go ahead and open up this one,
which isn't something we've seen,

1498
01:05:27,470 --> 01:05:31,330
but it's kind of an interactive game
like this made by a former student.

1499
01:05:31,330 --> 01:05:35,540
[MUSIC PLAYING]

1500
01:05:35,540 --> 01:05:36,790
Should we have an apple?

1501
01:05:36,790 --> 01:05:38,840
Yes.

1502
01:05:38,840 --> 01:05:40,240
A little animation.

1503
01:05:44,420 --> 01:05:44,920
OK.

1504
01:05:44,920 --> 01:05:46,720
That didn't end well.

1505
01:05:46,720 --> 01:05:47,770
Let's try again.

1506
01:05:47,770 --> 01:05:49,660
Play again.

1507
01:05:49,660 --> 01:05:51,960
And notice the say block is happening.

1508
01:05:51,960 --> 01:05:54,250
There's some kind of ask block.

1509
01:05:54,250 --> 01:05:56,730
The student was checking if
the human typed in yes or no.

1510
01:05:56,730 --> 01:05:57,730
Let's type no this time.

1511
01:05:57,730 --> 01:05:58,600
No apple.

1512
01:05:58,600 --> 01:05:59,100
Ooh.

1513
01:05:59,100 --> 01:05:59,740
Cupcake.

1514
01:05:59,740 --> 01:06:00,240
OK.

1515
01:06:00,240 --> 01:06:00,890
Yes.

1516
01:06:00,890 --> 01:06:01,390
Enter.

1517
01:06:06,790 --> 01:06:08,030
OK.

1518
01:06:08,030 --> 01:06:09,660
Don't do that.

1519
01:06:09,660 --> 01:06:10,380
One more life.

1520
01:06:10,380 --> 01:06:10,880
Here we go.

1521
01:06:10,880 --> 01:06:13,200
[MUSIC PLAYING]

1522
01:06:13,200 --> 01:06:13,700
OK.

1523
01:06:13,700 --> 01:06:15,590
No apple.

1524
01:06:15,590 --> 01:06:16,160
No cupcake.

1525
01:06:18,950 --> 01:06:21,660
Little variable.

1526
01:06:21,660 --> 01:06:24,590
[SCREAMING]

1527
01:06:26,050 --> 01:06:28,970
[LAUGHTER]

1528
01:06:28,970 --> 01:06:29,470
OK.

1529
01:06:29,470 --> 01:06:32,620
So I won the game.

1530
01:06:32,620 --> 01:06:37,780
In our final moments here, let me
go ahead and open one final example.

1531
01:06:37,780 --> 01:06:41,760
As you know, CS50 is offered not only
at Harvard, but at Yale, as well,

1532
01:06:41,760 --> 01:06:43,810
so it seems fitting to
perhaps end on a note that

1533
01:06:43,810 --> 01:06:47,320
pits one campus perhaps against
the other by way of another game

1534
01:06:47,320 --> 01:06:50,110
that a former student wrote
called Ivy's Hardest Game.

1535
01:06:50,110 --> 01:06:53,450
But for this, I think we need one final
volunteer who's company coming up.

1536
01:06:53,450 --> 01:06:53,950
OK.

1537
01:06:53,950 --> 01:06:54,590
First hand.

1538
01:06:54,590 --> 01:06:55,090
Right there.

1539
01:06:55,090 --> 01:06:56,920
Come on down.

1540
01:06:56,920 --> 01:07:00,140
So in Ivy's Hardest Game, it's
a game played with the keyboard.

1541
01:07:00,140 --> 01:07:02,980
And even though it might look a
little overwhelming at first glance,

1542
01:07:02,980 --> 01:07:06,310
just like Oscar Time did and just
like the gingerbread animation might,

1543
01:07:06,310 --> 01:07:09,520
realize that if you decompose it in
just your mind's eye thinking about what

1544
01:07:09,520 --> 01:07:11,770
those individual building
blocks are, you can probably

1545
01:07:11,770 --> 01:07:13,190
guess what the puzzle pieces are.

1546
01:07:13,190 --> 01:07:13,450
Hi.

1547
01:07:13,450 --> 01:07:13,780
What's your name?

1548
01:07:13,780 --> 01:07:14,050
ANDREA: Hi.

1549
01:07:14,050 --> 01:07:14,610
I'm Andrea.

1550
01:07:14,610 --> 01:07:14,950
DAVID MALAN: Andrea.

1551
01:07:14,950 --> 01:07:15,440
David.

1552
01:07:15,440 --> 01:07:16,270
Nice to meet you.

1553
01:07:16,270 --> 01:07:20,010
Here is Ivy's Hardest Game.

1554
01:07:20,010 --> 01:07:22,500
Pits you against all of the Ivies here.

1555
01:07:22,500 --> 01:07:26,300
And then right after this will we
adjourn for cupcakes in the transept.

1556
01:07:26,300 --> 01:07:28,770
Ready?

1557
01:07:28,770 --> 01:07:31,480
[MUSIC PLAYING]

1558
01:07:31,480 --> 01:07:33,230
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]

1559
01:07:33,230 --> 01:07:36,690
MC HAMMER: (SINGING)
You can't touch this.

1560
01:07:36,690 --> 01:07:42,940
You can't touch this.

1561
01:07:42,940 --> 01:07:43,690
DAVID MALAN: Nice.

1562
01:07:43,690 --> 01:07:44,130
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]

1563
01:07:44,130 --> 01:07:46,440
MC HAMMER: (SINGING)
You can't touch this.

1564
01:07:46,440 --> 01:07:48,460
My, my, my, my.

1565
01:07:48,460 --> 01:07:50,250
Music hits you so hard.

1566
01:07:50,250 --> 01:07:52,250
Makes me say, oh my lord.

1567
01:07:52,250 --> 01:07:55,740
Thank you for blessing me with a
mind to rhyme and two hype feet.

1568
01:07:55,740 --> 01:07:57,650
It feels good when you know you're down.

1569
01:07:57,650 --> 01:07:59,240
A super dope homeboy from the Oaktown.

1570
01:07:59,240 --> 01:08:01,230
And I'm known as such.

1571
01:08:01,230 --> 01:08:04,230
And this is the beat you can't touch.

1572
01:08:04,230 --> 01:08:07,470
I told you, homeboy,
you can't touch this.

1573
01:08:07,470 --> 01:08:07,970
Yeah.

1574
01:08:07,970 --> 01:08:11,210
That's how we're living and
you know you can't touch this.

1575
01:08:11,210 --> 01:08:12,210
Look in my eyes.

1576
01:08:12,210 --> 01:08:14,710
Man, you can't touch this.

1577
01:08:14,710 --> 01:08:15,620
[APPLAUSE]

1578
01:08:15,620 --> 01:08:17,370
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]

1579
01:08:17,370 --> 01:08:19,200
(SINGING) Fresh new kicks and pants.

1580
01:08:19,200 --> 01:08:21,530
You got it like that and now
you know you want to dance.

1581
01:08:21,530 --> 01:08:23,520
So move out of your
seat and get a fly girl

1582
01:08:23,520 --> 01:08:25,680
and catch this beat while it's rolling.

1583
01:08:25,680 --> 01:08:26,180
Hold on.

1584
01:08:26,180 --> 01:08:29,180
Pump a little bit and let me
know it's going on like that.

1585
01:08:29,180 --> 01:08:30,170
Like that.

1586
01:08:30,170 --> 01:08:31,670
Cold on a mission so pull on back.

1587
01:08:31,670 --> 01:08:36,660
Let them know that you're too much
and this is the beat you can't touch.

1588
01:08:36,660 --> 01:08:40,650
Yo, I told you, you can't touch this.

1589
01:08:40,650 --> 01:08:42,150
Why you standing there, man?

1590
01:08:42,150 --> 01:08:43,150
You can't touch this.

1591
01:08:43,150 --> 01:08:44,650
Yo, sound the bells.

1592
01:08:44,650 --> 01:08:45,640
School is in, sucker.

1593
01:08:45,640 --> 01:08:46,520
You can't touch this.

1594
01:08:46,520 --> 01:08:48,140
Give me a song or rhythm.

1595
01:08:48,140 --> 01:08:48,890
Making them sweat.

1596
01:08:48,890 --> 01:08:50,130
That's what I'm giving them.

1597
01:08:50,130 --> 01:08:52,630
Now they know when you
talk about the Hammer,

1598
01:08:52,630 --> 01:08:55,120
you talking about a show
that's hyped and tight.

1599
01:08:55,120 --> 01:08:58,120
Singers are sweating so
pass them a wipe or a tape

1600
01:08:58,120 --> 01:09:01,390
to learn what it's going to take
in the 90s to burn the charts.

1601
01:09:01,390 --> 01:09:02,810
DAVID MALAN: Second to last level.

1602
01:09:02,810 --> 01:09:03,310
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]

1603
01:09:03,310 --> 01:09:05,970
MC HAMMER: (SINGING) Either work
hard or you might as well quit.

1604
01:09:05,970 --> 01:09:10,810
That's word because you
know you can't touch this.

1605
01:09:10,810 --> 01:09:14,310
You can't touch this.

1606
01:09:14,310 --> 01:09:15,310
Break it down.

1607
01:09:28,310 --> 01:09:28,810
Stop.

1608
01:09:28,810 --> 01:09:29,810
Hammer time.

1609
01:09:29,810 --> 01:09:31,010
Go with the flow, it is said.

1610
01:09:31,010 --> 01:09:33,350
If you can't groove to this,
then you probably are dead.

1611
01:09:33,350 --> 01:09:34,810
So wave your hands in the air.

1612
01:09:34,810 --> 01:09:35,810
Bust a few moves.

1613
01:09:35,810 --> 01:09:37,260
Run your fingers through your hair.

1614
01:09:37,260 --> 01:09:38,390
This is it for a winner.

1615
01:09:38,390 --> 01:09:40,310
Dance to this and you're
going to get thinner.

1616
01:09:40,310 --> 01:09:41,310
Move.

1617
01:09:41,310 --> 01:09:42,310
Slide your rump.

1618
01:09:42,310 --> 01:09:45,810
Just for a minute, let's
all do the bump, bump, bump.

1619
01:09:45,810 --> 01:09:46,810
Yeah.

1620
01:09:46,810 --> 01:09:49,310
You can't touch this.

1621
01:09:49,310 --> 01:09:50,310
Look, man.

1622
01:09:50,310 --> 01:09:52,310
You can't touch this.

1623
01:09:52,310 --> 01:09:54,310
You better get hype,
boy, because you know--

1624
01:09:54,310 --> 01:09:56,180
[CROWD YELLING]

1625
01:09:56,180 --> 01:09:57,930
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]

1626
01:09:57,930 --> 01:10:00,310
(SINGING) Break it down.

1627
01:10:00,310 --> 01:10:04,310
DAVID MALAN: [INAUDIBLE]

1628
01:10:04,310 --> 01:10:05,100
ANDREA: [INAUDIBLE]

1629
01:10:05,100 --> 01:10:05,760
DAVID MALAN: No.

1630
01:10:05,760 --> 01:10:06,310
It's OK.

1631
01:10:06,310 --> 01:10:09,810
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]

1632
01:10:09,810 --> 01:10:10,810
One more life.

1633
01:10:10,810 --> 01:10:11,310
[MUSIC - MC HAMMER, "U CAN'T TOUCH
THIS"]

1634
01:10:11,310 --> 01:10:12,390
MC HAMMER: (SINGING) Stop.

1635
01:10:12,390 --> 01:10:12,920
Hammer time.

1636
01:10:18,420 --> 01:10:19,380
DAVID MALAN: All right.

1637
01:10:19,380 --> 01:10:21,210
A round of applause for
Andrea, if we could.

1638
01:10:21,210 --> 01:10:23,950
[APPLAUSE]

1639
01:10:23,950 --> 01:10:25,830
OK.

1640
01:10:25,830 --> 01:10:27,600
That's it for CS50.

1641
01:10:27,600 --> 01:10:28,950
See the website for details.

1642
01:10:28,950 --> 01:10:31,090
We'll see you for cake in the transept.

1643
01:10:31,090 --> 01:10:32,940
Welcome aboard.