>> [MUSIC PLAYING]

DAVID MALAN: All right.

This is CS50.

[MUSIC PLAYING Tritonal, Cash Cash, "Untouchable"]

[MUSIC PLAYING]

SPEAKER 1: I'm going to France, and you're going, too.

[MUSIC PLAYING]

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

to the intellectual enterprises of computer science

and the arts of program-- and for the first time in history,

Yale University's as well.

Indeed, whether you're here in Cambridge or in New Haven or Miami or St. Louis

or Amsterdam or anywhere around the world

taking CS50, computer science E50, CS50X, CS50 AP,

we are all one and the same.

Welcome to CS50.

>> What we have--

>> [APPLAUSE]

>> [LAUGHS]

>> [APPLAUSE]

>> So I made a mistake myself some time ago when I started off college.

And I got to college, and I decided to frankly, stick within my comfort zone.

I ended up declaring a concentration, or a major, of government.

Ant that was mostly a function of me being pretty familiar with government

or at least history or I really liked constitutional law in high school.

And so when I got here, I kind of gravitated toward things

with which I was already familiar.

Right?

God forbid I do poorly in the class.

I certainly wanted to stay within my comfort zone,

and it wasn't until sophomore year that I finally

got up the nerve to step foot in a classroom called CS50.

And at that point, did I finally realize that, my God, homework could actually

be fun.

>> Indeed, I was one of those kids that on Friday evenings when

the P-SETS would be released, I would go back to my room and dive

into the night's P-SETS.

And for me, that was a sign that this was a field for me.

But what was more important was the fact that I did get up this nerve

to explore waters unfamiliar to me and get beyond my own comfort zone

and frankly, I only was able to do that sophomore year by taking this class

pass/fail.

>> Indeed, it was the very last day that I finally switched over and finally

declared CS as my concentration, putting gov at that point behind me.

And so we're not setting out in this course to turn all of you

into CS majors or concentrators, but rather to give you an opportunity

to hopefully go beyond the world with which you're currently familiar

and bring back from this world skills and knowledge and savvy

that you can apply to your own world, whether that's

in the humanities, social sciences, natural sciences, or beyond.

>> Indeed, if you're feeling a little intrepid

about being in this room let alone in this class,

realize that if history is any indication, 72% of you

have never taken a CS course before.

So it is by all means not the case that the student sitting to the left

or to the right or in front or behind you knows far more about CS

or programming in particular than you.

That's not in fact the case.

And indeed, much of the support structure

that we've set up in this course over the past many years

has been for exactly that reason-- to provide an on ramp that still exits

just as rigorously and just as high as ever--

but the slope of which allow students less comfortable and more comfortable

alike to succeed irrespective of his or her prior background.

>> Indeed, what ultimately matters in this class is not

so much where you end up relative to your classmates

but where you in week 12 end up relative to yourself

in week zero, which is where we are here today.

>> Indeed and this may very well and probably does look like Greek

to many of you.

But rest assured, that this and so much more

is going to be completely within your grasp in just a little bit of time.

>> But today, we focus on some of the higher level ideas

to give you a taste of CS50 and computer science

in a sense of what you're signing up for.

And indeed, computer science might be distilled more

simply as computational thinking-- thinking like a computer, if you will.

And there's so many different things ingredients that go into that,

but let's propose just three for today.

If the goal of the class ultimately is not to teach you programming,

is not to teach you C or PHP or SQL or any number of the words

and acronyms in the course's description,

but rather to teach you to solve problems more effectively

and to think more methodically and more algorithmically, so to speak.

Let's see what exactly this means.

>> So I would propose that thinking computationally boils down

to solving problems.

What do you need to solve a problem?

You need to input-- like the input to the problem--

you need an output, which is hopefully the solution,

and then you need a process by which to solve that problem, which

we'll call an algorithm-- a set of instructions for solving some problem.

>> But first, let's focus on the first and the last of these inputs and outputs.

Computers after all, apparently only understands zeros and ones.

But how can that possibly be?

Even if you're not familiar at all with what's underneath the hood,

you probably at least heard that computers understand binary--

just zeros and ones-- but how can you possibly do anything interesting?

>> Well, one of the themes of the class is going

to be this layering-- where today, we'll take a quick glance at the lowest level

details, but with each passing day, where we layer

or abstract on top of those details to actually solve higher level

problems of interest to us.

>> So here is what we might call binary-- with just an alphabet of 0 and 1.

But we humans are mostly familiar with decimal.

Dec meaning 10.

Bi meaning two.

And so in the decimal system, we have 10 digits

at our disposal-- of course, zero through nine.

So if you look at a number like this, most of you

intuitively just grasp that is 123.

There's nothing really hard about that.

But why is it 123?

Well, if you think back to grade school-- or at least

the way I learned this kind of world-- you

might recall that we treated these things in columns, or places.

>> So we have the ones place on the right.

The tens place in the middle.

The hundreds place on the left.

And then how do we get from this pattern symbols--

1 2 3-- to this higher level idea that we know as 123?

Well, it's just some simple arithmetic.

Right?

>> The one there is essentially means give us 100 times 1 plus 10 times

2 plus 1 times 3.

And of course if we do out the math there, it's 100 plus 20

plus 3-- otherwise known as 123.

>> So if you're on the same page as that right

now and are comfortable with the so-called decimal system as a human,

it's actually well within your scope of comfort

to consider now the binary system.

Take a wild guess-- this represents, in the world of computers

in binary-- what number?

Zero.

>> But why is that?

Well, it turns out that the columns or places here-- they're not powers of 10.

1, 10, 100, 1,000, and so forth.

They're instead, quite simply, powers of 2.

So, 1, 2, 4, 8, 16, 32, and so on.

And so now we of course get to 0 here simply because we have 4 times

0 plus 2 times 0 plus 1 times 0, which of course gives us 0.

>> But how do I go about representing the number 1?

What's the pattern of zeros and ones to represent

the number we humans know as 1?

001.

And 2?

010.

>> And now the pattern starts to repeats.

Now it's 011.

And again, 0 fours, one 2, one 1.

So 2 plus 1.

That's 3.

>> And now to represent 4, we don't just change that 0 to a 1.

You sort of have to carry, so to speak, and the numbers

start flipping around just like in the decimal world.

>> So this is 4.

This is 5.

This is 6.

This is 7.

And so we've counted as high as 7.

>> Now all we just need is more a bits-- more zero's and one's.

And indeed "bits", if you've heard this term-- binary digit.

Bit is where that comes from.

And so if we want to represent bigger numbers, we need more bits.

But let's move away from slides now to something a little more real.

Suppose that we want to actually represent this thing.

>> Well let's take a look now at a little demonstration.

So this is a web based application that one of CS50's own, Michael G,

put together this summer to help us elucidate exactly this idea.

And would someone like to venture up on stage

in front of all his or her classmates?

Right there in front.

Come on up.

>> You have to be comfortable on camera and the internet.

Oh, right here.

OK.

We're OK.

All right.

Come on up.

What's your name?

Emily come on up.

So this is Emily.

What year are you?

>> Freshman.

>> Emily, nice to meet you.

David.

>> All right.

So up on the screen here, we have this touch screen

which is going to allow us to actually interact with this program,

and it's just a browser.

It's Chrome full screened at the moment, but it's

been programmed by Michael to respond in a way that allows

us to play around with binary digits.

>> So for instance, here we have not three but eight bits-- zeros and ones.

Right now, we're looking at the number 0.

And indeed, all eight zeros in decimal means zero.

So that's all that's being hinted at here.

>> So if you wanted to represent the number 8,

what's the pattern of zeros and ones that you want?

You can simply tap up or down or the numbers themselves.

All right.

So that of course is 8, as you can see up there.

And if we wanted to do 16, what do we do?

>> Yep, just touch it again.

16.

All right.

So this is all fine and good, it's still very low level.

We need a way in the real world for Emily

of actually representing these things.

And so suppose that we turn these zeros and ones, which is very

conceptual, into actual light bulbs.

Right?

>> A computer is a physical, mechanical, electrical device.

And its input-- at least if you plug it in or charge it--

is to have battery power and electrons flowing in and out.

>> So now, why don't we stop thinking about bits as zeros and ones,

but something more physical like light bulbs here.

And if Dan Armendariz could join me for just a moment-- come on up--

we're going to queue up an application.

>> Come on over, Emily.

Sorry this is the most awkward demo for you ever.

Come on over here.

We're going to queue up with thanks to Dan

Armendariz, another member of our staff, an application known as binary bulb.

>> So what we have here is an iPad application

that has the following user interface on the screen for Emily.

It's just got the same exact UI essentially that's over there.

And if you now want to represent the number, say 8,

how would you go about doing this noticing at the right,

the light bulbs that we have here?

Ah-ha.

Magical.

So if we want to now turn this into something a little more challenging,

and let's go ahead and pick a random number like the number 50 here.

Input this.

And if you can now be challenged to come up with the number 50,

we'll have a fabulous prize for you.

>> EMILY: OK.

Oh my God.

DAVID MALAN: Arithmetic is indeed hard in front

of hundreds of your classmates.

But 50 has been the answer here.

>> [APPLAUSE]

>> And so now, this is meant to be demonstrative for Emily.

So, in here, is some light bulbs quite like these,

but it's actually the little magnetic strips.

And what's cool about these and the reason we use them in CS50

is that they support something called an API-- an application programming

interface, which is just a fancy way of saying that what one of our staff

did over the summer was create an iPad application here

that talks over the internet to the light bulbs over