>> [APPLAUSE]

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

to the intellectual enterprises of computer science

and the art of programming.

Now if you are among those that every year are sitting here

with a bit of nerves in your mind, such that you don't think you belong here,

you think that most anyone sitting around you

knows far more than you, is indeed more comfortable than you at computer

science or computers more generally, realize

that 78% of the students who now take CS50 have no prior experience.

>> Indeed, there's 100 dots there on the display, 78 of which

are solid green, which means you, if you're among that demographic,

are in very good company here on out.

And if you are instead among the 22% of CS50 students who do indeed

have prior experience, whether in high school or some other program,

realize that you, too, will be challenged in the course.

>> Not only do we have different tracks for students less comfortable and more

comfortable alike in sections, we also have so-called hacker editions

of most problem sets that will challenge those students

with that additional experience to explore similar material

but from a more sophisticated perspective.

>> But what is computer science?

Well, ultimately, what's going to matter as you explore this field is not

so much where you end up relative to your classmates,

but where you yourself end up in week 12 versus where you begin here

in week zero.

Now computer science-- well, let's call it the science of computation--

where computation is really just a fancy way of saying, taking some input,

producing some output, and doing so by running algorithms,

sets of instructions for solving some problem on those inputs

in order to produce some output or solution in which you are interested.

>> So we recently had occasion to travel out

to California to meet with an alumna.

Her name is Susan Wojcicki.

And she'd like to speak to you here on video

to testify to just how applicable even just a taste of computer

science at the introductory level can be.

Even if you don't go on to pursue computer science as a field,

or even engineering, or STEM more generally,

you'll see, in fact, how a certain course so influenced her life.

And she only just took it when she was a senior here at Harvard College.

>> If we could dim the lights for Susan.

SUSAN WOJCICKI: Hello, world.

I'm Susan Wojcicki.

I'm the CEO of YouTube.

And I took CS50 when I was a senior at Harvard in 1990.

I was actually a history and literature major.

>> And my junior summer, I realized that maybe I

wanted to learn something about computers.

And so, I came back.

I took CS50.

It was hard, but it was the most amazing class I took.

>> It changed how I think about everything.

And when I graduated from Harvard in 1990, I went to Silicon Valley.

And I got a job.

And I've been working in tech ever since.

DAVID J. MALAN: Now what Susan didn't mention in this video,

that it was actually in her garage that Google itself was

founded by Larry and Sergey.

>> Now we also reached out to our friends at code.org, an organization that

over the past year has been getting people particularly

excited about computer science and programming, in particular.

But it's worth noting that programming is not computer science per se.

Computer science is not programming.

Rather programming is just a tool-- with which all of you

will be all too well familiar by semester's end--

such that you can apply not just to future courses in CS

but to whatever fields from whence you're coming, in humanities,

social sciences, natural science, or the like.

>> Indeed, allow a few other alumni and their colleagues

to speak to the applicability of the field that awaits.

>> BILL GATES: I was 13 when I first got access to a computer.

>> JACK DORSEY: My parents bought me a Macintosh in 1984

when I was eight-years-old.

>> MARK ZUCKERBERG: I was in the sixth grade.

>> SPEAKER 1: I learned to code in college.

>> RUCHI SANGHVI: Freshman year, first semester, Intro to Computer Science.

>> BILL GATES: I wrote a program that played tic-tac-toe.

>> DREW HOUSTON: I think it was pretty humble beginnings.

I think the first program I wrote asked things like,

what's your favorite color?

Or how old are you?

ELENA SILENOK: I first learned how to make a green circle

and a red square appear on the screen.

GABE NEWELL: The first time I actually had

something come up and say, hello, world.

And I made a computer do that.

It was just astonishing.

>> MARK ZUCKERBERG: Learning how to program didn't start off

as wanting to learn all of computer science

or trying to master this discipline or anything like that.

It just started off because I wanted to do this one simple thing.

I wanted to make something that was fun for myself and my sisters.

>> And I wrote this little program.

And then basically just added a little bit to it.

And then when I needed to learn something new,

I looked it up, either in a book or on the internet,

and then added a little bit to it.

>> DREW HOUSTON: It's really not unlike playing an instrument or something

or playing a sport.

DAVID J. MALAN: All right.

So let us now actually dive in a little deeper.

What are these inputs and outputs that we're talking about here?

>> So how about something simple?

You probably know, even if you have no familiarity with computer science

whatsoever, that computers somehow use and understands only zeros and ones.

But how can that possibly be given how much today's desktops and laptops alike

can do?

>> The DNA of the day, the only alphabet that they understand

is a zero or a one.

Well, consider this.

We, humans, tend to use the decimal system. "Dec" meaning 10.

And that's 10 because we have 10 digits, 0 through nine.

>> Now computers, by contrast, tend to use binary.

"Bi" meaning two.

So they tend to use only zero and one.

But it turns out, that even just with zeros and ones, that

is a sufficiently large alphabet with which to represent most

any piece of data you want, whether it's a number,

whether it's a letter, whether it's a graphic or video on the screen.

>> Consider, for instance, how we humans typically interpret this number here.

This is just three digits, one, two, three.

But we know this number innately now as 123.

But why is that?

>> Well, if you think back to perhaps grade school,

you probably were taught to think of these numbers as being in columns,

where the one is in the hundreds place, the two is in the tens place,

and the three is in the ones place.

Why is that actually useful?

Well, think about the super simple arithmetic

that we all have been doing for years now.

Effectively, if you've got a one in the hundreds place,

you do the quick math 100 times 1 plus 10 times 2--

because two is in the tens place-- plus 1 times 3--

because three is in the ones place.

So, of course, if we actually multiply this out,

what we're really representing with this pattern-- one

two three-- is 100 plus 20 plus 3, which, of course, is 123.

>> Now binary, and computers really, fundamentally speak the same language

that we do.

They just have a smaller alphabet.

So computers only have zeros and ones at their disposal.

So whereas we humans have essentially powers of 10 in each of these places--

10 to the zero, 10 to the one, ten to the two, giving you 110 and 100

respectively.

>> Because computers only have two values they can understand, zero and one,

they have to use different values in these columns, one, two, four.

And if we kept going, eight, 16, 32, 64, and so forth.

But the pattern and the mentality is exactly the same.

>> So by this logic, anyone, how would I go about representing the number

one in binary?

If you've never even thought about this before, what's your gut say?

>> AUDIENCE: One.

DAVID J. MALAN: One.

Exactly.

We just need a one in the ones place because the zeros

suffice to give us neither a four nor a two.

So one times one equals one.

Now things get a little interesting.

If I want to represent in binary the number two-- but,

again, even if you've never spoken this language before,

how do we represent in binary the value we humans know as two?

Zero one zero.

Just put the one in the column that you want it.

>> Now it's getting pretty easy probably now.

So if I want to represent three-- there is no three's column.

So, again, I can now add these values together by putting a one here.

So 2 times 1 plus 1 times 1 is, of course, 3.

>> Now things get a little fun in that the ones now become zeros.

And to represent four, I get this.

And if we increment slowly here-- that would be five.

This would be six.

This would be seven.

>> But now I seem to have run into a problem.

How might I go about representing eight-- would be the next value.

Yeah, so we need a new bits.

And, indeed, if you've heard this phrase before,

bits, that's just short for binary digit, zero or one.

>> And so I happen to be representing only three such bits here.

But if I had a way of storing not three different bits, but four,

surely I could represent eight, and then nine, and then

10, and even higher and higher.

>> But that then calls into question how we can

go about representing these things in the first place.

It's one thing to draw them up here on a slide,

but how do you represent them if you're a mechanical device?

What is a computer doing to represent the inputs and outputs that

fundamentally define computation at the end of the day?

>> Well, what about something super simple like this?

It's just a light bulb.

And I can trigger this light bulb to go on

by turning some electricity on and allowing electrons

to flow through, which changes its state or its value, so to speak.

For instance, this is an old school desk lamp

here with one such light bulb inside of it.

And right now it's not really doing anything useful.

But as soon as I plug it into an electrical socket

and then use this switch-- or we can even call it a transistor

or think of it as such-- I can now represent either

this value, where the light bulb's obviously off, or this value.

This value or this value.

This value and so forth.

>> So inside of a computer, presumably, are much smaller pieces of hardware,

but that at the end of the day simply have

to use electricity-- perhaps capture it--

and then either keep something on or keep something off.

Of course, this isn't particularly interesting to do

with just a single light bulb.

>> In fact, how high can I count in binary with this desk lamp here?

>> AUDIENCE: One.

>> DAVID J. MALAN: One, right?

I need more desk lamps if I actually want to count higher.

But we can do better than that.

Because the light bulbs that we've put in these things

are actually fancier light bulbs than yesteryear would allow.

And they're actually networked light bulbs.

And bunches of companies make these things these days.

>> But it turns out that this one in particular

comes with a feature whereby you can change its colors.

So for instance, if you adorned your dorm room

with a few of these light bulbs, depending on your mood,

depending on who comes in, depending on the weather,

depending on the time of day, you can actually

change the colors of the bulbs in your room.

And that's because these light bulbs and others like it have what's

called an API, an application programming interface, which

is a topic with which you'll be well familiar with by semester's end.