>> [CAR DOOR SHUTTING]

>> [CAR ENGINE STARTING]

>> [MUSIC PLAYING]

>> -Moving right along in search of good times and good news.

With good friends you can't lose.

>> -This could become a habit.

>> -Opportunity knocks Let's just reach out and grab it.

-Yeah!

-Together we'll nab it.

-We'll hitch hike, bus, or yellow cab it.

-Dammit.

-We're moving right along.

-Footloose and fancy free.

-Getting there is half the fun.

Come share it with me.

>> -We're moving right along.

>> -We're going to share the load.

>> -We don't need a map to get this show on the road.

-Hey Fozzy, I want you to turn left if you come to a fork in the road.

-Yes, sir.

Turn left at the fork in the road.

Turn left.

>> -I don't believe that.

>> -Cruising right along we found the lights on the highway.

>> -And your way or my way.

>> -So trust my navigation.

>> -Movie stars with flashy cars and life with the top down.

>> -We're storming the big hills.

>> -Yeah, storm is right.

Should it be snowing?

>> -No, I don't think so.

>> -Moving right along.

>> -Footloose and fancy free.

>> -You're ready for the big time.

>> -Is it ready for me?

>> -Moving right along.

Moving right along.

Moving right along.

Moving right along.

-I think you better pull over.

-Yes, sir.

>> [END PLAYBACK]

>> [APPLAUSE]

>> DAVID J. MALAN: This is CS50.

And this is the end of Week 0, but the start of a beautiful friendship.

We are so excited to be here at Yale University

for the very first time with our friend Scaz, and Jason, and Andy.

All 40 of CS50's TF's and CA's here at Yale.

And all of you.

And in fact, it's probably time to--

>> [APPLAUSE]

>> So, even though most of the lectures will indeed be in Cambridge this year,

the reality is that by design, and very much deliberately in CS50,

I do think we're nearing the point technologically, and pedagogically,

where it can be a superior educational experience to engage with some

of the course's material online.

Indeed, the reality-- and if I believe this philosophically-- is that lectures

are not a particularly effective means for delivering

fairly complex information.

Certainly over hour plus long spans of time.

And indeed every few minutes-- I remember myself in college--

you zone out for a moment, you miss some complex topic,

and you're gone pretty much for the next 45 minutes.

And the reality is that whether you're here in New Haven or Cambridge

or beyond, the simplicity of having the ability to pause and fast forward,

rewind, hyperlink to related resources, search full text transcripts

and the like is an opportunity that I dare say for our online students

well beyond New Haven, offers them an opportunity

to really grasp the conceptual material that we

introduce in lectures all the better.

>> But ultimately in CS50, is the students' experience

is characterized so, so much more by the course's problem sets,

or weekly programming projects, the course's sections led by the teaching

fellows, the nightly office hours led by the course assistants and the teaching

fellows, and indeed such events as CS50 Puzzle

Day, CS50's Hack-a-thon, the CS50 Fair, weekly lunches, and so much more.

And so whether you're here in New Haven, or tuning in from afar,

today in Cambridge, and we'll see you again soon,

the reality is this is one and the same class.

And we are so excited to be here all together today.

And so at the end of the semester, whether here or afar, well

hopefully you wear proudly, happily, and perhaps with a bit of relief,

the shirt that bears witness to you having taken CS50.

>> So where did we leave off on Wednesday?

We took a look at computational thinking.

And that's toward what we distilled computer science unto, at least

for now.

But we distilled it little further into at least three components elements.

>> Inputs, so what is the problem we're trying to solve?

Outputs, what is the answer we're hoping to get?

And the representation there of.

And we won't dwell here on after on binary, or even ASCII,

and the like so much, but rather take for granted

that we can represent this information, because the far more interesting parts

of those problems are not just the inputs and outputs, but the algorithms

that go into solving those problems.

>> And you might recall from the other day that we

took a fairly traditional concept of looking someone up

in a fairly large phone book, or more generally digitally these days,

just a very large data set.

A really big contacts list, with lots of names alphabetically sorted.

And we realized that while I could approach this problem simply

by using a linear approach-- page by page,

or even two's at a time-- we realized that the more intuitive algorithm--

just kind of dividing and conquering the problem again, and again,

and again-- halving it with each time, gave us this green result.

And it's so much flatter because it suggests

that even as this problem gets bigger and bigger,

as is the case these days with data sets and reality-- Facebook and Google

search results and the like-- does our algorithm

perform just as effectively with those bigger bites

as it did even with the smaller bites.

>> Now it begs the question, what can we actually solve similarly

with this kind of intuition?

This sort of division and conquering?

Well, we could do something like this today here.

We could take attendance.

So maybe like 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11--

not going to be all that efficient.

So wait, grade school.

2, 4, 6, 8, 10, 12, 14, 16, 18 and so forth.

But could we do a little better?

>> I'm pretty sure I can't do this by myself.

So if you'll humor us for just a moment, we brought with us

an algorithm that's just got three steps,

but it does require-- if you don't mind-- all of us stand up,

if you would.

So, with all of us now standing, step one of this algorithm is the following.

Stand up and assign yourself the number 1.

So at this point in time, literally everyone in this room

hopefully is thinking to themselves, however awkwardly, the number 1.

>> Now step two of this algorithm is going to involve the following.

Step two, pair off with someone standing,

add your numbers together, and adopt the sum as your new number.

Quick sanity check.

What number is everyone thinking of now?

>> So 2, except maybe for one, awkward, lonesome person.

If we have an odd number of people in the room.

So one person might still be the number 1, that's fine.

But step three here, one of you should sit down.

The other should go back to step two, and repeat, if you would.

>> So if still standing, you should be among those going back to step two.

Keep going.

A few people still standing.

>> So, if still standing, pair up with someone.

All right, dwindling down.

A few people still standing.

I'll help if need be.

Remember the key takeaway here, is how much faster this is than me counting.

>> So let's see.

I can help out.

So what number are you still thinking of?

>> AUDIENCE: I'm at 44.

>> DAVID J. MALAN: 44, so you go ahead and sit down.

What number are you thinking of?

AUDIENCE: 74.

>> DAVID J. MALAN: 74.

All right, go ahead and sit down.

Who else is still standing?

86.

And is anyone else still standing?

>> What number?

I heard 67.

And then in top?

32.

Anyone else still standing and thinking of a number?

>> Oh, hi.

Sorry.

Be right back.

42.

Anyone else?

>> AUDIENCE: 47.

>> DAVID J. MALAN: 47.