>> -Alright!

>> -This is CS50.

>> -This is CS50.

>> -This is CS50.

[MUSIC -- IGGY AZALEA, "FANCY"]

>> -My favorite memory from CS50 was when I went to Puzzle Day.

>> -Probably just the time spent working on problem sets

with my friends and people who wold eventually become my blockmates.

>> -My best memory from CS50 is the Hackathon.

-The CS50 Hackathon.

>> -Hackathon.

>> -Hackathon.

-Hackathon.

-The Hackathon

-Rob Bowden.

Just everything about him.

>> [MUSIC -- IGGY AZALEA, "FANCY"]

>> -My favorite memory is when I was on stage and I played the prestigious role

of a node [? in the Linked ?] [? List. ?]

>> -When we all got free Dropbox space and David was like, look under your seats.

And it was like, space for everyone!

>> [MUSIC -- IGGY AZALEA, "FANCY"]

>> -My advice for any incoming student would

be to really work on P-sets with friends.

>> -Office hours is very much your friend.

>> -Make the most of your experience and meet as many people as you can.

>> -Don't be afraid to ask for help.

-Start the P-sets early in the week.

-I think the biggest thing is to take advantage of all the resources

that CS50 has.

>> -Go to office hours early in the week.

>> -Definitely watch the shorts.

>> -Don't procrastinate on your P-sets.

-Make sure you find a great group of people to work on P-sets with.

You can have a lot of fun and get work done together.

>> -Don't be afraid to push yourself.

Go for the hacker edition sometimes.

>> -Write things on paper before you ever touch your computer.

>> -CS50 is really great at providing ways to get help.

>> -My one piece of advice is sleep.

Has anybody said that?

Sleep, for sure.

It's easy not to do, but you've got to do it, I think.

>> -I would say really be mentally prepared because you're going to love it.

[MUSIC -- IGGY AZALEA, "FANCY"]

>> -This is CS50.

>> -This is CS50.

>> -This is CS50.

[MUSIC -- IGGY AZALEA, "FANCY"]

>> -This is CS50.

>> [APPLAUSE]

>> DAVID J. MALAN: So this is CS50 and this is the end of Week 0.

And that was just some of CS50's staff who

await you not only in sections and office hours, but,

also, this coming weekend at CS50 Puzzle Day.

Which, again, is not all about programming.

Indeed, it's expected that you won't have to program anything,

but rather solve problems using wits and friends alongside you.

>> We will be joined by some of our friends at Facebook--

if you register here-- who for the past several years,

have actually been writing these challenges with us.

And so, they will be the ones ultimately running Puzzle Day.

And so, you will be challenged with precisely the kinds of things

and problems that folks at Facebook like to think about.

So that is tomorrow.

Register at cs50.harvard.edu/register.

>> Now a word on a couple of staff in particular.

This here is Ansel Duff, who is actually one

of the co-authors of these binary bulbs that we saw on Wednesday,

in addition to CS50's own Dan Bradley.

Ansel Duff was also a former freshman advisee of mine 3 years ago

and he actually even built this lectern.

He's gone on to do engineering sciences and more.

Now, his picture here is actually Ansel 3 years ago at the CS50 Hackathon

when he borrowed one of our balloons, stuck it to his laptop,

and, for the next 12 some odd hours, focused on his final project,

taking breaks only to open bags of candy at the Hackathon.

>> But he went on more recently to spend this past summer with us,

since CS50 for its staff, and now students this semester,

has its own 3D printer.

And in a nutshell a 3D printers is a device that looks quite like this.

You fill it with a plastic spool that is melted down by the device

and you build things literally from nothing.

Much like an inkjet printer, you start spitting out little dots of plastic

that form together to form whole objects.

And so Ansel for instance, earlier this summer, has an iPhone 5

and decided he really wanted to prop it up on his desk.

But he didn't want to go out and buy something

from the Apple store or the like, so he sat down and started drawing something.

He took a few measurements as to how thick

and how wide his iPhone was, he drew this image here,

he decided that he wanted to have a 75 degree tilt

as it was staring at him on his desk there.

He then turned this, using software, into a 3D CAD model

that looked a little something like this.

And then he proceeded, ultimately, to actually create it.

So in fact, if any of you here, perhaps in a row that I can throw to, have

an-- there we have folks with iPhone 5 , and here we have two more.

>> Now, not to be outdone, CS50's own Cheng Gong also set out this summer

to build quite a few things and, in fact, for reasons that are still

unclear, has been slowly printing an army of elephants

with articulating arms and trunks.

A couple of which are actually here if anyone would now like-- an elephant.

All right, . but what Cheng also did for us is he very kindly set up a camera

because that elephant, believe it or not,

takes some two and a half hours to print.

Even the iPhone stand took an hour and a half to print.

And what Cheng went ahead and did was set up a nice camera in front

of this 3D printer, filmed for an hour and a half as Ansel's design printed.

We overlaid some sexy music to it in order

to give you this to look at how 3D printing works.

And even though this is actually in plastic,

realize that if this is an area of interest to you academically,

there are folks, among them Jennifer Lewis here

at the School of Engineering, who are actually

working on 3D printing of plastic objects.

But even, increasingly, biological materials to solve

physiological problems for humans.

But here is a little something from CS50.

>> [LOUD MECHANICAL NOISES]

DAVID J. MALAN: It doesn't sound anything like that in reality,

but it's much cooler to watch it at that speed, and with that sound.

>> Now, on Wednesday, how did we first get here?

We started talking about computer science and we asked what it was.

And it's about a number of things, and there's so many different directions

in which you can head after a course like CS50.

In fact, if you picked up one of those unofficial guides

to CS outside, the booklet that we've provided, whether you're

thinking of taking just CS50, or maybe doing a secondary,

or maybe even concentrating in CS, do flip through that.

And you'll see a diagram toward the end that

shows you the many different directions in CS that you can go off in.

>> But for today, we'll focus, again, on really one of the fundamental views,

perhaps, where you have inputs to problems,

you have outputs from problems, and you have

algorithms with which to create those outputs from those inputs.

And one such example, was of course, this phone book here.

And we used as an example to go through an algorithm that was correct.

And then another one was correct, but a little faster.

And then another one that was a little more dramatic, but fundamentally

faster.

>> Right, this phone book we claimed had about 1,000 pages.

And how many times did I have to tear the phone book in half

to find someone like Mike Smith, maximally, in 1,000 page book?

So, 10 give or take.

And so once I tore this thing in half, or simply, more maturely,

divided in half, it's only 10 pages out of 1,000.

And if you extrapolate, a little unrealistically for a phone book,

but if this phone book had some 4 billion pages in it, so completely

unwieldy physically, how many times do you divide a 4 billion

page phone book in half?

So it's actually 32, give or take.

And so 32 times only, out of 4 billion pages, can

you find someone like Mike Smith.

And that's efficiency.

That's a good algorithm, daresay.

>> But then we moved from that to try to formalize it.

And I proposed this pseudocode code.

Pseudocode code is not anything formal.

It's not something you memorize.

It's just something you express fairly intuitively using English,

or any language really, that conveys your ideas succinctly.

But what's key about pseudocode code is that you

try to anticipate all of the possible cases that might happen.

And indeed, in this pseudocode code, there were really three cases

every time I divided the phone book.

Mike might