DAVID MALAN: All right.

This is CS50.

And just eight weeks ago, 2/3 of you had never

studied any computer science before.

And what we thought we'd begin with today is really a look back of truly

how far you've come over just those several weeks.

In fact, it was just eight weeks ago when you first

started tinkering with Scratch, this graphical programming language

by which you could drag and drop puzzle pieces and make the computer

do what you want.

And then a week later, you might have struggled

with just getting Mario to ascend a pyramid by way of hashes

that you were printing.

Which at the time was perhaps quite non obvious,

but it's ever since has just been what?

A pair of nested four loops.

And then the week after that, you explored cryptography

among other fields.

And you learned how to encrypt and you decrypted information,

like this snippet here.

After that, you explored some electoral processes and the implementation

of algorithms that you might be familiar with from the real world, but perhaps

didn't think about the trade offs in choosing one algorithm or another.

And then ultimately, implementing it in code.

Just a week later did we transition to filters on Instagram,

implementing things like your own sepia filters.

So that moving forward, hopefully, you won't

take for granted when you click that button on the screen on your phone,

you actually know or can presume, or infer,

what's going on underneath the hood as it changes from one setting to another.

And then, of course, Big Board, where we handed you some 140,000 English words

and a really large file.

And you had to implement your own spell check,

or a dictionary so to speak, with using as little amount of time

and as little space as you could, implementing ultimately your own hash

table.

Thankfully since then, we can now take for granted that those things exist.

But you, indeed, built that from scratch.

Just a week later, we started handing you large files like this.

And of all things, you were able to deduce by writing some Python code

that this is lavender's DNA, among others

in the class whose files we handed you.

And then most recently, or perhaps quite soon,

when you learn to apply another language.

In this case, SQL.

To questions like who has starred in films with Kevin Bacon.

And so, a number of tools are now in your toolkit.

And again, I would just emphasize that eight weeks ago, 2/3 of you

had never done any of that before.

So even if it's felt like it has never let up

and each week feels all the more challenging than the last,

consider the delta.

Consider what we promised in the first week of the class.

That what ultimately matters in this course

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

but where you end up relative to yourself when you began.

And it is, indeed, those eight weeks ago that you began.

And it's today and in the courses final weeks that you will ultimately

be able to compare yourself.

So in trying to think today about what we hope your own takeaways are from,

not only the course, but computer science more generally,

I actually pulled up some of my own notes.

You might recall from week zero that I mentioned I took this course myself

back in 1996 and it happened to be the one that turned me from a government

major to a computer science major.

And I didn't take many notes at first, it seems in that first class.

Now, there were a couple of other pages.

And so, I started flipping through these and in fact,

this was my second page of notes.

And if you focus on what's up here at the top, apparently, one of my first

notes to myself that a function, for those unfamiliar today,

are instructions that do the work.

And indeed, if I looked down further on my notes, arguments

we introduced that same first day.

It's info that's passed from one function to another

to have something done by or performed on it.

All right.

So that actually maps wonderfully cleanly to what

it is we indeed started ourselves doing those eight weeks ago.

Just talking about what CS is, problem solving.

And a problem is just something that's got input.

And the goal, of course, is to produce something with output.

And we had to agree from that very first week

how we're going to represent these inputs and outputs.

And so, we might do it symbolically with letters of the alphabet.

We know since then with that we might do it numerically as with decimal digits

or below that, we might just use binary, a different base

system using only zeros and ones.

Or we could even represent that same capital A just using

some pattern of light bulbs or switches that you turn on in some order.

In fact, if you think back then to these first principles

that we introduced those weeks ago, to any of the students

now in the audience, what does this say?

AUDIENCE: Hi.

DAVID MALAN: Hi.

So it indeed says hi.

Why is that?

Well, we just agreed weeks ago on what numbers map to what letters.

And so as long as all of the humans in the world

agree on that when they're sending emails, or text messages, or the like,

our computer systems can all present information

that we ultimately understand.

Another review question from week zero.

What did this say way back when?

If you need a hint, it was it was in decimal 128,514.

A little louder.

AUDIENCE: Emoji.

DAVID MALAN: Emoji.

This was the face with tears of joy, right?

Taking all of the fun out of sending emojis.

Anytime you receive or send an emoji, really all you're sending

is a pattern of zeros and ones, or worse the number 128,514.

But your computer or phone are presenting it to you

in a certain way based on that context.

All based on just how we humans or those before us decided

how to represent this information.

But of course, we don't have just those inputs and outputs.

There is another piece to the puzzle.

And so, I actually looked back at my own third page of notes

here and focused on my first line here back in 1996.

An algorithm is apparently a precise sequence

of steps for getting something done and programming more generally

is a process of taking an algorithm and putting it

into a language a computer can process.

And indeed, that really was the additional puzzle piece

we first focused on in week zero.

It was what was inside the proverbial black box?

The sort of secret sauce that you provide your input to,

you get your output from.

Those are the algorithms that we've been talking about ever since now, not

just in pseudocode or in English.

But in C, and in Python, and now SQL.

And then in the coming weeks, a few other languages as well.

But of course, we used algorithms to solve problems

and one problem is and was rather old school.

And we thought we'd reflect just a bit and perhaps

reinforce that same week zero if by taking a couple of volunteers.

Perhaps a student and one of their family members

voluntarily or forcibly with them.

You have to be comfortable, though, appearing on stage

and in turn, the internet in perpetuity is the catch.

I see-- OK.

I see one volunteer and either mom or dad

finding out who is not going to go up.

OK

[LAUGHTER]

Come on up.

A round of applause if we could for our volunteers.

[APPLAUSE]

DAVID MALAN: Come on over this way.

And what is your name?

DANIELLA: Daniella.

DAVID MALAN: Daniella.

Nice to meet you.

Come on over here.

And what is your name?

MARIANO: Mariano.

DAVID MALAN: Mariano.

And your dad, I presume.

MARIANO: Yes.

DAVID MALAN: All right.

Well, so nice to see you both.

You'll recall from week zero that one of the first problems we solved

was looking up someone in a phone book.

Well, here I have a pretty thick yellow pages wherein

there's a whole bunch of products and services alphabetized from A to Z.

And we can, of course, look up the number for someone in this book.

For instance, suppose today we need a plumber.

Could we ask Mariano to find us a plumber in this phone book?

Flipping through a few pages.

A lot of pages.

Yeah.

OK.

All right.

And we have a page full of plumbers.

And now, could your dad have done better do you think?

DANIELLA: Yes.

DAVID MALAN: I think so.

So would you like to show dad and other parents here

what you might have done instead?

Oh my goodness.

[LAUGHTER]

OK.

Thank you.

[APPLAUSE]

[LAUGHTER]

DAVID MALAN: And for those wondering, let's continue the algorithm,

if you will.

What's your next step?

[LAUGHTER]

Nice.

And why are you doing this, though?

DANIELLA: So I open halfway and then if it's not on the page

and it looks like, for example, if it's n, I know p is afterwards.

So I need to get rid of the first half and just look at the second half.

DAVID MALAN: Well, need to is a little strong.

[LAUGHTER]

But you can get rid of--

yes, indeed.

So if you keep this process going, halving, and halving, and halving,

what should you be left with ultimately?

DANIELLA: Just the letter p.

DAVID MALAN: Just the letter p.

Round of applause would be good for both of our volunteers here.

[APPLAUSE]

DAVID MALAN: Thank you.

A CS50 stress ball for you both.

Thank you so much.

Oh, thank you.

Thanks.

So that was one of the first lessons with which we

began focusing on algorithms.

And the point back then, recall, was to really emphasize the familiar, right?

We might not use that particular technology all that much anymore.

But it really is fundamentally the same thing that's now in our iPhones

and Android devices and the like.

It just so happens that we search for things by scrolling with our finger

up and down.

But consider too, even these days in 2019

when you type into the auto-complete to search

for someone by first name or last name, how

is your phone finding that information?

It's exactly like that.

Not quite as physically as that.

But most likely for efficiency, your phone

is looking roughly in the middle of all of those names then jumping

up or down in the blink of an eye in order to find that person for you

ever so quickly.

And so, all of us might have an intuitive understanding,

just like Mariano did, for finding plumbers by jumping to the p section.

But then, he came close to the p section, but then started flipping.

And I dare say you flipped through far more pages

than you ultimately needed to using our more efficient divide

and conquer if one time only algorithm instead.

So beyond algorithms, I recall that I had just a few other notes.

On my last page of notes, I also had this here.

That what was apparently important then and no less so

now is that in computer science and in turn programming,