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[MUSIC PLAYING]

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DAVID MALAN: All right.

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This is CS50.

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

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studied any computer science before.

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

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

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

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started tinkering with Scratch,
this graphical programming language

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

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do what you want.

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And then a week later,
you might have struggled

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

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that you were printing.

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Which at the time was
perhaps quite non obvious,

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but it's ever since has just been what?

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A pair of nested four loops.

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And then the week after that,
you explored cryptography

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among other fields.

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And you learned how to encrypt
and you decrypted information,

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like this snippet here.

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After that, you explored some electoral
processes and the implementation

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

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

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And then ultimately,
implementing it in code.

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

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implementing things like
your own sepia filters.

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So that moving forward,
hopefully, you won't

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

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you actually know or
can presume, or infer,

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

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

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and a really large file.

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And you had to implement
your own spell check,

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

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

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table.

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

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But you, indeed, built
that from scratch.

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

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

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that this is lavender's
DNA, among others

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in the class whose files we handed you.

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And then most recently,
or perhaps quite soon,

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when you learn to
apply another language.

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In this case, SQL.

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

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

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

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had never done any of that before.

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

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

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consider the delta.

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

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That what ultimately
matters in this course

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

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

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

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

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be able to compare yourself.

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

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not only the course, but
computer science more generally,

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I actually pulled up
some of my own notes.

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

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

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major to a computer science major.

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

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Now, there were a couple of other pages.

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

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this was my second page of notes.

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

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

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are instructions that do the work.

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

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we introduced that same first day.

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

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to have something done
by or performed on it.

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All right.

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So that actually maps
wonderfully cleanly to what

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

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Just talking about what
CS is, problem solving.

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

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

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And we had to agree from
that very first week

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

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

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

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

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system using only zeros and ones.

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Or we could even represent
that same capital A just using

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

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

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

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now in the audience, what does this say?

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AUDIENCE: Hi.

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DAVID MALAN: Hi.

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So it indeed says hi.

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Why is that?

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

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

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

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our computer systems can
all present information

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that we ultimately understand.

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Another review question from week zero.

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What did this say way back when?

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

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A little louder.

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AUDIENCE: Emoji.

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DAVID MALAN: Emoji.

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This was the face with
tears of joy, right?

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Taking all of the fun
out of sending emojis.

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

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

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But your computer or phone
are presenting it to you

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in a certain way based on that context.

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

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how to represent this information.

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

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There is another piece to the puzzle.

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

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

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An algorithm is apparently
a precise sequence

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of steps for getting something
done and programming more generally

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is a process of taking an
algorithm and putting it

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into a language a computer can process.

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And indeed, that really was
the additional puzzle piece

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we first focused on in week zero.

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It was what was inside
the proverbial black box?

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

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you get your output from.

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

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just in pseudocode or in English.

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

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

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But of course, we used
algorithms to solve problems

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and one problem is and
was rather old school.

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

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

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Perhaps a student and one
of their family members

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voluntarily or forcibly with them.

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You have to be comfortable,
though, appearing on stage

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

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I see-- OK.

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I see one volunteer
and either mom or dad

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finding out who is not going to go up.

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OK

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[LAUGHTER]

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Come on up.

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A round of applause if we
could for our volunteers.

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[APPLAUSE]

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DAVID MALAN: Come on over this way.

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And what is your name?

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DANIELLA: Daniella.

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DAVID MALAN: Daniella.

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Nice to meet you.

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Come on over here.

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And what is your name?

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MARIANO: Mariano.

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DAVID MALAN: Mariano.

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And your dad, I presume.

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MARIANO: Yes.

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DAVID MALAN: All right.

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Well, so nice to see you both.

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

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was looking up someone in a phone book.

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Well, here I have a pretty
thick yellow pages wherein

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

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

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For instance, suppose
today we need a plumber.

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

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Flipping through a few pages.

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A lot of pages.

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Yeah.

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OK.

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All right.

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And we have a page full of plumbers.

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

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DANIELLA: Yes.

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DAVID MALAN: I think so.

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So would you like to show
dad and other parents here

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what you might have done instead?

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Oh my goodness.

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[LAUGHTER]

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OK.

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Thank you.

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[APPLAUSE]

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[LAUGHTER]

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

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if you will.

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What's your next step?

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[LAUGHTER]

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Nice.

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And why are you doing this, though?

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

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

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

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

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[LAUGHTER]

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But you can get rid of--

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yes, indeed.

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

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what should you be left with ultimately?

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DANIELLA: Just the letter p.

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DAVID MALAN: Just the letter p.

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

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[APPLAUSE]

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DAVID MALAN: Thank you.

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A CS50 stress ball for you both.

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Thank you so much.

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Oh, thank you.

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Thanks.

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So that was one of the
first lessons with which we

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began focusing on algorithms.

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

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

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

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and Android devices and the like.

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

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up and down.

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But consider too, even
these days in 2019

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when you type into the
auto-complete to search

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for someone by first
name or last name, how

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is your phone finding that information?

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It's exactly like that.

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Not quite as physically as that.

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But most likely for
efficiency, your phone

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

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

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ever so quickly.

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

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

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

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And I dare say you flipped
through far more pages

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than you ultimately needed to
using our more efficient divide

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and conquer if one time
only algorithm instead.

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

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

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That what was apparently
important then and no less so

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now is that in computer science
and in turn programming,

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precision is important and
correctness is important.

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And indeed, one of
the things you perhaps

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gleaned in programming, whether
in Scratch or C or Python

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or SQL over the past few weeks, is that
the computers are really unforgiving.

247
00:10:47,600 --> 00:10:49,310
They can't just infer like we humans do.

248
00:10:49,310 --> 00:10:52,580
And frankly, I'm not sure we would want
our computers to just infer sometimes

249
00:10:52,580 --> 00:10:56,120
what we mean if the goal is
precision and correctness.

250
00:10:56,120 --> 00:10:57,590
You need to be ever so precise.

251
00:10:57,590 --> 00:11:00,200
So let's see, perhaps, with
one more demonstration just how

252
00:11:00,200 --> 00:11:03,020
much this one has perhaps sunk in.

253
00:11:03,020 --> 00:11:07,670
If we can get perhaps two more
volunteers, a student and family member

254
00:11:07,670 --> 00:11:08,720
as well.

255
00:11:08,720 --> 00:11:09,860
Student and family member.

256
00:11:09,860 --> 00:11:11,960
Here and I don't see a
family member's hand up.

257
00:11:11,960 --> 00:11:13,140
Can you convince one?

258
00:11:13,140 --> 00:11:13,640
OK.

259
00:11:13,640 --> 00:11:14,660
Come on up.

260
00:11:14,660 --> 00:11:15,230
Wonderful.

261
00:11:15,230 --> 00:11:18,100
A round of applause for our
other pair of volunteers here.

262
00:11:18,100 --> 00:11:21,500
[APPLAUSE]

263
00:11:21,500 --> 00:11:24,377
So now for this one, not
everyone's off the hook.

264
00:11:24,377 --> 00:11:26,960
If all the students in the room,
if they have a piece of paper

265
00:11:26,960 --> 00:11:30,002
and/or a pen or pencil could take that
out and share with anyone next you

266
00:11:30,002 --> 00:11:31,700
if you don't have.

267
00:11:31,700 --> 00:11:34,407
Emma and Brian are also going
to pass out some paper and pens

268
00:11:34,407 --> 00:11:35,990
if you don't have a writing apparatus.

269
00:11:35,990 --> 00:11:37,740
The goal at hand is
actually for all of us

270
00:11:37,740 --> 00:11:39,710
to participate, thanks
to our two volunteers.

271
00:11:39,710 --> 00:11:40,160
And what's your name?

272
00:11:40,160 --> 00:11:40,700
DAVID: David.

273
00:11:40,700 --> 00:11:41,492
DAVID MALAN: David.

274
00:11:41,492 --> 00:11:42,220
Nice to see you.

275
00:11:42,220 --> 00:11:42,680
SPEAKER: [INAUDIBLE]

276
00:11:42,680 --> 00:11:44,210
DAVID MALAN: [INAUDIBLE]
Nice to see you, as well.

277
00:11:44,210 --> 00:11:45,600
Come on over in here.

278
00:11:45,600 --> 00:11:49,010
And this will be a two
part exercise as well.

279
00:11:49,010 --> 00:11:51,660
And the first goal at
hand is for you to choose.

280
00:11:51,660 --> 00:11:53,540
Do you want to go first or dad, is it?

281
00:11:53,540 --> 00:11:55,040
Would you like your dad to go first?

282
00:11:55,040 --> 00:11:55,540
DAVID: OK.

283
00:11:55,540 --> 00:11:56,240
I'll go first.

284
00:11:56,240 --> 00:11:57,060
DAVID MALAN: You're going to go first.

285
00:11:57,060 --> 00:11:57,560
All right.

286
00:11:57,560 --> 00:11:58,670
So come on over here.

287
00:11:58,670 --> 00:12:03,200
And what I'm going to show David in
just a moment is an image on the screen

288
00:12:03,200 --> 00:12:07,440
that I'm going to ask that
he program you, the audience,

289
00:12:07,440 --> 00:12:10,310
to draw giving verbal instructions only.

290
00:12:10,310 --> 00:12:14,030
The goal of which is for him
to be as precise as possible

291
00:12:14,030 --> 00:12:18,470
and as correct as possible to compel
the audience, much like a computer,

292
00:12:18,470 --> 00:12:20,570
to follow his instructions.

293
00:12:20,570 --> 00:12:23,000
And in turn, implement your algorithm.

294
00:12:23,000 --> 00:12:27,050
So in just a moment, David
is going to rattle off step

295
00:12:27,050 --> 00:12:31,268
by step instructions for having
everyone in the audience draw this.

296
00:12:31,268 --> 00:12:33,560
You are the only one that
can see what's on the screen.

297
00:12:33,560 --> 00:12:37,640
Everyone in the audience just about has
a piece of paper and a pen or pencil.

298
00:12:37,640 --> 00:12:41,540
And I just need you very
confidently, clearly,

299
00:12:41,540 --> 00:12:45,410
to recite step by step instructions by
which everyone with their pen or pencil

300
00:12:45,410 --> 00:12:48,260
can draw what you see
on the screen here.

301
00:12:48,260 --> 00:12:48,940
Makes sense?

302
00:12:48,940 --> 00:12:49,440
DAVID: Yes.

303
00:12:49,440 --> 00:12:53,174
DAVID MALAN: You may use any
verbal instructions you like.

304
00:12:53,174 --> 00:12:54,100
DAVID: OK.

305
00:12:54,100 --> 00:12:55,300
DAVID MALAN: All right.

306
00:12:55,300 --> 00:12:57,530
Just about ready to begin?

307
00:12:57,530 --> 00:12:59,400
The goal, precision correctness.

308
00:12:59,400 --> 00:12:59,900
Begin.

309
00:12:59,900 --> 00:13:00,400
DAVID: OK.

310
00:13:00,400 --> 00:13:01,450
Step 1.

311
00:13:01,450 --> 00:13:05,120
Draw a sideways square.

312
00:13:05,120 --> 00:13:05,620
[LAUGHTER]

313
00:13:05,620 --> 00:13:06,850
Sideways.

314
00:13:06,850 --> 00:13:08,430
45 degrees rotated.

315
00:13:08,430 --> 00:13:08,930
Yeah.

316
00:13:08,930 --> 00:13:11,990


317
00:13:11,990 --> 00:13:13,130
DAVID MALAN: OK.

318
00:13:13,130 --> 00:13:17,630
DAVID: From the bottom three
corners, draw a straight line down

319
00:13:17,630 --> 00:13:21,530
that's approximately the length
of each side of the square.

320
00:13:21,530 --> 00:13:25,850


321
00:13:25,850 --> 00:13:28,910
DAVID MALAN: Unfortunately, programs
cannot ask questions of programmer.

322
00:13:28,910 --> 00:13:31,280
So we continue on with the next step.

323
00:13:31,280 --> 00:13:35,750
DAVID: And finally, between
the first and second line,

324
00:13:35,750 --> 00:13:38,060
draw a line connecting
the two at the end.

325
00:13:38,060 --> 00:13:41,420
And between the second and third
line, draw a line connecting the end.

326
00:13:41,420 --> 00:13:43,902


327
00:13:43,902 --> 00:13:44,860
DAVID MALAN: All right.

328
00:13:44,860 --> 00:13:46,527
How do you feel about your instructions?

329
00:13:46,527 --> 00:13:47,550
Precise and correct?

330
00:13:47,550 --> 00:13:48,490
DAVID: Wishy-washy.

331
00:13:48,490 --> 00:13:49,170
DAVID MALAN: Wishy-washy.

332
00:13:49,170 --> 00:13:49,830
OK.

333
00:13:49,830 --> 00:13:50,130
All right.

334
00:13:50,130 --> 00:13:50,850
So let's go ahead.

335
00:13:50,850 --> 00:13:52,020
Hang on to the mic for just a moment.

336
00:13:52,020 --> 00:13:54,395
I'm going to hop down into
the audience with our TFs just

337
00:13:54,395 --> 00:13:56,040
to grab a few representative solutions.

338
00:13:56,040 --> 00:13:58,290
If you wouldn't mind my
grabbing a few sheets of paper

339
00:13:58,290 --> 00:14:00,060
from folks who have participated.

340
00:14:00,060 --> 00:14:02,940
Let me take a few over
here, just a random sample.

341
00:14:02,940 --> 00:14:06,180
Welcome to volunteer proactively
or keep it to yourself.

342
00:14:06,180 --> 00:14:07,600
Let me go in over here.

343
00:14:07,600 --> 00:14:08,100
OK.

344
00:14:08,100 --> 00:14:10,530
Thank you so much.

345
00:14:10,530 --> 00:14:11,750
Any takers over here?

346
00:14:11,750 --> 00:14:12,790
OK.

347
00:14:12,790 --> 00:14:13,290
All right.

348
00:14:13,290 --> 00:14:13,790
Good.

349
00:14:13,790 --> 00:14:14,640
Good.

350
00:14:14,640 --> 00:14:15,510
Good.

351
00:14:15,510 --> 00:14:16,027
Thank you.

352
00:14:16,027 --> 00:14:16,610
And all right.

353
00:14:16,610 --> 00:14:18,330
The TFs are grabbing a couple too.

354
00:14:18,330 --> 00:14:19,140
See some over here.

355
00:14:19,140 --> 00:14:19,860
All right.

356
00:14:19,860 --> 00:14:23,460
That should do.

357
00:14:23,460 --> 00:14:24,420
That should do.

358
00:14:24,420 --> 00:14:27,637
Let me grab these from
Brian over here and Emma.

359
00:14:27,637 --> 00:14:28,470
Oh, those are blank.

360
00:14:28,470 --> 00:14:28,970
All right.

361
00:14:28,970 --> 00:14:32,200
So I think we have plenty over here.

362
00:14:32,200 --> 00:14:35,910
So let's take a look before we
do part two of two, if I may,

363
00:14:35,910 --> 00:14:39,080
at how well David
programmed the audience.

364
00:14:39,080 --> 00:14:41,820
I'm going to go ahead in
just a moment and pull up

365
00:14:41,820 --> 00:14:45,960
a projection of some of
these drawings here that

366
00:14:45,960 --> 00:14:47,730
are the results of these operations.

367
00:14:47,730 --> 00:14:49,200
So let me flip through.

368
00:14:49,200 --> 00:14:51,150
Get a sample here.

369
00:14:51,150 --> 00:14:55,930
And I see a lot of good options here.

370
00:14:55,930 --> 00:15:01,830
I see this picture here, which
perhaps resembles what you drew.

371
00:15:01,830 --> 00:15:06,067
See this one here the top
corner, very similar in spirit.

372
00:15:06,067 --> 00:15:08,150
This one left a lot of
room for other things but--

373
00:15:08,150 --> 00:15:11,100
[LAUGHTER]

374
00:15:11,100 --> 00:15:13,950
This one was a little
more abstract, if I may.

375
00:15:13,950 --> 00:15:15,310
[LAUGHTER]

376
00:15:15,310 --> 00:15:18,990
And so, let me go over to a
spoiler to show you what it

377
00:15:18,990 --> 00:15:22,500
is David was programming you to draw.

378
00:15:22,500 --> 00:15:25,140
And with some suspense,
he was compelling you

379
00:15:25,140 --> 00:15:30,180
to draw we hope this here.

380
00:15:30,180 --> 00:15:30,680
All right.

381
00:15:30,680 --> 00:15:32,220
So close or not close perhaps.

382
00:15:32,220 --> 00:15:32,720
All right.

383
00:15:32,720 --> 00:15:34,130
Round of applause for
David, if we could.

384
00:15:34,130 --> 00:15:34,580
[APPLAUSE]

385
00:15:34,580 --> 00:15:35,000
DAVID MALAN: Thank you.

386
00:15:35,000 --> 00:15:35,720
One moment.

387
00:15:35,720 --> 00:15:37,550
And so, you gave pretty--

388
00:15:37,550 --> 00:15:40,020
you used, if you would,
abstractions in discussing this.

389
00:15:40,020 --> 00:15:43,040
You said a sideways square,
used 45 degree angles

390
00:15:43,040 --> 00:15:46,380
hoping that folks would presumably
know what you mean by that.

391
00:15:46,380 --> 00:15:48,505
Why did you not just say
draw a cube, for instance?

392
00:15:48,505 --> 00:15:50,172
DAVID: I thought that would be cheating.

393
00:15:50,172 --> 00:15:51,050
DAVID MALAN: OK.

394
00:15:51,050 --> 00:15:52,490
Well, it would not have
been cheating, but it would

395
00:15:52,490 --> 00:15:54,220
have been a wonderful abstraction.

396
00:15:54,220 --> 00:15:57,812
If everyone in the room,
assuming, knows what in a cube is,

397
00:15:57,812 --> 00:16:00,020
you might then be compelled
to draw it quite quickly.

398
00:16:00,020 --> 00:16:02,600
But that, too, leaves
ambiguity and lacks precision.

399
00:16:02,600 --> 00:16:03,955
Well, how is the cube oriented?

400
00:16:03,955 --> 00:16:04,580
Is it this way?

401
00:16:04,580 --> 00:16:05,205
Is it that way?

402
00:16:05,205 --> 00:16:06,260
Is a curved this way?

403
00:16:06,260 --> 00:16:08,802
So sometimes, these abstractions
aren't sufficiently helpful.

404
00:16:08,802 --> 00:16:11,520
So I probably would have
done what you did as well.

405
00:16:11,520 --> 00:16:14,600
Now let's do one other
example here, if we could.

406
00:16:14,600 --> 00:16:17,870
I'm going to go ahead in just a moment
and project an image onto the screen

407
00:16:17,870 --> 00:16:21,110
that everyone in the audience
can see except you two.

408
00:16:21,110 --> 00:16:23,790
Let me go ahead and re
angle this a little bit.

409
00:16:23,790 --> 00:16:28,490
And if father and son would like
to get together or solo draw

410
00:16:28,490 --> 00:16:31,688
a picture that the audience is
going to tell you how to draw.

411
00:16:31,688 --> 00:16:33,230
So we're going to flip the roles now.

412
00:16:33,230 --> 00:16:35,330
You all will see the
drawing on the screen.

413
00:16:35,330 --> 00:16:38,720
We ask that you tell our
volunteers what to draw.

414
00:16:38,720 --> 00:16:41,840
You can use any words that you want,
but you cannot ask questions and no

415
00:16:41,840 --> 00:16:44,360
physical gestures to explain.

416
00:16:44,360 --> 00:16:45,200
All right.

417
00:16:45,200 --> 00:16:47,480
Unfortunately, it's a little
hard technologically here

418
00:16:47,480 --> 00:16:48,860
in that the solution
is going to be there.

419
00:16:48,860 --> 00:16:50,420
The solution is going to be there.

420
00:16:50,420 --> 00:16:53,870
So we're going to have to put
some visors on you, if we could--

421
00:16:53,870 --> 00:16:54,560
[LAUGHTER]

422
00:16:54,560 --> 00:16:56,393
--so that you can only
see straight forward.

423
00:16:56,393 --> 00:16:59,437
And if you don't mind hugging
the board as close as possible,

424
00:16:59,437 --> 00:17:02,270
but occasionally back up so that
people can see what you're drawing,

425
00:17:02,270 --> 00:17:06,020
but resist the temptation to
look up, over left or right.

426
00:17:06,020 --> 00:17:06,890
All right.

427
00:17:06,890 --> 00:17:09,858
So for our audience then, the images.

428
00:17:09,858 --> 00:17:11,900
And if you two could focus
only on the board now.

429
00:17:11,900 --> 00:17:14,450
Only on the board and definitely
not facing that screen.

430
00:17:14,450 --> 00:17:14,950
OK.

431
00:17:14,950 --> 00:17:16,369
[LAUGHTER]

432
00:17:16,369 --> 00:17:20,619
The audience is about to
see the picture in question.

433
00:17:20,619 --> 00:17:23,200
And so, we need a volunteer
first from the audience

434
00:17:23,200 --> 00:17:25,329
to call out an instruction.

435
00:17:25,329 --> 00:17:26,859
Any step ones?

436
00:17:26,859 --> 00:17:28,810
Over here.

437
00:17:28,810 --> 00:17:29,580
Make a circle.

438
00:17:29,580 --> 00:17:32,898


439
00:17:32,898 --> 00:17:37,640
[LAUGHTER]

440
00:17:37,640 --> 00:17:40,280
I heard a small--

441
00:17:40,280 --> 00:17:42,770
I heard draw a smaller circle.

442
00:17:42,770 --> 00:17:45,540
I didn't hear use the eraser, but OK.

443
00:17:45,540 --> 00:17:46,080
OK.

444
00:17:46,080 --> 00:17:46,580
OK.

445
00:17:46,580 --> 00:17:47,450
No looking at me.

446
00:17:47,450 --> 00:17:48,260
No looking at me.

447
00:17:48,260 --> 00:17:48,760
All right.

448
00:17:48,760 --> 00:17:51,120
So I'll take a third step.

449
00:17:51,120 --> 00:17:52,920
So let's go to someone else.

450
00:17:52,920 --> 00:17:54,758
Yeah.

451
00:17:54,758 --> 00:17:56,050
AUDIENCE: Draw a vertical line.

452
00:17:56,050 --> 00:17:57,592
DAVID MALAN: Draw a vertical line.

453
00:17:57,592 --> 00:17:59,259
AUDIENCE: From the center of the circle.

454
00:17:59,259 --> 00:18:01,444
DAVID MALAN: From the
center of the circle.

455
00:18:01,444 --> 00:18:02,320
AUDIENCE: From the bottom
of the circle and down.

456
00:18:02,320 --> 00:18:04,715
DAVID MALAN: From the bottom
of the circle and down.

457
00:18:04,715 --> 00:18:07,048
AUDIENCE: Large stick figure
that appears to be walking.

458
00:18:07,048 --> 00:18:08,940
[LAUGHTER]

459
00:18:08,940 --> 00:18:11,500
DAVID MALAN: I hear an abstraction.

460
00:18:11,500 --> 00:18:13,780
So we were also given
a fourth instruction.

461
00:18:13,780 --> 00:18:17,818
Draw a stick figure that appears
to be walking, if that helps.

462
00:18:17,818 --> 00:18:20,110
But I think we're going to
need to be more precise here

463
00:18:20,110 --> 00:18:23,320
because I can imagine a stick figure
doing multiple things in multiple--

464
00:18:23,320 --> 00:18:29,730
[LAUGHTER]

465
00:18:29,730 --> 00:18:30,230
With--

466
00:18:30,230 --> 00:18:31,910
[LAUGHTER]

467
00:18:31,910 --> 00:18:32,660
OK.

468
00:18:32,660 --> 00:18:33,680
Maybe step five.

469
00:18:33,680 --> 00:18:36,413
Do we want to make any tweaks?

470
00:18:36,413 --> 00:18:38,330
AUDIENCE: It's an almost
upside down triangle.

471
00:18:38,330 --> 00:18:40,792
DAVID MALAN: Draw an almost
upside down triangle, I heard.

472
00:18:40,792 --> 00:18:41,750
AUDIENCE: For the legs.

473
00:18:41,750 --> 00:18:42,995
DAVID MALAN: Through the legs.

474
00:18:42,995 --> 00:18:43,310
AUDIENCE: For the legs.

475
00:18:43,310 --> 00:18:44,393
DAVID MALAN: For the legs.

476
00:18:44,393 --> 00:18:46,934
AUDIENCE: After the
vertical line coming down.

477
00:18:46,934 --> 00:18:48,533
AUDIENCE: You need to erase the legs.

478
00:18:48,533 --> 00:18:49,200
DAVID MALAN: OK.

479
00:18:49,200 --> 00:18:50,370
I heard erase the legs.

480
00:18:50,370 --> 00:18:51,620
AUDIENCE: Get rid of the arms.

481
00:18:51,620 --> 00:18:53,334
DAVID MALAN: And get rid of the arms.

482
00:18:53,334 --> 00:18:56,970
[LAUGHTER]

483
00:18:56,970 --> 00:18:59,810
OK and step five was?

484
00:18:59,810 --> 00:19:03,156
AUDIENCE: From the bottom of
the vertical line, the body,

485
00:19:03,156 --> 00:19:07,232
make the two legs are like a
triangle without the bottom.

486
00:19:07,232 --> 00:19:08,940
DAVID MALAN: From the
bottom of the body,

487
00:19:08,940 --> 00:19:12,032
draw like a triangle
to represent the legs.

488
00:19:12,032 --> 00:19:12,824
AUDIENCE: But not--

489
00:19:12,824 --> 00:19:14,640
[LAUGHTER]

490
00:19:14,640 --> 00:19:16,560
DAVID MALAN: But not that way.

491
00:19:16,560 --> 00:19:17,440
That's good.

492
00:19:17,440 --> 00:19:18,780
Let's move on to step six.

493
00:19:18,780 --> 00:19:20,100
Step six.

494
00:19:20,100 --> 00:19:22,208
Someone over here.

495
00:19:22,208 --> 00:19:24,688
AUDIENCE: So once they
erase that triangle,

496
00:19:24,688 --> 00:19:27,002
from the bottom of the
vertical line, draw

497
00:19:27,002 --> 00:19:30,392
an upside V, where the
center of the V is touching

498
00:19:30,392 --> 00:19:32,150
the bottom of that bottom line.

499
00:19:32,150 --> 00:19:37,857
DAVID MALAN: From the bottom of the
straight line, draw an upside down V.

500
00:19:37,857 --> 00:19:38,690
I think we did that.

501
00:19:38,690 --> 00:19:41,770
Can you step aside just
so the audience can see?

502
00:19:41,770 --> 00:19:42,960
Now we have a tripod.

503
00:19:42,960 --> 00:19:43,460
OK.

504
00:19:43,460 --> 00:19:45,150
[LAUGHTER]

505
00:19:45,150 --> 00:19:46,320
OK.

506
00:19:46,320 --> 00:19:46,820
OK.

507
00:19:46,820 --> 00:19:47,730
So I think we're there.

508
00:19:47,730 --> 00:19:48,200
I think we're there.

509
00:19:48,200 --> 00:19:49,380
One other instruction.

510
00:19:49,380 --> 00:19:51,170
Let's see if we can take this home.

511
00:19:51,170 --> 00:19:52,199
Yes, right here.

512
00:19:52,199 --> 00:19:55,951
AUDIENCE: Draw a less than symbol,
starting from the base of the middle

513
00:19:55,951 --> 00:19:57,360
of the circle.

514
00:19:57,360 --> 00:20:00,000
DAVID MALAN: Draw a less
than symbol from the base

515
00:20:00,000 --> 00:20:04,650
of the middle of the end of
the circle on the left side.

516
00:20:04,650 --> 00:20:09,047


517
00:20:09,047 --> 00:20:10,380
DAVID: Wait, a less than symbol.

518
00:20:10,380 --> 00:20:11,630
DAVID MALAN: Less than symbol.

519
00:20:11,630 --> 00:20:12,200
Yep.

520
00:20:12,200 --> 00:20:14,400
DAVID: Less than.

521
00:20:14,400 --> 00:20:15,480
DAVID MALAN: Yes.

522
00:20:15,480 --> 00:20:15,980
DAVID: Here?

523
00:20:15,980 --> 00:20:18,022
DAVID MALAN: So this is
what we call a condition.

524
00:20:18,022 --> 00:20:18,550
So, yes.

525
00:20:18,550 --> 00:20:19,300
AUDIENCE: Correct.

526
00:20:19,300 --> 00:20:20,180
DAVID MALAN: Correct, I hear.

527
00:20:20,180 --> 00:20:20,680
OK.

528
00:20:20,680 --> 00:20:21,550
I think we're close.

529
00:20:21,550 --> 00:20:23,530
Two more steps, maybe.

530
00:20:23,530 --> 00:20:26,890
AUDIENCE: To the top of the
circle, write the word Hi

531
00:20:26,890 --> 00:20:28,933
with a capital H and lowercase I.

532
00:20:28,933 --> 00:20:29,600
DAVID MALAN: OK.

533
00:20:29,600 --> 00:20:35,390
From the top left of the head, draw the
word hi, capital H lowercase I with--

534
00:20:35,390 --> 00:20:36,960
I think did you say a line to it?

535
00:20:36,960 --> 00:20:37,700
AUDIENCE: Yes, with a line.

536
00:20:37,700 --> 00:20:38,992
DAVID MALAN: With a line to it.

537
00:20:38,992 --> 00:20:39,744
Yeah.

538
00:20:39,744 --> 00:20:42,010
AUDIENCE: [INAUDIBLE]

539
00:20:42,010 --> 00:20:42,910
DAVID MALAN: OK.

540
00:20:42,910 --> 00:20:44,830
And can you step aside, just
so the audience can see?

541
00:20:44,830 --> 00:20:46,288
I think we need just one more step.

542
00:20:46,288 --> 00:20:47,270
One more step.

543
00:20:47,270 --> 00:20:47,770
OK.

544
00:20:47,770 --> 00:20:48,726
Back here.

545
00:20:48,726 --> 00:20:54,350
AUDIENCE: From the bottom of the
circle on the right hand side--

546
00:20:54,350 --> 00:20:57,142
DAVID MALAN: From the bottom of
the circle on the right hand side--

547
00:20:57,142 --> 00:21:00,930
AUDIENCE: --touching the base of the
circle and the vertical line going

548
00:21:00,930 --> 00:21:01,430
down--

549
00:21:01,430 --> 00:21:04,513
DAVID MALAN: --touching the base of
the circle and the vertical line going

550
00:21:04,513 --> 00:21:05,030
down--

551
00:21:05,030 --> 00:21:07,010
AUDIENCE: --make the letter L--

552
00:21:07,010 --> 00:21:09,179
DAVID MALAN: --make the letter L--

553
00:21:09,179 --> 00:21:11,840
AUDIENCE: --at a 15 degree angle.

554
00:21:11,840 --> 00:21:14,330
DAVID MALAN: I heard
at a 15 degree angle?

555
00:21:14,330 --> 00:21:14,830
Sure.

556
00:21:14,830 --> 00:21:16,620
[LAUGHTER]

557
00:21:16,620 --> 00:21:17,730
OK.

558
00:21:17,730 --> 00:21:22,457
I think-- I think could we have you
take several steps back and look up?

559
00:21:22,457 --> 00:21:24,165
And round of applause
for our volunteers.

560
00:21:24,165 --> 00:21:26,808
[APPLAUSE]

561
00:21:26,808 --> 00:21:27,600
That's pretty good.

562
00:21:27,600 --> 00:21:29,500
Thank you both so much.

563
00:21:29,500 --> 00:21:31,930
Please keep the hats.

564
00:21:31,930 --> 00:21:32,910
Congrats.

565
00:21:32,910 --> 00:21:36,940
So clearly, abstractions can be useful,
but they can also be challenging.

566
00:21:36,940 --> 00:21:38,220
Like programming is hard.

567
00:21:38,220 --> 00:21:41,670
And even though this went a bit off
the rails here and there verbally,

568
00:21:41,670 --> 00:21:43,050
this really is just programming.

569
00:21:43,050 --> 00:21:46,050
And we're all trying to agree on
a common language or common syntax

570
00:21:46,050 --> 00:21:48,570
to use to have the
computer, or in this case,

571
00:21:48,570 --> 00:21:51,420
our human volunteers
execute those instructions.

572
00:21:51,420 --> 00:21:53,390
And sometimes, these
abstractions are great.

573
00:21:53,390 --> 00:21:55,380
Draw a stick figure who
appears to be walking.

574
00:21:55,380 --> 00:21:57,840
I think we can all
visualize what that is.

575
00:21:57,840 --> 00:22:00,090
Unfortunately, we're
all probably visualizing

576
00:22:00,090 --> 00:22:02,210
slightly different stick figures.

577
00:22:02,210 --> 00:22:05,550
And that's, again, where precision
comes into play and correctness too.

578
00:22:05,550 --> 00:22:09,130
Of course, sometimes the chalk
went off in the wrong direction.

579
00:22:09,130 --> 00:22:10,320
So we might have to undo.

580
00:22:10,320 --> 00:22:14,500
And of course, we've spent also the
past eight weeks debugging code as well.

581
00:22:14,500 --> 00:22:17,850
And so, I wish I could say that
this gets easier or just gets

582
00:22:17,850 --> 00:22:19,140
absolutely easy.

583
00:22:19,140 --> 00:22:21,970
But it never does because the
problems you aspire to solve,

584
00:22:21,970 --> 00:22:24,720
whether it's in the real world or
in the world of computer science

585
00:22:24,720 --> 00:22:26,760
and programming, are
going to be ever changing.

586
00:22:26,760 --> 00:22:29,170
And your aspirations
are going to increase.

587
00:22:29,170 --> 00:22:31,740
And so, this frustration
you might feel now never

588
00:22:31,740 --> 00:22:34,680
really goes away, if I can
say as much some 20 plus years

589
00:22:34,680 --> 00:22:35,850
after doing this myself.

590
00:22:35,850 --> 00:22:39,000
But the problems you're solving
with those same challenges

591
00:22:39,000 --> 00:22:42,935
and those same hurdles get so much more
powerful, so much more interesting.

592
00:22:42,935 --> 00:22:45,060
And again, it all started
from just a few weeks ago

593
00:22:45,060 --> 00:22:48,870
when Mario's pyramid was perhaps
the first problem to solve.

594
00:22:48,870 --> 00:22:52,650
If we go back to where we were here,
we had of course our drawings, and then

595
00:22:52,650 --> 00:22:54,120
ultimately this process.

596
00:22:54,120 --> 00:22:55,703
This, I dare say, is computer science.

597
00:22:55,703 --> 00:22:56,370
You have inputs.

598
00:22:56,370 --> 00:22:58,170
You have outputs and
algorithms in between.

599
00:22:58,170 --> 00:23:00,670
But really, what are we talking
about at the end of the day?

600
00:23:00,670 --> 00:23:03,828
Well, the creation of, the output
of, the management of information.

601
00:23:03,828 --> 00:23:06,870
I mean, that really is what we've been
talking about for these past eight

602
00:23:06,870 --> 00:23:07,920
weeks is information.

603
00:23:07,920 --> 00:23:09,210
How do you process it?

604
00:23:09,210 --> 00:23:11,190
How do you represent it?

605
00:23:11,190 --> 00:23:13,980
How do you transform it into
something more and ultimately, solve

606
00:23:13,980 --> 00:23:15,190
problems with it?

607
00:23:15,190 --> 00:23:18,060
But, with this manipulation
of, this creation of,

608
00:23:18,060 --> 00:23:20,970
this storage of information
ever more so these days

609
00:23:20,970 --> 00:23:23,190
comes increasing responsibility.

610
00:23:23,190 --> 00:23:25,680
And I daresay one of the
things that we encourage

611
00:23:25,680 --> 00:23:28,860
you, as you exit a course like
this, is not just what you can do,

612
00:23:28,860 --> 00:23:30,840
but frankly whether you should do it.

613
00:23:30,840 --> 00:23:34,830
And indeed, all the more relevant
today in societies of course stories.

614
00:23:34,830 --> 00:23:37,770
Horrifying stories of
where our data has ended up

615
00:23:37,770 --> 00:23:40,260
or what has happened to our
data, or good intentions

616
00:23:40,260 --> 00:23:42,870
perhaps gone awry because we
haven't considered implications.

617
00:23:42,870 --> 00:23:45,180
And what we wanted to
do today is to point out

618
00:23:45,180 --> 00:23:48,450
a few examples of
opportunities to think harder

619
00:23:48,450 --> 00:23:51,990
about what it is you do in the
real world as just a citizen.

620
00:23:51,990 --> 00:23:55,770
And what you do in the
keyboard as a programmer,

621
00:23:55,770 --> 00:23:59,580
and how you might solve problems with
one's privacy and the security of one's

622
00:23:59,580 --> 00:24:00,720
data in mind.

623
00:24:00,720 --> 00:24:02,712
Consider, for instance, passwords.

624
00:24:02,712 --> 00:24:04,920
These, of course, are perhaps
the most familiar thing

625
00:24:04,920 --> 00:24:08,580
that you and I use probably
every day to secure our accounts,

626
00:24:08,580 --> 00:24:10,932
and in turn maintain the
privacy of our information.

627
00:24:10,932 --> 00:24:13,140
Whether it's our photographs,
or financial documents,

628
00:24:13,140 --> 00:24:16,620
or emails, or text
messages, or the like.

629
00:24:16,620 --> 00:24:19,470
Unfortunately, passwords really
aren't the best mechanism.

630
00:24:19,470 --> 00:24:22,770
And most of us in this room
probably should be more thoughtful

631
00:24:22,770 --> 00:24:25,290
when it comes to making these
kinds of decisions, as well.

632
00:24:25,290 --> 00:24:29,760
For instance, in 2019 already,
based on a very large data breach

633
00:24:29,760 --> 00:24:31,920
and in the analysis
thereof, it turns out

634
00:24:31,920 --> 00:24:37,790
that the top 10 passwords in the world
as of this year are number one, 123456.

635
00:24:37,790 --> 00:24:39,180
[LAUGHTER]

636
00:24:39,180 --> 00:24:41,580
Apparently, the result of
some website or applications

637
00:24:41,580 --> 00:24:44,042
requiring a password of
at least six characters.

638
00:24:44,042 --> 00:24:47,250
And this is about the least amount of
effort you can do to satisfy that goal.

639
00:24:47,250 --> 00:24:50,080
Number two password was 123456789.

640
00:24:50,080 --> 00:24:51,180
[LAUGHTER]

641
00:24:51,180 --> 00:24:52,090
Slightly better.

642
00:24:52,090 --> 00:24:54,630
Slightly more secure
insofar as it's longer.

643
00:24:54,630 --> 00:24:56,003
Number three is qwerty.

644
00:24:56,003 --> 00:24:58,170
And if you don't know what
that means, that actually

645
00:24:58,170 --> 00:24:59,550
describes the type of keyboard.

646
00:24:59,550 --> 00:25:03,270
But because if you go to the top left
hand corner of your keyboard and type

647
00:25:03,270 --> 00:25:05,910
one, two, three, four, five,
six from left to right,

648
00:25:05,910 --> 00:25:09,330
you will end up spelling Q-W-E-R-T-Y.

649
00:25:09,330 --> 00:25:10,470
Easy to remember.

650
00:25:10,470 --> 00:25:13,530
Also, pretty easy for other
people to remember as well.

651
00:25:13,530 --> 00:25:16,380
Password is the number
four password out there.

652
00:25:16,380 --> 00:25:20,670
And number five is, as
of this year, 6 ones.

653
00:25:20,670 --> 00:25:23,580
So another way of satisfying the
constraints that these people

654
00:25:23,580 --> 00:25:25,110
did, but with even less effort.

655
00:25:25,110 --> 00:25:28,140
Just hitting the same key
again, and again, and again.

656
00:25:28,140 --> 00:25:29,700
12345678.

657
00:25:29,700 --> 00:25:31,890
So, slightly behind the other two.

658
00:25:31,890 --> 00:25:32,603
ABC 123.

659
00:25:32,603 --> 00:25:34,770
Starting to get a little
more interesting that we're

660
00:25:34,770 --> 00:25:36,510
combining letters and numbers.

661
00:25:36,510 --> 00:25:39,000
1234567 slips in there as well.

662
00:25:39,000 --> 00:25:42,780
Number nine is password1,
clearly satisfying websites

663
00:25:42,780 --> 00:25:44,700
that require that you
not use an English word

664
00:25:44,700 --> 00:25:47,360
but you include at least one
symbol or letter here too.

665
00:25:47,360 --> 00:25:48,630
Dare say the bare minimum.

666
00:25:48,630 --> 00:25:52,020
And number 10 as of this year is 12345.

667
00:25:52,020 --> 00:25:56,040
So it's easy to poke fun at these and
we won't call for a show of hands.

668
00:25:56,040 --> 00:25:59,550
But odds are some of
us in this room maybe--

669
00:25:59,550 --> 00:26:00,720
don't make eye contact--

670
00:26:00,720 --> 00:26:04,830
have at least one of these
passwords as your own.

671
00:26:04,830 --> 00:26:08,100
It's just all too easy, then,
to guess certainly these.

672
00:26:08,100 --> 00:26:13,410
But even if you're practicing adhering
to better practices than these,

673
00:26:13,410 --> 00:26:15,446
and you're not as
simplistic as I'm going

674
00:26:15,446 --> 00:26:17,620
to pick an easy word or
a very simple number.

675
00:26:17,620 --> 00:26:19,540
Suppose you're actually
being more thoughtful.

676
00:26:19,540 --> 00:26:21,820
It doesn't really matter these days.

677
00:26:21,820 --> 00:26:25,600
I'm going to go ahead and turn my
attention to just a moment to a text

678
00:26:25,600 --> 00:26:27,430
editing program here on my computer.

679
00:26:27,430 --> 00:26:30,542
The students in the room will
recognize this is just representative

680
00:26:30,542 --> 00:26:31,750
of a programming environment.

681
00:26:31,750 --> 00:26:33,100
And today, this just
happens to be something

682
00:26:33,100 --> 00:26:35,440
called Visual Studio
Code, or VS code that you

683
00:26:35,440 --> 00:26:37,200
can download on your own Mac or PC.

684
00:26:37,200 --> 00:26:41,140
And I'm going to go ahead and
just save a file called pin.py.

685
00:26:41,140 --> 00:26:43,283
Many of us have not only
passwords, but pins.

686
00:26:43,283 --> 00:26:46,450
Personal identification numbers that
you're supposed to keep secret and that

687
00:26:46,450 --> 00:26:48,460
are usually, say, six digits long.

688
00:26:48,460 --> 00:26:52,330
Well, even if you've protected your
financial account or some other account

689
00:26:52,330 --> 00:26:55,330
with a six digit code, it might
take a human quite a while

690
00:26:55,330 --> 00:26:58,570
to guess all of the possible
codes that you might have chosen.

691
00:26:58,570 --> 00:27:00,740
But it's not terribly
hard for a computer.

692
00:27:00,740 --> 00:27:02,740
In fact, we'll do it even
more simply like a lot

693
00:27:02,740 --> 00:27:05,080
of times with a four digit code.

694
00:27:05,080 --> 00:27:07,630
I can simply go in a
program like this, if I

695
00:27:07,630 --> 00:27:09,220
know a bit of programming in Python.

696
00:27:09,220 --> 00:27:12,640
And I can say from time import sleep.

697
00:27:12,640 --> 00:27:15,190
I can say something like for I in range.

698
00:27:15,190 --> 00:27:21,070
Well, let's start from 0000
and go all the way up to 9,999.

699
00:27:21,070 --> 00:27:22,210
A four digit number.

700
00:27:22,210 --> 00:27:25,328
But it turns out, we need
to iterate up to 10,000

701
00:27:25,328 --> 00:27:28,120
because the students in the room
will recall that we iterate up to,

702
00:27:28,120 --> 00:27:29,680
but not through that value.

703
00:27:29,680 --> 00:27:32,055
And then in here, I'm going
to go ahead and say something

704
00:27:32,055 --> 00:27:35,980
like, I am checking
this value I. So this

705
00:27:35,980 --> 00:27:39,310
is to say I'm proposing to
write a program in Python that

706
00:27:39,310 --> 00:27:43,990
will try to crack someone's pin
to show how easy it is to generate

707
00:27:43,990 --> 00:27:49,210
as a computer all of the possible PINs
in the world that are four digits long.

708
00:27:49,210 --> 00:27:50,960
So and just to demonstrate
this, I'm going

709
00:27:50,960 --> 00:27:55,090
to do it a little slowly
by sleeping for 1/10

710
00:27:55,090 --> 00:27:57,200
of a second between each iteration.

711
00:27:57,200 --> 00:27:58,990
So I've gone ahead and saved my file.

712
00:27:58,990 --> 00:28:02,890
I'm going to go ahead down here
now and run Python of pin.py.

713
00:28:02,890 --> 00:28:07,807
And I'll go ahead and make my screen a
little taller here so we can see more.

714
00:28:07,807 --> 00:28:09,140
AUDIENCE: Close the parentheses.

715
00:28:09,140 --> 00:28:11,130
DAVID MALAN: Thank you.

716
00:28:11,130 --> 00:28:12,080
Thank you.

717
00:28:12,080 --> 00:28:12,740
Thank you.

718
00:28:12,740 --> 00:28:13,450
New here.

719
00:28:13,450 --> 00:28:14,420
OK.

720
00:28:14,420 --> 00:28:15,530
So here we go.

721
00:28:15,530 --> 00:28:19,880
We've now written a program that,
granted is not cracking anything,

722
00:28:19,880 --> 00:28:24,260
but it is demonstrating how easily and
how quickly we can generate numbers

723
00:28:24,260 --> 00:28:26,630
from 0 on up to 9,999.

724
00:28:26,630 --> 00:28:27,710
It's not quite right.

725
00:28:27,710 --> 00:28:30,680
These aren't four digit codes because
they're being treated as numbers.

726
00:28:30,680 --> 00:28:32,840
But there's actually syntax
with which we can solve this.

727
00:28:32,840 --> 00:28:34,548
And if you've never
seen this before, you

728
00:28:34,548 --> 00:28:37,010
can actually just say
something like this colon 0 4.

729
00:28:37,010 --> 00:28:39,750
And that's just going to reform
my output as being four digits.

730
00:28:39,750 --> 00:28:41,630
And it's going to pat
it with zeros instead.

731
00:28:41,630 --> 00:28:42,200
So boom.

732
00:28:42,200 --> 00:28:45,920
I've written a program that generates
all possible four digit codes.

733
00:28:45,920 --> 00:28:49,250
Of course, I've been deliberately
sleeping, that is pausing,

734
00:28:49,250 --> 00:28:51,400
each time I'm printing something out.

735
00:28:51,400 --> 00:28:52,940
A hacker is not going to do that.

736
00:28:52,940 --> 00:28:55,310
They are going to try to do
this as quickly as possible.

737
00:28:55,310 --> 00:28:57,300
No need for sleep whatsoever.

738
00:28:57,300 --> 00:29:00,470
So let me just rerun this code,
getting rid of those lines.

739
00:29:00,470 --> 00:29:01,460
Boom.

740
00:29:01,460 --> 00:29:04,190
That's every possible
four digit code that you

741
00:29:04,190 --> 00:29:07,220
might have on your bank account or
on some email account or the like,

742
00:29:07,220 --> 00:29:10,903
assuming the system allows you to have
a relatively short number like that.

743
00:29:10,903 --> 00:29:11,570
You might think.

744
00:29:11,570 --> 00:29:12,070
OK.

745
00:29:12,070 --> 00:29:13,850
Well, what's better than four digits?

746
00:29:13,850 --> 00:29:14,700
Five digits.

747
00:29:14,700 --> 00:29:15,800
Let's just increase it.

748
00:29:15,800 --> 00:29:19,100
But I think that adversary is going
to be able to crack that pin as well.

749
00:29:19,100 --> 00:29:22,500
I'm not even over at the screen
yet and it's done already.

750
00:29:22,500 --> 00:29:24,500
It is not hard once you
know how to program, not

751
00:29:24,500 --> 00:29:28,577
only to use these systems for good, but
in this case, for some form of evil.

752
00:29:28,577 --> 00:29:31,160
Now maybe you're off the hook
because you're not as simplistic

753
00:29:31,160 --> 00:29:35,240
as I claim as to use only,
for instance, an alphabetical

754
00:29:35,240 --> 00:29:36,560
or rather a numeric code.

755
00:29:36,560 --> 00:29:40,910
Maybe you're using a word and
not a word as simple as password.

756
00:29:40,910 --> 00:29:44,120
It's a more arcane word from English
or perhaps some other language.

757
00:29:44,120 --> 00:29:46,310
That, too, doesn't really matter.

758
00:29:46,310 --> 00:29:51,170
Let me go ahead and copy from our source
directory today, which for the students

759
00:29:51,170 --> 00:29:55,520
is available online, that large
dictionary that we had from problem

760
00:29:55,520 --> 00:29:58,610
sets five wherein you
loaded all of these words

761
00:29:58,610 --> 00:29:59,870
into a dictionary of your own.

762
00:29:59,870 --> 00:30:02,390
There was 140,000 some odd words here.

763
00:30:02,390 --> 00:30:04,640
I'm going to go ahead and
create another file, though.

764
00:30:04,640 --> 00:30:05,807
This one called password.py.

765
00:30:05,807 --> 00:30:09,740
And recall from some of our building
blocks over the past few weeks

766
00:30:09,740 --> 00:30:14,828
how easy it is to similarly
manipulate words from a dictionary.

767
00:30:14,828 --> 00:30:16,870
Let me go ahead and again,
initially import sleep

768
00:30:16,870 --> 00:30:18,500
just so that we can see this happening.

769
00:30:18,500 --> 00:30:20,450
I'm then going to say with open.

770
00:30:20,450 --> 00:30:22,610
Large in read mode.

771
00:30:22,610 --> 00:30:24,470
And I'm going to call this as file.

772
00:30:24,470 --> 00:30:28,640
And over here I'm going to go
ahead and say now for word in.

773
00:30:28,640 --> 00:30:29,612
I want to say file.

774
00:30:29,612 --> 00:30:31,820
And we've not perhaps seen
this one, but it turns out

775
00:30:31,820 --> 00:30:35,600
there's a really easy Python function
where you can say file dot read lines.

776
00:30:35,600 --> 00:30:39,230
And it's just going to slurp in all
of the 140,000 words from the file

777
00:30:39,230 --> 00:30:41,670
and allow you to iterate
over them one at a time.

778
00:30:41,670 --> 00:30:45,330
And if I want to go ahead and print this
out, I'll use print again, as before.

779
00:30:45,330 --> 00:30:47,330
And I'm going to go and
claim that I'm checking,

780
00:30:47,330 --> 00:30:52,218
for instance, that word dot, dot, dot
just as I claim to be checking a pin.

781
00:30:52,218 --> 00:30:53,760
Now again, I'm not checking anything.

782
00:30:53,760 --> 00:30:56,750
I'm just demonstrating that I could
be checking this fast as by simply

783
00:30:56,750 --> 00:30:58,283
printing out these words instead.

784
00:30:58,283 --> 00:31:00,200
And I'm going to go ahead
and as before, sleep

785
00:31:00,200 --> 00:31:02,420
for a tenth of a second
on each iteration.

786
00:31:02,420 --> 00:31:06,290
But I'm also in anticipation
going to call this function, which

787
00:31:06,290 --> 00:31:08,240
some students will
recall is to strip off

788
00:31:08,240 --> 00:31:10,050
any whitespace at the end of the line.

789
00:31:10,050 --> 00:31:13,130
Recall that in large our big
dictionary, there was a new line

790
00:31:13,130 --> 00:31:15,350
character at the end of every word.

791
00:31:15,350 --> 00:31:17,630
So this line of code will
we'll get rid of this here.

792
00:31:17,630 --> 00:31:18,130
Yeah.

793
00:31:18,130 --> 00:31:19,165
Question over here.

794
00:31:19,165 --> 00:31:21,440
AUDIENCE: What's the
significance of different colors?

795
00:31:21,440 --> 00:31:23,000
DAVID MALAN: What's the significance
of the different colors?

796
00:31:23,000 --> 00:31:25,150
This is happening
automatically in my text editor

797
00:31:25,150 --> 00:31:27,040
the program I'm using to write code.

798
00:31:27,040 --> 00:31:31,060
Each type of word or
blocks of words that I type

799
00:31:31,060 --> 00:31:33,462
have a different semantic
meaning to the computer.

800
00:31:33,462 --> 00:31:35,170
And so, the computer
is highlighting them

801
00:31:35,170 --> 00:31:37,975
in that way to draw my attention
to different types of words here.

802
00:31:37,975 --> 00:31:40,600
For instance, we have this
preposition from that does something

803
00:31:40,600 --> 00:31:43,120
like load someone else's code.

804
00:31:43,120 --> 00:31:47,230
Time, though, and sleep are names of
things that other humans have actually

805
00:31:47,230 --> 00:31:48,160
invented.

806
00:31:48,160 --> 00:31:52,000
So I'm going to go ahead now and
run this as Python of password.py.

807
00:31:52,000 --> 00:31:54,255
We'll see, as the
students will recall, all

808
00:31:54,255 --> 00:31:57,130
of the words in that dictionary
starting with the A words, then the B

809
00:31:57,130 --> 00:31:59,920
words, then the C words, and so
forth all the way down through F.

810
00:31:59,920 --> 00:32:01,960
Now, of course, I'm
sleeping unnecessarily.

811
00:32:01,960 --> 00:32:06,520
But if during this demonstration you
see your own password fly by the screen,

812
00:32:06,520 --> 00:32:10,570
it's not that hard for an adversary
to be writing code like this

813
00:32:10,570 --> 00:32:12,700
and not just printing out
the possible passwords,

814
00:32:12,700 --> 00:32:15,040
but trying to log into your
account again and again

815
00:32:15,040 --> 00:32:18,100
and again until they gain
access to the system.

816
00:32:18,100 --> 00:32:21,700
And in fact, if we deliberately speed
this up, let's not bother sleeping.

817
00:32:21,700 --> 00:32:24,130
We'll get rid of all of the
sleep related lines as before.

818
00:32:24,130 --> 00:32:25,420
Now, run this code.

819
00:32:25,420 --> 00:32:31,083
Now, I'm not even over at the board
and it's already done 140,000 words.

820
00:32:31,083 --> 00:32:33,250
Which is to say if you're
choosing a password that's

821
00:32:33,250 --> 00:32:36,400
just a word in the dictionary, whether
it's English or some other language,

822
00:32:36,400 --> 00:32:37,400
you're not off the hook.

823
00:32:37,400 --> 00:32:40,120
You're not being so clever
because anyone with a computer

824
00:32:40,120 --> 00:32:41,500
can check all of those codes.

825
00:32:41,500 --> 00:32:45,130
Now you might think, well,
this is why someone had abc123.

826
00:32:45,130 --> 00:32:46,420
That's not in the dictionary.

827
00:32:46,420 --> 00:32:47,500
That's not just numbers.

828
00:32:47,500 --> 00:32:48,748
That's not just letters.

829
00:32:48,748 --> 00:32:49,290
But, come on.

830
00:32:49,290 --> 00:32:52,373
If you give me a few more minutes, I
bet we could write a program together

831
00:32:52,373 --> 00:32:55,300
that starts with letters and
ends with numbers or vise versa.

832
00:32:55,300 --> 00:32:57,212
We could try to concoct these patterns.

833
00:32:57,212 --> 00:32:59,170
And granted, it's going
to get a little slower.

834
00:32:59,170 --> 00:33:01,270
A little slower, the
more complicated it gets.

835
00:33:01,270 --> 00:33:01,930
But it can.

836
00:33:01,930 --> 00:33:05,710
The adversary, if the goal is to
get your money, get your data,

837
00:33:05,710 --> 00:33:09,130
might have all the time in the
world to actually wage that attack.

838
00:33:09,130 --> 00:33:12,850
And so, I would consider moving forward
exactly what the implications are

839
00:33:12,850 --> 00:33:15,130
of putting your data
in one place or another

840
00:33:15,130 --> 00:33:18,820
and not thinking to actually
secure it because in the best case,

841
00:33:18,820 --> 00:33:21,220
someone nosily might end up getting in.

842
00:33:21,220 --> 00:33:24,340
At worst case, more than that,
whether it's financial or personal,

843
00:33:24,340 --> 00:33:26,350
might be compromised.

844
00:33:26,350 --> 00:33:28,900
Well, what other forms
does information come

845
00:33:28,900 --> 00:33:33,490
that might hint at potential
threats for us too?

846
00:33:33,490 --> 00:33:36,502
Well you've all probably heard of
cookies, when it comes to the web.

847
00:33:36,502 --> 00:33:39,460
And indeed, if you pursue our web
track in a couple of weeks' time will

848
00:33:39,460 --> 00:33:41,590
you dive in deeper to
what these things can do.

849
00:33:41,590 --> 00:33:43,570
But cookies are little
pieces of information

850
00:33:43,570 --> 00:33:47,350
that websites plant on your
Mac or PC or your phone anytime

851
00:33:47,350 --> 00:33:48,453
you visit the website.

852
00:33:48,453 --> 00:33:50,620
Now, that might seem a
little creepy at first glance

853
00:33:50,620 --> 00:33:52,390
and it can be used for creepy purposes.

854
00:33:52,390 --> 00:33:55,780
Advertising and other such
tracking applications among them.

855
00:33:55,780 --> 00:33:59,200
But this is actually a very useful
primitive and computer science and web

856
00:33:59,200 --> 00:34:01,210
programming, in that
if you have the ability

857
00:34:01,210 --> 00:34:03,940
to plant a little file
on someone's computer,

858
00:34:03,940 --> 00:34:06,673
it's like the digital
version of a hand stamp.

859
00:34:06,673 --> 00:34:08,590
You can remember that
you've seen them before.

860
00:34:08,590 --> 00:34:11,007
And that's actually useful
because when you log into Gmail

861
00:34:11,007 --> 00:34:13,659
or whatever email account
you have, you don't really

862
00:34:13,659 --> 00:34:17,440
want to be typing in your
username and password every time

863
00:34:17,440 --> 00:34:19,120
you look at a new message.

864
00:34:19,120 --> 00:34:21,159
Gmail or Google would
be a little obnoxious

865
00:34:21,159 --> 00:34:22,780
if they kept asking you
wait a minute, who are you?

866
00:34:22,780 --> 00:34:23,905
Wait a minute, who are you?

867
00:34:23,905 --> 00:34:26,920
These cookies are stored on your
computer so that, unbeknownst to you,

868
00:34:26,920 --> 00:34:30,460
it's sent from you, the
browser, to them, the server,

869
00:34:30,460 --> 00:34:34,201
to just remind them perpetually who you
are because you've already logged in.

870
00:34:34,201 --> 00:34:35,409
And we can actually see this.

871
00:34:35,409 --> 00:34:38,440
Let me actually go over to
my browser and you, too,

872
00:34:38,440 --> 00:34:41,350
might be in the habit of using
Chrome just as I am here.

873
00:34:41,350 --> 00:34:47,050
And I can go in Chrome go to View,
Developer, and go to Developer Tools.

874
00:34:47,050 --> 00:34:50,770
And you can do this, too, at
home on your own Mac or PC.

875
00:34:50,770 --> 00:34:54,350
You can do this with Firefox, or
Edge, or other browsers as well.

876
00:34:54,350 --> 00:34:57,048
And you'll see among the various
things that just popped up

877
00:34:57,048 --> 00:34:58,340
are not terribly user friendly.

878
00:34:58,340 --> 00:35:01,210
This is really the domain of
indeed developers or programmers.

879
00:35:01,210 --> 00:35:04,870
But I've clicked on this network tab
here because on my own Mac or PC,

880
00:35:04,870 --> 00:35:09,400
if I visit some url like
www.google.com and hit Enter,

881
00:35:09,400 --> 00:35:14,440
I can actually see inside of Chrome all
of the requests my computer has just

882
00:35:14,440 --> 00:35:17,007
made over the internet
from me to Google.com.

883
00:35:17,007 --> 00:35:19,840
And I'm going to go ahead and click
on the very first of these here.

884
00:35:19,840 --> 00:35:22,632
And this, too, is going to look
pretty arcane for today's purposes,

885
00:35:22,632 --> 00:35:26,050
but it's only meant to reveal
what it is that's going on here.

886
00:35:26,050 --> 00:35:28,480
Let me scroll down,
down, down, down, down.

887
00:35:28,480 --> 00:35:31,072
And you'll see a few things
here request headers.

888
00:35:31,072 --> 00:35:34,030
These are little pieces of information
that my Mac, and in term Chrome,

889
00:35:34,030 --> 00:35:37,540
just sent to Google.com com simply
because I visited the website.

890
00:35:37,540 --> 00:35:40,400
And then if I scroll down here,
we'll see response headers.

891
00:35:40,400 --> 00:35:44,680
This is what's coming back from
Google.com to my own Mac or PC.

892
00:35:44,680 --> 00:35:47,180
And this is the line that's interesting.

893
00:35:47,180 --> 00:35:49,900
If you've ever heard
about a cookie, all it

894
00:35:49,900 --> 00:35:53,380
means when I say a server is
planting some information or a file

895
00:35:53,380 --> 00:35:55,720
on your computer, it's
really just doing this.

896
00:35:55,720 --> 00:35:57,850
The response you're
getting from Google.com

897
00:35:57,850 --> 00:36:02,883
simply says literally in English set
dash cookie colon, and then some value.

898
00:36:02,883 --> 00:36:05,050
And to be fair, that value
is pretty arcane looking.

899
00:36:05,050 --> 00:36:06,305
It looks a little weird.

900
00:36:06,305 --> 00:36:08,180
It apparently is going
to expire in December.

901
00:36:08,180 --> 00:36:10,030
So I can infer from
this that Google wants

902
00:36:10,030 --> 00:36:12,550
to be able to remember
me at least for a couple

903
00:36:12,550 --> 00:36:14,380
more months to the end of the year.

904
00:36:14,380 --> 00:36:18,730
And this value here collectively
would seem to be like a hand stamp

905
00:36:18,730 --> 00:36:21,590
they've put on my computer so that
if I now proceed to do searches,

906
00:36:21,590 --> 00:36:25,220
if I proceed to log into Gmail,
use Google Calendar, or the like,

907
00:36:25,220 --> 00:36:29,690
they know who I am or they know
that it's me again and again.

908
00:36:29,690 --> 00:36:30,750
So what does this mean?

909
00:36:30,750 --> 00:36:33,290
This means that any website
you visit can certainly

910
00:36:33,290 --> 00:36:36,140
be tracking what it is you're
doing and where it is you're going.

911
00:36:36,140 --> 00:36:39,980
And because big companies like
Google have advertising networks

912
00:36:39,980 --> 00:36:44,090
as part of their portfolio, if
a website that's not Google.com

913
00:36:44,090 --> 00:36:47,600
is something else dot com, or
something even else dot com,

914
00:36:47,600 --> 00:36:50,670
and they are using Google's
advertising, well long story short,

915
00:36:50,670 --> 00:36:53,240
these cookies unfortunately are sent.

916
00:36:53,240 --> 00:36:58,160
These digital hand stamps are presented,
not just to those individual websites,

917
00:36:58,160 --> 00:37:01,477
but also to Google again, and
again, and again because they

918
00:37:01,477 --> 00:37:02,810
are the middleman in this story.

919
00:37:02,810 --> 00:37:07,040
The advertising network that's using
ads on all of these different websites.

920
00:37:07,040 --> 00:37:12,740
So here a very fundamentally useful and
compelling computer science principle

921
00:37:12,740 --> 00:37:15,440
can be used certainly for
wonderfully useful applications,

922
00:37:15,440 --> 00:37:17,090
just remembering that I'm logged in.

923
00:37:17,090 --> 00:37:18,963
But also, if you don't
think twice about it

924
00:37:18,963 --> 00:37:22,130
or if you want to make money off of it,
can be used for these other purposes

925
00:37:22,130 --> 00:37:22,830
as well.

926
00:37:22,830 --> 00:37:24,622
Now we won't go down
this rabbit hole today

927
00:37:24,622 --> 00:37:26,780
of well this is largely
advertising what has helped

928
00:37:26,780 --> 00:37:28,280
make the internet become what it is.

929
00:37:28,280 --> 00:37:31,668
So there are trade certainly here,
but that too has been thematic.

930
00:37:31,668 --> 00:37:33,710
But again, I would encourage
you, as you discover

931
00:37:33,710 --> 00:37:37,340
more and more of these principles
these things you can do through code,

932
00:37:37,340 --> 00:37:41,930
that you consider whether indeed
you should be doing so as well.

933
00:37:41,930 --> 00:37:43,978
Well, what else might
you do to mitigate this?

934
00:37:43,978 --> 00:37:46,270
Well some of you might be in
the habit of using at work

935
00:37:46,270 --> 00:37:49,250
or at home incognito mode or private
mode, which most browsers today

936
00:37:49,250 --> 00:37:49,970
support.

937
00:37:49,970 --> 00:37:51,620
Well what is that actually doing?

938
00:37:51,620 --> 00:37:53,328
Well, according to
Google, it says Chrome

939
00:37:53,328 --> 00:37:56,162
won't save the following information,
your browsing history, cookies

940
00:37:56,162 --> 00:37:58,890
and site data, information
entered in forms, and so forth.

941
00:37:58,890 --> 00:38:00,320
Well, that means literally that.

942
00:38:00,320 --> 00:38:03,080
Even if Google or Facebook
or some other website

943
00:38:03,080 --> 00:38:06,230
sends you a set cookie
value saying please

944
00:38:06,230 --> 00:38:09,930
store this, Chrome, in this case,
it's just going to throw it away.

945
00:38:09,930 --> 00:38:11,990
It's going to empty the
cookie jar, so to speak,

946
00:38:11,990 --> 00:38:14,610
once you close that incognito window.

947
00:38:14,610 --> 00:38:17,900
However, your browser is still
sending to Google or Facebook

948
00:38:17,900 --> 00:38:21,380
or whatever website you're
visiting those values again,

949
00:38:21,380 --> 00:38:22,610
and again, and again.

950
00:38:22,610 --> 00:38:25,670
You're not private within
that window, per se.

951
00:38:25,670 --> 00:38:28,250
You're only private within
that window with respect

952
00:38:28,250 --> 00:38:30,110
to everything else you have opened.

953
00:38:30,110 --> 00:38:34,070
So for instance, if I go now here and
go to View, Developer, and Developer

954
00:38:34,070 --> 00:38:34,990
Tools.

955
00:38:34,990 --> 00:38:37,190
And I have my network
tab open again and I

956
00:38:37,190 --> 00:38:42,620
visit something like
https://www.google.com and hit Enter,

957
00:38:42,620 --> 00:38:45,740
you'll see that Google is
indeed still planting a cookie.

958
00:38:45,740 --> 00:38:48,650
This time, I got two cookies
from Google on my computer.

959
00:38:48,650 --> 00:38:52,910
And those cookies will exist for as long
as I have this incognito window open.

960
00:38:52,910 --> 00:38:54,980
So Google might be
able to infer who I am

961
00:38:54,980 --> 00:38:57,823
or that I am the same person
based on these cookies.

962
00:38:57,823 --> 00:38:59,240
But they know even more than that.

963
00:38:59,240 --> 00:39:02,180
If I scroll back down to where
we began to these requests

964
00:39:02,180 --> 00:39:05,810
headers, the information my
browser is sending to the server,

965
00:39:05,810 --> 00:39:09,020
you'll notice that among these
values is something like this.

966
00:39:09,020 --> 00:39:10,640
This one, too, looks a little arcane.

967
00:39:10,640 --> 00:39:12,440
Let me go ahead and zoom in on it here.

968
00:39:12,440 --> 00:39:13,698
But it's called user agent.

969
00:39:13,698 --> 00:39:14,990
It's formatted in the same way.

970
00:39:14,990 --> 00:39:16,950
User dash agent colon.

971
00:39:16,950 --> 00:39:18,500
And then an arcane string here.

972
00:39:18,500 --> 00:39:22,070
But it looks like for
whatever reason, my computer

973
00:39:22,070 --> 00:39:26,620
is telling Google without my even
asking it to that I'm using a Mac,

974
00:39:26,620 --> 00:39:28,085
that I'm running Mac OS 10.14.

975
00:39:28,085 --> 00:39:29,243
6 at the moment.

976
00:39:29,243 --> 00:39:31,160
And if I keep scrolling,
it's going to tell it

977
00:39:31,160 --> 00:39:33,920
further that this is a specific
version of Google Chrome.

978
00:39:33,920 --> 00:39:37,670
And this is just some of the information
that is leaked deliberately and often

979
00:39:37,670 --> 00:39:38,930
for good purposes.

980
00:39:38,930 --> 00:39:42,260
But these are the kinds of traces we
all leave when we're using the internet.

981
00:39:42,260 --> 00:39:45,260
And we won't even go down this rabbit
hole too, but all of our computers

982
00:39:45,260 --> 00:39:46,220
have unique addresses.

983
00:39:46,220 --> 00:39:48,350
You might have heard of
things called IP addresses.

984
00:39:48,350 --> 00:39:51,230
You can't just get rid of
those much like you can't just

985
00:39:51,230 --> 00:39:54,590
remove your postal address and
expect mail to still arrival.

986
00:39:54,590 --> 00:39:58,340
All of us have unique addresses that are
still being presented to these servers.

987
00:39:58,340 --> 00:40:01,640
And so, through computer science,
through courses like this,

988
00:40:01,640 --> 00:40:04,850
and through reading up on these
kinds of topics hereinafter realize

989
00:40:04,850 --> 00:40:09,230
that in understanding these
primitives, these things like cookies

990
00:40:09,230 --> 00:40:11,390
and how they are set
can you at least then,

991
00:40:11,390 --> 00:40:14,420
we hope, make a more informed
decision as to whether to use them

992
00:40:14,420 --> 00:40:17,780
in that way or not or to
visit websites in some way

993
00:40:17,780 --> 00:40:22,490
that you know to be using
them for some purpose.

994
00:40:22,490 --> 00:40:26,390
Well what do we have besides this
to perhaps worry about in the world?

995
00:40:26,390 --> 00:40:29,220
I daresay Snapchat is
pretty popular these days.

996
00:40:29,220 --> 00:40:31,220
And of course, with
Snapchat there's this notion

997
00:40:31,220 --> 00:40:35,000
that they popularized it being able
to delete photos after 1 second or 10

998
00:40:35,000 --> 00:40:36,060
seconds or the like.

999
00:40:36,060 --> 00:40:39,350
And in fact, you all may
recall from problems set four.

1000
00:40:39,350 --> 00:40:41,870
You implemented recover
in a language called C

1001
00:40:41,870 --> 00:40:44,430
and you recovered photos
that had been deleted.

1002
00:40:44,430 --> 00:40:48,740
So clearly already, deleted doesn't
necessarily have to mean deleted.

1003
00:40:48,740 --> 00:40:51,560
And that's certainly the case
even for third party services.

1004
00:40:51,560 --> 00:40:55,818
Now to be fair, we can only guess how it
is Snapchat works underneath the hood.

1005
00:40:55,818 --> 00:40:58,610
They claim to be deleting your
photos after some number of seconds,

1006
00:40:58,610 --> 00:41:01,070
but let's consider for a
moment what that might actually

1007
00:41:01,070 --> 00:41:03,680
mean because it's our information
we're putting out there,

1008
00:41:03,680 --> 00:41:05,107
in this case in pictorial form.

1009
00:41:05,107 --> 00:41:07,190
If you've never use Snapchat,
when taking a photo,

1010
00:41:07,190 --> 00:41:09,690
you're able to see a screen
like this and allow your message

1011
00:41:09,690 --> 00:41:12,800
to expire either never or
infinitely many seconds from now,

1012
00:41:12,800 --> 00:41:14,690
or 1 to 10 seconds somewhere in between.

1013
00:41:14,690 --> 00:41:16,738
After which, the photo is deleted.

1014
00:41:16,738 --> 00:41:17,780
Well what does that mean?

1015
00:41:17,780 --> 00:41:21,340
In the context of last week, those
of you with now background in SQL

1016
00:41:21,340 --> 00:41:24,190
might hope that what Snapchat
is doing on their servers

1017
00:41:24,190 --> 00:41:25,830
is executing a query like this.

1018
00:41:25,830 --> 00:41:28,730
Delete from snaps where
ID equals something.

1019
00:41:28,730 --> 00:41:32,170
And the question mark represents
the idea of the snap you just sent,

1020
00:41:32,170 --> 00:41:34,240
the photograph that you just took.

1021
00:41:34,240 --> 00:41:35,710
But maybe they're doing that.

1022
00:41:35,710 --> 00:41:38,410
We, as outsiders as users,
have no way to audit this.

1023
00:41:38,410 --> 00:41:39,948
We can only trust what they say.

1024
00:41:39,948 --> 00:41:42,240
What if they're instead just
doing something like this?

1025
00:41:42,240 --> 00:41:45,400
Update snaps, set
deleted equals true where

1026
00:41:45,400 --> 00:41:49,690
ID equals question mark, where that
again is the identifier of your snap.

1027
00:41:49,690 --> 00:41:51,253
Now what is this representative of?

1028
00:41:51,253 --> 00:41:53,170
Well in the world of
computer science, there's

1029
00:41:53,170 --> 00:41:55,720
this notion of hard
deletion and soft deletion.

1030
00:41:55,720 --> 00:41:57,790
Hard delete means truly
just delete the data,

1031
00:41:57,790 --> 00:42:01,510
like throw the bits away so that
they really shouldn't be recoverable.

1032
00:42:01,510 --> 00:42:04,720
Soft delete is generally implemented
like this, where you literally

1033
00:42:04,720 --> 00:42:09,430
change a 0 to a 1, a false to a true
in your database to just remember oh

1034
00:42:09,430 --> 00:42:11,302
the user deleted this.

1035
00:42:11,302 --> 00:42:13,510
But that doesn't mean we
have to throw the data away.

1036
00:42:13,510 --> 00:42:15,385
This might have value
for marketing purposes,

1037
00:42:15,385 --> 00:42:17,718
for analytical purposes,
business purposes, or the like.

1038
00:42:17,718 --> 00:42:19,593
So for all intents and
purposes, it's deleted

1039
00:42:19,593 --> 00:42:22,510
because you the user can't get it
back and no one else can see it.

1040
00:42:22,510 --> 00:42:26,260
But perhaps one or more employees
at Snap can see that data.

1041
00:42:26,260 --> 00:42:28,090
And perhaps, that's
the kind of data that's

1042
00:42:28,090 --> 00:42:30,310
at risk of being leaked out longer term.

1043
00:42:30,310 --> 00:42:33,357
And we've not even discussed things
like backups, which too have upsides.

1044
00:42:33,357 --> 00:42:35,440
You don't want your data
to get lost, but you also

1045
00:42:35,440 --> 00:42:40,120
don't want your data to get stuck on
some backup in perpetuity as well.

1046
00:42:40,120 --> 00:42:43,090
So the lesson here we would
propose is just consider.

1047
00:42:43,090 --> 00:42:45,760
Don't just trust what some tool
or some application or developer

1048
00:42:45,760 --> 00:42:46,900
says something does.

1049
00:42:46,900 --> 00:42:50,350
Decide for yourself just how
much weight to give those claims

1050
00:42:50,350 --> 00:42:53,620
and whether or not this kind
of risk is worth taking photos

1051
00:42:53,620 --> 00:42:56,530
that you might regret, or wish
to unsend because there really

1052
00:42:56,530 --> 00:43:02,020
is no notion of unsending, no
matter what it is these applications

1053
00:43:02,020 --> 00:43:03,280
describe it as.

1054
00:43:03,280 --> 00:43:06,280
One last one that we saw
just a couple of weeks ago.

1055
00:43:06,280 --> 00:43:09,410
Photo, the ability to tagged photos
using artificial intelligence

1056
00:43:09,410 --> 00:43:12,190
and machine learning these days,
or just simple pattern matching

1057
00:43:12,190 --> 00:43:15,400
was incredibly easy and a few lines
of Python code to pick me out,

1058
00:43:15,400 --> 00:43:20,080
for instance, out of all of CS50 staff
based on relatively little information.

1059
00:43:20,080 --> 00:43:22,120
This is enhanced, the closest thereof.

1060
00:43:22,120 --> 00:43:23,780
This is a fuzzy picture.

1061
00:43:23,780 --> 00:43:28,150
Didn't represent it really
just by two eyes, a nose,

1062
00:43:28,150 --> 00:43:31,180
and a mouth that apparently
resembles my particular face.

1063
00:43:31,180 --> 00:43:32,692
This was really easy to do.

1064
00:43:32,692 --> 00:43:35,650
And I don't have to tell you these
days that if you're on social media,

1065
00:43:35,650 --> 00:43:38,800
all of us have probably been
uploading or have had upload

1066
00:43:38,800 --> 00:43:42,190
on our behalf photographs
of us that are then tagged

1067
00:43:42,190 --> 00:43:45,790
and we have for better or for worse
been just telling the world what

1068
00:43:45,790 --> 00:43:48,010
we look like from all sorts of angles.

1069
00:43:48,010 --> 00:43:50,237
And here too, can the
data perhaps be misused.

1070
00:43:50,237 --> 00:43:53,320
Yes, it's wonderfully useful when you
upload an album because your friends

1071
00:43:53,320 --> 00:43:55,120
can see oh, that was a great night out.

1072
00:43:55,120 --> 00:43:57,170
We can now reminisce
about those memories.

1073
00:43:57,170 --> 00:43:59,457
But someone else can be
scraping all that data,

1074
00:43:59,457 --> 00:44:01,540
as we have read about
having happened in the past.

1075
00:44:01,540 --> 00:44:04,275
And someone else might use that
data to find you in a crowd

1076
00:44:04,275 --> 00:44:05,650
where you don't want to be found.

1077
00:44:05,650 --> 00:44:08,860
Or governments could do this
too, based on driver's licenses

1078
00:44:08,860 --> 00:44:09,850
photos or the like.

1079
00:44:09,850 --> 00:44:12,460
Here, too, just because the
software can do something

1080
00:44:12,460 --> 00:44:15,400
doesn't necessarily
mean we should do that.

1081
00:44:15,400 --> 00:44:18,160
So we have all of these building
blocks underneath our belts

1082
00:44:18,160 --> 00:44:19,795
now from week zero on through up eight.

1083
00:44:19,795 --> 00:44:21,712
We have a number of
languages under our belts.

1084
00:44:21,712 --> 00:44:24,487
And for the final weeks of the
class is the goal ultimately,

1085
00:44:24,487 --> 00:44:26,320
to take off the last
of the training wheels.

1086
00:44:26,320 --> 00:44:29,278
In fact, the training wheels we
literally took off just a few weeks ago

1087
00:44:29,278 --> 00:44:30,640
are sitting over there now.

1088
00:44:30,640 --> 00:44:33,040
And now, is there an
opportunity for you in the weeks

1089
00:44:33,040 --> 00:44:36,130
ahead as the students in the room
to decide on your follow on tracks.

1090
00:44:36,130 --> 00:44:38,080
All toward an end of
CS50's final project.

1091
00:44:38,080 --> 00:44:40,540
And among the tracks, for
instance, that are on the horizon

1092
00:44:40,540 --> 00:44:41,980
for your web programming.

1093
00:44:41,980 --> 00:44:44,110
This is taking primitives
like we just saw there,

1094
00:44:44,110 --> 00:44:47,482
using JavaScript, HTML, CSS,
plus some more Python and SQL

1095
00:44:47,482 --> 00:44:50,440
which you've seen over the past couple
of weeks, to build applications.

1096
00:44:50,440 --> 00:44:53,170
This particular track will
be led by CS50's own Brian.

1097
00:44:53,170 --> 00:44:56,230
And among the projects you'll
tackle over the coming weeks

1098
00:44:56,230 --> 00:44:57,230
are something like this.

1099
00:44:57,230 --> 00:45:00,790
CS50 finance, so to speak,
which is a web based application

1100
00:45:00,790 --> 00:45:05,050
via which you can buy quote unquote
and sell quote unquote stocks using

1101
00:45:05,050 --> 00:45:09,220
real time data from an actual API,
application programming interface,

1102
00:45:09,220 --> 00:45:12,160
to get the latest prices for
stock symbols in the world.

1103
00:45:12,160 --> 00:45:15,310
So you and this track will build this
application complete with the ability

1104
00:45:15,310 --> 00:45:19,720
to log in, to log out, to buy stocks,
sell stocks, all using virtual dollars

1105
00:45:19,720 --> 00:45:23,890
and using a SQL database underneath the
hood to keep track of that information.

1106
00:45:23,890 --> 00:45:27,460
If you pursue the mobile track
instead, applying your past eight

1107
00:45:27,460 --> 00:45:30,970
weeks of skills and concepts to a
domain familiar to you and the phone

1108
00:45:30,970 --> 00:45:34,720
you have in your own pocket can
you explore programming on iOS

1109
00:45:34,720 --> 00:45:36,730
or iPhones with a language called Swift.

1110
00:45:36,730 --> 00:45:40,060
Or if you're an Android user exploring
that domain instead, using a language

1111
00:45:40,060 --> 00:45:41,020
called Java.

1112
00:45:41,020 --> 00:45:44,680
In this track, you'll be led by CS50's
own Tommy McWilliam, who will guide you

1113
00:45:44,680 --> 00:45:47,020
to a number of projects,
one of which implementing

1114
00:45:47,020 --> 00:45:50,267
your own Pokemon decks, a sort of
Rolodex for Pokemon characters,

1115
00:45:50,267 --> 00:45:52,600
including all of the native
user interface elements that

1116
00:45:52,600 --> 00:45:54,790
come with both iOS and Android.

1117
00:45:54,790 --> 00:45:57,970
You'll be able to explore an Instagram
like application reminiscent of some

1118
00:45:57,970 --> 00:46:00,160
of the filters that you
did in problem set four.

1119
00:46:00,160 --> 00:46:04,240
But this one will be interactive and
on your own device or in a simulator

1120
00:46:04,240 --> 00:46:08,380
much closer to the app, whether it's
Instagram or Snapchat that you yourself

1121
00:46:08,380 --> 00:46:09,130
are familiar with.

1122
00:46:09,130 --> 00:46:11,713
And then lastly in this track
will you implement your own note

1123
00:46:11,713 --> 00:46:12,610
taking application.

1124
00:46:12,610 --> 00:46:15,520
The ability to add and remove
information to keep track of todo's,

1125
00:46:15,520 --> 00:46:19,820
whether it's a grocery list, class
schedule, or something else altogether.

1126
00:46:19,820 --> 00:46:22,380
And then lastly, you have the
choice of CS50's games track

1127
00:46:22,380 --> 00:46:23,970
using a language called Lua.

1128
00:46:23,970 --> 00:46:26,400
This is a language that
facilitates implementing games

1129
00:46:26,400 --> 00:46:28,240
and many other applications as well.

1130
00:46:28,240 --> 00:46:30,240
And CS50's own Colton
Ogden will introduce you

1131
00:46:30,240 --> 00:46:33,450
to a couple of applications, some
of which our parents in the room

1132
00:46:33,450 --> 00:46:34,230
might recall.

1133
00:46:34,230 --> 00:46:37,140
This was one of the first games
ever implemented on a computer,

1134
00:46:37,140 --> 00:46:37,710
for instance.

1135
00:46:37,710 --> 00:46:39,130
Not to date people in the room.

1136
00:46:39,130 --> 00:46:40,530
But this is a game called Pong.

1137
00:46:40,530 --> 00:46:42,660
And it's relatively simple mechanically.

1138
00:46:42,660 --> 00:46:46,450
You move these white paddles up, down,
up, down, on the left and on the right.

1139
00:46:46,450 --> 00:46:49,990
And this little ball back in my day was
represented with a pixel, if you will.

1140
00:46:49,990 --> 00:46:52,740
That ball is going to bounce back
and forth against those paddles.

1141
00:46:52,740 --> 00:46:55,710
So you have to figure out
exactly how to do the reflections

1142
00:46:55,710 --> 00:46:57,780
and how to implement that interface.

1143
00:46:57,780 --> 00:47:01,290
And then, as is consistent with the
CS50's theme of beginning with Mario,

1144
00:47:01,290 --> 00:47:03,610
you can end the semester
with Mario again,

1145
00:47:03,610 --> 00:47:07,230
implementing your own version of Super
Mario Brothers, which is representative

1146
00:47:07,230 --> 00:47:11,040
of a two dimensional side
scrolling game, so to speak,

1147
00:47:11,040 --> 00:47:14,920
implementing this on your own Mac
or PC using your own computer.

1148
00:47:14,920 --> 00:47:19,140
Indeed, for each of these tracks do
you no longer need to use CS50 IDE.

1149
00:47:19,140 --> 00:47:21,990
And while you're welcome to
for at least the web track,

1150
00:47:21,990 --> 00:47:26,160
you can now use tools that are freely
available and are the types of tools

1151
00:47:26,160 --> 00:47:28,530
you'll use after this
course, in the real world.

1152
00:47:28,530 --> 00:47:32,228
The goal of the final few weeks of the
class is indeed this final project.

1153
00:47:32,228 --> 00:47:34,020
The goal of which is
to apply these lessons

1154
00:47:34,020 --> 00:47:36,240
learned now to a project
of your own design.

1155
00:47:36,240 --> 00:47:39,030
In the coming weeks when you
propose and then design, and then

1156
00:47:39,030 --> 00:47:43,200
implement your final project inspired
perhaps by any of those tracks,

1157
00:47:43,200 --> 00:47:46,800
you'll be able, for instance, in the web
track to download Python, the language,

1158
00:47:46,800 --> 00:47:49,740
and in turn, the interpreter, onto
your own Mac or PC and use it.

1159
00:47:49,740 --> 00:47:51,960
You can use a popular tool
like I used briefly today,

1160
00:47:51,960 --> 00:47:54,780
Visual Studio Code, which is a
free and open source tool that's

1161
00:47:54,780 --> 00:47:57,600
gaining momentum in industry
because it's relatively accessible

1162
00:47:57,600 --> 00:48:01,060
and has lots of features many
more than CS50's own IDE.

1163
00:48:01,060 --> 00:48:03,750
Xcode, by contrast, is what
you'll use if you're using a Mac

1164
00:48:03,750 --> 00:48:08,080
and developing an iOS project
for your iPhone because of that

1165
00:48:08,080 --> 00:48:10,080
being the software that
Apple provides for that.

1166
00:48:10,080 --> 00:48:12,780
Or in the worlds of
Google's, Android Studio,

1167
00:48:12,780 --> 00:48:16,622
a free tool that you can use to build
those Android applications as well.

1168
00:48:16,622 --> 00:48:18,330
But how are we going
to get there and how

1169
00:48:18,330 --> 00:48:20,860
will we support you along the the way?

1170
00:48:20,860 --> 00:48:24,990
So there's this tradition in CS50 for
the past decade of a CS50 hackathon.

1171
00:48:24,990 --> 00:48:29,130
This is an event that will begin
at 7:00 PM sometime in December

1172
00:48:29,130 --> 00:48:31,440
and will end at 7 AM the next night.

1173
00:48:31,440 --> 00:48:34,140
But unlike most, for
instance, all night affairs,

1174
00:48:34,140 --> 00:48:38,100
this one will be entirely focused
on building something and creating

1175
00:48:38,100 --> 00:48:41,500
something alongside all of your
classmates and all of your staff,

1176
00:48:41,500 --> 00:48:43,800
including some of our
friends from Yale as well.

1177
00:48:43,800 --> 00:48:47,240
Upon arrival, we'll all CS50 staff
greet you at the top of the stairs

1178
00:48:47,240 --> 00:48:49,260
where you check in, get
settled for the night,

1179
00:48:49,260 --> 00:48:52,530
and ultimately spend those 12
hours working theoretically

1180
00:48:52,530 --> 00:48:54,450
on your final project.

1181
00:48:54,450 --> 00:48:56,940
Perhaps, even putting the
finishing touches on it here.

1182
00:48:56,940 --> 00:49:00,630
One of the rooms from across the river
where the event is held, this I think

1183
00:49:00,630 --> 00:49:03,300
is shortly after a bug was
fixed later that night.

1184
00:49:03,300 --> 00:49:06,580
But along the way, there will be
made several meal times as well.

1185
00:49:06,580 --> 00:49:10,530
We're in the habit of providing 100 of
burritos from Philippe's around 9 PM.

1186
00:49:10,530 --> 00:49:13,410
Domino's kindly brings a few
100 pizzas around 1:00 AM.

1187
00:49:13,410 --> 00:49:15,780
And then, if you're
still awake at 5:00 AM,

1188
00:49:15,780 --> 00:49:19,260
will some Harvard shuttles take us too.

1189
00:49:19,260 --> 00:49:21,850
And our therapy dog who
will also be in attendance

1190
00:49:21,850 --> 00:49:26,240
will take us down the road
to Ihop for pancakes at 5 AM

1191
00:49:26,240 --> 00:49:28,350
if you would like to join us there.

1192
00:49:28,350 --> 00:49:29,520
[LAUGHTER]

1193
00:49:29,520 --> 00:49:33,180
And after that is the final, the
one final capstone to which everyone

1194
00:49:33,180 --> 00:49:35,310
in this room parents
and families as well are

1195
00:49:35,310 --> 00:49:38,850
invited as are all faculty and staff
across campus, is the CS50 fair.

1196
00:49:38,850 --> 00:49:42,960
A campus wide exhibition of all
students final projects at which you'll

1197
00:49:42,960 --> 00:49:44,580
be to which you'll bring your laptop.

1198
00:49:44,580 --> 00:49:46,710
We'll have music, and popcorn,
and candy, and friends,

1199
00:49:46,710 --> 00:49:47,760
and alumni from industry.

1200
00:49:47,760 --> 00:49:50,730
And it really is meant to be
this exhibition of in celebration

1201
00:49:50,730 --> 00:49:54,580
of the final projects you by that
point a month hence will have created.

1202
00:49:54,580 --> 00:49:57,300
Its at central Harvard
Square in the Smith center

1203
00:49:57,300 --> 00:50:00,250
and will you be greeted
by such visuals as these.

1204
00:50:00,250 --> 00:50:02,760
Our friends at Yale will be
doing the same in parallel.

1205
00:50:02,760 --> 00:50:05,640
And it's just an opportunity to
see what you and your friends

1206
00:50:05,640 --> 00:50:08,910
have done to invite them as well to
see what you've accomplished this term.

1207
00:50:08,910 --> 00:50:11,100
And to ultimately share
in the kinds of projects

1208
00:50:11,100 --> 00:50:14,392
that you've both created with
a few passers by here, as well.

1209
00:50:14,392 --> 00:50:17,100
And here is where I wanted to
thank the staff, not only those who

1210
00:50:17,100 --> 00:50:18,750
helped run the show in the room here.

1211
00:50:18,750 --> 00:50:22,470
But also a few of the course's
heads, literally heads.

1212
00:50:22,470 --> 00:50:25,530
This here is CS50 staff here
in Cambridge this past year.

1213
00:50:25,530 --> 00:50:29,250
All of our teaching fellows, course
assistants, producers, and the like.

1214
00:50:29,250 --> 00:50:31,020
These are our team
members at Yale, where

1215
00:50:31,020 --> 00:50:32,965
the course is indeed held in parallel.

1216
00:50:32,965 --> 00:50:35,340
And there is a few faces we
wanted to call in particular.

1217
00:50:35,340 --> 00:50:38,610
Rodrigo, our head teaching fellow
who sadly is graduating, but has

1218
00:50:38,610 --> 00:50:40,380
been with the course for so many years.

1219
00:50:40,380 --> 00:50:43,770
Emma, who is currently our head CA,
will take over as head teaching fellow

1220
00:50:43,770 --> 00:50:44,670
this coming year.

1221
00:50:44,670 --> 00:50:46,680
And of course, Brian,
many of whom you know

1222
00:50:46,680 --> 00:50:49,170
as the course's preceptor
who himself holds

1223
00:50:49,170 --> 00:50:53,320
both of those roles over
the past couple of years.

1224
00:50:53,320 --> 00:50:55,320
But this is where we'll
end just as we began.

1225
00:50:55,320 --> 00:50:58,500
2/3 of you eight weeks ago
had never taken CS before.

1226
00:50:58,500 --> 00:51:00,690
And when we surveyed you
as to your comfort levels,

1227
00:51:00,690 --> 00:51:02,340
you might recall this breakdown.

1228
00:51:02,340 --> 00:51:05,820
That over some 49% percent
of you described yourselves

1229
00:51:05,820 --> 00:51:09,210
as among those less comfortable
just with the idea of taking,

1230
00:51:09,210 --> 00:51:12,480
shopping, let alone staying
in the course like CS50.

1231
00:51:12,480 --> 00:51:15,150
16% of you described
yourselves as more comfortable.

1232
00:51:15,150 --> 00:51:18,270
And 35% described yourselves
as somewhere in between.

1233
00:51:18,270 --> 00:51:21,400
As you now embark on your final
projects of your own choice,

1234
00:51:21,400 --> 00:51:26,110
I can officially decree that you
are all now more comfortable.

1235
00:51:26,110 --> 00:51:28,270
This then, was just the
beginning of your journey

1236
00:51:28,270 --> 00:51:30,220
toward the end of these final projects.

1237
00:51:30,220 --> 00:51:33,730
Thank you all, too, for coming today
and for joining us this semester.

1238
00:51:33,730 --> 00:51:34,870
This was CS50.

1239
00:51:34,870 --> 00:51:36,720
[APPLAUSE]

1240
00:51:36,720 --> 00:51:38,000