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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 this is the end.

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All that remains here on out
is really your final projects.

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And we cannot wait to
see what you create.

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Today what we thought we would
do is take a bit of a look back,

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but also we'll look forward
so that you know exactly where

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and what you can do beyond CS50 itself.

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But first a word of thanks.

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We are, of course, here
in the Loeb Drama Center

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with The American Repertory Theater,
who have been our amazing hosts

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this whole semester.

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And truly, they have breathed new life,
new lights, new animation, new sounds

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into CS50.

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And we are so grateful to
have had such a privilege

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to work with the amazingly
talented team here to indeed bring

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this whole stage to life and evolve
it over the course of the semester.

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And then, of course,
there's CS50'S team.

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And though I'm the
only one here on stage

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with everyone else spread
quite far apart this semester,

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it would not be without CS50's team
that we have the videos that we have,

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the technology that we
have, and all of the visuals

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that supplement, hopefully,
everything that you yourselves

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have been doing hands on.

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So thank you truly to both teams for
having made this semester all possible.

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These are suffice it to say, among
the more unusual and difficult times.

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And we hope, if you're watching this
now live, or in some time from now,

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that this finds everyone
healthy and well.

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And that, indeed, we have
helped you find your way

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along this path of
learning something new.

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Of course, there's more
folks even than that

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behind the scenes, CS50'S whole team.

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And when I look out on the
crowd here, really, there

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is no crowd here in person.

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And if you've wondered what it
looks like behind the scenes,

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pictured here is a
photograph of exactly what

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it is I am seeing when we
hold each of these classes.

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And indeed, if we Zoom in, when
we're having these conversations

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or answering or asking
questions, it really is just us

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and some TV screens here this year.

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But we do look forward to
all reuniting before long.

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Now behind the scenes, there's
indeed this whole team.

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In fact, pictured here are
just most of, but not even

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all, CS50'S teaching Fellows, teaching
assistants, and course assistants,

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both at Harvard and at Yale, without
whom this semester would also not

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be possible.

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Because they are, indeed, the
backbone of and the support structure

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for getting everyone
ultimately to the finish

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line with problem sets, labs, and more.

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But it's worth noting
that we are all fallible.

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And, indeed, I'm told it's
fairly instructive when

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I do something completely wrong, or
get a little befuddled here on stage

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and can't quite figure out
why my own code isn't working,

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or can't quite answer a
question off the top of my head.

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This all happens,
certainly, to all of us.

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So even if you are feeling
here, toward the end

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of the semester, that not
everything quite clicked

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and you're still struggling sometimes
to find that bug in your code,

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or you're still googling or searching
for some answer to some smaller

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technical problem, rest assured,
or take comfort in knowing,

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that that is never
really going to go away.

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And in fact, to reinforce that,
besides all of the mistakes

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I here have made on stage, we thought
we would share a little bit of a clip,

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some bloopers if you will, from when the
teaching staff some weeks ago prepared

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that passing of TCP/IP packets on video,
which worked out wonderfully well,

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where folks were passing
up, down, left and right.

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The goal of which was to
get three TCP/IP packets

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from the bottom
right-hand corner of Zoom

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to the top left-hand corner of Zoom.

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But we thought we would give you
a glimpse of what actually went on

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behind the scenes and just
how many takes it took us

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to get even that demonstration right.

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I give you some of CS50'S team.

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[VIDEO PLAYBACK]

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- There we go.

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Buffering.

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OK.

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Josh?

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- Hi.

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- Helen, oh!

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[LAUGHTER]

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- [INAUDIBLE].

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No, wait.

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That was amazing, Josh.

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Um, Sophie?

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[LAUGHTER]

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Amazing.

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That was perfect.

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Moni?

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[LAUGHTER]

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I think I--

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- [INAUDIBLE].

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- Amazing.

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- Guy?

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That was amazing.

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Thank you all.

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- So good!

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[APPLAUSE]

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[END PLAYBACK]

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DAVID MALAN: So suffice it to say,
computer science is hard for all of us.

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And so some of these feelings,
some of these frustrations

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are never going to go away.

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But hopefully you have, indeed,
all the more tools in your toolkit,

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all the more of a
foundation now to build on

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so that you can take comfort in being a
little uncomfortable as you forge ahead

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and solve new problems,
learn new languages,

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and ultimately pick up
new ideas and skills.

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But remember, for CS50
alone, what ultimately

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matters in this course
is not so much where

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you end up relative to your
classmates, but where you end

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up relative to yourself when you began.

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And consider, it wasn't all
that long ago that you began.

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In fact, just some weeks
ago was this perhaps,

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the biggest of your problems
in CS50, just trying

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to figure out how to get
the pyramid to align right,

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whether you did the less
comfortable version or the more

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comfortable version, figuring
out how to print spaces,

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how to shift the pyramid over and the
like, figuring out how to nest loops,

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let alone getting all of the
semicolons and compilation steps right.

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And then, fast forward
to just a week or two ago

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when you built your very
own web application, one

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that used a third party API and
pulled in nearly real time data

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and generated views for the
user, had a controller governing

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with the model exactly all of the
data you were reading and writing

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and the like.

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Like, that is a huge way to have gone
over the course of just a few months.

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So take comfort in that, too.

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Especially as you dive
into your final project

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and might bump up against
some more walls, those, too,

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will you ultimately push through.

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So what have we focused on over
the course of this semester?

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A lot of the times we've spent time
talking about and doing programming.

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But really, we'd like to think
it's some of the higher level

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ideas and the takeaways
that are what last you

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far longer than the
particulars of these languages.

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Whether it's Scratch or C or
Python or JavaScript or SQL,

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or any of the other practical tools
that we looked at, all of those

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are eventually in some form
going to be out of date.

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Or they might remain with
us is older languages,

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but newer and better
things will come along.

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And what we hope, then, is
that over the past few months,

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you've walked away with the fundamentals
and sort of a foundation on which you

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can bootstrap yourself to learn
new things as they come out

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and really reduce new things
to their basic building blocks,

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the puzzle pieces with which we
began, first principles from which you

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can infer how some new system,
some new piece of hardware,

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how some new language must surely work.

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Because underneath the hood, at the
end of the day, it's still going to be,

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for some time, just 0's and 1's.

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And so we introduced in Week 0,
recall, computational thinking,

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encouraging you to think more
methodically, more algorithmically.

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But really, computational thinking is
just a computer scientist's incarnation

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of what we might otherwise think
of as just critical thinking.

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This process is taking
as input information

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and producing as output some solution.

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And in between there, of course, are
our algorithms, the black box that's

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doing something interesting and perhaps
difficult. But at the end of the day,

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this is problem solving.

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And this isn't going anywhere,
irrespective of the languages

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that you use or pick up or even
forget somewhere along the way.

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And indeed, today, too, whether this
is input and output in binary form,

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or it's just information and
decisions or facts and conclusions,

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this process of taking
input and producing

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as output correct answers,
correct conclusions,

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correct decisions is hopefully
going to be with you far

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longer than the
particulars of C or Python

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or any of the more hands-on skills
that we've spent time on this term.

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And recall, too, that
at least within CS50,

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the tools with which we propose that you
evaluate the quality of your approach

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to problem solving are these
three axes, the first and foremost

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of which is surely correctness.

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Because if it doesn't work, what's the
point of it all in the first place?

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So getting your code, your algorithm,
your process from input to output

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to be correct is certainly paramount.

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But after that comes
questions of design.

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If you actually want to
build more complex systems,

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or solve more sophisticated
problems, you really

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do want to design your solutions
to those problems cleanly.

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You don't want them to
be slow or inefficient.

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You don't want them to
be a mess in real terms.

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You don't want your code to
be completely undecipherable.

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Because that's just going to hamper you
longer term to using those same tools,

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those same libraries to
solve more interesting,

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more sophisticated problems.

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And it's surely going to
make it harder to interface

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with other people, other
collaborators, and other systems.

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And indeed along those lines,
is style still important?

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It's perhaps the third
in this trio for us.

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But it's the aesthetics of your code,
and the indentation in the variables

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and all that, much like you might
convey in our human language,

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put your best foot forward
with punctuation and the like.

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It just helps other
people understand you.

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And indeed, even though we spent
a lot of our time interacting

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with computers, in a course like
this, at the end of the day,

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you're really just communicating.

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And whether you're communicating
to a machine or to another human,

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doing that cleanly and in a
way that helps your ideas,

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your solutions, become adopted
is surely no less important

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than some of these other ideas as well.

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But what about other
basic building blocks

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that transcend the particular
languages and pieces that we did?

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Well, abstraction, this idea of
taking fairly complicated ideas

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and simplifying them so you don't
have to worry about the lower level

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implementation details, you
can focus only on the solution

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that that puzzle piece or that
building block actually provides.

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And abstraction is everywhere around us.

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Certainly in code, we saw functions
like get_string as an abstraction.

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I don't know.

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I don't really remember
exactly how get_string

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is implemented underneath the
hood, let alone a function

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that comes with C, like printf.

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But I know that it works.

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And I know that it takes input.

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I know that it produces output.

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And I can therefore build my
own ideas, my own software,

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on top of that building block,
abstracting away those particulars.

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And in the real world,
too, we abstract things--

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we abstract away things all of the
time, taking a complex idea or a process

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and assume that that will be done.

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Someone else might do that.

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And I can therefore build on
the output of that process,

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even if I am myself am no expert
in the underlying implementation

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details thereof.

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But then there's
precision, this other idea

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where it's super important,
certainly when writing code,

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but also when talking to another human,
to be precise and make super clear

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what you mean.

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And to consider the corner
cases, to consider the inputs

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that you might not otherwise
expect, but might nonetheless

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happen so that you don't err and
have some unexpected behavior, as we

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certainly did more than
once in actual code.

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And sometimes,
abstractions and precisions

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are kind of at odds with one another.

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Because abstraction would
sort of have you think at

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and talk at a fairly high
level, whereas precision

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00:11:25,198 --> 00:11:27,490
would suggest that you really
should get into the weeds

243
00:11:27,490 --> 00:11:31,450
and really go step by step by step
when it comes to giving someone

244
00:11:31,450 --> 00:11:33,490
else or a computer instructions.

245
00:11:33,490 --> 00:11:36,010
And we thought we would
bring this to life, perhaps,

246
00:11:36,010 --> 00:11:37,630
with a couple of examples.

247
00:11:37,630 --> 00:11:40,630
And we thought we would try to involve
as many people as we can in this,

248
00:11:40,630 --> 00:11:43,420
albeit from afar, by having
everyone, if you can,

249
00:11:43,420 --> 00:11:45,872
take out a piece of paper
and a pen or pencil.

250
00:11:45,872 --> 00:11:47,830
It's OK if you don't
quite have that available.

251
00:11:47,830 --> 00:11:49,955
You can do this on a
computer, too, if you'd rather

252
00:11:49,955 --> 00:11:53,390
draw on a notepad or a tablet, or
something like that totally fine.

253
00:11:53,390 --> 00:11:56,350
But ideally, taking out now something
with which to draw and something

254
00:11:56,350 --> 00:11:57,940
to draw on.

255
00:11:57,940 --> 00:12:00,700
We're going to go ahead and
try to apply some principles

256
00:12:00,700 --> 00:12:03,950
of computational thinking and
see just how helpful or hurtful

257
00:12:03,950 --> 00:12:09,680
it is to use abstraction or
precision at one level or another.

258
00:12:09,680 --> 00:12:12,250
So I think to do this,
Brian, we're going to need

259
00:12:12,250 --> 00:12:14,740
a helping hand from the audience.

260
00:12:14,740 --> 00:12:17,650
I think we're going
to need for one person

261
00:12:17,650 --> 00:12:22,570
out there to volunteer to
write instructions verbally

262
00:12:22,570 --> 00:12:23,593
for everyone else.

263
00:12:23,593 --> 00:12:25,510
We're going to treat
everyone in the audience,

264
00:12:25,510 --> 00:12:27,640
or really, n minus 1
people in the audience

265
00:12:27,640 --> 00:12:30,340
as the computers today who
are going to be programmed.

266
00:12:30,340 --> 00:12:33,850
And we need one human
volunteer to be the programmer.

267
00:12:33,850 --> 00:12:36,550
And that programmer
is going to be Daniel.

268
00:12:36,550 --> 00:12:38,740
So Daniel, thank you for volunteering.

269
00:12:38,740 --> 00:12:42,280
Brian, could we go ahead and share
with Daniel, and only Daniel,

270
00:12:42,280 --> 00:12:46,163
a picture of something that
we want everyone else to draw?

271
00:12:46,163 --> 00:12:48,580
So Daniel, what you should see
on your screen in a moment,

272
00:12:48,580 --> 00:12:50,122
if you haven't already, is a picture.

273
00:12:50,122 --> 00:12:51,880
Don't tell anyone else what it is.

274
00:12:51,880 --> 00:12:55,000
You may use any words in
a moment that you want.

275
00:12:55,000 --> 00:12:57,653
You should not use your hands
or any gestures like that.

276
00:12:57,653 --> 00:13:00,070
But the goal is going to be
to write an algorithm verbally

277
00:13:00,070 --> 00:13:02,200
for everyone else in
the room, step by step,

278
00:13:02,200 --> 00:13:05,530
so that ideally they
draw what it is you see.

279
00:13:05,530 --> 00:13:08,800
And you can say anything you want,
but just no physical gestures.

280
00:13:08,800 --> 00:13:10,420
Does that make sense?

281
00:13:10,420 --> 00:13:10,960
- Got it.

282
00:13:10,960 --> 00:13:13,043
And do you want to say a
little bit about yourself

283
00:13:13,043 --> 00:13:14,727
first to the group before we begin?

284
00:13:14,727 --> 00:13:15,310
STUDENT: Sure.

285
00:13:15,310 --> 00:13:16,150
My name's Daniel.

286
00:13:16,150 --> 00:13:20,590
I'm from Ezra Stiles
College at Yale University.

287
00:13:20,590 --> 00:13:22,915
And I really CS50 this semester.

288
00:13:22,915 --> 00:13:24,040
DAVID MALAN: Oh, wonderful.

289
00:13:24,040 --> 00:13:25,600
Thank you for volunteering.

290
00:13:25,600 --> 00:13:29,290
And let's go ahead and have
everyone else with their paper

291
00:13:29,290 --> 00:13:30,640
and pencil or pen ready.

292
00:13:30,640 --> 00:13:33,650
Daniel, what should be the first
thing everyone does, step one.

293
00:13:33,650 --> 00:13:34,150
STUDENT: OK.

294
00:13:34,150 --> 00:13:38,680
So the first thing we're
going to draw is a hexagon.

295
00:13:38,680 --> 00:13:40,780
So it's a regular hexagon.

296
00:13:40,780 --> 00:13:50,828
And we're going to make sure that we
draw it so that one of the vertices

297
00:13:50,828 --> 00:13:53,370
is on the very bottom of the
hexagon, and one of the vertices

298
00:13:53,370 --> 00:13:55,990
is at the very top of the hexagon.

299
00:13:55,990 --> 00:13:58,210
So one side is not laying flat.

300
00:13:58,210 --> 00:13:58,958
You're doing--

301
00:13:58,958 --> 00:14:00,250
DAVID MALAN: Wup, dut, dut dut.

302
00:14:00,250 --> 00:14:01,120
No hand gestures.

303
00:14:01,120 --> 00:14:02,287
STUDENT: No hands, no hands.

304
00:14:02,287 --> 00:14:03,400
Right.

305
00:14:03,400 --> 00:14:06,940
One vertice at the very top, one
vertice is at the very bottom.

306
00:14:06,940 --> 00:14:10,850
And you've got your other
four vertices on the sides.

307
00:14:10,850 --> 00:14:13,870
So once you've got your
hexagon, your next step

308
00:14:13,870 --> 00:14:18,580
is going to be find your
midpoint of the hexagon.

309
00:14:18,580 --> 00:14:20,740
And so once you've found
your midpoint, you're

310
00:14:20,740 --> 00:14:26,080
going to draw three lines from
a vertices to that midpoint.

311
00:14:26,080 --> 00:14:29,440
The vertices that you're
going to choose to draw from

312
00:14:29,440 --> 00:14:37,510
are the very bottom to the midpoint and
then from the midpoint to the vertices

313
00:14:37,510 --> 00:14:42,142
that are on the left and
right of the top vertices.

314
00:14:42,142 --> 00:14:43,100
DAVID MALAN: All right.

315
00:14:43,100 --> 00:14:46,720
Any final instructions?

316
00:14:46,720 --> 00:14:48,858
STUDENT: I think, hopefully,
that should be it.

317
00:14:48,858 --> 00:14:49,900
DAVID MALAN: [INAUDIBLE].

318
00:14:49,900 --> 00:14:51,670
Those were very long steps one and two.

319
00:14:51,670 --> 00:14:53,230
But yes.

320
00:14:53,230 --> 00:14:55,690
All right, well, let's
go ahead and reveal.

321
00:14:55,690 --> 00:14:57,310
This will be a little bit of a hack.

322
00:14:57,310 --> 00:14:59,890
But if anyone everyone
is comfortable picking up

323
00:14:59,890 --> 00:15:02,310
their piece of paper or
their tablet and holding it

324
00:15:02,310 --> 00:15:07,980
in front of their Zoom camera
steadily for five or 10 seconds,

325
00:15:07,980 --> 00:15:11,100
we'll see exactly what
everyone has drawn.

326
00:15:11,100 --> 00:15:13,950
If you go into gallery view, you'll
be able to see everyone else.

327
00:15:13,950 --> 00:15:17,220
Daniel, hopefully you're
seeing some familiar pictures?

328
00:15:17,220 --> 00:15:19,940
I think we definitely have range.

329
00:15:19,940 --> 00:15:22,547
Are you seeing one or more that
match what you had in mind?

330
00:15:22,547 --> 00:15:23,130
STUDENT: Yeah.

331
00:15:23,130 --> 00:15:24,210
They all look pretty good.

332
00:15:24,210 --> 00:15:24,900
DAVID MALAN: They all?

333
00:15:24,900 --> 00:15:25,900
All right, so good.

334
00:15:25,900 --> 00:15:28,020
Let me go ahead, then,
and share on my screen

335
00:15:28,020 --> 00:15:31,190
in just a moment what it
is Daniel was describing.

336
00:15:31,190 --> 00:15:34,260
So what Brian had shared with
Daniel in advance was this picture

337
00:15:34,260 --> 00:15:38,490
here, which I dare say is a cube.

338
00:15:38,490 --> 00:15:42,360
But indeed, it's composed of a
hexagon and then the additional lines

339
00:15:42,360 --> 00:15:43,410
that Daniel described.

340
00:15:43,410 --> 00:15:47,670
And Daniel, I did happen to see, maybe
on pages two and three of the Zoom

341
00:15:47,670 --> 00:15:52,590
window, there were definitely
some that weren't quite cubes.

342
00:15:52,590 --> 00:15:57,330
What was going through your
mind as to how you approached

343
00:15:57,330 --> 00:15:59,420
the algorithm that you provided?

344
00:15:59,420 --> 00:16:02,250
STUDENT: I wanted to--

345
00:16:02,250 --> 00:16:05,040
to me, the first thing that
went through my head was a cube.

346
00:16:05,040 --> 00:16:08,040
But I knew that there's so
many ways to draw a cube.

347
00:16:08,040 --> 00:16:09,980
I didn't want to describe it as a cube.

348
00:16:09,980 --> 00:16:11,730
Because if I said just
draw a cube, I knew

349
00:16:11,730 --> 00:16:13,990
that we would get tons
of different results.

350
00:16:13,990 --> 00:16:16,260
So I wanted to be as clear as possible.

351
00:16:16,260 --> 00:16:21,030
And I knew that if I could describe
it in sort of a mathematical way,

352
00:16:21,030 --> 00:16:23,505
describing it with a
hexagon and describing it

353
00:16:23,505 --> 00:16:27,690
with the vertices in the midpoint,
that hopefully more people would

354
00:16:27,690 --> 00:16:29,790
be able to draw a precise shape.

355
00:16:29,790 --> 00:16:30,540
DAVID MALAN: Yeah.

356
00:16:30,540 --> 00:16:31,350
Really well said.

357
00:16:31,350 --> 00:16:33,420
Now if we had everyone's
volume on, odd are,

358
00:16:33,420 --> 00:16:35,520
you'd hear a bit of
chuckling now, perhaps,

359
00:16:35,520 --> 00:16:37,383
or maybe a little bit of awkwardness.

360
00:16:37,383 --> 00:16:40,050
And I daresay not all of the
pictures quite turned out that way,

361
00:16:40,050 --> 00:16:43,110
but that's a perfect example of where
maybe abstractions can kind of get us

362
00:16:43,110 --> 00:16:43,652
into trouble.

363
00:16:43,652 --> 00:16:47,190
Because if Daniel had just said,
draw a cube, right, some of you

364
00:16:47,190 --> 00:16:48,833
might start drawing immediately a cube.

365
00:16:48,833 --> 00:16:52,000
But many of you would have a question,
well, what should the orientation be?

366
00:16:52,000 --> 00:16:52,900
What should the size be?

367
00:16:52,900 --> 00:16:53,820
What should the position be?

368
00:16:53,820 --> 00:16:56,350
And so there, precision
becomes increasingly important.

369
00:16:56,350 --> 00:16:58,562
But the more precise it
gets, odds are some of you

370
00:16:58,562 --> 00:17:01,770
just kind of got overwhelmed with the
amount of detail and sort of lost track

371
00:17:01,770 --> 00:17:04,440
where your pen or pencil was
supposed to be at one point

372
00:17:04,440 --> 00:17:07,190
because you were operating
at a much lower level.

373
00:17:07,190 --> 00:17:08,910
So there's this tension, then.

374
00:17:08,910 --> 00:17:11,410
But I think we did get some
of you to that finish line.

375
00:17:11,410 --> 00:17:13,530
Let's see if we can't now take
the pressure off of all of you,

376
00:17:13,530 --> 00:17:15,405
and thank you to Daniel,
in particular, let's

377
00:17:15,405 --> 00:17:19,035
see if we can't now have all of you
collectively program me, if you will.

378
00:17:19,035 --> 00:17:21,160
So I'm going to go ahead
and pull up my screen here

379
00:17:21,160 --> 00:17:25,900
where I have the ability to draw with
my mouse and cursor on my screen here.

380
00:17:25,900 --> 00:17:29,190
And Brian, if you don't mind,
could you share with everyone

381
00:17:29,190 --> 00:17:34,560
else a picture that I promise
I have not seen in advance.

382
00:17:34,560 --> 00:17:35,890
So we will see how this goes.

383
00:17:35,890 --> 00:17:38,280
So I'm the only one right
now in the Zoom room

384
00:17:38,280 --> 00:17:39,990
that has not seen this picture.

385
00:17:39,990 --> 00:17:42,930
But Brian has gone and provided
only you all with the URL.

386
00:17:42,930 --> 00:17:44,640
So pull that up on your screen.

387
00:17:44,640 --> 00:17:47,760
And then, Brian, if we could perhaps
iteratively call on some volunteers.

388
00:17:47,760 --> 00:17:52,477
Why don't I try to draw what
people tell me to do, step by step?

389
00:17:52,477 --> 00:17:53,310
BRIAN YU: All right.

390
00:17:53,310 --> 00:17:54,330
David has not seen this.

391
00:17:54,330 --> 00:17:56,760
I just picked this out,
like, five minutes ago.

392
00:17:56,760 --> 00:17:59,677
And you're all going to raise your
hand if you want to give him, like,

393
00:17:59,677 --> 00:18:01,560
one instruction for what to do next.

394
00:18:01,560 --> 00:18:03,780
And let's start with George.

395
00:18:03,780 --> 00:18:06,570
STUDENT: So you're going
to start by drawing

396
00:18:06,570 --> 00:18:09,593
a circle near the top of the screen.

397
00:18:09,593 --> 00:18:10,260
DAVID MALAN: OK.

398
00:18:10,260 --> 00:18:11,970
A circle near the top of the screen.

399
00:18:11,970 --> 00:18:15,100
And let me make clear, I have no
delete abilities on the computer.

400
00:18:15,100 --> 00:18:16,650
So once I commit, we're in.

401
00:18:16,650 --> 00:18:20,940
So drawing a circle near
the top of the screen.

402
00:18:20,940 --> 00:18:21,720
OK.

403
00:18:21,720 --> 00:18:22,710
Thank you, George.

404
00:18:22,710 --> 00:18:23,820
Brian, step two?

405
00:18:23,820 --> 00:18:25,750
BRIAN YU: All right,
let's go to Sophia next.

406
00:18:25,750 --> 00:18:32,800
STUDENT: Then, in the very center of the
screen, door a black, filled in circle,

407
00:18:32,800 --> 00:18:36,233
which is approximately a tenth of
the size of the circle at the top.

408
00:18:36,233 --> 00:18:36,900
DAVID MALAN: OK.

409
00:18:36,900 --> 00:18:41,710
A black, filled in circle, I
heard, that's a tenth of the size.

410
00:18:41,710 --> 00:18:46,580
So I'm going to do something like this,
and then just kind of shade it in.

411
00:18:46,580 --> 00:18:48,110
All right, thank you, Sophia.

412
00:18:48,110 --> 00:18:48,800
Step three?

413
00:18:48,800 --> 00:18:50,270
BRIAN YU: Let's go to Santiago.

414
00:18:50,270 --> 00:18:55,290
STUDENT: You're going
to draw another circle.

415
00:18:55,290 --> 00:18:57,440
But it's not actually
going to be a circle,

416
00:18:57,440 --> 00:19:04,190
it's more of an ellipse, that's going
to be bigger than the first one.

417
00:19:04,190 --> 00:19:05,610
So it's going to be in the middle.

418
00:19:05,610 --> 00:19:08,360
And it's going to enclose
that filled in circle

419
00:19:08,360 --> 00:19:10,723
and leave some room in the bottom.

420
00:19:10,723 --> 00:19:11,390
DAVID MALAN: OK.

421
00:19:11,390 --> 00:19:12,182
So it's an ellipse.

422
00:19:12,182 --> 00:19:14,670
It's bigger than the first circle.

423
00:19:14,670 --> 00:19:18,320
But it encloses the smaller one?

424
00:19:18,320 --> 00:19:20,105
All right, so I heard kind of this.

425
00:19:20,105 --> 00:19:23,560


426
00:19:23,560 --> 00:19:26,260
OK.

427
00:19:26,260 --> 00:19:27,910
Step 4?

428
00:19:27,910 --> 00:19:30,280
STUDENT: Under that
smaller ellipse, you're

429
00:19:30,280 --> 00:19:35,110
going to want to draw a
bigger circle underneath it

430
00:19:35,110 --> 00:19:38,020
and act as if the circle is
going through that ellipse,

431
00:19:38,020 --> 00:19:41,320
but don't actually show the
lines going through the ellipse.

432
00:19:41,320 --> 00:19:44,590
So that is, we draw a bigger circle
underneath, but without having

433
00:19:44,590 --> 00:19:46,240
the lines go through.

434
00:19:46,240 --> 00:19:50,250
It looks like it will kind of
be going through the edge of it.

435
00:19:50,250 --> 00:19:51,270
DAVID MALAN: OK.

436
00:19:51,270 --> 00:20:01,850
I'm a little worried here, but
what I heard was like this, maybe?

437
00:20:01,850 --> 00:20:03,620
Step five?

438
00:20:03,620 --> 00:20:07,190
BRIAN YU: All right, let's
go to [INAUDIBLE] next.

439
00:20:07,190 --> 00:20:11,060
STUDENT: So in that
kind of middle ellipse,

440
00:20:11,060 --> 00:20:14,002
you know, like, when kids
act like they're an airplane,

441
00:20:14,002 --> 00:20:15,710
and then they make,
like, airplane noise,

442
00:20:15,710 --> 00:20:17,627
then they do that weird
thing with their arms?

443
00:20:17,627 --> 00:20:18,830
DAVID MALAN: Uh huh.

444
00:20:18,830 --> 00:20:21,890
STUDENT: So draw those kind of,
like, arms in that middle ellipse,

445
00:20:21,890 --> 00:20:24,152
coming out of the big middle ellipse.

446
00:20:24,152 --> 00:20:25,610
DAVID MALAN: In the middle ellipse?

447
00:20:25,610 --> 00:20:28,010
This lower one?

448
00:20:28,010 --> 00:20:29,960
STUDENT: The one outside of it.

449
00:20:29,960 --> 00:20:32,720
DAVID MALAN: Oh, this big ellipse?

450
00:20:32,720 --> 00:20:34,050
STUDENT: Yeah, the outer bound.

451
00:20:34,050 --> 00:20:34,550
Yeah.

452
00:20:34,550 --> 00:20:37,883
DAVID MALAN: All right, so I should draw
some hands like a kid would have when--

453
00:20:37,883 --> 00:20:42,400


454
00:20:42,400 --> 00:20:43,570
OK.

455
00:20:43,570 --> 00:20:45,527
I'm not sure this is going to end well.

456
00:20:45,527 --> 00:20:46,360
BRIAN YU: All right.

457
00:20:46,360 --> 00:20:49,180
We need some more volunteers
to help David finish this.

458
00:20:49,180 --> 00:20:51,420
Let's go to Gabrielle.

459
00:20:51,420 --> 00:20:52,590
STUDENT: OK.

460
00:20:52,590 --> 00:20:55,147
Try to draw a--

461
00:20:55,147 --> 00:20:56,772
DAVID MALAN: Try is the operative word.

462
00:20:56,772 --> 00:20:58,500
[LAUGHTER]

463
00:20:58,500 --> 00:21:00,345
STUDENT: You've got a
bigger ellipse that's

464
00:21:00,345 --> 00:21:04,110
at the very bottom, that's bigger
than both the top and middle one,

465
00:21:04,110 --> 00:21:07,620
but showing no overlapping lines
between the middle one and the one

466
00:21:07,620 --> 00:21:08,580
that you're trying.

467
00:21:08,580 --> 00:21:10,440
DAVID MALAN: So show
no overlapping lines.

468
00:21:10,440 --> 00:21:12,460
So I heard an even bigger ellipse.

469
00:21:12,460 --> 00:21:14,640
So, like, oops, sorry.

470
00:21:14,640 --> 00:21:15,540
This?

471
00:21:15,540 --> 00:21:16,710
OK?

472
00:21:16,710 --> 00:21:17,550
STUDENT: Good job.

473
00:21:17,550 --> 00:21:18,150
DAVID MALAN: Thank you.

474
00:21:18,150 --> 00:21:18,570
OK.

475
00:21:18,570 --> 00:21:18,870
Good.

476
00:21:18,870 --> 00:21:20,610
Keep the positive reinforcement coming.

477
00:21:20,610 --> 00:21:21,862
Final couple steps?

478
00:21:21,862 --> 00:21:22,695
BRIAN YU: All right.

479
00:21:22,695 --> 00:21:23,637
[? Ika? ?]

480
00:21:23,637 --> 00:21:25,470
STUDENT: One other step
you would have to do

481
00:21:25,470 --> 00:21:30,840
is draw a small, filled in circle,
slightly smaller than the one

482
00:21:30,840 --> 00:21:34,230
you already drew, right in
the center of the first circle

483
00:21:34,230 --> 00:21:35,883
you drew right at the top.

484
00:21:35,883 --> 00:21:36,550
DAVID MALAN: OK.

485
00:21:36,550 --> 00:21:38,217
Right in the center of the first circle.

486
00:21:38,217 --> 00:21:38,820
OK.

487
00:21:38,820 --> 00:21:41,160
And I think this is starting
to take shape for me.

488
00:21:41,160 --> 00:21:44,013
And I regret some of
my earlier decisions.

489
00:21:44,013 --> 00:21:46,930
BRIAN YU: [INAUDIBLE],, you want to
provide an additional instruction?

490
00:21:46,930 --> 00:21:49,472
STUDENT: Another circle in
between the last one you just drew

491
00:21:49,472 --> 00:21:53,520
and in between the
edge of the circle, so

492
00:21:53,520 --> 00:21:56,490
to the left of that circle you're
going to draw another circle.

493
00:21:56,490 --> 00:21:59,550
DAVID MALAN: To the left of this circle?

494
00:21:59,550 --> 00:22:00,390
STUDENT: Mhm.

495
00:22:00,390 --> 00:22:01,057
DAVID MALAN: OK.

496
00:22:01,057 --> 00:22:03,380


497
00:22:03,380 --> 00:22:05,150
BRIAN YU: And Ryan?

498
00:22:05,150 --> 00:22:08,730
STUDENT: Underneath, you're going
to want to repeat the same process,

499
00:22:08,730 --> 00:22:10,850
except draw a circle on the right side.

500
00:22:10,850 --> 00:22:12,280
DAVID MALAN: OK.

501
00:22:12,280 --> 00:22:13,258
Little loop.

502
00:22:13,258 --> 00:22:15,800
BRIAN YU: I think we've got
maybe one or two more steps left.

503
00:22:15,800 --> 00:22:17,200
Let's go back to Sophia.

504
00:22:17,200 --> 00:22:22,130
STUDENT: Underneath the filled in
circle, that's in the middle ellipse,

505
00:22:22,130 --> 00:22:25,360
you want to draw two
replicas of that circle

506
00:22:25,360 --> 00:22:28,630
below the original one
in the middle ellipse.

507
00:22:28,630 --> 00:22:33,310
DAVID MALAN: In the middle
ellipse, so here, OK.

508
00:22:33,310 --> 00:22:35,530
I think I know what this is.

509
00:22:35,530 --> 00:22:37,870
BRIAN YU: And Jason, how
about one last instruction?

510
00:22:37,870 --> 00:22:39,610
DAVID MALAN: All right.

511
00:22:39,610 --> 00:22:42,100
STUDENT: Underneath,
in the top most circle,

512
00:22:42,100 --> 00:22:46,840
under the three, so the row of
three circles, draw a wide V,

513
00:22:46,840 --> 00:22:48,850
sort of shaped with two straight lines.

514
00:22:48,850 --> 00:22:52,000
DAVID MALAN: A wide V
with two straight lines.

515
00:22:52,000 --> 00:22:52,750
OK.

516
00:22:52,750 --> 00:22:54,930
That part, I think, I nailed.

517
00:22:54,930 --> 00:22:57,640
Shall I switch over and reveal?

518
00:22:57,640 --> 00:23:00,130
So this is the URL I believe
all of you were given.

519
00:23:00,130 --> 00:23:01,510
I have not visited it yet.

520
00:23:01,510 --> 00:23:04,780
But if I go and visit this now.

521
00:23:04,780 --> 00:23:06,910
Hey, that's not all that bad.

522
00:23:06,910 --> 00:23:09,245
All right, I definitely took
a detour partway through.

523
00:23:09,245 --> 00:23:10,370
But here's another example.

524
00:23:10,370 --> 00:23:13,840
Had you just started with draw
a snowman as follows, like,

525
00:23:13,840 --> 00:23:16,600
that might have helped orient
me, truthfully, similar in spirit

526
00:23:16,600 --> 00:23:20,197
to Daniel's design that would have
given you a mental model of what I,

527
00:23:20,197 --> 00:23:23,030
or given me a mental model of what
it is I should have been drawing.

528
00:23:23,030 --> 00:23:25,090
So here, too, abstraction is hard.

529
00:23:25,090 --> 00:23:28,840
And even precision is hard and
figuring out the right level of detail

530
00:23:28,840 --> 00:23:32,230
to operate at is kind of part of
the process of problem solving.

531
00:23:32,230 --> 00:23:34,780
Though, now that I look at it,
that's actually not half bad.

532
00:23:34,780 --> 00:23:36,822
Like, I definitely did
the wrong thing over here.

533
00:23:36,822 --> 00:23:40,390
But very well done to all
of our volunteers online.

534
00:23:40,390 --> 00:23:43,060
So remember these kinds
of details when you're

535
00:23:43,060 --> 00:23:46,178
trying to explain some process to
someone, when you're giving someone

536
00:23:46,178 --> 00:23:48,970
instructions, even if it's for
something mundane in the real world,

537
00:23:48,970 --> 00:23:53,470
like going to run errands or pick up
supplies at the market, being precise

538
00:23:53,470 --> 00:23:56,830
is certainly important, but the more
precision you provide, the much easier

539
00:23:56,830 --> 00:23:59,870
it is for the person to
get lost in those weeds.

540
00:23:59,870 --> 00:24:05,030
And so sometimes a higher level list of
details is all that someone might need.

541
00:24:05,030 --> 00:24:07,947
So now that you have this ability
to program and to do things

542
00:24:07,947 --> 00:24:10,030
that we've shown you in
lectures and problem sets,

543
00:24:10,030 --> 00:24:11,800
and indeed, have the
ability to figure out

544
00:24:11,800 --> 00:24:13,870
how to do things that we
haven't even shown you,

545
00:24:13,870 --> 00:24:17,350
we wanted to take a moment to consider
just whether you should do those things

546
00:24:17,350 --> 00:24:19,220
and, if you should,
how you should do them.

547
00:24:19,220 --> 00:24:21,970
But beyond just answering these
questions with your own instincts,

548
00:24:21,970 --> 00:24:24,012
we thought we would invite
some of our colleagues

549
00:24:24,012 --> 00:24:27,280
from the Philosophy department to
propose a more formal framework,

550
00:24:27,280 --> 00:24:29,740
a thought process by which
we can approach problems

551
00:24:29,740 --> 00:24:32,050
when it comes to technology
and the writing of code

552
00:24:32,050 --> 00:24:36,080
to help us decide, ultimately,
just because I can code something,

553
00:24:36,080 --> 00:24:39,820
should I, and if so, indeed,
how should I do that?

554
00:24:39,820 --> 00:24:43,090
So allow me to introduce our colleagues
from the Philosophy department, Meica

555
00:24:43,090 --> 00:24:45,610
Magnani and also Susan Kennedy.

556
00:24:45,610 --> 00:24:46,600
Meica?

557
00:24:46,600 --> 00:24:47,590
MEICA MAGNANI: So hi.

558
00:24:47,590 --> 00:24:49,000
I'm Meica Magnani.

559
00:24:49,000 --> 00:24:52,690
I am a philosophy postdoc with
the Embedded Ethics Program

560
00:24:52,690 --> 00:24:53,380
here at Harvard.

561
00:24:53,380 --> 00:24:54,130
SUSAN KENNEDY: Hi.

562
00:24:54,130 --> 00:24:54,910
I'm Susan Kennedy.

563
00:24:54,910 --> 00:24:59,020
And I'm also a philosophy postdoc with
the Embedded Ethics Program at Harvard.

564
00:24:59,020 --> 00:25:01,040
MEICA MAGNANI: And
before we get started,

565
00:25:01,040 --> 00:25:03,860
I'll just say a few things about
the Embedded Ethics Program.

566
00:25:03,860 --> 00:25:08,380
So we are an interdisciplinary
team of philosophers and computer

567
00:25:08,380 --> 00:25:12,250
scientists working together to integrate
ethics into the computer science

568
00:25:12,250 --> 00:25:13,570
curriculum.

569
00:25:13,570 --> 00:25:17,920
The idea behind this approach is
to embed tools of ethical reasoning

570
00:25:17,920 --> 00:25:21,040
into computer science
courses themselves.

571
00:25:21,040 --> 00:25:25,540
The reason for this is that when making
decisions about the design, deployment,

572
00:25:25,540 --> 00:25:30,130
or development of a piece of technology,
one is, whether or not one realizes it,

573
00:25:30,130 --> 00:25:31,990
making ethical decisions.

574
00:25:31,990 --> 00:25:35,210
That is, making decisions which
stand to have social, political,

575
00:25:35,210 --> 00:25:36,612
or human impact.

576
00:25:36,612 --> 00:25:39,070
At Harvard, we think it's
important for computer scientists

577
00:25:39,070 --> 00:25:42,730
to be equipped with tools for
thinking through these implications.

578
00:25:42,730 --> 00:25:45,730
SUSAN KENNEDY: Technology holds a
lot of power and influence over us.

579
00:25:45,730 --> 00:25:49,938
And that means, by extension, that the
people who design technology do, too.

580
00:25:49,938 --> 00:25:52,480
Now that you're starting to
think about what responsibilities

581
00:25:52,480 --> 00:25:54,940
you might have as computer
scientists, so you

582
00:25:54,940 --> 00:25:58,570
can avoid notable mishaps,
like face mash, for instance,

583
00:25:58,570 --> 00:26:01,960
we're going to turn your attention to
the topic of social media platforms

584
00:26:01,960 --> 00:26:04,420
and how they affect the
distribution of and engagement

585
00:26:04,420 --> 00:26:07,010
with news and information.

586
00:26:07,010 --> 00:26:09,260
It would seem that this
topic is especially relevant

587
00:26:09,260 --> 00:26:13,370
now, given the recent US presidential
election, where political content has

588
00:26:13,370 --> 00:26:17,120
been dominating the internet and
television broadcasts and controversy

589
00:26:17,120 --> 00:26:19,880
has played out on social
media, garnering attention

590
00:26:19,880 --> 00:26:21,930
from around the world.

591
00:26:21,930 --> 00:26:24,840
Undoubtedly, technology has
completely revolutionized

592
00:26:24,840 --> 00:26:28,740
the way information and news is
both disseminated and consumed.

593
00:26:28,740 --> 00:26:32,370
Instead of a paper boy shouting, get
your news here on the street corner,

594
00:26:32,370 --> 00:26:34,590
just about everyone uses
the internet to stay up

595
00:26:34,590 --> 00:26:39,250
to date with what's happening, not
just locally, but around the world.

596
00:26:39,250 --> 00:26:42,790
And in the past few years, social
media platforms, in particular,

597
00:26:42,790 --> 00:26:46,240
have started to play a huge role
in how people access, share,

598
00:26:46,240 --> 00:26:48,460
and engage with information.

599
00:26:48,460 --> 00:26:52,630
For instance, research
shows that 44% of US adults

600
00:26:52,630 --> 00:26:55,850
report getting the news from Facebook.

601
00:26:55,850 --> 00:26:58,850
It's safe to say a lot has
changed in recent years,

602
00:26:58,850 --> 00:27:01,230
owing to developments in technology.

603
00:27:01,230 --> 00:27:05,030
And this matters when we consider
what's at stake, namely, the ability

604
00:27:05,030 --> 00:27:07,010
for the public to
engage in discourse that

605
00:27:07,010 --> 00:27:09,950
supports a well-functioning democracy.

606
00:27:09,950 --> 00:27:13,280
So I'll first present you a brief
overview of where we came from

607
00:27:13,280 --> 00:27:16,430
and where we are now, owing
to technological developments

608
00:27:16,430 --> 00:27:20,220
and then consider what challenges
we're faced with today.

609
00:27:20,220 --> 00:27:22,530
Before the internet,
news and information

610
00:27:22,530 --> 00:27:26,670
was almost entirely in the hands of a
few major broadcast stations and print

611
00:27:26,670 --> 00:27:31,020
media outlets, otherwise known
as the mass media sphere.

612
00:27:31,020 --> 00:27:34,110
Since a few organizations were
responsible for disseminating

613
00:27:34,110 --> 00:27:37,230
all the news, information
was essentially

614
00:27:37,230 --> 00:27:40,110
filtered through a narrow
lens or narrow aperture

615
00:27:40,110 --> 00:27:44,080
from organizations to
a wide public audience.

616
00:27:44,080 --> 00:27:46,900
The journalists who are responsible
for researching and writing

617
00:27:46,900 --> 00:27:51,680
the content for these organizations
all shared a professional ethos.

618
00:27:51,680 --> 00:27:55,520
They were concerned with truth,
representation of social groups,

619
00:27:55,520 --> 00:28:00,400
creating a forum for criticism,
clarifying public values,

620
00:28:00,400 --> 00:28:03,190
and offering comprehensive coverage.

621
00:28:03,190 --> 00:28:05,380
And notably, since
the aim was to produce

622
00:28:05,380 --> 00:28:07,720
content that appealed
to a wide audience,

623
00:28:07,720 --> 00:28:12,810
there was less polarization and
extremist commentary than we see today.

624
00:28:12,810 --> 00:28:15,270
But the journalists
responsible for news coverage

625
00:28:15,270 --> 00:28:17,740
were very uniform in a lot of ways.

626
00:28:17,740 --> 00:28:22,380
There were relatively affluent,
highly educated, mostly white, male,

627
00:28:22,380 --> 00:28:24,040
and so forth.

628
00:28:24,040 --> 00:28:28,170
And this had effects on the coverage
of racial politics, economic policy,

629
00:28:28,170 --> 00:28:31,660
and views about the role
of the US in the world.

630
00:28:31,660 --> 00:28:34,720
Moreover, there were seldom
opportunities for the audience

631
00:28:34,720 --> 00:28:37,810
to respond, to develop
new themes or topics,

632
00:28:37,810 --> 00:28:41,110
or level criticism against
the mass media sphere.

633
00:28:41,110 --> 00:28:43,210
There weren't any likes
and comment sections

634
00:28:43,210 --> 00:28:46,270
for the newspaper or
television broadcasts.

635
00:28:46,270 --> 00:28:49,570
If you didn't like it, well, tough luck.

636
00:28:49,570 --> 00:28:51,640
This all started to
change in recent years,

637
00:28:51,640 --> 00:28:56,290
as news coverage not only moved online,
but onto social media platforms.

638
00:28:56,290 --> 00:28:59,830
We now live in a digitally
networked public sphere.

639
00:28:59,830 --> 00:29:03,070
So instead of having a narrow
aperture of communications,

640
00:29:03,070 --> 00:29:06,940
or just a few organizations to
disseminate information to the public,

641
00:29:06,940 --> 00:29:10,600
we now have a digital sphere with a
wide aperture, where lots of people

642
00:29:10,600 --> 00:29:13,170
can share news and information.

643
00:29:13,170 --> 00:29:15,450
More specifically,
the sources of content

644
00:29:15,450 --> 00:29:19,140
are not just organizations and the
professional journalists they employed,

645
00:29:19,140 --> 00:29:22,920
but the public and particularly,
social media users.

646
00:29:22,920 --> 00:29:26,010
Anyone can tweet or post
on Facebook, and anyone

647
00:29:26,010 --> 00:29:28,590
can read those tweets and posts.

648
00:29:28,590 --> 00:29:30,630
It's not only resulted
in greater diversity

649
00:29:30,630 --> 00:29:34,250
of content, but greater
access to information as well.

650
00:29:34,250 --> 00:29:36,900
If you want to follow the news,
there are a ton of options

651
00:29:36,900 --> 00:29:41,500
and free places online you can
access with just a few mouse clicks.

652
00:29:41,500 --> 00:29:43,990
These prospects of increased
diversity and access

653
00:29:43,990 --> 00:29:47,230
are what led many people to believe
that the digital sphere held

654
00:29:47,230 --> 00:29:49,990
great promise for improving
the public discourse that

655
00:29:49,990 --> 00:29:52,540
supports a well-functioning democracy.

656
00:29:52,540 --> 00:29:55,460
And in some ways, this has been true.

657
00:29:55,460 --> 00:29:57,930
For example, thanks to
Twitter and Facebook,

658
00:29:57,930 --> 00:30:00,560
we saw the mobilization of
social justice movements,

659
00:30:00,560 --> 00:30:03,440
like Me, Too and Black Lives Matter.

660
00:30:03,440 --> 00:30:05,780
And the increased
diversity of perspectives

661
00:30:05,780 --> 00:30:08,930
made it possible for individual
researchers and scientists

662
00:30:08,930 --> 00:30:12,350
to weigh in on the CDC'S
claims about coronavirus.

663
00:30:12,350 --> 00:30:16,160
So while the CDC did not initially
say coronavirus was characterized

664
00:30:16,160 --> 00:30:19,363
by airborne transmission,
leading to community spread,

665
00:30:19,363 --> 00:30:21,530
they ended up revising their
stance after scientists

666
00:30:21,530 --> 00:30:25,700
took to Twitter with evidence
proving that this was the case.

667
00:30:25,700 --> 00:30:28,260
While the digital sphere has
brought about some improvements,

668
00:30:28,260 --> 00:30:32,680
it's also exacerbated some problems
and created new challenges.

669
00:30:32,680 --> 00:30:37,120
For example, since anyone can create
content, fact checking and monitoring

670
00:30:37,120 --> 00:30:39,455
have become much more
difficult. People are

671
00:30:39,455 --> 00:30:42,580
left to fend for themselves when it
comes to figuring out whether something

672
00:30:42,580 --> 00:30:45,390
they read online is trustworthy.

673
00:30:45,390 --> 00:30:48,240
We've also seen increased
personalization with respect

674
00:30:48,240 --> 00:30:51,600
to news and information,
where specific content could

675
00:30:51,600 --> 00:30:54,870
be targeted to specific users
by the means of curated news

676
00:30:54,870 --> 00:30:58,260
feeds on social media and
cable news stations cropping up

677
00:30:58,260 --> 00:31:02,310
that take a particular angle
on the news that they cover.

678
00:31:02,310 --> 00:31:03,510
This is significant.

679
00:31:03,510 --> 00:31:06,660
Because we end up with a
somewhat paradoxical effect.

680
00:31:06,660 --> 00:31:09,720
Despite a greater diversity in
the content that's available,

681
00:31:09,720 --> 00:31:12,390
there's less diversity in the
news and information people

682
00:31:12,390 --> 00:31:16,650
actually end up consuming, with the
personalization of information having

683
00:31:16,650 --> 00:31:19,110
a tendency to reinforce
a person's viewpoints,

684
00:31:19,110 --> 00:31:22,070
rather than challenge or broaden them.

685
00:31:22,070 --> 00:31:25,150
Additionally, in the absence
of centralized sources of news,

686
00:31:25,150 --> 00:31:29,650
we've also seen different aims expressed
by those creating and sharing content.

687
00:31:29,650 --> 00:31:33,550
Some have bypassed a concern for truth
in an effort to garner more views

688
00:31:33,550 --> 00:31:37,540
and likes with extremist
content or fake news.

689
00:31:37,540 --> 00:31:39,820
And fake news became a
huge issue around the time

690
00:31:39,820 --> 00:31:43,270
of the 2016 presidential
election, as there were concerns

691
00:31:43,270 --> 00:31:46,330
that the massive spread of
misinformation on social media

692
00:31:46,330 --> 00:31:50,590
could influence or sway
individuals political views.

693
00:31:50,590 --> 00:31:53,380
While the spread of misinformation
has always been an issue,

694
00:31:53,380 --> 00:31:56,740
it's truly been exacerbated
by the digital public sphere,

695
00:31:56,740 --> 00:32:00,910
with social media platforms essentially
pouring gasoline on the fire.

696
00:32:00,910 --> 00:32:04,210
The dissemination of fake
news explodes on social media

697
00:32:04,210 --> 00:32:07,810
because the structure of digital
environments, from likes to retweets,

698
00:32:07,810 --> 00:32:10,900
allows a single post on
fake news to go viral,

699
00:32:10,900 --> 00:32:13,570
reaching the screens of
millions around the world.

700
00:32:13,570 --> 00:32:16,690
And there are serious worries about
how fake news has played a role

701
00:32:16,690 --> 00:32:20,200
in amplifying political polarization.

702
00:32:20,200 --> 00:32:23,350
So while technology has made
possible unique advantages,

703
00:32:23,350 --> 00:32:26,080
it's also brought on unique challenges.

704
00:32:26,080 --> 00:32:28,420
One major question that
we're faced with now

705
00:32:28,420 --> 00:32:32,800
is figuring out how content should be
regulated on social media platforms,

706
00:32:32,800 --> 00:32:34,270
if at all.

707
00:32:34,270 --> 00:32:37,180
Given the scale of the problem,
some might be skeptical,

708
00:32:37,180 --> 00:32:40,760
believing that any form of content
regulation would be impossible.

709
00:32:40,760 --> 00:32:43,660
There's just too many people posting
online to fact check them all.

710
00:32:43,660 --> 00:32:46,990
And fake news spreads so quickly,
it's hard to stop before it's already

711
00:32:46,990 --> 00:32:49,280
reached a huge audience.

712
00:32:49,280 --> 00:32:51,260
There's also worries
that attempts to regulate

713
00:32:51,260 --> 00:32:54,740
content could end up becoming a
form of censorship that violates

714
00:32:54,740 --> 00:32:57,380
the right to freedom of speech.

715
00:32:57,380 --> 00:33:00,020
But some people are more
optimistic about the possibilities

716
00:33:00,020 --> 00:33:03,620
of designing social media platforms
in a way that promotes and preserves

717
00:33:03,620 --> 00:33:05,300
democracy.

718
00:33:05,300 --> 00:33:08,510
In particular, there's a possibility
that with responsibly designed

719
00:33:08,510 --> 00:33:11,300
algorithms and user
interface choices, we

720
00:33:11,300 --> 00:33:13,310
might be able to slow
the spread of fake news

721
00:33:13,310 --> 00:33:17,150
and more generally improve the ways
information is disseminated and engaged

722
00:33:17,150 --> 00:33:19,410
with on social media.

723
00:33:19,410 --> 00:33:23,340
For example, some people believe
that companies like Facebook, Twitter

724
00:33:23,340 --> 00:33:26,340
and YouTube have a responsibility
to regulate content

725
00:33:26,340 --> 00:33:29,460
because of the enormous
influence they have over us.

726
00:33:29,460 --> 00:33:33,300
In particular, it's thought that social
media platforms have a responsibility

727
00:33:33,300 --> 00:33:37,140
to police fake news and reduce the
power of data driven algorithms that

728
00:33:37,140 --> 00:33:40,590
personalize the user experience,
even if doing these things

729
00:33:40,590 --> 00:33:42,960
would come at the cost
of user engagement,

730
00:33:42,960 --> 00:33:48,330
resulting in less time spent on the
platform and less advertising revenue.

731
00:33:48,330 --> 00:33:50,370
It's clear that the path
going forward in terms

732
00:33:50,370 --> 00:33:54,630
of content regulation on social media
platforms is going to be tricky.

733
00:33:54,630 --> 00:33:57,600
Whether or not we promote
democratic ideals or undermine

734
00:33:57,600 --> 00:34:01,670
them will come down to the
particular design choices we make.

735
00:34:01,670 --> 00:34:04,070
In order to use technology
to create solutions

736
00:34:04,070 --> 00:34:06,320
to the problems we're
facing today, we'll

737
00:34:06,320 --> 00:34:08,969
need to make informed
decisions about design choices.

738
00:34:08,969 --> 00:34:12,620
And this requires some critical
thinking about ethics and philosophy

739
00:34:12,620 --> 00:34:15,130
to figure out the best way to do this.

740
00:34:15,130 --> 00:34:17,815
But we're hoping that students
like you, taking CS50,

741
00:34:17,815 --> 00:34:21,594
can harness your creativity, technical
knowledge, and ethical reasoning

742
00:34:21,594 --> 00:34:25,703
to design technology
in a responsible way.

743
00:34:25,703 --> 00:34:27,620
So I'm now going to pass
things over to Meica,

744
00:34:27,620 --> 00:34:30,400
who will tell you about some
philosophical concepts that'll

745
00:34:30,400 --> 00:34:32,679
help you think proactively
about particular design

746
00:34:32,679 --> 00:34:36,550
choices and algorithmic tools that
can be implemented to structure

747
00:34:36,550 --> 00:34:40,960
social media platforms in a way that
promotes democratic public discourse.

748
00:34:40,960 --> 00:34:43,420
MEICA MAGNANI: In Democracy
and The Digital Public Sphere,

749
00:34:43,420 --> 00:34:46,900
an article which offers a fantastic
diagnosis of our situation,

750
00:34:46,900 --> 00:34:49,000
and from which Susan and
I are drawing heavily

751
00:34:49,000 --> 00:34:53,080
upon for this lecture, the authors
Joshua Cohen and Archon Fung,

752
00:34:53,080 --> 00:34:56,409
tell us that the bloom
is off the digital rose.

753
00:34:56,409 --> 00:34:58,840
As Susan was describing,
we had such high hopes

754
00:34:58,840 --> 00:35:02,200
for the democratizing potential
of social media and the internet.

755
00:35:02,200 --> 00:35:05,740
But now we face an environment
in which fake news runs rampant,

756
00:35:05,740 --> 00:35:08,380
citizens appear to be
dramatically polarized,

757
00:35:08,380 --> 00:35:11,020
information swirls in
its own isolated bubbles,

758
00:35:11,020 --> 00:35:14,140
and hate speech reaches
appalling levels of vitriol.

759
00:35:14,140 --> 00:35:17,440
All of which stand to threaten,
or so people speculate,

760
00:35:17,440 --> 00:35:20,290
the conditions required
for an effective democracy.

761
00:35:20,290 --> 00:35:23,500
So the following questions
arise, in what ways

762
00:35:23,500 --> 00:35:25,720
are the conditions of
democracy threatened?

763
00:35:25,720 --> 00:35:27,580
What can or should be done about it?

764
00:35:27,580 --> 00:35:29,920
Is the structure of our
technology responsible?

765
00:35:29,920 --> 00:35:33,610
Or is it just us, as human
beings, creating these problems?

766
00:35:33,610 --> 00:35:35,830
In this module, we're
focusing specifically

767
00:35:35,830 --> 00:35:38,140
on the issue of content regulation.

768
00:35:38,140 --> 00:35:40,960
Social media companies like
Twitter, Facebook, and YouTube

769
00:35:40,960 --> 00:35:44,200
are now all in the game of trying to
address these problems through platform

770
00:35:44,200 --> 00:35:45,970
design and features.

771
00:35:45,970 --> 00:35:49,750
From one angle then, they are acting
in the service of protecting democracy

772
00:35:49,750 --> 00:35:52,960
by trying to get control over
the spread of misinformation,

773
00:35:52,960 --> 00:35:56,530
the amplification of hate speech,
and the deepening of polarization.

774
00:35:56,530 --> 00:35:58,810
However, from another
angle, they're stepping in

775
00:35:58,810 --> 00:36:01,220
to shape the distribution
of information.

776
00:36:01,220 --> 00:36:03,520
And depending on the
particular design choices,

777
00:36:03,520 --> 00:36:05,560
might be set to be
regulating or silencing

778
00:36:05,560 --> 00:36:09,310
speech, which of course, is at
odds with democratic commitments

779
00:36:09,310 --> 00:36:11,590
to free speech and discourse.

780
00:36:11,590 --> 00:36:14,170
The point of this module,
then, is to give you some tools

781
00:36:14,170 --> 00:36:18,040
to think through these issues,
tools for understanding the problem,

782
00:36:18,040 --> 00:36:22,510
diagnosing the sources of the
problem, and brainstorming solutions.

783
00:36:22,510 --> 00:36:24,640
In the remaining 10 or
15 minutes, I'm going

784
00:36:24,640 --> 00:36:27,080
to provide an overview
of the main tools which

785
00:36:27,080 --> 00:36:29,200
you will find detailed in the readings.

786
00:36:29,200 --> 00:36:33,350
They are also the tools you will be
asked to analyze in this week's lab.

787
00:36:33,350 --> 00:36:35,620
So first then, we need to
think clearly about what

788
00:36:35,620 --> 00:36:37,870
is required for a healthy democracy.

789
00:36:37,870 --> 00:36:42,040
If we're going to be making claims
about how tech threatens democracy,

790
00:36:42,040 --> 00:36:46,750
we better understand A, what a democracy
is, and B, what sort of conditions

791
00:36:46,750 --> 00:36:50,860
support democracy, such that those
conditions could come under threat.

792
00:36:50,860 --> 00:36:54,610
In their article, Archon Fung, who
is a professor in political science

793
00:36:54,610 --> 00:36:58,900
here at Harvard, and Joshua Cohen, who
is a political philosopher now working

794
00:36:58,900 --> 00:37:03,800
with the faculty at Apple University,
provide us with these tools.

795
00:37:03,800 --> 00:37:06,070
So behind the idea of
democracy is an ideal

796
00:37:06,070 --> 00:37:08,170
of what political society should be.

797
00:37:08,170 --> 00:37:11,810
Fung and Cohen reduce this
ideal to three elements.

798
00:37:11,810 --> 00:37:15,610
First, the idea of a
democratic society, a society

799
00:37:15,610 --> 00:37:19,982
in which the political culture
views individuals as free and equal.

800
00:37:19,982 --> 00:37:21,940
Even though it is likely
that these people have

801
00:37:21,940 --> 00:37:25,900
different interests, identities,
and systems of belief, as citizens,

802
00:37:25,900 --> 00:37:30,040
they are committed to arriving m through
reflection and discourse, principles

803
00:37:30,040 --> 00:37:33,040
that will enable them to work together
while respecting their freedom

804
00:37:33,040 --> 00:37:35,040
and equality.

805
00:37:35,040 --> 00:37:38,400
Second is the idea of a
democratic political regime, which

806
00:37:38,400 --> 00:37:42,060
is characterized by regular
elections, rights of participation,

807
00:37:42,060 --> 00:37:44,160
along with associative
and expressive rights

808
00:37:44,160 --> 00:37:48,220
that make participation
both informed and effective.

809
00:37:48,220 --> 00:37:52,030
Third and lastly is the idea
of a deliberative democracy,

810
00:37:52,030 --> 00:37:54,130
according to which
political discussion should

811
00:37:54,130 --> 00:37:56,620
appeal to reasons that are
suitable for cooperation

812
00:37:56,620 --> 00:38:00,070
amongst free and equal persons.

813
00:38:00,070 --> 00:38:04,330
So in justifying a policy, you cannot
appeal to, say, your own religion,

814
00:38:04,330 --> 00:38:07,660
given that others do not
necessarily hold those same beliefs.

815
00:38:07,660 --> 00:38:10,900
You can appeal to the notion of,
say, religious freedom, but not

816
00:38:10,900 --> 00:38:14,990
the particular beliefs contained
within the religion itself.

817
00:38:14,990 --> 00:38:17,440
So democracy, then,
is basically an ideal

818
00:38:17,440 --> 00:38:20,920
that we govern ourselves by collective
decision making, decision making that

819
00:38:20,920 --> 00:38:23,200
respects our freedom and equality.

820
00:38:23,200 --> 00:38:26,980
This decision making consists not only
of the formal procedures of voting,

821
00:38:26,980 --> 00:38:31,750
elections, and legislation, it is also
informed by the informal public sphere,

822
00:38:31,750 --> 00:38:35,140
that is, citizens identifying
problems and concerns,

823
00:38:35,140 --> 00:38:38,890
discussing and debating problems,
expressing opinions, challenging

824
00:38:38,890 --> 00:38:41,590
viewpoints, and
organizing around causes.

825
00:38:41,590 --> 00:38:45,850
This is an absolutely critical part of
the democratic decision-making process.

826
00:38:45,850 --> 00:38:51,010
It is where we, as the public,
form, test, disperse, exchange,

827
00:38:51,010 --> 00:38:53,290
challenge, and revise our views.

828
00:38:53,290 --> 00:38:56,080
The flow of information,
along with user engagement

829
00:38:56,080 --> 00:38:58,510
on Facebook, YouTube,
and Twitter, are all part

830
00:38:58,510 --> 00:39:01,740
of this informal public sphere.

831
00:39:01,740 --> 00:39:03,840
In order that individuals
can participate

832
00:39:03,840 --> 00:39:07,200
as free and equal citizens in
this arena of public discourse,

833
00:39:07,200 --> 00:39:10,140
Cohen and Fung lay out a set
of rights and opportunities

834
00:39:10,140 --> 00:39:12,930
that a well-functioning
democracy will require.

835
00:39:12,930 --> 00:39:15,900
And these are the tools
of analysis on offer.

836
00:39:15,900 --> 00:39:18,030
So first, rights.

837
00:39:18,030 --> 00:39:20,610
"As citizens of a
democracy, we have rights

838
00:39:20,610 --> 00:39:24,930
to basic liberties, liberties
of expression and association.

839
00:39:24,930 --> 00:39:27,090
The right to expressive
liberty is important

840
00:39:27,090 --> 00:39:29,160
not only for the freedom
of the individual,

841
00:39:29,160 --> 00:39:33,520
so that he or she will not be censored,
but also for democracy itself.

842
00:39:33,520 --> 00:39:37,410
It enables citizens to bring their
ideas into conversation with one another

843
00:39:37,410 --> 00:39:41,250
and to criticize and hold accountable
those who exercise power."

844
00:39:41,250 --> 00:39:44,700
Second is the opportunity
for expression.

845
00:39:44,700 --> 00:39:47,040
"Not only should we be
free of censorship, but

846
00:39:47,040 --> 00:39:50,940
we should have fair opportunity to
participate in public discussion.

847
00:39:50,940 --> 00:39:53,190
It shouldn't be the case
that because someone is, say,

848
00:39:53,190 --> 00:39:58,260
wealthier or more powerful, that they
have more opportunity to participate."

849
00:39:58,260 --> 00:40:00,390
Third is access.

850
00:40:00,390 --> 00:40:03,150
"Each person should have
good and equal access

851
00:40:03,150 --> 00:40:06,540
to quality and reliable
information on public matters.

852
00:40:06,540 --> 00:40:08,640
That is, if we make
the effort, we should

853
00:40:08,640 --> 00:40:10,690
be able to acquire this information.

854
00:40:10,690 --> 00:40:16,200
Effective participation in decision
making requires being informed."

855
00:40:16,200 --> 00:40:18,540
Fourth is diversity.

856
00:40:18,540 --> 00:40:23,070
"Each person should have good and equal
chances to hear a wide range of views.

857
00:40:23,070 --> 00:40:25,440
We need access to competing
views in order to have

858
00:40:25,440 --> 00:40:28,410
a more informed and reasoned position"

859
00:40:28,410 --> 00:40:31,620
And lastly, number five,
communicative power.

860
00:40:31,620 --> 00:40:35,880
"Citizens should have good and equal
chances to explore interests and ideas

861
00:40:35,880 --> 00:40:37,570
in association with others.

862
00:40:37,570 --> 00:40:40,710
And through these associations,
to develop new concerns that

863
00:40:40,710 --> 00:40:44,120
might challenge the mainstream view."

864
00:40:44,120 --> 00:40:47,360
These rights and opportunities
together provide critical conditions

865
00:40:47,360 --> 00:40:50,360
for enabling participation
in public discussion.

866
00:40:50,360 --> 00:40:53,120
They might seem like a lot
to keep track of initially,

867
00:40:53,120 --> 00:40:56,420
but if we're going to think through
how social media threatens democracy,

868
00:40:56,420 --> 00:41:00,210
and more concretely, how platform design
might promote or hinder democracy,

869
00:41:00,210 --> 00:41:01,910
these are valuable tools.

870
00:41:01,910 --> 00:41:04,700
We can use, say, the
access condition, the idea

871
00:41:04,700 --> 00:41:07,280
that we should all have access
to reliable information,

872
00:41:07,280 --> 00:41:09,470
as a lens of analysis.

873
00:41:09,470 --> 00:41:11,840
Does our platform prevent
certain groups or users

874
00:41:11,840 --> 00:41:14,270
from accessing reliable information?

875
00:41:14,270 --> 00:41:17,270
Or we can use the diversity
condition, the idea

876
00:41:17,270 --> 00:41:20,180
that we should all have access to
a plurality of conflicting views

877
00:41:20,180 --> 00:41:22,020
as a lens of analysis.

878
00:41:22,020 --> 00:41:25,640
So for example, we might ask
ourselves, does our platform

879
00:41:25,640 --> 00:41:28,880
create a filter bubble in which
individuals are no longer confronted

880
00:41:28,880 --> 00:41:31,610
with opposing views?

881
00:41:31,610 --> 00:41:34,100
In addition to understanding
what conditions support

882
00:41:34,100 --> 00:41:38,690
a democratic society, we also need
to understand the purported problems

883
00:41:38,690 --> 00:41:42,010
before we can propose
effective interventions.

884
00:41:42,010 --> 00:41:43,720
Consider fake news.

885
00:41:43,720 --> 00:41:47,080
Why are people so gullible
when it comes to fake news?

886
00:41:47,080 --> 00:41:51,490
Why do they often repost without
proper critical assessment?

887
00:41:51,490 --> 00:41:54,490
Regina Rini, in the reading,
proposes that in order

888
00:41:54,490 --> 00:41:56,620
to understand the
phenomenon of fake news,

889
00:41:56,620 --> 00:41:59,780
we should think about it
as a form of testimony.

890
00:41:59,780 --> 00:42:01,780
When another person shares
information with you,

891
00:42:01,780 --> 00:42:04,310
you typically take it to be true.

892
00:42:04,310 --> 00:42:07,240
This is because of the norms
governing our practice of testimony.

893
00:42:07,240 --> 00:42:09,940
When you assert something,
passing it on to others,

894
00:42:09,940 --> 00:42:12,580
you typically take
responsibility for its truth.

895
00:42:12,580 --> 00:42:15,160
It is assumed that you have
either acquired evidence

896
00:42:15,160 --> 00:42:18,460
for yourself or you've received
this information from a source

897
00:42:18,460 --> 00:42:20,440
that you deem reliable.

898
00:42:20,440 --> 00:42:23,710
Most of our knowledge about the
world comes through this practice.

899
00:42:23,710 --> 00:42:27,280
We could not possibly acquire
evidence for all the beliefs we hold.

900
00:42:27,280 --> 00:42:31,860
So we often have to rely on sources
we deem and hope to be credible.

901
00:42:31,860 --> 00:42:36,690
But social media, Rini points out,
has unsettled testimonial norms.

902
00:42:36,690 --> 00:42:41,670
When someone posts a piece of news,
we seem to hold two conflicting views.

903
00:42:41,670 --> 00:42:44,730
On the one hand, we see it
as an active endorsement.

904
00:42:44,730 --> 00:42:48,660
The person posting has taken
some degree of responsibility

905
00:42:48,660 --> 00:42:51,210
for the accuracy of their
post, the same way one

906
00:42:51,210 --> 00:42:54,480
would before passing on
information in a conversation.

907
00:42:54,480 --> 00:42:57,240
On the other hand,
though, it's just a share.

908
00:42:57,240 --> 00:43:00,480
We see this attitude coming through
when Donald Trump, called out

909
00:43:00,480 --> 00:43:06,060
on one of his questionable tweets,
retorts with, eh, it's just a tweet.

910
00:43:06,060 --> 00:43:09,090
To fight fake news,
then, Rini argues that we

911
00:43:09,090 --> 00:43:13,440
need to stabilize social media's norms
of testimony so that, as she says,

912
00:43:13,440 --> 00:43:15,870
the same norms that keep
us honest over cocktails

913
00:43:15,870 --> 00:43:18,150
will keep us honest in our posts.

914
00:43:18,150 --> 00:43:20,340
We need people to be
held accountable for,

915
00:43:20,340 --> 00:43:22,920
or to have a sense of
responsibility, for the information

916
00:43:22,920 --> 00:43:24,840
that they share with others.

917
00:43:24,840 --> 00:43:29,650
Her concrete proposal, give
users a credibility score.

918
00:43:29,650 --> 00:43:33,300
So in practice, this would be an
amendment to Facebook's system,

919
00:43:33,300 --> 00:43:35,910
using independent fact
checking organizations,

920
00:43:35,910 --> 00:43:41,460
Facebook flags problematic news and
warns users before they repost it.

921
00:43:41,460 --> 00:43:44,280
When a user tries to post
something that has been identified

922
00:43:44,280 --> 00:43:47,760
as false or misleading, a pop up
appears that explains the problem

923
00:43:47,760 --> 00:43:49,930
and identifies the original source.

924
00:43:49,930 --> 00:43:51,930
It then asks the user to
confirm that they would

925
00:43:51,930 --> 00:43:54,620
like to continue with their repost.

926
00:43:54,620 --> 00:43:57,690
A user's credibility
score, for Rini, would

927
00:43:57,690 --> 00:44:00,450
depend on how often they
choose to ignore these warnings

928
00:44:00,450 --> 00:44:02,820
and pass on misleading information.

929
00:44:02,820 --> 00:44:05,220
Quote, "a green dot by
the user's name could

930
00:44:05,220 --> 00:44:08,820
indicate that the user hasn't
chosen to share much disputed news.

931
00:44:08,820 --> 00:44:11,250
A yellow dot could indicate
that they do it sometimes.

932
00:44:11,250 --> 00:44:13,350
And a red could indicate
that they do it often."

933
00:44:13,350 --> 00:44:14,670
Unquote.

934
00:44:14,670 --> 00:44:16,710
The idea, then, is that
a credibility score

935
00:44:16,710 --> 00:44:18,960
would incentivize users
to take responsibility

936
00:44:18,960 --> 00:44:22,470
for what they share and would also give
others a sense of their reliability

937
00:44:22,470 --> 00:44:24,690
as sources.

938
00:44:24,690 --> 00:44:27,540
So Rini comes up with this
solution through a careful analysis

939
00:44:27,540 --> 00:44:30,240
of why we are so gullible to fake news.

940
00:44:30,240 --> 00:44:33,090
I will leave it up to you
to consider this proposal

941
00:44:33,090 --> 00:44:35,730
in light of the various rights
and opportunities required

942
00:44:35,730 --> 00:44:38,010
for a democratic public sphere.

943
00:44:38,010 --> 00:44:41,190
Does Rini's proposal violate or
threaten freedom of expression?

944
00:44:41,190 --> 00:44:45,000
Does it promote or hinder our access
to reliable information, our access

945
00:44:45,000 --> 00:44:48,300
to diversity of views, or
does it promote or hinder

946
00:44:48,300 --> 00:44:50,190
our communicative power?

947
00:44:50,190 --> 00:44:51,990
It is these sorts of
questions that we hope

948
00:44:51,990 --> 00:44:54,032
that you will start to
ask yourself when thinking

949
00:44:54,032 --> 00:44:56,760
through the following sorts of issues.

950
00:44:56,760 --> 00:45:00,520
What problems do fake news, hate
speech, polarization, et cetera

951
00:45:00,520 --> 00:45:02,550
pose to democracy?

952
00:45:02,550 --> 00:45:05,830
How successful are various attempts
by companies like Twitter, YouTube,

953
00:45:05,830 --> 00:45:08,370
and Facebook to address these problems?

954
00:45:08,370 --> 00:45:11,400
And how might particular design
features of social media platforms

955
00:45:11,400 --> 00:45:15,180
promote or hinder these particular
rights and opportunities?

956
00:45:15,180 --> 00:45:18,750
Whether as a future computer
scientist, a tech industry leader,

957
00:45:18,750 --> 00:45:22,950
or just as a user of these technologies,
we hope asking these sorts of questions

958
00:45:22,950 --> 00:45:25,783
will help you navigate these tricky
issues with a more critical eye.

959
00:45:25,783 --> 00:45:28,867
SUSAN KENNEDY: We're really looking
forward to the sorts of design choices

960
00:45:28,867 --> 00:45:30,850
that you'll be making in the future.

961
00:45:30,850 --> 00:45:31,200
MEICA MAGNANI: Great.

962
00:45:31,200 --> 00:45:32,867
Thanks so much for having us here today.

963
00:45:32,867 --> 00:45:36,052
And best of luck to everybody.

964
00:45:36,052 --> 00:45:39,010
DAVID MALAN: Well, thank you so much
to Susan and Meica for joining us.

965
00:45:39,010 --> 00:45:41,510
Indeed, in this coming week's
lab, we'll have an opportunity

966
00:45:41,510 --> 00:45:44,050
to consider some of these issues
in the context of some very

967
00:45:44,050 --> 00:45:46,730
specific real world scenarios.

968
00:45:46,730 --> 00:45:49,690
So we now thought we would take
a look forward at what you can do

969
00:45:49,690 --> 00:45:52,300
and how you can do it
after CS50 when it comes

970
00:45:52,300 --> 00:45:56,530
to the more practical side of things
beyond computational thinking alone.

971
00:45:56,530 --> 00:45:58,910
So programming, of
course, for many of you,

972
00:45:58,910 --> 00:46:02,127
this will be by design the only
computer science or programming course

973
00:46:02,127 --> 00:46:02,710
that you take.

974
00:46:02,710 --> 00:46:04,030
And that's certainly OK.

975
00:46:04,030 --> 00:46:06,130
Indeed, we hope that you'll
be able now to return

976
00:46:06,130 --> 00:46:09,970
to your own domains of interest in the
arts and humanities, social sciences,

977
00:46:09,970 --> 00:46:13,840
or sciences and actually be able to
have a concrete set of practical skills

978
00:46:13,840 --> 00:46:16,630
be it in Python or C or any of
the other technical languages

979
00:46:16,630 --> 00:46:20,470
we looked at and can actually solve
problems in your own preferred domain.

980
00:46:20,470 --> 00:46:23,140
And if you're interested in
learning more about computer science

981
00:46:23,140 --> 00:46:25,330
itself and moving on
in that world, we hope

982
00:46:25,330 --> 00:46:27,400
that you'll walk away
with a solid foundation

983
00:46:27,400 --> 00:46:32,800
for further theoretical and systematic
explorations of this particular field.

984
00:46:32,800 --> 00:46:34,720
But very practically
speaking, we hope now

985
00:46:34,720 --> 00:46:38,230
that you can not only program,
but also ask questions better,

986
00:46:38,230 --> 00:46:41,240
whether that's in the technical
world or even in just the real world.

987
00:46:41,240 --> 00:46:44,810
Odds are, if you've ever asked a
question on CS50's discussion forums,

988
00:46:44,810 --> 00:46:48,040
the teaching fellows or I might have
very well responded with questions

989
00:46:48,040 --> 00:46:49,210
asking you to clarify.

990
00:46:49,210 --> 00:46:52,090
Or better yet, you would have
provided us, in anticipation,

991
00:46:52,090 --> 00:46:55,000
with answers to all of the
questions that we might have.

992
00:46:55,000 --> 00:46:56,950
And if you've noticed
on Ed, we deliberately

993
00:46:56,950 --> 00:46:59,230
have this sort of
template via which you're

994
00:46:59,230 --> 00:47:02,242
coaxed to answer, well, what are
the symptoms that you are seeing?

995
00:47:02,242 --> 00:47:04,450
What's the error message
that you're struggling with?

996
00:47:04,450 --> 00:47:06,670
What steps have you tried
to resolve the problem?

997
00:47:06,670 --> 00:47:09,295
Because if we imagine in the real
world, even just reaching out

998
00:47:09,295 --> 00:47:11,860
to some random company's
customer service line,

999
00:47:11,860 --> 00:47:14,450
those are exactly the kinds
of questions that someone else

1000
00:47:14,450 --> 00:47:16,450
is going to have to ask
you to better understand

1001
00:47:16,450 --> 00:47:18,710
a problem from your own perspective.

1002
00:47:18,710 --> 00:47:22,270
And so we would encourage you to think
about, as you emerge from CS50 itself,

1003
00:47:22,270 --> 00:47:24,550
just how to ask better
questions of people.

1004
00:47:24,550 --> 00:47:26,420
If you've got more
information than they,

1005
00:47:26,420 --> 00:47:29,950
how can you succinctly but correctly
convey that information to them

1006
00:47:29,950 --> 00:47:32,710
so that they can help
you more efficiently.

1007
00:47:32,710 --> 00:47:34,120
But also, finding answers.

1008
00:47:34,120 --> 00:47:39,970
Like, we absolutely understand that many
of CS50's weeks, all of CS50's weeks,

1009
00:47:39,970 --> 00:47:42,118
maybe, have been quite the frustration.

1010
00:47:42,118 --> 00:47:45,160
Because you quite often feel like,
well, we didn't cover that in lecture.

1011
00:47:45,160 --> 00:47:46,720
Or I didn't see that in section.

1012
00:47:46,720 --> 00:47:48,280
And I see some noddings of the head.

1013
00:47:48,280 --> 00:47:49,690
So this seems to be the case.

1014
00:47:49,690 --> 00:47:52,870
And much as I would love to
reassure otherwise, like,

1015
00:47:52,870 --> 00:47:54,580
that was very much the intent.

1016
00:47:54,580 --> 00:47:57,790
Because the last of the training
wheels of any course like this

1017
00:47:57,790 --> 00:47:59,640
now really do officially come off.

1018
00:47:59,640 --> 00:48:01,390
And in the coming
weeks, while we'll still

1019
00:48:01,390 --> 00:48:03,782
be with you to lend a
hand with final projects

1020
00:48:03,782 --> 00:48:05,740
and answer questions
along those lines, there's

1021
00:48:05,740 --> 00:48:09,970
of course no specification for the final
project telling you exactly what to do,

1022
00:48:09,970 --> 00:48:12,780
or in what language to do
it, or what libraries to use.

1023
00:48:12,780 --> 00:48:14,530
Undoubtedly, in the
coming weeks, you will

1024
00:48:14,530 --> 00:48:16,870
run into error messages you
haven't even seen before.

1025
00:48:16,870 --> 00:48:20,380
And frankly, maybe I, maybe Brian,
maybe the teaching assistants,

1026
00:48:20,380 --> 00:48:23,260
and the course assistants haven't
even seen those errors before.

1027
00:48:23,260 --> 00:48:26,470
But the goal, of course, is to get
you over those hurdles in a way

1028
00:48:26,470 --> 00:48:29,050
that you can figure out how to
do those things on your own.

1029
00:48:29,050 --> 00:48:32,380
And so when it comes to just using
the internet, be it Google, or Stack

1030
00:48:32,380 --> 00:48:34,360
Overflow, or interacting
with other humans,

1031
00:48:34,360 --> 00:48:37,870
just finding answers when it
comes to the world of programming

1032
00:48:37,870 --> 00:48:41,800
or really just the world of
problem solving more generally,

1033
00:48:41,800 --> 00:48:44,450
we hope that is actually
a lasting skill.

1034
00:48:44,450 --> 00:48:48,040
And we hope that you've been able to
do that with admittedly frustration,

1035
00:48:48,040 --> 00:48:51,320
but with the safety net of the course
underneath you all these months.

1036
00:48:51,320 --> 00:48:55,180
But here on out, we hope you'll be more
comfortable, again, being uncomfortable

1037
00:48:55,180 --> 00:48:56,972
as you figure out new things.

1038
00:48:56,972 --> 00:48:58,930
And part of that is just
reading documentation.

1039
00:48:58,930 --> 00:49:02,440
And here, too, this is a frustration
that may very well never go away.

1040
00:49:02,440 --> 00:49:06,430
Like, some documentation out there for
certain languages or libraries, just

1041
00:49:06,430 --> 00:49:07,150
isn't good.

1042
00:49:07,150 --> 00:49:10,630
It was written by people
that just don't think

1043
00:49:10,630 --> 00:49:14,780
like you or I do, don't think with the
same form of empathy as you might hope.

1044
00:49:14,780 --> 00:49:18,100
And therefore, it's written at a
very low level of technical detail,

1045
00:49:18,100 --> 00:49:20,470
and they don't just tell you
what does the function do.

1046
00:49:20,470 --> 00:49:23,140
Or conversely, it's written at
such a high level that, my God,

1047
00:49:23,140 --> 00:49:25,990
you have to start looking at
the source code of the library

1048
00:49:25,990 --> 00:49:27,670
to even figure out how to use it.

1049
00:49:27,670 --> 00:49:29,770
And you will see both extremes.

1050
00:49:29,770 --> 00:49:33,280
But getting comfortable with reading
things like Python's documentation,

1051
00:49:33,280 --> 00:49:36,670
like some API's documentation is
just going to empower you, we hope,

1052
00:49:36,670 --> 00:49:39,070
all the more to just
do much cooler things

1053
00:49:39,070 --> 00:49:42,910
and solve more powerful problems
on your own, ultimately.

1054
00:49:42,910 --> 00:49:46,030
And then lastly, and this is perhaps
the biggest one, teaching you

1055
00:49:46,030 --> 00:49:47,860
how to teach yourself new languages.

1056
00:49:47,860 --> 00:49:51,250
There is a reason we didn't
spend that much time on Python.

1057
00:49:51,250 --> 00:49:53,680
And we spent even less
time on JavaScript.

1058
00:49:53,680 --> 00:49:55,840
And about an equal
amount of time on SQL.

1059
00:49:55,840 --> 00:49:58,603
We spent a number of
weeks on C, not because C

1060
00:49:58,603 --> 00:50:00,520
is more important than
any of those languages,

1061
00:50:00,520 --> 00:50:02,740
but because along the way,
many of you, most of you

1062
00:50:02,740 --> 00:50:04,750
were just learning programming itself.

1063
00:50:04,750 --> 00:50:08,320
And even as the language has changed
and evolved as the course went on,

1064
00:50:08,320 --> 00:50:09,520
the ideas didn't go away.

1065
00:50:09,520 --> 00:50:12,400
There were still functions, and
conditions, and loops, and even

1066
00:50:12,400 --> 00:50:13,550
events by terms.

1067
00:50:13,550 --> 00:50:16,690
And, again, so we hope that you
walk away from a class like this

1068
00:50:16,690 --> 00:50:19,450
not thinking that, oh, I
learned how to program in C.

1069
00:50:19,450 --> 00:50:21,490
Or oh, I learned how
to program in Python.

1070
00:50:21,490 --> 00:50:24,610
Because none of us have been
experts at those things yet.

1071
00:50:24,610 --> 00:50:28,390
But you certainly are now more
expert at just being a programmer

1072
00:50:28,390 --> 00:50:31,360
and figuring out what holes you need
to fill in in your knowledge, what

1073
00:50:31,360 --> 00:50:33,740
gaps you need to fill
in order to figure out,

1074
00:50:33,740 --> 00:50:36,880
oh, what is the syntax for this
same approach in this language

1075
00:50:36,880 --> 00:50:38,738
as I've already seen in another.

1076
00:50:38,738 --> 00:50:41,530
And that's, indeed, why we compared
so many of these languages side

1077
00:50:41,530 --> 00:50:45,640
by side to just reinforce that
the ideas are no different, even

1078
00:50:45,640 --> 00:50:49,797
though the syntax is going to require
a bunch of Googling, a bunch of asking.

1079
00:50:49,797 --> 00:50:52,630
And that, too, is something we hope
you'll be able to do on your own

1080
00:50:52,630 --> 00:50:56,832
as the next and best thing comes
along well after these languages.

1081
00:50:56,832 --> 00:50:58,540
Well, speaking of
training wheels, you're

1082
00:50:58,540 --> 00:51:02,410
welcome and encouraged to keep using
CS50 IDE for your final project.

1083
00:51:02,410 --> 00:51:05,510
And heck, you can use it even after
that for other courses or projects.

1084
00:51:05,510 --> 00:51:08,260
But at the end of the day, this,
too, is probably a training wheel

1085
00:51:08,260 --> 00:51:10,360
that you should take off for yourself.

1086
00:51:10,360 --> 00:51:14,720
The IDE is designed to be representative
of a real world programming

1087
00:51:14,720 --> 00:51:15,220
environment.

1088
00:51:15,220 --> 00:51:17,620
But we definitely did a
lot of things for you.

1089
00:51:17,620 --> 00:51:20,392
We installed all the
libraries you might need over

1090
00:51:20,392 --> 00:51:21,850
the course of the semester for you.

1091
00:51:21,850 --> 00:51:24,580
We've got these nice commands
that end in the number 50.

1092
00:51:24,580 --> 00:51:27,678
Those don't tend to exist in the real
world when you're at your first job,

1093
00:51:27,678 --> 00:51:29,470
or you're going back
to your own department

1094
00:51:29,470 --> 00:51:31,210
and solving some
problem in code, there's

1095
00:51:31,210 --> 00:51:32,937
not going to be a help50 longer term.

1096
00:51:32,937 --> 00:51:35,770
And so what we thought we would do,
too, is spend just a few minutes

1097
00:51:35,770 --> 00:51:38,890
giving you a sense of what are
some of the more industry standard

1098
00:51:38,890 --> 00:51:42,010
tools that you should consider
using, playing with, perhaps

1099
00:51:42,010 --> 00:51:45,820
over break or in the months to come,
so that you know exactly how to do

1100
00:51:45,820 --> 00:51:48,880
the same kinds of things you did
this term, but on your own Mac

1101
00:51:48,880 --> 00:51:51,380
or PC or some other device.

1102
00:51:51,380 --> 00:51:55,870
So for instance, if you would like to
install a set of command line tools

1103
00:51:55,870 --> 00:51:59,200
on your Mac or PC, turns out
some of them are already there.

1104
00:51:59,200 --> 00:52:01,510
Indeed, I mentioned at
one point that Mac OS

1105
00:52:01,510 --> 00:52:05,170
has under its applications folder
utility's terminal a terminal window.

1106
00:52:05,170 --> 00:52:07,180
And Windows has an analog as well.

1107
00:52:07,180 --> 00:52:10,810
But there's other commands that don't
necessarily come with your Mac or PC,

1108
00:52:10,810 --> 00:52:13,900
for instance, a compiler
for C or some other tools.

1109
00:52:13,900 --> 00:52:17,650
And so we would encourage you to visit
URLs like these on your Mac or PC,

1110
00:52:17,650 --> 00:52:21,490
respectively, if you'd like to just
install more of the command line tools

1111
00:52:21,490 --> 00:52:26,120
that you saw and used in
CS50 in your own environment.

1112
00:52:26,120 --> 00:52:29,380
Another tool we would recommend
that you read up on, or in this case

1113
00:52:29,380 --> 00:52:31,650
watch, a video by Brian, is Git.

1114
00:52:31,650 --> 00:52:35,410
Git is an example of version
control, a fundamental building

1115
00:52:35,410 --> 00:52:38,650
block of any good software
practice these days.

1116
00:52:38,650 --> 00:52:42,460
We kind of use Git in CS50, but
we hide this detail from you.

1117
00:52:42,460 --> 00:52:46,465
Any time you have run check50 or
submit50, we underneath the hood,

1118
00:52:46,465 --> 00:52:51,230
have been running an open source command
called Git, which pushes your code,

1119
00:52:51,230 --> 00:52:54,550
in this case from CS50 IDE to
GitHub.com, which is just one

1120
00:52:54,550 --> 00:52:56,980
of several popular
websites via which you

1121
00:52:56,980 --> 00:53:00,400
can host code, share code, collaborate
on code, run automated tests,

1122
00:53:00,400 --> 00:53:01,220
and the like.

1123
00:53:01,220 --> 00:53:05,020
But Git itself can be used to
put an end to the convention

1124
00:53:05,020 --> 00:53:08,473
that you probably have, even with things
like Microsoft Word or Google Docs,

1125
00:53:08,473 --> 00:53:11,140
where when you want to save
something or another copy of a file,

1126
00:53:11,140 --> 00:53:14,020
maybe you just changed the
end of the file name to 2,

1127
00:53:14,020 --> 00:53:16,720
and then the next time to 3, or to 4.

1128
00:53:16,720 --> 00:53:19,568
Or maybe you do dash Sunday
night, dash Monday morning.

1129
00:53:19,568 --> 00:53:21,610
I mean, I'm still guilty
of this sometimes when I

1130
00:53:21,610 --> 00:53:22,960
want to version my files.

1131
00:53:22,960 --> 00:53:24,770
There are better ways to do that.

1132
00:53:24,770 --> 00:53:26,770
And so if you find
yourself in the future,

1133
00:53:26,770 --> 00:53:29,650
doing something that you think
there's got to be a better way,

1134
00:53:29,650 --> 00:53:33,007
Git is an example of one
of those better ways.

1135
00:53:33,007 --> 00:53:34,840
And if you watch this
particular video, read

1136
00:53:34,840 --> 00:53:38,950
up a bit more, it will help you not only
maintain multiple versions, in essence,

1137
00:53:38,950 --> 00:53:42,010
backups of your own code, it
will also empower you ultimately

1138
00:53:42,010 --> 00:53:44,230
to collaborate with others.

1139
00:53:44,230 --> 00:53:46,390
As for text editors,
the tool that you might

1140
00:53:46,390 --> 00:53:50,225
use to actually write code, perhaps
one of the latest and greatest and most

1141
00:53:50,225 --> 00:53:52,600
popular out there these days
is something called VS Code.

1142
00:53:52,600 --> 00:53:56,260
This is an open source tool that you
can download on your own Mac and PCs.

1143
00:53:56,260 --> 00:53:58,840
Increasingly, it's available
on the web as well.

1144
00:53:58,840 --> 00:54:02,170
But this is one of the most popular
tools, certainly, out there today.

1145
00:54:02,170 --> 00:54:03,410
But it's just a text editor.

1146
00:54:03,410 --> 00:54:06,700
And there are absolutely alternatives
to each and every one of these tools

1147
00:54:06,700 --> 00:54:09,352
that you're certainly welcome
to take a look at as well.

1148
00:54:09,352 --> 00:54:11,560
Well, if you're interested
in the web side of things,

1149
00:54:11,560 --> 00:54:14,200
and you want to host a website,
like a static website, just

1150
00:54:14,200 --> 00:54:17,560
your own personal homepage,
GitHub pages is a thing.

1151
00:54:17,560 --> 00:54:18,700
Netlify is a thing.

1152
00:54:18,700 --> 00:54:21,800
And dot dot dot, there are so
many other web hosts out there,

1153
00:54:21,800 --> 00:54:24,640
many of which offer
free or student level

1154
00:54:24,640 --> 00:54:28,240
accounts so that they don't
necessarily need to even cost anything.

1155
00:54:28,240 --> 00:54:30,010
But static is different from dynamic.

1156
00:54:30,010 --> 00:54:33,370
And if you actually want to host a
web application that actually takes

1157
00:54:33,370 --> 00:54:35,560
user input, stores things
in a database, does

1158
00:54:35,560 --> 00:54:37,650
more interesting things
than a static website,

1159
00:54:37,650 --> 00:54:40,150
you might want to use something
called Heroku, which is just

1160
00:54:40,150 --> 00:54:43,930
a popular third party service that also
has a free entry level account that you

1161
00:54:43,930 --> 00:54:46,030
can use to start playing
with, quite commonly used

1162
00:54:46,030 --> 00:54:47,560
by students for final projects.

1163
00:54:47,560 --> 00:54:50,630
And then there's other providers
out there, bigger cloud providers,

1164
00:54:50,630 --> 00:54:52,888
so to speak, like Amazon,
and Microsoft, and Google,

1165
00:54:52,888 --> 00:54:55,180
for which the learning curve's
perhaps a little higher.

1166
00:54:55,180 --> 00:54:58,540
But they, too, are really good
typically about providing discounts

1167
00:54:58,540 --> 00:55:01,690
or free accounts for
student uses as well.

1168
00:55:01,690 --> 00:55:04,180
How to stay abreast of
topics in technology.

1169
00:55:04,180 --> 00:55:07,180
We focus, of course, in a class
like this really on fundamentals.

1170
00:55:07,180 --> 00:55:10,600
But you're not going to be able to pick
up the news in any form down the road

1171
00:55:10,600 --> 00:55:12,790
and not see something
that's technology related.

1172
00:55:12,790 --> 00:55:15,850
And if you'd just like to keep
your fingers on the pulse of things

1173
00:55:15,850 --> 00:55:18,940
in the tech world more generally,
here's just a few places

1174
00:55:18,940 --> 00:55:21,010
that you might enjoy staying abreast of.

1175
00:55:21,010 --> 00:55:24,285
So Reddit has a couple of different
communities, or subreddit, specifically

1176
00:55:24,285 --> 00:55:26,410
about programming, both
for experienced programmers

1177
00:55:26,410 --> 00:55:28,035
and those of us who are still learning.

1178
00:55:28,035 --> 00:55:29,785
Stack Overflow, of
course, you've probably

1179
00:55:29,785 --> 00:55:32,380
used to solve small problems
over the course of the past term.

1180
00:55:32,380 --> 00:55:34,610
Server Fault is similar
in spirit to that,

1181
00:55:34,610 --> 00:55:38,770
but it's focused more on administration,
Linux-type stuff as well.

1182
00:55:38,770 --> 00:55:41,718
Techcrunch is a popular place, not
just for consumer-focused news,

1183
00:55:41,718 --> 00:55:43,760
but just really anything
that's trending in tech.

1184
00:55:43,760 --> 00:55:47,650
And then a website called Hacker
News on YCombinator's site

1185
00:55:47,650 --> 00:55:49,982
that also is a place to just
glance at once in a while

1186
00:55:49,982 --> 00:55:52,690
because you'll see the latest and
greatest libraries or something

1187
00:55:52,690 --> 00:55:53,770
that's quite nascent.

1188
00:55:53,770 --> 00:55:56,230
So if in general you just want
to get a sense of what's new

1189
00:55:56,230 --> 00:55:58,150
and what's trending out there
in the tech world, things

1190
00:55:58,150 --> 00:56:00,442
that you should just be aware
of even if you don't care

1191
00:56:00,442 --> 00:56:02,860
to get into the weeds of
doing those things hands on,

1192
00:56:02,860 --> 00:56:06,740
these are all good sites and
surely others out there as well.

1193
00:56:06,740 --> 00:56:10,180
And then, CS50, of course, has its
own online community, some of which

1194
00:56:10,180 --> 00:56:13,420
some of you have been part for some
time, in high school or even prior.

1195
00:56:13,420 --> 00:56:15,700
Please feel free to keep in
touch with us in some way,

1196
00:56:15,700 --> 00:56:17,950
or give back a little something
to your successors who

1197
00:56:17,950 --> 00:56:20,140
might take this or another
course down the road

1198
00:56:20,140 --> 00:56:23,290
and participate not only in asking
questions in these communities here,

1199
00:56:23,290 --> 00:56:27,470
but also in answering
others' questions as well.

1200
00:56:27,470 --> 00:56:30,100
So we thought we would do a
little less of the talking

1201
00:56:30,100 --> 00:56:34,790
now and turn things around for a sort
of final community activity together.

1202
00:56:34,790 --> 00:56:37,320
Thanks to many of you who
have contributed questions

1203
00:56:37,320 --> 00:56:38,570
over the past couple of weeks.

1204
00:56:38,570 --> 00:56:41,950
Thanks to Brian, we thought we'd
put together a CS50 quiz show

1205
00:56:41,950 --> 00:56:43,960
on which to end this final lecture.

1206
00:56:43,960 --> 00:56:47,200
These are questions written by
you, by the staff, by Brian.

1207
00:56:47,200 --> 00:56:49,900
And it'll be an opportunity
for everyone to buzz in

1208
00:56:49,900 --> 00:56:54,250
with their answers to some 20 questions
that we have prepared in advance.

1209
00:56:54,250 --> 00:56:55,640
Time is of the essence.

1210
00:56:55,640 --> 00:56:59,060
So your score will be higher
if you buzz in more quickly.

1211
00:56:59,060 --> 00:57:01,150
So it's important not
only to be correct,

1212
00:57:01,150 --> 00:57:04,820
but also to be fast for
this particular one as well.

1213
00:57:04,820 --> 00:57:08,060
And in just a moment, I'm going
to go ahead and share my screen.

1214
00:57:08,060 --> 00:57:12,250
And, again, we'll have some 20
questions here, all of them drawn from,

1215
00:57:12,250 --> 00:57:14,530
inspired by CS50 in some form.

1216
00:57:14,530 --> 00:57:18,640
And after each question, depending on
how many people get it right or wrong,

1217
00:57:18,640 --> 00:57:21,970
we'll take a moment to at least
explain where it is you went right

1218
00:57:21,970 --> 00:57:23,920
or where it is you went wrong.

1219
00:57:23,920 --> 00:57:24,700
All right, Brian.

1220
00:57:24,700 --> 00:57:26,172
Ready on your end?

1221
00:57:26,172 --> 00:57:27,380
BRIAN YU: We are ready to go.

1222
00:57:27,380 --> 00:57:29,880
DAVID MALAN: All right, well,
let's go ahead and take a look

1223
00:57:29,880 --> 00:57:31,150
with the first question here.

1224
00:57:31,150 --> 00:57:35,860
What are the steps for compiling
source code into machine code?

1225
00:57:35,860 --> 00:57:38,290
Preprocessing, compiling,
assembling, and linking?

1226
00:57:38,290 --> 00:57:40,810
Writing, compiling,
debugging, and testing?

1227
00:57:40,810 --> 00:57:43,270
Processing, creating,
asserting, and clang?

1228
00:57:43,270 --> 00:57:45,410
Or make?

1229
00:57:45,410 --> 00:57:49,310
Go ahead and buzz in on your
phone or laptop or desktop,

1230
00:57:49,310 --> 00:57:51,500
using that same URL that Brian provided.

1231
00:57:51,500 --> 00:57:55,040
You've got 20 seconds for each
question, two of which now remain.

1232
00:57:55,040 --> 00:57:55,970
That's it for time.

1233
00:57:55,970 --> 00:57:58,095
Let's go ahead and take a
look at the results here.

1234
00:57:58,095 --> 00:58:02,000
It looks like 70% of you said
preprocessing, compiling, assembling,

1235
00:58:02,000 --> 00:58:02,750
and linking.

1236
00:58:02,750 --> 00:58:04,987
Brian, would you like to
tell us if that's right?

1237
00:58:04,987 --> 00:58:05,570
BRIAN YU: Yes.

1238
00:58:05,570 --> 00:58:06,800
That is the correct answer.

1239
00:58:06,800 --> 00:58:10,010
Preprocessing first, compiling,
assembling, and linking, all of that

1240
00:58:10,010 --> 00:58:11,440
is behind the scenes.

1241
00:58:11,440 --> 00:58:13,940
So you don't necessarily think
about every time you compile.

1242
00:58:13,940 --> 00:58:15,590
But those are, indeed, the steps.

1243
00:58:15,590 --> 00:58:18,245
DAVID MALAN: And to be fair,
make is arguably an abstraction

1244
00:58:18,245 --> 00:58:20,870
for all of that insofar as it
just kicks off the whole process.

1245
00:58:20,870 --> 00:58:24,170
But I think a little more precisely,
an answer to steps would be,

1246
00:58:24,170 --> 00:58:25,543
indeed, those four things there.

1247
00:58:25,543 --> 00:58:27,710
All right, let's take a
look at the scoreboard here.

1248
00:58:27,710 --> 00:58:29,418
We have a whole number
of guest accounts.

1249
00:58:29,418 --> 00:58:33,500
Guest number 200 is in the lead, but
tied with several other guests here.

1250
00:58:33,500 --> 00:58:36,240
So those of you with 1,000
points buzzed in really quickly.

1251
00:58:36,240 --> 00:58:37,910
So again time is of the essence.

1252
00:58:37,910 --> 00:58:42,560
Next question, what is the
runtime of binary search?

1253
00:58:42,560 --> 00:58:47,330
Is it O of 1, O of log n,
O of n, or O of n squared?

1254
00:58:47,330 --> 00:58:52,700
15 seconds remain, the
runtime of binary search.

1255
00:58:52,700 --> 00:58:55,280
Recall, this was one of the
first algorithms we looked at.

1256
00:58:55,280 --> 00:58:57,350
It was first incarnated
with a phone book,

1257
00:58:57,350 --> 00:59:00,500
even if we didn't call
it that by name early on.

1258
00:59:00,500 --> 00:59:02,780
Brian, let's take a look at the results.

1259
00:59:02,780 --> 00:59:05,690
Looks like 61% of you say log n.

1260
00:59:05,690 --> 00:59:06,840
Brian?

1261
00:59:06,840 --> 00:59:08,670
BRIAN YU: Log n is the correct answer.

1262
00:59:08,670 --> 00:59:10,040
If you remember that
phone book, the question

1263
00:59:10,040 --> 00:59:11,900
really came down to
how many times can we

1264
00:59:11,900 --> 00:59:13,940
divide that phone book
and half again and again

1265
00:59:13,940 --> 00:59:16,590
and again, until we get
down to just one page.

1266
00:59:16,590 --> 00:59:20,120
And that turns out to be log of n if
three are n pages in the phone book.

1267
00:59:20,120 --> 00:59:23,240
DAVID MALAN: Indeed, and sort of pro
tip moving forward in life, any time

1268
00:59:23,240 --> 00:59:26,150
you see something happening in
half and half and half and half,

1269
00:59:26,150 --> 00:59:28,670
odds are there's going to
be an algorithm involved

1270
00:59:28,670 --> 00:59:31,070
somewhere in the analysis thereof.

1271
00:59:31,070 --> 00:59:34,710
All right, next leaderboard here,
guest 200 slipped down a little bit.

1272
00:59:34,710 --> 00:59:39,200
But we have a whole bunch of people tied
in first place for 2,000 points now.

1273
00:59:39,200 --> 00:59:42,500
Next question, which of
these animals was the first

1274
00:59:42,500 --> 00:59:45,620
to be mentioned in a CS50 lecture?

1275
00:59:45,620 --> 00:59:50,250
Llama, python, duck, cat.

1276
00:59:50,250 --> 00:59:52,750
15 seconds remain.

1277
00:59:52,750 --> 00:59:57,930
Which was mentioned
first in a CS50 lecture?

1278
00:59:57,930 --> 00:59:59,910
And let's see the results.

1279
00:59:59,910 --> 01:00:04,500
Looks like cat just barely
eked out duck with 51%.

1280
01:00:04,500 --> 01:00:05,177
Brian?

1281
01:00:05,177 --> 01:00:07,260
BRIAN YU: And cat is, in
fact, the correct answer.

1282
01:00:07,260 --> 01:00:10,520
Llamas showed up in Lab 1, but
they were not mentioned in lecture.

1283
01:00:10,520 --> 01:00:12,270
The duck didn't show
up until a little bit

1284
01:00:12,270 --> 01:00:13,890
later when we talked about debugging.

1285
01:00:13,890 --> 01:00:17,280
And Python was briefly mentioned
at the end of the lecture.

1286
01:00:17,280 --> 01:00:19,590
But it was after we introduced
ourselves to Scratch.

1287
01:00:19,590 --> 01:00:22,350
And the main character in
Scratch is, of course, the cat.

1288
01:00:22,350 --> 01:00:25,600
DAVID MALAN: All right, we're probably
going to see a bit of spread here soon.

1289
01:00:25,600 --> 01:00:27,852
We have a whole bunch of
people with 3,000, though.

1290
01:00:27,852 --> 01:00:29,310
But the names are starting to vary.

1291
01:00:29,310 --> 01:00:30,990
Let's move on to the next question.

1292
01:00:30,990 --> 01:00:34,920
Every time you malloc memory,
you must also be sure to--

1293
01:00:34,920 --> 01:00:39,690
realloc, return, free, or exit?

1294
01:00:39,690 --> 01:00:44,310
Every time you malloc memory, you
should also be sure to realloc,

1295
01:00:44,310 --> 01:00:48,270
return, free, or exit?

1296
01:00:48,270 --> 01:00:51,420
Recall that malloc was the source
of a lot of segmentation faults

1297
01:00:51,420 --> 01:00:52,230
mid-semester.

1298
01:00:52,230 --> 01:00:55,950
The responses now are 78% said free.

1299
01:00:55,950 --> 01:00:57,042
Brian, do you concur?

1300
01:00:57,042 --> 01:00:58,500
BRIAN YU: And they are all correct.

1301
01:00:58,500 --> 01:01:01,542
Whenever you malloc memory, ask the
computer for some memory dynamically.

1302
01:01:01,542 --> 01:01:04,250
When you're done with it, you
should give it back to the computer

1303
01:01:04,250 --> 01:01:05,010
by calling free.

1304
01:01:05,010 --> 01:01:06,927
DAVID MALAN: Indeed, and
Brian, as a teachable

1305
01:01:06,927 --> 01:01:10,140
moment, why is it that we never had a
call free forget_string, which we now

1306
01:01:10,140 --> 01:01:12,090
know underneath the
hood is using something

1307
01:01:12,090 --> 01:01:14,070
like malloc to allocate memory?

1308
01:01:14,070 --> 01:01:16,500
BRIAN YU: So get_string was
a function in CS50's library.

1309
01:01:16,500 --> 01:01:19,890
And CS50's library takes care of that
memory management process for you.

1310
01:01:19,890 --> 01:01:23,070
So you didn't have to worry about
freeing all of that memory yourself.

1311
01:01:23,070 --> 01:01:26,250
DAVID MALAN: Indeed, but anytime
you call malloc, you must call free.

1312
01:01:26,250 --> 01:01:28,440
All right, the leaderboard
here looks like we have

1313
01:01:28,440 --> 01:01:30,840
guest 600 still in the lead with 4,000.

1314
01:01:30,840 --> 01:01:31,860
Next question.

1315
01:01:31,860 --> 01:01:34,550
What is a race condition?

1316
01:01:34,550 --> 01:01:37,040
When conditions are nice out for racing?

1317
01:01:37,040 --> 01:01:40,670
When two things happen at the same
time and produce an unexpected result?

1318
01:01:40,670 --> 01:01:43,170
When a line of code is
executed too quickly?

1319
01:01:43,170 --> 01:01:45,830
When a line of code is
executed too slowly?

1320
01:01:45,830 --> 01:01:49,130
What is a race condition?

1321
01:01:49,130 --> 01:01:50,850
Ah, things just escalated quickly.

1322
01:01:50,850 --> 01:01:54,590
But you'll recall this came
up in the context of SQL.

1323
01:01:54,590 --> 01:01:57,020
And databases, 0 seconds, let's see.

1324
01:01:57,020 --> 01:02:00,170
85% said when two things
happen at the same time

1325
01:02:00,170 --> 01:02:02,138
and produce an unexpected result. Brian?

1326
01:02:02,138 --> 01:02:03,680
BRIAN YU: That is the correct answer.

1327
01:02:03,680 --> 01:02:06,650
I appreciate that at least 1% of
people said when conditions outside

1328
01:02:06,650 --> 01:02:07,640
are nice for racing.

1329
01:02:07,640 --> 01:02:10,078
But in the context of
computer science, at least,

1330
01:02:10,078 --> 01:02:13,370
when two things happen at the same time
and could produce an unexpected result,

1331
01:02:13,370 --> 01:02:15,680
that is what we would refer
to as a race condition.

1332
01:02:15,680 --> 01:02:17,330
DAVID MALAN: Indeed, recall
that's how Brian and I ended up

1333
01:02:17,330 --> 01:02:18,973
with too much milk in the refrigerator.

1334
01:02:18,973 --> 01:02:21,140
Because we both inspected
the state of that variable

1335
01:02:21,140 --> 01:02:22,955
at essentially the same time.

1336
01:02:22,955 --> 01:02:24,330
All right, the leader board here.

1337
01:02:24,330 --> 01:02:26,630
Now we have a whole bunch
of people with 5,000 points.

1338
01:02:26,630 --> 01:02:27,590
Let's move on.

1339
01:02:27,590 --> 01:02:32,660
Does zooming in on a photo let you
enhance it to generate more detail?

1340
01:02:32,660 --> 01:02:35,210
Yes, just like in CSI.

1341
01:02:35,210 --> 01:02:39,370
No, a photo only has a
certain amount of detail.

1342
01:02:39,370 --> 01:02:45,120
Does zooming in on a photo let you
enhance it to generate more detail?

1343
01:02:45,120 --> 01:02:48,390
And I will admit, I was watching
some show recently and thought of you

1344
01:02:48,390 --> 01:02:50,340
all when they literally said, enhance.

1345
01:02:50,340 --> 01:02:52,490
All right, 0 seconds.

1346
01:02:52,490 --> 01:02:56,970
Looks like 93% of you said, no, a photo
only has a certain amount of detail.

1347
01:02:56,970 --> 01:02:59,010
7% of you said yes, just like in CSI.

1348
01:02:59,010 --> 01:03:01,290
Brian, can you help
us reconcile the two?

1349
01:03:01,290 --> 01:03:03,480
BRIAN YU: The 93%, in
this case, are correct.

1350
01:03:03,480 --> 01:03:05,800
A photo only has a
certain number of pixels.

1351
01:03:05,800 --> 01:03:08,100
And if you keep zooming
in on one pixel, you're

1352
01:03:08,100 --> 01:03:10,470
not going to be able to
generate additional detail that

1353
01:03:10,470 --> 01:03:11,528
wasn't there before.

1354
01:03:11,528 --> 01:03:14,070
DAVID MALAN: And to be fair,
that's kind of sort of changing.

1355
01:03:14,070 --> 01:03:15,840
Or at least the answer is
getting a little harder nowadays

1356
01:03:15,840 --> 01:03:18,450
with machine learning or
artificial intelligence,

1357
01:03:18,450 --> 01:03:21,210
where algorithms sort of
figure out what level of detail

1358
01:03:21,210 --> 01:03:22,750
could or should be there.

1359
01:03:22,750 --> 01:03:24,810
But that really is just
statistical inference,

1360
01:03:24,810 --> 01:03:27,810
that is not actually recovering
information that was ever stored

1361
01:03:27,810 --> 01:03:29,820
on the camera or some other device.

1362
01:03:29,820 --> 01:03:34,290
All right, the leaderboard now is at
6,000 points with these folks tied.

1363
01:03:34,290 --> 01:03:38,670
Which of the following is not a
characteristic of a good hash function?

1364
01:03:38,670 --> 01:03:44,280
Deterministic output, randomness,
uniform distribution, efficiency.

1365
01:03:44,280 --> 01:03:45,780
Things just got real again.

1366
01:03:45,780 --> 01:03:50,160
Which of the following is not a
characteristic of a good hash function?

1367
01:03:50,160 --> 01:03:54,600
Recall we used hash functions
in the context of hash tables

1368
01:03:54,600 --> 01:03:57,070
when talking about data structures.

1369
01:03:57,070 --> 01:03:57,570
All right?

1370
01:03:57,570 --> 01:03:58,770
One second.

1371
01:03:58,770 --> 01:04:00,930
The answers are more spread this time.

1372
01:04:00,930 --> 01:04:03,270
62% don't like randomness.

1373
01:04:03,270 --> 01:04:04,172
Brian, should they?

1374
01:04:04,172 --> 01:04:05,880
BRIAN YU: And that is
the correct answer.

1375
01:04:05,880 --> 01:04:08,670
Randomness is not a characteristic
of a good hash function.

1376
01:04:08,670 --> 01:04:10,650
You want your hash
function to always give you

1377
01:04:10,650 --> 01:04:12,330
the same output given the same input.

1378
01:04:12,330 --> 01:04:15,150
That way you can rely on
whatever the output of it is.

1379
01:04:15,150 --> 01:04:17,290
If it's random, it's
going to be hard to use.

1380
01:04:17,290 --> 01:04:19,123
DAVID MALAN: Indeed,
consider a spellchecker

1381
01:04:19,123 --> 01:04:20,550
that randomly says yes or no.

1382
01:04:20,550 --> 01:04:23,250
This is a word, probably
not a property you want.

1383
01:04:23,250 --> 01:04:26,430
All right, the leaderboard now, we're
eking our way up to 7,000 points,

1384
01:04:26,430 --> 01:04:28,590
but finally starting to see some spread.

1385
01:04:28,590 --> 01:04:31,260
So a few of you haven't been
quite quick or correct enough.

1386
01:04:31,260 --> 01:04:34,140
Next question, what does FIFO stand for?

1387
01:04:34,140 --> 01:04:35,550
FIFO.

1388
01:04:35,550 --> 01:04:40,620
Is it a common dog's name, your
credit score, first in, first out,

1389
01:04:40,620 --> 01:04:43,580
function input, file output?

1390
01:04:43,580 --> 01:04:46,700
What does FIFO stand for?

1391
01:04:46,700 --> 01:04:49,220
I'll be curious to see the spread here.

1392
01:04:49,220 --> 01:04:50,330
Let's see.

1393
01:04:50,330 --> 01:04:52,190
80% of you said first in, first out.

1394
01:04:52,190 --> 01:04:52,777
Brian?

1395
01:04:52,777 --> 01:04:53,860
BRIAN YU: That is correct.

1396
01:04:53,860 --> 01:04:56,090
And that was what we
were using to describe

1397
01:04:56,090 --> 01:04:58,490
what would be called like a queue,
where the first thing in the queue

1398
01:04:58,490 --> 01:05:00,448
is the first thing that
comes out of the queue.

1399
01:05:00,448 --> 01:05:02,443
So it obeys that FIFO ordering.

1400
01:05:02,443 --> 01:05:04,610
DAVID MALAN: Indeed, let's
see the leaderboard here.

1401
01:05:04,610 --> 01:05:07,880
All right, we have some 8,000s,
but more in the 7,000 range.

1402
01:05:07,880 --> 01:05:09,860
Next up is a more colorful question.

1403
01:05:09,860 --> 01:05:13,760
Which of the following would
represent pink using RGB values?

1404
01:05:13,760 --> 01:05:15,980
And I'll let you read these on your own.

1405
01:05:15,980 --> 01:05:19,730
And surely, there's some Googling
happening behind the scenes now.

1406
01:05:19,730 --> 01:05:21,050
But that's OK.

1407
01:05:21,050 --> 01:05:22,430
In fact, Google is pretty smart.

1408
01:05:22,430 --> 01:05:24,860
If you type in a hexadecimal
code, it might even

1409
01:05:24,860 --> 01:05:28,017
show you a little color wheel or swatch.

1410
01:05:28,017 --> 01:05:29,850
All right, let's take
a look at the results.

1411
01:05:29,850 --> 01:05:33,590
Looks like 55% of you said ffd0e0.

1412
01:05:33,590 --> 01:05:34,370
Brian?

1413
01:05:34,370 --> 01:05:35,670
BRIAN YU: And that is correct.

1414
01:05:35,670 --> 01:05:38,450
So those RGB values are
like six different values,

1415
01:05:38,450 --> 01:05:42,438
where each two correspond to one
color, two for red, two for green,

1416
01:05:42,438 --> 01:05:42,980
two for blue.

1417
01:05:42,980 --> 01:05:44,300
This is all in hexadecimal.

1418
01:05:44,300 --> 01:05:46,668
And pink would be a lot of each of them.

1419
01:05:46,668 --> 01:05:48,710
Because it's very close
to white, which is, like,

1420
01:05:48,710 --> 01:05:50,360
all red, all green, and all blue.

1421
01:05:50,360 --> 01:05:52,740
But it's more red than
it is green and blue.

1422
01:05:52,740 --> 01:05:58,130
And so that one, ffd0e0, is a lot
of red, a little bit less green,

1423
01:05:58,130 --> 01:05:59,460
and a little bit less blue.

1424
01:05:59,460 --> 01:06:00,080
DAVID MALAN: Indeed.

1425
01:06:00,080 --> 01:06:01,788
All right, let's see
where we're at here.

1426
01:06:01,788 --> 01:06:03,560
We're now up to-- interesting.

1427
01:06:03,560 --> 01:06:05,300
No one has a perfect score anymore.

1428
01:06:05,300 --> 01:06:09,800
But guest 200 is still in the lead
with just shy of 9,000 points.

1429
01:06:09,800 --> 01:06:12,330
In C, which of the following
lines of code allocates

1430
01:06:12,330 --> 01:06:17,090
enough memory for a
copy of the string s?

1431
01:06:17,090 --> 01:06:19,160
I'll let you read these.

1432
01:06:19,160 --> 01:06:21,680
In C, which of the following
lines of code allocates

1433
01:06:21,680 --> 01:06:25,970
enough memory for a
copy of the string s?

1434
01:06:25,970 --> 01:06:28,250
Bunch of viable choices
here, it would seem.

1435
01:06:28,250 --> 01:06:31,130
And time, let's take
a look at the results.

1436
01:06:31,130 --> 01:06:34,880
Looks like 46% said malloc of size s.

1437
01:06:34,880 --> 01:06:39,110
But Brian, 33% said malloc
of strlen of s plus 1.

1438
01:06:39,110 --> 01:06:40,490
Who is right?

1439
01:06:40,490 --> 01:06:43,610
BRIAN YU: And in this case, the
minority, the 33% are correct here.

1440
01:06:43,610 --> 01:06:46,940
Malloc, remember, takes as its
argument the number of bytes of memory

1441
01:06:46,940 --> 01:06:48,080
that you want to allocate.

1442
01:06:48,080 --> 01:06:50,850
And if you have a string and you want
to figure out how many bytes you need,

1443
01:06:50,850 --> 01:06:52,600
first thing you need
to know is figure out

1444
01:06:52,600 --> 01:06:54,680
how long is that string.
strlen will tell you

1445
01:06:54,680 --> 01:06:56,690
how many characters are in that string.

1446
01:06:56,690 --> 01:07:00,050
But you do need one additional byte,
because at the end of every string,

1447
01:07:00,050 --> 01:07:01,880
we have that null terminating character.

1448
01:07:01,880 --> 01:07:03,680
And we need one byte of memory for that.

1449
01:07:03,680 --> 01:07:07,040
So strlen of s will give you
the length of the string plus 1.

1450
01:07:07,040 --> 01:07:09,170
That's how many bytes
you need for memory.

1451
01:07:09,170 --> 01:07:09,590
DAVID MALAN: Indeed.

1452
01:07:09,590 --> 01:07:11,007
And see, you get nothing for free.

1453
01:07:11,007 --> 01:07:12,830
Anything you want you
need to do yourself.

1454
01:07:12,830 --> 01:07:15,740
And indeed, the plus 1 is
a problem for you to solve.

1455
01:07:15,740 --> 01:07:19,580
The distribution now, guest
200 still in the lead with just

1456
01:07:19,580 --> 01:07:20,900
shy of 10,000 points.

1457
01:07:20,900 --> 01:07:22,010
That was question 10.

1458
01:07:22,010 --> 01:07:23,750
We're in the second half of the game.

1459
01:07:23,750 --> 01:07:26,930
How should you organize
your clothes to be cool?

1460
01:07:26,930 --> 01:07:28,100
This is number 11.

1461
01:07:28,100 --> 01:07:32,120
Stack, queue, dictionary, binary tree.

1462
01:07:32,120 --> 01:07:35,840
How should you organize
your clothes to be cool?

1463
01:07:35,840 --> 01:07:39,830
You might recall Jack and
Lou, who taught us this one.

1464
01:07:39,830 --> 01:07:41,740
Two seconds remain.

1465
01:07:41,740 --> 01:07:45,000
And it looks like 48% said queue, Brian.

1466
01:07:45,000 --> 01:07:46,250
BRIAN YU: And that is correct.

1467
01:07:46,250 --> 01:07:48,560
So from that video with
Jack and Lou, there

1468
01:07:48,560 --> 01:07:50,442
were different ways of
organizing the clues.

1469
01:07:50,442 --> 01:07:52,150
But the conclusion of
that video was, you

1470
01:07:52,150 --> 01:07:53,830
want to put your clothes in a queue.

1471
01:07:53,830 --> 01:07:56,290
So that after you're done with one,
you put it at the end of the queue.

1472
01:07:56,290 --> 01:07:59,470
And you use something else before you
go back to the one you already wore.

1473
01:07:59,470 --> 01:08:00,070
DAVID MALAN: Indeed.

1474
01:08:00,070 --> 01:08:00,570
All right.

1475
01:08:00,570 --> 01:08:03,040
And the leaderboard now, looks
like guest 10 broke 10,000.

1476
01:08:03,040 --> 01:08:04,630
But so did a bunch of other people.

1477
01:08:04,630 --> 01:08:07,540
Next question, what is
a segmentation fault?

1478
01:08:07,540 --> 01:08:10,060
When a computer runs out of
memory, when our program tries

1479
01:08:10,060 --> 01:08:13,480
to read an empty file, when a program
tries to access memory that it

1480
01:08:13,480 --> 01:08:16,620
shouldn't, when an earthquake happens.

1481
01:08:16,620 --> 01:08:19,609
Looks like a lot of these
could be pretty close.

1482
01:08:19,609 --> 01:08:21,260
Two seconds.

1483
01:08:21,260 --> 01:08:22,963
And let's see.

1484
01:08:22,963 --> 01:08:26,130
Looks like 80% said when a program tries
to access memory that it shouldn't.

1485
01:08:26,130 --> 01:08:26,810
Brian?

1486
01:08:26,810 --> 01:08:28,160
BRIAN YU: That is the correct answer.

1487
01:08:28,160 --> 01:08:30,077
Segmentation fault can
happen if you're trying

1488
01:08:30,077 --> 01:08:33,790
to touch memory that you're not supposed
to have access to inside of a program.

1489
01:08:33,790 --> 01:08:35,540
DAVID MALAN: And for
the 13% of people who

1490
01:08:35,540 --> 01:08:38,927
said when a computer runs out of memory,
why is that not quite the answer here?

1491
01:08:38,927 --> 01:08:41,010
BRIAN YU: So the computer
could run out of memory.

1492
01:08:41,010 --> 01:08:43,640
Where when you call malloc,
malloc might return null,

1493
01:08:43,640 --> 01:08:46,040
because there's no available
memory to allocate.

1494
01:08:46,040 --> 01:08:48,707
But as long as you check for that,
and we try and encourage you,

1495
01:08:48,707 --> 01:08:52,100
whenever you're mallocing memory, to
check to see if the value you get back

1496
01:08:52,100 --> 01:08:52,760
is null.

1497
01:08:52,760 --> 01:08:54,805
That can help you to avoid
those types of errors.

1498
01:08:54,805 --> 01:08:56,930
DAVID MALAN: So let's take
a look at the board now.

1499
01:08:56,930 --> 01:08:59,569
11,000 something for guest 200.

1500
01:08:59,569 --> 01:09:01,670
Let's now proceed with this question.

1501
01:09:01,670 --> 01:09:05,689
Which of the following types of
overflow can result from recursion

1502
01:09:05,689 --> 01:09:07,340
without a base case?

1503
01:09:07,340 --> 01:09:13,359
Heap overflow, integer overflow,
stack overflow, buffer overflow.

1504
01:09:13,359 --> 01:09:16,370
And all forms of
overflow, indeed, came up.

1505
01:09:16,370 --> 01:09:19,510
One of them is also, of course,
the name of a popular website.

1506
01:09:19,510 --> 01:09:21,580
But all of these are actual things.

1507
01:09:21,580 --> 01:09:23,660
But which is correct?

1508
01:09:23,660 --> 01:09:25,149
All right, let's see the results.

1509
01:09:25,149 --> 01:09:29,319
Looks like 61%, 60% went
with stack overflow.

1510
01:09:29,319 --> 01:09:29,990
Brian?

1511
01:09:29,990 --> 01:09:31,240
BRIAN YU: And that is correct.

1512
01:09:31,240 --> 01:09:34,598
Every time you call a function, you
end up getting a little bit of memory

1513
01:09:34,598 --> 01:09:35,890
on the stack for that function.

1514
01:09:35,890 --> 01:09:38,515
And if you keep calling that
function recursively over and over

1515
01:09:38,515 --> 01:09:40,960
and never stop, because
there's no base case,

1516
01:09:40,960 --> 01:09:42,460
then you can run out of stack space.

1517
01:09:42,460 --> 01:09:43,997
And we call that a stack overflow.

1518
01:09:43,997 --> 01:09:44,830
DAVID MALAN: Indeed.

1519
01:09:44,830 --> 01:09:46,660
All right, let's see
the leaderboard now.

1520
01:09:46,660 --> 01:09:48,970
Guest 200, still the one to beat.

1521
01:09:48,970 --> 01:09:52,149
But guest 216 is not too far behind.

1522
01:09:52,149 --> 01:09:53,020
Next question.

1523
01:09:53,020 --> 01:09:55,630
In the town of Fiftyville,
what were the names

1524
01:09:55,630 --> 01:09:59,680
of the three people who witnessed
the rubber duck robbery?

1525
01:09:59,680 --> 01:10:01,720
I'll let you read these.

1526
01:10:01,720 --> 01:10:04,660
In the town of Fiftyville, what
were the names of the three people

1527
01:10:04,660 --> 01:10:06,430
who witnessed the rubber duck robbery.

1528
01:10:06,430 --> 01:10:10,810
A new problem this year, recall that
he disappeared altogether from the IDE

1529
01:10:10,810 --> 01:10:11,770
for that week.

1530
01:10:11,770 --> 01:10:13,540
All right, let's see the results.

1531
01:10:13,540 --> 01:10:15,460
Brian, this one is close.

1532
01:10:15,460 --> 01:10:17,875
33% said Ruth, Eugene, and Raymond.

1533
01:10:17,875 --> 01:10:20,000
BRIAN YU: And Ruth, Eugene,
and Raymond is correct.

1534
01:10:20,000 --> 01:10:22,083
They've got more responses
than any of the others.

1535
01:10:22,083 --> 01:10:22,930
It was tricky.

1536
01:10:22,930 --> 01:10:24,430
But yeah, that's the correct answer.

1537
01:10:24,430 --> 01:10:26,500
There wasn't a whole lot
of reason behind the names.

1538
01:10:26,500 --> 01:10:28,300
I put a lot of thought
into the story itself.

1539
01:10:28,300 --> 01:10:29,500
But not a lot of thought to the names.

1540
01:10:29,500 --> 01:10:31,630
Those were kind of
just randomly selected.

1541
01:10:31,630 --> 01:10:33,767
But those were the
names of the witnesses.

1542
01:10:33,767 --> 01:10:34,600
DAVID MALAN: Indeed.

1543
01:10:34,600 --> 01:10:38,380
And the leaderboard now, we still
have guest 200 is the one to beat.

1544
01:10:38,380 --> 01:10:39,530
This is question 15.

1545
01:10:39,530 --> 01:10:42,280
So we are nearing the end,
still chance to pull ahead.

1546
01:10:42,280 --> 01:10:46,180
Which of these command line programs
checks your code for memory leaks?

1547
01:10:46,180 --> 01:10:50,110
Valgrind, clang, mkdir, make.

1548
01:10:50,110 --> 01:10:53,860
Notice that none of these have 50 on
it, which means these are all real world

1549
01:10:53,860 --> 01:10:56,950
commands that you would continue
to see on your own Mac or PC

1550
01:10:56,950 --> 01:10:59,800
or some future Linux system.

1551
01:10:59,800 --> 01:11:02,380
And let's see the results.

1552
01:11:02,380 --> 01:11:05,020
Here we have valgrind,
the clear winner, 78%.

1553
01:11:05,020 --> 01:11:05,560
Brian?

1554
01:11:05,560 --> 01:11:06,760
BRIAN YU: And that's the correct answer.

1555
01:11:06,760 --> 01:11:09,302
That's the program you can use
in order to check your program

1556
01:11:09,302 --> 01:11:11,380
to see if you have any
memory leaks, to see

1557
01:11:11,380 --> 01:11:14,890
if you're touching memory you shouldn't,
if you're forgetting to free something.

1558
01:11:14,890 --> 01:11:16,100
Valgrind is useful for all of that.

1559
01:11:16,100 --> 01:11:19,030
DAVID MALAN: And if I may, I feel like
5% of you are just messing with us now.

1560
01:11:19,030 --> 01:11:20,320
Hopefully, but we shall see.

1561
01:11:20,320 --> 01:11:22,300
All right, last five questions to go.

1562
01:11:22,300 --> 01:11:26,210
After taking a look at the leaderboard
now, guest 200's still up at the top.

1563
01:11:26,210 --> 01:11:30,130
Which of the following
exists in C, but not Python.

1564
01:11:30,130 --> 01:11:32,800
Boolean expressions,
do-while loops, recursive

1565
01:11:32,800 --> 01:11:35,740
functions, floating-point numbers.

1566
01:11:35,740 --> 01:11:40,450
Which of the following
exists in C, but not Python?

1567
01:11:40,450 --> 01:11:44,740
An interesting comparison between two
languages that goes beyond syntax.

1568
01:11:44,740 --> 01:11:45,580
All right.

1569
01:11:45,580 --> 01:11:46,330
Time's up.

1570
01:11:46,330 --> 01:11:47,590
Let's take a look.

1571
01:11:47,590 --> 01:11:50,380
Looks like 68% went with do-while loops.

1572
01:11:50,380 --> 01:11:51,160
Brian?

1573
01:11:51,160 --> 01:11:52,360
BRIAN YU: That is correct.

1574
01:11:52,360 --> 01:11:53,560
Python has for loops.

1575
01:11:53,560 --> 01:11:54,910
Python has while loops.

1576
01:11:54,910 --> 01:11:57,730
But it doesn't have do-while
loops in the same way that C does.

1577
01:11:57,730 --> 01:12:00,190
You'd have to find some other
way of trying to achieve

1578
01:12:00,190 --> 01:12:01,810
that same kind of logical idea.

1579
01:12:01,810 --> 01:12:02,200
DAVID MALAN: Indeed.

1580
01:12:02,200 --> 01:12:04,408
And Brian, what was the
approach that we took when we

1581
01:12:04,408 --> 01:12:06,010
tried to recreate that some weeks ago?

1582
01:12:06,010 --> 01:12:06,250
BRIAN YU: Yeah.

1583
01:12:06,250 --> 01:12:09,070
So one approach to it is having
an infinite loop, while true,

1584
01:12:09,070 --> 01:12:10,780
that will just always repeat.

1585
01:12:10,780 --> 01:12:13,720
And then when you reach a point
where you can exit the loop,

1586
01:12:13,720 --> 01:12:16,000
you can use the command
break to get out of the loop

1587
01:12:16,000 --> 01:12:17,627
and move on to the rest of the program.

1588
01:12:17,627 --> 01:12:18,460
DAVID MALAN: Indeed.

1589
01:12:18,460 --> 01:12:20,210
All right, let's take
a look at the board.

1590
01:12:20,210 --> 01:12:25,810
Guest 200 still now at 15,938, but
still a few close folks behind.

1591
01:12:25,810 --> 01:12:30,310
What HTTP request method should you
use when sending private information

1592
01:12:30,310 --> 01:12:31,600
like a password?

1593
01:12:31,600 --> 01:12:35,200
GET, POST, SELECT, or TEXT?

1594
01:12:35,200 --> 01:12:37,900
Which HTTP request method
should use when sending

1595
01:12:37,900 --> 01:12:41,860
private information like a password?

1596
01:12:41,860 --> 01:12:43,510
Take a look at the results.

1597
01:12:43,510 --> 01:12:44,290
All right.

1598
01:12:44,290 --> 01:12:48,548
And the distribution is a lot
of people said POST, Brian, 74%

1599
01:12:48,548 --> 01:12:49,840
BRIAN YU: And they are correct.

1600
01:12:49,840 --> 01:12:52,007
Yeah, if it was a get
request, then you would end up

1601
01:12:52,007 --> 01:12:54,790
with sensitive information
inside the URL that might show up

1602
01:12:54,790 --> 01:12:56,695
in your browsing history, for example.

1603
01:12:56,695 --> 01:12:59,600
So to be secure, you want to
be sure to use the POST request

1604
01:12:59,600 --> 01:13:00,850
method for that type of stuff.

1605
01:13:00,850 --> 01:13:02,410
DAVID MALAN: And to be
clear, don't do this.

1606
01:13:02,410 --> 01:13:04,310
Get is possible, and
we saw how to do that.

1607
01:13:04,310 --> 01:13:07,420
But of course, that then ends up in
your history and other exposed places.

1608
01:13:07,420 --> 01:13:10,550
SELECT and TEXT were not HTTP verbs.

1609
01:13:10,550 --> 01:13:12,010
So POST is indeed spot on.

1610
01:13:12,010 --> 01:13:14,140
All right, only three questions remain.

1611
01:13:14,140 --> 01:13:17,830
Guest 200 is still the one to
beat, followed by guest 216.

1612
01:13:17,830 --> 01:13:22,870
What data structure allows for constant
time look up for words in a dictionary?

1613
01:13:22,870 --> 01:13:28,330
A linked list, a binary search
tree, an array, or a trie?

1614
01:13:28,330 --> 01:13:30,730
Recall that a dictionary
was an abstract data

1615
01:13:30,730 --> 01:13:33,880
type, insofar as you could
implement it in different ways.

1616
01:13:33,880 --> 01:13:37,000
But to get constant time
look up, you might want

1617
01:13:37,000 --> 01:13:40,780
to use one of these over the others.

1618
01:13:40,780 --> 01:13:42,570
Let's see the results.

1619
01:13:42,570 --> 01:13:43,800
Interesting.

1620
01:13:43,800 --> 01:13:45,870
Brian, 32% said trie.

1621
01:13:45,870 --> 01:13:47,010
Can you help us out here?

1622
01:13:47,010 --> 01:13:47,310
BRIAN YU: Yeah.

1623
01:13:47,310 --> 01:13:48,602
The trie is the correct answer.

1624
01:13:48,602 --> 01:13:52,020
For all of the others, the linked list,
the binary search tree, and the array,

1625
01:13:52,020 --> 01:13:54,220
as you have more and more
words in the dictionary,

1626
01:13:54,220 --> 01:13:56,580
it's going to take longer
and longer to find a word,

1627
01:13:56,580 --> 01:13:58,650
as you have to either
linear search through it

1628
01:13:58,650 --> 01:14:02,340
or you have to go down through various
nodes in the binary search tree.

1629
01:14:02,340 --> 01:14:05,580
The trie on the other hand, it only
depends upon the length of the word

1630
01:14:05,580 --> 01:14:06,540
that you're looking up.

1631
01:14:06,540 --> 01:14:08,970
It doesn't matter how many
words are in the dictionary.

1632
01:14:08,970 --> 01:14:12,090
You just follow one node for each
letter in the word you're looking up.

1633
01:14:12,090 --> 01:14:14,490
And you'll find that
word in constant time.

1634
01:14:14,490 --> 01:14:17,440
DAVID MALAN: And Brian, if constant
time, Big O of 1 is so good,

1635
01:14:17,440 --> 01:14:19,107
why not use tries, then, for everything?

1636
01:14:19,107 --> 01:14:21,273
BRIAN YU: Well, there are
trade offs for everything.

1637
01:14:21,273 --> 01:14:23,640
The trie gives you
theoretically constant time.

1638
01:14:23,640 --> 01:14:25,440
But one of the big trade offs is memory.

1639
01:14:25,440 --> 01:14:27,570
That tries end up using
much more memory to be

1640
01:14:27,570 --> 01:14:31,297
able to store a dictionary than many
of those other data structures would.

1641
01:14:31,297 --> 01:14:32,130
DAVID MALAN: Indeed.

1642
01:14:32,130 --> 01:14:33,450
Let's look at the results.

1643
01:14:33,450 --> 01:14:35,400
And guest 200 still at the lead.

1644
01:14:35,400 --> 01:14:38,310
But guest 752 is now
nipping at their heels.

1645
01:14:38,310 --> 01:14:40,500
We have two final questions.

1646
01:14:40,500 --> 01:14:42,900
And speed, again, does matter.

1647
01:14:42,900 --> 01:14:44,980
What is a cookie?

1648
01:14:44,980 --> 01:14:48,730
Data used to identify your computer
to websites, a delicious snack,

1649
01:14:48,730 --> 01:14:53,140
both of the above, or none of the above.

1650
01:14:53,140 --> 01:14:54,310
This is a tough one, Brian.

1651
01:14:54,310 --> 01:14:56,950
Especially if there's
only one right answer.

1652
01:14:56,950 --> 01:14:58,660
We might see a bit more of a split.

1653
01:14:58,660 --> 01:15:00,230
Which of these is a cookie?

1654
01:15:00,230 --> 01:15:02,590
All right, let's see the results.

1655
01:15:02,590 --> 01:15:05,530
Data used to identify your
computer to websites with 60%.

1656
01:15:05,530 --> 01:15:07,300
Both of the above was 35%.

1657
01:15:07,300 --> 01:15:09,340
Only 2% of you like cookies alone.

1658
01:15:09,340 --> 01:15:10,215
Brian?

1659
01:15:10,215 --> 01:15:12,340
BRIAN YU: Both of the above
was the correct answer.

1660
01:15:12,340 --> 01:15:15,465
I'll remind that all of these questions
were written originally by students

1661
01:15:15,465 --> 01:15:18,490
and the answer choice of students
selected as the correct one was

1662
01:15:18,490 --> 01:15:19,452
both of the above.

1663
01:15:19,452 --> 01:15:20,410
DAVID MALAN: All right.

1664
01:15:20,410 --> 01:15:24,400
And now the second to last leaderboard,
Guest 200 is still in the lead.

1665
01:15:24,400 --> 01:15:26,770
But there's been some variance
toward the bottom there.

1666
01:15:26,770 --> 01:15:30,580
Very last question of CS50 itself.

1667
01:15:30,580 --> 01:15:34,330
What's your comfort level now?

1668
01:15:34,330 --> 01:15:39,560
And we'll let you decide
among these answers, too.

1669
01:15:39,560 --> 01:15:40,760
All right.

1670
01:15:40,760 --> 01:15:41,720
Answers are all in.

1671
01:15:41,720 --> 01:15:43,520
Let's take a look at the distribution.

1672
01:15:43,520 --> 01:15:47,360
Looks like 43% of you said you're
among more of those more comfortable.

1673
01:15:47,360 --> 01:15:49,610
24% of you went with the second.

1674
01:15:49,610 --> 01:15:52,480
19%, very fascinating
distribution from top to bottom.

1675
01:15:52,480 --> 01:15:55,640
But the point is that you are all
indeed now officially inducted

1676
01:15:55,640 --> 01:15:57,410
into those more comfortable.

1677
01:15:57,410 --> 01:15:59,300
Thank you so much for
joining us in CS50.

1678
01:15:59,300 --> 01:16:01,610
We cannot wait to see
your final projects.

1679
01:16:01,610 --> 01:16:03,110
This, then, is the end.

1680
01:16:03,110 --> 01:16:04,490
And this was CS50.

1681
01:16:04,490 --> 01:16:14,090


1682
01:16:14,090 --> 01:16:17,440
[MUSIC PLAYING]

1683
01:16:17,440 --> 01:17:13,000