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So it is my greatest pleasure
to welcome you all here today

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for the first lecture
of, well, Introduction

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to Computing and Programming.

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Also known as CS50 or well, CPSC
100, officially here at Yale.

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>> So we couldn't be more excited
to welcome you all here.

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My name is Patrick Rebeschini.

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I'm the head instructor for the class.

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I am here representing a group
of about 60 staff members

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that will work with you
throughout the semester.

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This number is almost 60 of us.

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Yet along the extraordinary
level of commitments

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that we put into this
class, makes CS50 the class

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at Yale University that offers
the greatest level of support

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to all of you.

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And we couldn't be more proud of
offering this class here again.

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>> In fact, as you will soon experience,
CS50 is much more than a class.

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It's a community.

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And you will be part
soon of this community.

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This is the second year that
Yale is offering this class.

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We are building on the extreme
success of last year, where

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for the first time,
here at this university,

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undergraduate learning assistant
were adopted in classrooms.

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It all started with
this class last year.

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>> So as you know, the class is taught
jointly with Harvard University.

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To teach this course
we are relying-- we can

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count on the great expertise of
David Malan and the Harvard team.

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So David has been teaching
CS50 for well, 10 years now.

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And every year he has been
pushing the boundaries

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and improving the classroom experience.

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Again, we couldn't be more happy to
continue this collaboration with them.

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>> In fact, one of the
most interesting parts,

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I will say of running this class now,
both at Harvard and here at Yale,

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is the really incredible
cross-fertilization

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of ideas, aimed at improving the
learning experience to you all.

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So as a result of this extensive
collaboration between the two

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university, CS50 is proud to
announce the new version this year

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with noticeable changes.

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David will all tell us about them now.

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So please-- this being
said, please join me

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and welcome to give a
big round of applause

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to welcome David and
Harvard team here at Yale.

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>> [APPLAUSE]

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>> DAVID MALAN: Thank you.

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Thanks.

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This is CS50, Harvard University's
and Yale University's introduction

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to the intellectual
enterprises of computer science

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and the art of programming.

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And what that means is that this course
ultimately, is about problem solving.

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Indeed many of you might
have come out of high school

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or have spent the past couple of years
wondering what some of your friends

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did last year or in other classes.

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And yet, the reality
is, no matter what we

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do at the end of the day in this class,
it's going to be about problem solving.

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>> And as such, perhaps take
some reassurance in the fact

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that 73% of the students that
take this class, both here

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at Yale as well as at Harvard,
have never taken a CS class before.

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So if you're sitting here in
the audience today wondering

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why you are sitting here
in the audience today,

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or maybe you just followed
along with some friends,

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or maybe you've been a
little curious as to what

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computer science and
programming is, realize

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that most of your classmates to
the left and to the right of you

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are very much in that same demographic.

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>> And indeed, if we look
at last year statistics

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within the student body of CS50, both
here and at Harvard, 58% of students

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describe themselves as less comfortable.

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9% is more comfortable.

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And then 33% is somewhere in between.

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And there's no formal definition
of what these buckets means.

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You sort of know you're
less comfortable if you are.

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You're feeling a little uneasy
with maybe being in the class.

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You're not quite sure if a computer
science class is ultimately for you,

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and realize that you are
in very good company.

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And indeed the grading,
and the assessment,

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and the feedback, and all of that
support structure in the class

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is ultimately very much individualized.

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More so than most any
other class by design.

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And indeed, what ultimately
matters in this class is not

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so much where you end
up relative to others,

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but where you, in week 11 or last,
and relative to yourself in week

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0 here our first.

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So what does that mean?

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Well, this means of those 73% of
students last year that had never taken

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a CS class before, by the
start of the semester they

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were dabbling in a language
called Scratch, which we ourselves

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will see here today.

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And by the end of the
semester had they gone

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through this entire list of challenges.

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Starting with a language called c.

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Implementing, what's
at first glance, going

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to be a bit of a challenge for
some, but fairly gratifying once you

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get Super Mario bouncing
up and down a pyramid

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implemented, albeit, with just
something called ASCII art.

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Implementing last year-- what
the students last year then

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did after that was implement their
own Caesar cipher and vigenere cipher.

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So encryption algorithms
with which you could

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scramble information and then unscramble
information to send secret messages.

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The game of 15.

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If you remember from
childhood or some party favor,

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that little plastic game where you move
the numbers up, down, left and right

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to try to get them in order,
actually implementing that game

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and solving the logic required there.

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And then we dabbled in
forensics last year.

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>> So by mid-semester,
students who had never

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used their keyboards
for this purpose before,

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were writing software
to recover, so to speak,

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JPEGs or photographs
that we had accidentally

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deleted from a digital
memory card from a camera.

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Recovering secret messages
from inside of a bitmap image,

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and other such types
of graphics as well.

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>> We then transitioned to giving
the whole class a dictionary.

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Just a really big text file
with 150,000 English words.

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And everyone was challenged
to somehow read, so to speak,

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those words into memory.

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Into the computer's memory.

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And then answer questions
of the form, is this a word?

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Is this a word?

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Is this a word?

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Really just implementing
a spell checker.

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And then challenging each
other with a big board--

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a leader board to see who could
use the least amount of memory,

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in the least amount of time to
actually spell check large documents.

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>> We transitioned from then to
implementing ones own web server.

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So not making web pages in languages
like HTML and CSS, if you're familiar.

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But actually implementing
the server that

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listens on the internet
for requests from browsers

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and then responding to those requests.

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Then implementing our own
e-trade like website, where

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students could buy and sell stocks.

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Drawing in nearly real time
stock quotes from Yahoo Finance.

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And allowing students to see
how their portfolio develops.

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And then finally a mash up
of Google News and Google

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Maps whereby students
by term by terms end

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had the ability to click, and
round, and search on a Google map.

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And then see all of
the news articles that

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are proximal to those particular areas.

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So truly going from zero to 60.

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>> And along the way having what we had
last year called, hacker additions.

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That raise the bar
further for those of you

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who might very well have a good
amount of experience being in that 9%

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of more comfortable.

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So realize that there's
a very high ceiling

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even within those
challenges for students

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coming from a different background.

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Because at the end of
the day, we're ultimately

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focused quite simply on this.

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>> But what does this
mean, problem solving?

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So let's propose that
we distill it like this.

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So problem solving is really
just this kind of picture.

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So you've got inputs to some problem,
something you actually want to solve.

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The goal is to get outputs,
a solution to that problem.

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And then in the middle is
what we'll call a black box.

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You don't necessarily know or even
care what's inside that black box.

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All you know is that when
you feed input into it,

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you hopefully get output
or a solution from it.

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And while today we'll look
both at inputs and outputs,

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we'll long term, and over the
course of the whole semester,

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focus on what's inside that box.

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>> And therein will lie
something called algorithms.

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Step by step instructions for
actually solving some problems.

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But what's an example of some inputs?

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So maybe a simple thing at the
start of every school year, someone

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might want to take attendance.

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So we might do one, two,
three, four, five, six,

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and how would I keep
track of that information.

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I might just go one, two,
three, four, five, six.

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And just use sort of single digits.

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>> Or I could actually record
this a little longer term.

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And how do I represent all
the humans in this room?

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Well, I might do something like, OK.

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I see one person.

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All right.

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I see another person, a
third person, and so forth.

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But no one counts people like this.

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So literally, most of us if we're
even going to draw anything at all,

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are probably going to go
one, two, three, four,

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maybe get a little fancy, five, six,
seven, eight, nine, ten and so forth.

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>> And that's actually a
system called unary.

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Uno, like uno implying one, where you
just have one letter of the alphabet.

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You've just got this hash mark.

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And I, for efficiency, just drew
these hash marks, ultimately

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as straight lines.

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But I could have drawn them
as little stick figures.

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Where to represent
one person, one input,

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I just draw a stick
figure or a hash mark.

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But this isn't all that expressive.

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>> If all I have is these hash
marks, let alone stick figures,

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how might I represent
something like the number 15?

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Or 15 people in the room?

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I might have to do something like 1,
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,

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14, 15.

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It just does not scale very well.

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As the inputs get large, we
need a better system than this.

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>> And it turns out that the
system that computers use

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is not all that different
from what you and I know.

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In fact, most people in this room,
even if you are among those less

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comfortable, don't necessarily know
how your Mac or PC really works,

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you've probably at least heard, that
underneath the hood are 0's and 1's.

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The so-called binary system.

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So indeed, computers have more than
just hash marks in their vocabulary,

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but not as much of a
vocabulary as we humans.

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>> Indeed, we humans don't use binary.

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Bi meaning 2, 0 and 1.

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But decimal, deca
meaning 10, 0 through 9.

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So we have a lot more expressive
capabilities in our normal human world.

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But I'd argue that these systems,
binary, and decimal, and everything

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in between and beyond, are
actually all quite familiar.

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For instance, consider
this example here, 123.

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So this really is, of course,
a number we know as 123.

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But all I just drew was just this
pattern of symbols, glyphs so to speak.

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Sort of shapes on the board in chalk.

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>> But why do we immediately and
intuitively grasp this as 123?

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Well, if you were like
me in grade school,

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you probably learned that this is the
1s column, this is the 10s column,

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this is the 100s column.

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And why is that useful?

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Well, it's simple arithmetic you now
do to get from a pattern of symbols

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to a number we understand intuitively.

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Is what, 100 times 1, and then
10 times 2, and 1 times 3,

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which of course is just 100, and
this is 20, and this is three.

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And so if we add those together-- ah.

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So therein lies the sort of reasoning
behind why this set of symbols

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means something real and numeric.

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>> Well, computers do the exact same thing,
but they only can count as high as one.

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Whereas I was able to
count as high as three.

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And in fact, if I kept going I could
go as high as nine in this system.

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Computers only have zeros
and ones in their alphabet.

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>> So what does that mean?

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Well, it just means that if a computer
wants to represent, say the number 0,

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maybe using three characters-- three
letters of the alphabet so to speak,

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that's how a computer represents 0.

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So not all that scary so far.

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It's exactly what we humans would do.

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And in fact, most of us would just
ignore the leading zeros anyway.

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>> A computer, if it wants
to store the number 1,

238
00:11:01,640 --> 00:11:03,330
turns out is going to do this.

239
00:11:03,330 --> 00:11:05,630
And a computer to store
the number 2 is not

240
00:11:05,630 --> 00:11:09,280
going to do the unary system,
which I alluded to earlier.

241
00:11:09,280 --> 00:11:11,360
It's actually going to do this.

242
00:11:11,360 --> 00:11:13,570
And this is probably
where the pattern starts

243
00:11:13,570 --> 00:11:15,570
to become less obvious for most folks.

244
00:11:15,570 --> 00:11:18,580
That's 2, this is 3.

245
00:11:18,580 --> 00:11:21,600
Curiously, this is now 4.

246
00:11:21,600 --> 00:11:24,120
And now it really does
seem to be perhaps cryptic,

247
00:11:24,120 --> 00:11:26,762
but it's not if we consider
what binary really means.

248
00:11:26,762 --> 00:11:28,720
It means you have two
letters of your alphabet.

249
00:11:28,720 --> 00:11:31,940
So two possible characters
for each placeholder.

250
00:11:31,940 --> 00:11:35,970
>> So that really means we're going
to need a 1s place, or 2s place,

251
00:11:35,970 --> 00:11:40,800
a 4s place and then
8, and 16, 32, and 64.

252
00:11:40,800 --> 00:11:42,290
And what's the difference there?

253
00:11:42,290 --> 00:11:47,150
Like these are 1, 2, 4, 8, 16, 32, 64.

254
00:11:47,150 --> 00:11:50,690
And before we had 110, 100,000, 10,000.

255
00:11:50,690 --> 00:11:53,170
What's the similarity there?

256
00:11:53,170 --> 00:11:54,940
And what's the pattern?

257
00:11:54,940 --> 00:11:55,505
Yeah.

258
00:11:55,505 --> 00:11:57,380
STUDENT: Powers of 2
instead of powers of 10.

259
00:11:57,380 --> 00:11:57,660
DAVID MALAN: Yeah.

260
00:11:57,660 --> 00:11:59,290
Powers of 2 instead of powers of 10.

261
00:11:59,290 --> 00:12:02,160
And so if I wanted to keep
going, 8, 16s and so forth--

262
00:12:02,160 --> 00:12:05,822
but now if you have this sort
of clue, now the binary system

263
00:12:05,822 --> 00:12:07,280
is actually pretty straightforward.

264
00:12:07,280 --> 00:12:10,910
Why is this pattern of 0's
in the world of computers 0?

265
00:12:10,910 --> 00:12:14,910
Well because it's 4 times 0, 2
times 0, 1 times 0 and you get 0.

266
00:12:14,910 --> 00:12:16,650
>> Why is this the number 1?

267
00:12:16,650 --> 00:12:20,070
Same reasoning, but now we
have a 1 in the 1 column.

268
00:12:20,070 --> 00:12:21,840
Why is this 2?

269
00:12:21,840 --> 00:12:24,450
We have a 1 in the 2s column.

270
00:12:24,450 --> 00:12:30,190
And how then do I represent
say, the number 7 in binary?

271
00:12:30,190 --> 00:12:31,324
Say louder.

272
00:12:31,324 --> 00:12:32,170
>> STUDENT: Three 1s.

273
00:12:32,170 --> 00:12:33,086
>> DAVID MALAN: Three 1s.

274
00:12:33,086 --> 00:12:37,110
So 1, 1, 1 because we just need
4 plus 2 plus 1 gives me 7.

275
00:12:37,110 --> 00:12:37,610
All right.

276
00:12:37,610 --> 00:12:40,410
So from there how do we
represent 8 with 3 placeholders?

277
00:12:40,410 --> 00:12:43,310

278
00:12:43,310 --> 00:12:43,922
Yeah.

279
00:12:43,922 --> 00:12:45,070
>> STUDENT: 1, 0, 0, 0.

280
00:12:45,070 --> 00:12:46,970
>> DAVID MALAN: Yeah 1, 0, 0, 0.

281
00:12:46,970 --> 00:12:49,380
And yet maybe, I kind
of technically need

282
00:12:49,380 --> 00:12:52,380
to add another placeholder to the board.

283
00:12:52,380 --> 00:12:55,480
If I want to fit that I indeed
need to do something like this.

284
00:12:55,480 --> 00:12:58,990
So I actually need to use now
the 8s column, and that's fine.

285
00:12:58,990 --> 00:13:02,680
But the curious thing in computing is
that that's going to cost us something.

286
00:13:02,680 --> 00:13:04,390
You need more RAM in your computer now.

287
00:13:04,390 --> 00:13:06,348
You need more memory
because you need something

288
00:13:06,348 --> 00:13:09,650
physical to store that
additional bit, so to speak.

289
00:13:09,650 --> 00:13:10,700
Binary digits.

290
00:13:10,700 --> 00:13:13,260
And indeed all that's happened
here, like the decimal system,

291
00:13:13,260 --> 00:13:17,380
if we keep adding numbers up and up
and up, we go to 5 to 6 to 7 to 8

292
00:13:17,380 --> 00:13:19,330
it's like carrying the 1, literally.

293
00:13:19,330 --> 00:13:21,940
And then everything else
goes back down to zero.

294
00:13:21,940 --> 00:13:25,870
>> But how do we actually represent
these things physically in a computer?

295
00:13:25,870 --> 00:13:29,380
Well, at the end of the day, the only
physical input going into my computer

296
00:13:29,380 --> 00:13:33,630
here is this power cord, so
electricity or electrons from the wall.

297
00:13:33,630 --> 00:13:36,920
And so how do I get from something
physical like that to actually

298
00:13:36,920 --> 00:13:40,630
representing an idea like this instead.

299
00:13:40,630 --> 00:13:41,930
>> Well, what could we do?

300
00:13:41,930 --> 00:13:45,560
We could consider that, all right,
maybe if electricity is flowing

301
00:13:45,560 --> 00:13:47,420
I could store it and hold on to it.

302
00:13:47,420 --> 00:13:49,250
And if I'm holding on
to some electricity,

303
00:13:49,250 --> 00:13:51,700
that's just going to
arbitrarily represent a 1.

304
00:13:51,700 --> 00:13:53,997
And if I pull the plug
and there's nothing there,

305
00:13:53,997 --> 00:13:56,330
you know that's just going
to arbitrarily represent a 0.

306
00:13:56,330 --> 00:13:57,700
>> So if something's there, 1.

307
00:13:57,700 --> 00:13:59,280
If nothing's there, 0.

308
00:13:59,280 --> 00:14:01,420
Or you can make this
a little more visual.

309
00:14:01,420 --> 00:14:02,704
Here is a 0.

310
00:14:02,704 --> 00:14:05,370
There's nothing interesting going
on about the back of my phone.

311
00:14:05,370 --> 00:14:08,690
But if I allow a little
bit of electricity to flow,

312
00:14:08,690 --> 00:14:11,490
even though it's a little bright
in here, my flashlight went on.

313
00:14:11,490 --> 00:14:15,840
So I'm storing a charge and ergo,
this phone now represents a 1.

314
00:14:15,840 --> 00:14:17,700
So 0 1.

315
00:14:17,700 --> 00:14:23,630
>> So with 1 iPhone how high can I
count using this kind of approach?

316
00:14:23,630 --> 00:14:24,490
I mean to 1.

317
00:14:24,490 --> 00:14:25,900
It's not all that compelling.

318
00:14:25,900 --> 00:14:27,170
So what more could we do?

319
00:14:27,170 --> 00:14:31,180
Well let's see, is anyone on their
phone right now that I could borrow?

320
00:14:31,180 --> 00:14:33,800
Anyone who has a phone
with a flashlight built in?

321
00:14:33,800 --> 00:14:34,969
May I borrow?

322
00:14:34,969 --> 00:14:36,010
I don't need it unlocked.

323
00:14:36,010 --> 00:14:36,510
All right.

324
00:14:36,510 --> 00:14:37,150
Thank you.

325
00:14:37,150 --> 00:14:37,780
Let me borrow this.

326
00:14:37,780 --> 00:14:38,279
All right.

327
00:14:38,279 --> 00:14:45,570
So if I now scroll up and here,
what am I representing now?

328
00:14:45,570 --> 00:14:46,070
Yeah.

329
00:14:46,070 --> 00:14:49,350
So it's a three because this is in the
1s column, this is in the 2s column.

330
00:14:49,350 --> 00:14:50,480
So 1 plus 2 is 3.

331
00:14:50,480 --> 00:14:53,410
And then if we try to get
really creative-- oh, thank you.

332
00:14:53,410 --> 00:14:54,690
Very preemptive.

333
00:14:54,690 --> 00:14:55,560
All right.

334
00:14:55,560 --> 00:14:57,100
I now have three iPhones.

335
00:14:57,100 --> 00:14:58,810
All right.

336
00:14:58,810 --> 00:15:02,500
>> And now this-- I won't
do any further than this.

337
00:15:02,500 --> 00:15:04,380
What am I representing now?

338
00:15:04,380 --> 00:15:05,090
Just sevens.

339
00:15:05,090 --> 00:15:08,000
But I needed physically
more memory in this case.

340
00:15:08,000 --> 00:15:08,920
But that's all it is.

341
00:15:08,920 --> 00:15:12,870
You can think of what's going on--
thank you-- inside of your phone

342
00:15:12,870 --> 00:15:16,240
as just being a switch that's
being turned on and off.

343
00:15:16,240 --> 00:15:18,350
>> And if you've ever heard
the word transistor.

344
00:15:18,350 --> 00:15:21,312
Or if you've ever heard the
marketing speak Intel inside,

345
00:15:21,312 --> 00:15:24,270
that's speaking to the kind of hardware
that's inside of your computer.

346
00:15:24,270 --> 00:15:26,707
Intel makes CPUs,
central processing units,

347
00:15:26,707 --> 00:15:28,790
which are like the brains
inside of your computer.

348
00:15:28,790 --> 00:15:30,640
And these CPUs and
things they're connected

349
00:15:30,640 --> 00:15:32,740
to have lots and lots of tiny switches.

350
00:15:32,740 --> 00:15:36,440
Millions, billions of switches
that can either be on or off.

351
00:15:36,440 --> 00:15:38,840
>> So computers, thankfully,
like our Macs and PCs,

352
00:15:38,840 --> 00:15:43,830
can count way higher than 7 or 8
because they have way more than three

353
00:15:43,830 --> 00:15:44,660
or four bits.

354
00:15:44,660 --> 00:15:48,140
Way more than the equivalent of the
three flashlights that we just had.

355
00:15:48,140 --> 00:15:52,110
But now this starts to get
pretty uninteresting quickly.

356
00:15:52,110 --> 00:15:55,810
If I now want to actually be able
to do something more interesting,

357
00:15:55,810 --> 00:16:00,340
I want to be able to jump
to something like this.

358
00:16:00,340 --> 00:16:03,604
>> So ASCII, it's not really a useful
acronym, but American Standard Code

359
00:16:03,604 --> 00:16:04,770
for Information Interchange.

360
00:16:04,770 --> 00:16:07,450
It just means, some years
ago we humans decided,

361
00:16:07,450 --> 00:16:10,724
you know what, we want to be able to do
more with computers than just numbers.

362
00:16:10,724 --> 00:16:12,890
We don't want them to just
be expensive calculators,

363
00:16:12,890 --> 00:16:16,730
we'd like to be able to do things like
word processing, albeit very simply.

364
00:16:16,730 --> 00:16:20,030
Later we had email and other such media.

365
00:16:20,030 --> 00:16:23,700
>> And so the world decided some years
ago according to this system ASCII,

366
00:16:23,700 --> 00:16:24,770
you know what?

367
00:16:24,770 --> 00:16:26,970
In certain types of
programs any time you

368
00:16:26,970 --> 00:16:30,649
see the equivalent of the number
65, like the pattern of bits.

369
00:16:30,649 --> 00:16:32,440
And we could do the
math here on the board.

370
00:16:32,440 --> 00:16:34,450
The pattern of bits that represent 65.

371
00:16:34,450 --> 00:16:37,240
Don't think of it as 65 in decimal.

372
00:16:37,240 --> 00:16:41,620
Think of it as arbitrarily, but
globally, consistently as the capital

373
00:16:41,620 --> 00:16:42,201
A.

374
00:16:42,201 --> 00:16:43,950
And then the world
decided, you know what?

375
00:16:43,950 --> 00:16:45,630
Let's take another pattern of bits.

376
00:16:45,630 --> 00:16:47,770
And if we ever see the
number 66, let's just

377
00:16:47,770 --> 00:16:50,573
assume that that is the
capital B. Fast forward to H

378
00:16:50,573 --> 00:16:55,510
and I, if you see 72 or 73, that
should be an H and an I, respectively.

379
00:16:55,510 --> 00:16:57,780
And so as long as the whole
world agrees upon this.

380
00:16:57,780 --> 00:17:01,342
So that when you receive an email, or
you would get a file on a USB stick,

381
00:17:01,342 --> 00:17:03,800
or something like that-- when
you see that pattern of bits,

382
00:17:03,800 --> 00:17:06,980
you know that it should be this
letter or some other letter.

383
00:17:06,980 --> 00:17:08,620
>> But it's context specific, right.

384
00:17:08,620 --> 00:17:12,250
An email program might interpret
these things as characters,

385
00:17:12,250 --> 00:17:15,760
but a graphing calculator or
calculator might represent or interpret

386
00:17:15,760 --> 00:17:18,370
these things, of course, as letters.

387
00:17:18,370 --> 00:17:21,610
>> So with that said, quick little review.

388
00:17:21,610 --> 00:17:25,440
This is maybe a three character
e-mail that's been sent to me.

389
00:17:25,440 --> 00:17:27,990
Underneath the hood it's all in
0s and 1s, But we don't care.

390
00:17:27,990 --> 00:17:31,580
We're going to start to abstract
above the 0s and 1s to letters.

391
00:17:31,580 --> 00:17:36,830
And if I see a pattern of 0s and 1s that
really represent 72, hint, hint, 73,

392
00:17:36,830 --> 00:17:38,866
and then 33, what's the message?

393
00:17:38,866 --> 00:17:40,348
>> STUDENT: [INAUDIBLE]

394
00:17:40,348 --> 00:17:42,639
DAVID MALAN: So if you think
back just a moment ago, HI

395
00:17:42,639 --> 00:17:47,210
was the message I was trying to
communicate here because H is 72,

396
00:17:47,210 --> 00:17:50,925
I is 73, and now 33-- you wouldn't
necessarily know this in advance,

397
00:17:50,925 --> 00:17:53,800
but it turns out if you actually
see more of the chart and the system

398
00:17:53,800 --> 00:17:56,800
that humanity agreed upon years
ago, it's just an exclamation point.

399
00:17:56,800 --> 00:18:01,590
And indeed, there is a pattern of
symbols and numbers for every character

400
00:18:01,590 --> 00:18:03,849
that you might have on your keyboard.

401
00:18:03,849 --> 00:18:04,389
>> All right.

402
00:18:04,389 --> 00:18:05,660
Let's abstract further.

403
00:18:05,660 --> 00:18:09,380
If we don't want to just have
things like numbers and letters,

404
00:18:09,380 --> 00:18:11,420
we actually want to implement graphics.

405
00:18:11,420 --> 00:18:13,351
Well, if you've ever
heard the acronym RGB.

406
00:18:13,351 --> 00:18:15,559
It's kind of dated now, but
it's still kind of there.

407
00:18:15,559 --> 00:18:17,380
RGB is red, green, blue.

408
00:18:17,380 --> 00:18:19,380
And it's just a system
of saying, you know what,

409
00:18:19,380 --> 00:18:21,650
let's use three sets of bits.

410
00:18:21,650 --> 00:18:25,280
A set of 8 bits, another set of 8
bits, and another set of 8 bits.

411
00:18:25,280 --> 00:18:27,710
And let's use those bits
to store how much red we

412
00:18:27,710 --> 00:18:31,430
want on our screen, how much
green we want on our screen,

413
00:18:31,430 --> 00:18:34,070
and how much blue we want on our screen.

414
00:18:34,070 --> 00:18:37,860
And this just means that if you
have a lot-- a big number for red,

415
00:18:37,860 --> 00:18:39,200
that means give me a lot of red.

416
00:18:39,200 --> 00:18:41,658
If you have a big number for
green, give me a lot of green.

417
00:18:41,658 --> 00:18:44,660
And if you have just a little bit
of blue or a small number like 33,

418
00:18:44,660 --> 00:18:45,960
give me a little bit of blue.

419
00:18:45,960 --> 00:18:49,720
And if you happen to combine those
three magnitudes, so to speak,

420
00:18:49,720 --> 00:18:53,160
you get this-- you barely can see on
the projector here, but this murky

421
00:18:53,160 --> 00:18:54,630
shade of yellow or brown.

422
00:18:54,630 --> 00:18:58,590
>> But this is to say, using that
pattern of 8 plus 8 plus plus 8--

423
00:18:58,590 --> 00:19:01,710
that pattern of 24 bits
is how a computer would

424
00:19:01,710 --> 00:19:06,460
store that shade of yellow in one
tiny dot a pixel on the screen.

425
00:19:06,460 --> 00:19:10,570
So we've gone from 0s and 1s to decimal
numbers to letters of the alphabet.

426
00:19:10,570 --> 00:19:13,680
Or more interesting, colored dots.

427
00:19:13,680 --> 00:19:16,200
>> Well, what of course then comes next?

428
00:19:16,200 --> 00:19:19,040
Well, what is an image that you
see on Facebook or get in an email?

429
00:19:19,040 --> 00:19:20,172
Or the like?

430
00:19:20,172 --> 00:19:22,130
What is the definition
technically of an image?

431
00:19:22,130 --> 00:19:25,090

432
00:19:25,090 --> 00:19:27,710
Yeah.

433
00:19:27,710 --> 00:19:31,270
What is an image composed of if you
look really close at your screen?

434
00:19:31,270 --> 00:19:31,770
Yeah.

435
00:19:31,770 --> 00:19:33,186
It's just a whole bunch of pixels.

436
00:19:33,186 --> 00:19:35,220
In fact, if you take your
laptop maybe later on,

437
00:19:35,220 --> 00:19:37,180
and look really closely
at it-- depending

438
00:19:37,180 --> 00:19:40,370
on how expensive the laptop is and
how high quality the screen is,

439
00:19:40,370 --> 00:19:43,480
you might very well see all of
the little dots on the screen.

440
00:19:43,480 --> 00:19:45,890
>> And those dots or pixels,
which means there's

441
00:19:45,890 --> 00:19:49,810
24 bits representing every pixel in that
photograph that you see on Facebook,

442
00:19:49,810 --> 00:19:52,049
or that you just took
on your iPhone recently.

443
00:19:52,049 --> 00:19:54,090
And so that's how we get
to things like graphics.

444
00:19:54,090 --> 00:19:55,120
Well, what's a video?

445
00:19:55,120 --> 00:19:58,660
A video is just a set of graphics
flying by the screen again

446
00:19:58,660 --> 00:19:59,780
and again and again.

447
00:19:59,780 --> 00:20:04,080
And so videos really, are just patterns
of bits representing grids, rows

448
00:20:04,080 --> 00:20:06,880
and columns of dots,
flying by the screen image,

449
00:20:06,880 --> 00:20:09,240
after image, after image, a.k.a.

450
00:20:09,240 --> 00:20:10,650
Motion pictures.

451
00:20:10,650 --> 00:20:12,340
So that's it for inputs and outputs.

452
00:20:12,340 --> 00:20:14,420
>> All we have now is an
assumption that, you

453
00:20:14,420 --> 00:20:17,190
know what, if we want a computer
to represent information,

454
00:20:17,190 --> 00:20:18,560
we have a system for doing it.

455
00:20:18,560 --> 00:20:20,780
We can do it with 0s and
1s at the end of the day.

456
00:20:20,780 --> 00:20:23,160
But we can abstract, so
to speak, on top of that

457
00:20:23,160 --> 00:20:25,140
so as to represent more
interesting things.

458
00:20:25,140 --> 00:20:28,790
And here on out in CS50, and in
computer science more generally,

459
00:20:28,790 --> 00:20:31,250
we now stand on the shoulders
of all the people who

460
00:20:31,250 --> 00:20:33,530
came before us who figured that out.

461
00:20:33,530 --> 00:20:38,170
And now just assume that computers
can represent inputs and outputs.

462
00:20:38,170 --> 00:20:41,140
>> But now let's actually
do something with them.

463
00:20:41,140 --> 00:20:44,850
So an algorithm is just a set
of instructions, step by step,

464
00:20:44,850 --> 00:20:46,340
for solving some problem.

465
00:20:46,340 --> 00:20:48,730
And what might one such problem be.

466
00:20:48,730 --> 00:20:52,440
So this is an old school
technology, a phone book.

467
00:20:52,440 --> 00:20:55,534
And inside of a phone book is a
whole bunch of names and numbers.

468
00:20:55,534 --> 00:20:57,700
And those names are generally
sorted alphabetically.

469
00:20:57,700 --> 00:21:01,310
>> So if I wanted to find someone in
this phone book like Mike Smith,

470
00:21:01,310 --> 00:21:02,930
what's a typical human going to do?

471
00:21:02,930 --> 00:21:06,000
Well, you could simply open
it up, look at the first page.

472
00:21:06,000 --> 00:21:07,390
I don't see Mike Smith.

473
00:21:07,390 --> 00:21:09,740
Turn to the second page,
I don't see Mike Smith.

474
00:21:09,740 --> 00:21:11,830
And just keep going and going.

475
00:21:11,830 --> 00:21:14,397
Is this step by step approach correct?

476
00:21:14,397 --> 00:21:17,380

477
00:21:17,380 --> 00:21:18,310
Yeah.

478
00:21:18,310 --> 00:21:20,080
It's kind of stupid, right.

479
00:21:20,080 --> 00:21:21,646
It's inefficient, right.

480
00:21:21,646 --> 00:21:24,520
Because it's going to take forever
to get to Mike, but it is correct.

481
00:21:24,520 --> 00:21:26,620
Because if Mike is here
I will indeed find him.

482
00:21:26,620 --> 00:21:29,030
>> So what's a slightly more
reasonable person going to do?

483
00:21:29,030 --> 00:21:32,180
They might still open to the front,
and maybe fly through the phone book

484
00:21:32,180 --> 00:21:33,250
two pages at a time.

485
00:21:33,250 --> 00:21:34,840
Two, four, six, eight.

486
00:21:34,840 --> 00:21:36,830
I can't actually
physically do it very well.

487
00:21:36,830 --> 00:21:39,560
But in theory, this should be
twice as fast, two pages at a time.

488
00:21:39,560 --> 00:21:41,152
Is this algorithm correct?

489
00:21:41,152 --> 00:21:43,252
>> STUDENT: [INAUDIBLE]

490
00:21:43,252 --> 00:21:44,460
DAVID MALAN: Not necessarily.

491
00:21:44,460 --> 00:21:44,960
Good.

492
00:21:44,960 --> 00:21:46,112
Why that caveat?

493
00:21:46,112 --> 00:21:50,020
>> STUDENT: Because he could be on one
of the pages that you're skipping.

494
00:21:50,020 --> 00:21:50,770
DAVID MALAN: Yeah.

495
00:21:50,770 --> 00:21:52,260
So even if I get closer and closer.

496
00:21:52,260 --> 00:21:56,150
What if he's just accidentally, by bad
luck, sandwiched between the two pages

497
00:21:56,150 --> 00:21:57,290
that I'm flying over?

498
00:21:57,290 --> 00:21:59,920

499
00:21:59,920 --> 00:22:01,730
So we need a fix for this.

500
00:22:01,730 --> 00:22:03,590
We actually need to
then say, wait a minute,

501
00:22:03,590 --> 00:22:06,240
maybe if we go too far, maybe
if we hit the T section,

502
00:22:06,240 --> 00:22:09,670
for T coming after Smith, then we should
at least double back at least one page.

503
00:22:09,670 --> 00:22:12,630
So fixable, but there is
a conditional issue there.

504
00:22:12,630 --> 00:22:16,159
So it's twice as fast, but you might
have to double back just a little bit.

505
00:22:16,159 --> 00:22:19,200
But no one in his room, even if you
don't really use phone books anymore,

506
00:22:19,200 --> 00:22:20,740
is going to start at the beginning.

507
00:22:20,740 --> 00:22:23,317
What are you going to do
looking for Mike Smith?

508
00:22:23,317 --> 00:22:24,900
You're going to go roughly to the S's.

509
00:22:24,900 --> 00:22:26,820
Or if you don't really have
the cheat sheet on the paper,

510
00:22:26,820 --> 00:22:28,365
you're going to go at least
roughly to the middle.

511
00:22:28,365 --> 00:22:30,220
And certainly not to
the front of the book.

512
00:22:30,220 --> 00:22:31,130
You're going to look down.

513
00:22:31,130 --> 00:22:33,770
And mathematically you're probably
going to see the M section, which

514
00:22:33,770 --> 00:22:34,890
is roughly in the middle.

515
00:22:34,890 --> 00:22:36,848
And then you're going to
realize, what is true?

516
00:22:36,848 --> 00:22:37,968
Where is Mike?

517
00:22:37,968 --> 00:22:39,170
>> STUDENT: [INAUDIBLE]

518
00:22:39,170 --> 00:22:39,920
DAVID MALAN: Yeah.

519
00:22:39,920 --> 00:22:41,530
So he's over on this side.

520
00:22:41,530 --> 00:22:42,710
And so what can you do?

521
00:22:42,710 --> 00:22:48,870
Well, both figuratively and literally
can you tear the problem in half once?

522
00:22:48,870 --> 00:22:54,260
And then know that you can throw
this half of the problem away.

523
00:22:54,260 --> 00:22:58,170
And now we're left with fundamentally
the same problem, but it's half as big.

524
00:22:58,170 --> 00:22:59,920
And so now what's the
set of instructions?

525
00:22:59,920 --> 00:23:01,753
What's the algorithm
for finding Mike Smith?

526
00:23:01,753 --> 00:23:03,030
It's the exact same thing.

527
00:23:03,030 --> 00:23:05,750
>> Now this happens to be the M
section and this is the Z section,

528
00:23:05,750 --> 00:23:08,650
but the fundamental
formula is still the same.

529
00:23:08,650 --> 00:23:10,800
Go roughly to the middle,
look down, oh, darn it.

530
00:23:10,800 --> 00:23:13,160
Now I'm in the T section,
I've gone too far.

531
00:23:13,160 --> 00:23:16,640
But here too can you
apply that same logic.

532
00:23:16,640 --> 00:23:19,189
Throw half of the problem
away and now we're

533
00:23:19,189 --> 00:23:21,230
left with a problem that's
a quarter of the size.

534
00:23:21,230 --> 00:23:28,140
And we can repeat, and we can repeat,
and we can repeat until theoretically

535
00:23:28,140 --> 00:23:32,190
there's just one page left on
which Mike either is or isn't.

536
00:23:32,190 --> 00:23:34,014
>> So what's so powerful about this idea?

537
00:23:34,014 --> 00:23:35,680
I mean after all, it's pretty intuitive.

538
00:23:35,680 --> 00:23:37,390
No one's going to start at the
beginning of the phone book

539
00:23:37,390 --> 00:23:39,320
and flip 1,000 pages to find Mike Smith.

540
00:23:39,320 --> 00:23:42,319
Most everyone in this room is going
to do roughly that kind of algorithm

541
00:23:42,319 --> 00:23:43,220
save for the tearing.

542
00:23:43,220 --> 00:23:45,480
>> And so why did we do that?

543
00:23:45,480 --> 00:23:47,810
Well, consider the efficiency.

544
00:23:47,810 --> 00:23:51,500
Consider just how much better this
algorithm was by breaking it down

545
00:23:51,500 --> 00:23:52,890
into its component parts.

546
00:23:52,890 --> 00:23:53,980
So what did I first do?

547
00:23:53,980 --> 00:23:55,389
I picked up the phone book.

548
00:23:55,389 --> 00:23:57,180
And a computer scientist,
and a programmer,

549
00:23:57,180 --> 00:24:00,090
more generally it turns out, is going
to start counting everything at 0.

550
00:24:00,090 --> 00:24:00,610
>> Why?

551
00:24:00,610 --> 00:24:03,240
Well, it's a little strange
that we humans count, generally,

552
00:24:03,240 --> 00:24:04,400
starting from one.

553
00:24:04,400 --> 00:24:08,110
Because what's the smallest number
we can clearly represent based

554
00:24:08,110 --> 00:24:09,930
even on our old grade school math?

555
00:24:09,930 --> 00:24:12,400
Well, it was 0, whether
it's in decimal or binary.

556
00:24:12,400 --> 00:24:14,900
And so you'll see in the world
of computing and programming,

557
00:24:14,900 --> 00:24:17,620
specifically, we start
counting everything from 0.

558
00:24:17,620 --> 00:24:19,690
>> So I picked up the phone book step 0.

559
00:24:19,690 --> 00:24:21,924
I'm going to open to the
middle of the phone book.

560
00:24:21,924 --> 00:24:23,840
And that's indeed an
expression of what I did.

561
00:24:23,840 --> 00:24:25,900
And then step two was look at the names.

562
00:24:25,900 --> 00:24:29,110
Step three is a little
different conceptually.

563
00:24:29,110 --> 00:24:30,600
I'm asking myself a question.

564
00:24:30,600 --> 00:24:33,610
If Smith is among the names,
I'm going to make a decision.

565
00:24:33,610 --> 00:24:36,550
If he's among the names,
then I'm going to call Mike.

566
00:24:36,550 --> 00:24:39,590
And I'm going to make a decision
based on that piece of information.

567
00:24:39,590 --> 00:24:44,100
>> However, if not, if Smith is
earlier in the book to the left,

568
00:24:44,100 --> 00:24:48,200
I'm going to open to the middle
of the left half of the book.

569
00:24:48,200 --> 00:24:51,674
And then here's the cleverness,
I'm going to go back to step two.

570
00:24:51,674 --> 00:24:53,590
I'm going to sort of
stand on my own shoulders

571
00:24:53,590 --> 00:24:55,110
and just repeat the past work I did.

572
00:24:55,110 --> 00:24:58,062
But the work I have left is
less, and less, and less.

573
00:24:58,062 --> 00:24:59,270
But it's still going to work.

574
00:24:59,270 --> 00:25:02,110
But if Mike, instead, is later
in the book to the right,

575
00:25:02,110 --> 00:25:04,980
I'm going to open to the middle
of the right half of the book,

576
00:25:04,980 --> 00:25:07,240
then go back to step two.

577
00:25:07,240 --> 00:25:09,330
>> But there's actually a fourth scenario.

578
00:25:09,330 --> 00:25:13,336
Mike's either here,
or here, or here, or--

579
00:25:13,336 --> 00:25:14,152
>> STUDENT: Not there.

580
00:25:14,152 --> 00:25:15,110
DAVID MALAN: Not there.

581
00:25:15,110 --> 00:25:18,580
And indeed, if we don't anticipate
this fourth and final scenario

582
00:25:18,580 --> 00:25:21,510
our program might be buggy
or flawed in some way.

583
00:25:21,510 --> 00:25:25,020
Else, quit in the case that
we haven't found Mike at all.

584
00:25:25,020 --> 00:25:27,720
And indeed, if you've ever noticed
your computer hanging, or all

585
00:25:27,720 --> 00:25:30,490
of a sudden word or some other
program just quits unexpectedly,

586
00:25:30,490 --> 00:25:32,614
and sometimes thee error
message is literally that.

587
00:25:32,614 --> 00:25:34,070
This program quit unexpectedly.

588
00:25:34,070 --> 00:25:35,570
It can be for any number of reasons.

589
00:25:35,570 --> 00:25:38,120
But sometimes it's
something as simple as this.

590
00:25:38,120 --> 00:25:40,440
The human programmer
who wrote that software

591
00:25:40,440 --> 00:25:43,610
didn't realize that, oh, there's a
forth thing that can actually happen.

592
00:25:43,610 --> 00:25:46,480
And if you don't write code to
capture that fourth scenario,

593
00:25:46,480 --> 00:25:51,342
it is indeed unexpected sometimes
what the computer might actually do.

594
00:25:51,342 --> 00:25:53,050
Now let's call out a
few of these things.

595
00:25:53,050 --> 00:25:55,465
So in yellow here, I
have highlighted terms

596
00:25:55,465 --> 00:25:57,590
that henceforth we're just
going to call functions.

597
00:25:57,590 --> 00:26:00,700
Functions in the world of
programming are just like actions,

598
00:26:00,700 --> 00:26:01,850
statements of actions.

599
00:26:01,850 --> 00:26:05,720
So pick up, open to, look
at, call, open, open, quit.

600
00:26:05,720 --> 00:26:09,350
That's a function, a procedure,
an action, any number of synonyms

601
00:26:09,350 --> 00:26:10,184
would work as well.

602
00:26:10,184 --> 00:26:11,850
Now what are these things now in yellow?

603
00:26:11,850 --> 00:26:14,210
If else, if else, if
else, these are what

604
00:26:14,210 --> 00:26:16,500
we're going to call
conditions in programming,

605
00:26:16,500 --> 00:26:19,270
or branches, decision
points, if you will.

606
00:26:19,270 --> 00:26:22,759
But how do you know which fork
in the road to take, so to speak?

607
00:26:22,759 --> 00:26:24,550
We need to highlight
the terms to the right

608
00:26:24,550 --> 00:26:27,570
there, which are these
yes, no questions.

609
00:26:27,570 --> 00:26:29,040
These true false questions.

610
00:26:29,040 --> 00:26:30,880
Smith among names?

611
00:26:30,880 --> 00:26:32,190
Smith earlier in book?

612
00:26:32,190 --> 00:26:33,260
Smith later in book?

613
00:26:33,260 --> 00:26:35,490
These are questions to
which there is a yes, or no,

614
00:26:35,490 --> 00:26:42,110
or equivalently true, or false, or
equivalently, one or zero answer.

615
00:26:42,110 --> 00:26:44,260
>> And meanwhile there's
just one last piece.

616
00:26:44,260 --> 00:26:45,940
This here has what kind of effect?

617
00:26:45,940 --> 00:26:48,230
Whether or not you program
before, how would you

618
00:26:48,230 --> 00:26:51,030
describe what step
seven and 10 are doing?

619
00:26:51,030 --> 00:26:54,110

620
00:26:54,110 --> 00:26:55,027
What did you say?

621
00:26:55,027 --> 00:26:56,110
STUDENT: A recursive step.

622
00:26:56,110 --> 00:26:56,980
DAVID MALAN: A recursive step.

623
00:26:56,980 --> 00:26:58,000
Yes, essentially.

624
00:26:58,000 --> 00:27:00,384
It's technically iterative
here if you're familiar.

625
00:27:00,384 --> 00:27:01,550
But we'll come back to that.

626
00:27:01,550 --> 00:27:03,930
But it's doing something clearly.

627
00:27:03,930 --> 00:27:06,370
Again, it's inducing a
cycle, a loop, right.

628
00:27:06,370 --> 00:27:08,770
You're literally going
back to some earlier step.

629
00:27:08,770 --> 00:27:12,592
And so indeed, this is going to
implement some kind of cycle.

630
00:27:12,592 --> 00:27:15,050
But you're not going to get
stuck in this endlessly, right.

631
00:27:15,050 --> 00:27:19,290
Because if you're constantly checking is
Mike here, or to the left, or not here,

632
00:27:19,290 --> 00:27:21,360
eventually he's not going to be there.

633
00:27:21,360 --> 00:27:24,590
And you can just quit altogether
as per that last line.

634
00:27:24,590 --> 00:27:25,834
>> So that's it for vocabulary.

635
00:27:25,834 --> 00:27:28,250
And this was what we would
generally call pseudocode code.

636
00:27:28,250 --> 00:27:29,570
It's not an actual language.

637
00:27:29,570 --> 00:27:32,405
It's just very terse English,
but it communicates the point.

638
00:27:32,405 --> 00:27:33,780
There's no formal structure here.

639
00:27:33,780 --> 00:27:35,738
You just use it's few
words, but as clear words

640
00:27:35,738 --> 00:27:37,870
as you can to communicate your idea.

641
00:27:37,870 --> 00:27:40,580
>> Now how good is that algorithm
and how much better is it?

642
00:27:40,580 --> 00:27:43,570
Well, we don't have to get into the
specifics of numbers or anything

643
00:27:43,570 --> 00:27:44,069
like that.

644
00:27:44,069 --> 00:27:46,300
But we can look at the
shape of this solution.

645
00:27:46,300 --> 00:27:50,194
So if we just draw some xy plot
here on the horizontal axis here.

646
00:27:50,194 --> 00:27:51,860
Let's just call the size of the problem.

647
00:27:51,860 --> 00:27:55,540
And a computer scientist would
typically use n as the variable here.

648
00:27:55,540 --> 00:27:59,690
So n pages, or n people in the room, or
whatever it is you're trying to count.

649
00:27:59,690 --> 00:28:03,039
>> And then on the vertical axis on the
left, that would be the time to solve.

650
00:28:03,039 --> 00:28:05,330
So how many seconds does it
take me to find Mike Smith?

651
00:28:05,330 --> 00:28:06,621
Or how many steps does it take?

652
00:28:06,621 --> 00:28:08,100
How many page turns does it take?

653
00:28:08,100 --> 00:28:11,370
So that's how much it costs
me in time to solve a problem.

654
00:28:11,370 --> 00:28:15,030
And we might draw the first
algorithms slope, if you will,

655
00:28:15,030 --> 00:28:16,910
as just this straight line in red.

656
00:28:16,910 --> 00:28:18,450
And I'll call it n.

657
00:28:18,450 --> 00:28:19,117
>> Why n?

658
00:28:19,117 --> 00:28:20,950
Why is it just this one
to one relationship?

659
00:28:20,950 --> 00:28:22,700
Well, if Verizon or
whatever phone company

660
00:28:22,700 --> 00:28:25,130
adds one more page to
the phone book next year,

661
00:28:25,130 --> 00:28:28,370
that might push Mike one
more step closer to the end,

662
00:28:28,370 --> 00:28:29,950
depending on where that page is.

663
00:28:29,950 --> 00:28:32,380
And so the effect might just
be to add one more second.

664
00:28:32,380 --> 00:28:33,520
Or one more page turn.

665
00:28:33,520 --> 00:28:34,970
A one to one ratio.

666
00:28:34,970 --> 00:28:37,410
>> By contrast, the second algorithm.

667
00:28:37,410 --> 00:28:41,406
How much faster was that intuitively?

668
00:28:41,406 --> 00:28:42,780
Where I went two pages at a time?

669
00:28:42,780 --> 00:28:43,664
Yeah.

670
00:28:43,664 --> 00:28:44,497
>> STUDENT: [INAUDIBLE]

671
00:28:44,497 --> 00:28:47,255

672
00:28:47,255 --> 00:28:48,080
>> DAVID MALAN: Yeah.

673
00:28:48,080 --> 00:28:49,220
So it's going to be twice as fast.

674
00:28:49,220 --> 00:28:51,344
And we would draw that here
depending on the scale.

675
00:28:51,344 --> 00:28:54,220
It still is a straight line,
but lower than the red line.

676
00:28:54,220 --> 00:28:56,410
Because for some number
of pages, if it takes

677
00:28:56,410 --> 00:28:58,360
you this many steps with
the first algorithm,

678
00:28:58,360 --> 00:29:00,570
it's going to take you half
as many steps with the second.

679
00:29:00,570 --> 00:29:02,770
And so the yellow line
describing the second algorithm

680
00:29:02,770 --> 00:29:03,990
is just going to be below it.

681
00:29:03,990 --> 00:29:07,250
>> But what's really powerful is to
think about the third and final,

682
00:29:07,250 --> 00:29:10,480
and amazingly most intuitive
algorithm, that has this shape.

683
00:29:10,480 --> 00:29:12,720
Technically we would call
this a logarithmic curve.

684
00:29:12,720 --> 00:29:14,309
Log base 2 of n in this case.

685
00:29:14,309 --> 00:29:15,600
But that doesn't really matter.

686
00:29:15,600 --> 00:29:19,140
What matters really is the fundamentally
different shape that it has.

687
00:29:19,140 --> 00:29:22,810
And you can consider just how
much shorter this line really

688
00:29:22,810 --> 00:29:24,620
is in the long run.

689
00:29:24,620 --> 00:29:25,870
It's constantly increasing.

690
00:29:25,870 --> 00:29:27,620
It doesn't flatten out perfectly.

691
00:29:27,620 --> 00:29:34,030
But it grows ever so much more slowly
as the problem gets bigger and bigger.

692
00:29:34,030 --> 00:29:36,710
And you can think of it this
way-- if Verizon doesn't just

693
00:29:36,710 --> 00:29:40,410
add one page next year but doubles
the number of pages in the phone book,

694
00:29:40,410 --> 00:29:42,740
the first algorithm might
take twice as many steps.

695
00:29:42,740 --> 00:29:45,360
If it's 1,000 pages this
year, 2,000 pages next year,

696
00:29:45,360 --> 00:29:47,180
Mike might be that much farther away.

697
00:29:47,180 --> 00:29:49,380
So it's 1,000 extra steps to find him.

698
00:29:49,380 --> 00:29:51,610
The second algorithm
might be only 500 more

699
00:29:51,610 --> 00:29:55,490
steps to find him because again,
I'm flying through it two at a time.

700
00:29:55,490 --> 00:29:56,950
>> But what about the third algorithm?

701
00:29:56,950 --> 00:29:59,010
If Verizon doubles the
size of the phone book

702
00:29:59,010 --> 00:30:02,620
next year from 1,000 to 2000
pages, how many more steps

703
00:30:02,620 --> 00:30:05,540
is my third algorithm going to take?

704
00:30:05,540 --> 00:30:06,630
Yeah, it's just one.

705
00:30:06,630 --> 00:30:08,600
And that's the powerful idea.

706
00:30:08,600 --> 00:30:12,780
You can take 1,000 page bite
out of that problem at once.

707
00:30:12,780 --> 00:30:14,510
And now if you consider
a silly scenario,

708
00:30:14,510 --> 00:30:17,320
but it kind of speaks to the
power of this kind of intuition--

709
00:30:17,320 --> 00:30:21,494
if a phone book had, like, four billion
pages, feels like a really big problem.

710
00:30:21,494 --> 00:30:23,410
And indeed, it might
take me four billion page

711
00:30:23,410 --> 00:30:26,549
turns to find Mike Smith in that
case with the first algorithm.

712
00:30:26,549 --> 00:30:28,840
But how many steps would it
take in the third algorithm

713
00:30:28,840 --> 00:30:31,585
to find Mike among four
billion pieces of paper?

714
00:30:31,585 --> 00:30:34,865

715
00:30:34,865 --> 00:30:36,240
So four billion you tear in half.

716
00:30:36,240 --> 00:30:37,220
You get two billion.

717
00:30:37,220 --> 00:30:42,810
Then one billion, then 500 million,
250 million, 125 million-- but it

718
00:30:42,810 --> 00:30:44,780
feels like this is
going to take a while.

719
00:30:44,780 --> 00:30:47,340
I might need 32 fingers
to count up that high.

720
00:30:47,340 --> 00:30:51,340
But it is indeed as
few as 32 page tears.

721
00:30:51,340 --> 00:30:54,060
You can go from four
billion to one page dividing

722
00:30:54,060 --> 00:30:57,030
the original number of
pages in half 32 times

723
00:30:57,030 --> 00:30:59,290
until you're left with
just that single page.

724
00:30:59,290 --> 00:31:01,320
>> Now, of course, I'm cheating here.

725
00:31:01,320 --> 00:31:06,470
It's not that we are just being sort
of stupid entirely with the first two

726
00:31:06,470 --> 00:31:07,470
algorithms.

727
00:31:07,470 --> 00:31:11,500
I am cheating in some sense, or
really I'm leveraging an assumption.

728
00:31:11,500 --> 00:31:15,610
What was true about the phone book
in its original form that allowed

729
00:31:15,610 --> 00:31:17,450
me to even use that third algorithm?

730
00:31:17,450 --> 00:31:18,110
Yeah?

731
00:31:18,110 --> 00:31:19,360
AUDIENCE: It was alphabetized.

732
00:31:19,360 --> 00:31:21,130
DAVID MALAN: It was alphabetized, right?

733
00:31:21,130 --> 00:31:23,500
If it were just in random
order, this is a waste

734
00:31:23,500 --> 00:31:24,880
of time, this whole conversation.

735
00:31:24,880 --> 00:31:27,500
I have to look at every
page if it's in random order

736
00:31:27,500 --> 00:31:30,350
to find Mike Smith before I
can conclude he's there or not.

737
00:31:30,350 --> 00:31:32,880
And so the corner we
have cut is that I have

738
00:31:32,880 --> 00:31:35,752
assumed that someone else in
this case did the work for me.

739
00:31:35,752 --> 00:31:38,460
And so that ultimately invites
the question, well, wait a minute.

740
00:31:38,460 --> 00:31:41,284
How do you sort 1,000
pages of names and numbers?

741
00:31:41,284 --> 00:31:43,200
That's actually a different
problem, something

742
00:31:43,200 --> 00:31:44,574
we'll come back to in the future.

743
00:31:44,574 --> 00:31:48,130
But when you think about websites
like Facebook and Google for Gmail

744
00:31:48,130 --> 00:31:50,450
and things like Google's
own search indexes,

745
00:31:50,450 --> 00:31:53,800
when you have millions or billions of
pieces of data being stored these days,

746
00:31:53,800 --> 00:31:56,660
searching-- and not to mention
sorting those problems--

747
00:31:56,660 --> 00:31:59,450
is ultimately a challenge unto itself.

748
00:31:59,450 --> 00:32:02,250
And indeed, this then is
just one of those challenges

749
00:32:02,250 --> 00:32:03,460
that we'll be looking at.

750
00:32:03,460 --> 00:32:05,590
>> So now let's take a moment
and take a look at CS50

751
00:32:05,590 --> 00:32:09,082
itself and give you a sense of
what's in store this semester.

752
00:32:09,082 --> 00:32:11,540
Indeed, if you haven't already,
do take a look at this URL.

753
00:32:11,540 --> 00:32:13,880
And as Patrick alluded
to, this year we're

754
00:32:13,880 --> 00:32:17,130
making a significant investment all
the more in the course's support

755
00:32:17,130 --> 00:32:20,170
structure in terms of the TAs
and the CAs, office hours,

756
00:32:20,170 --> 00:32:23,520
sections availability, and
digital materials online, as well.

757
00:32:23,520 --> 00:32:27,489
Indeed, in terms of the course's
lecture, we're here today.

758
00:32:27,489 --> 00:32:29,780
And the expectations this
year officially of the course

759
00:32:29,780 --> 00:32:32,370
are attend to today, the course's
last lecture, and a course

760
00:32:32,370 --> 00:32:35,161
roughly in the middle of the semester
with every lecture in between

761
00:32:35,161 --> 00:32:37,280
made available generally
on a Friday afternoon

762
00:32:37,280 --> 00:32:40,240
online, both for Yale students
and Harvard students this year.

763
00:32:40,240 --> 00:32:42,090
Indeed, one of the
fundamental changes is

764
00:32:42,090 --> 00:32:44,310
that we're adopting at
Harvard a paradigm very much

765
00:32:44,310 --> 00:32:46,570
like we did here last
year and now this year,

766
00:32:46,570 --> 00:32:50,300
so that similarly, we still film most
of the course's lectures in Cambridge

767
00:32:50,300 --> 00:32:52,900
but make them available earlier
than we have in the past

768
00:32:52,900 --> 00:32:55,540
so that those of you-- if you
would like to, for instance,

769
00:32:55,540 --> 00:32:58,400
get a head start on materials
on the the first weekend rather

770
00:32:58,400 --> 00:33:02,000
than the second weekend, you'll have
access to these kinds of materials,

771
00:33:02,000 --> 00:33:06,561
searchable, embeddable, hyperlinkable
to related resources all the earlier.

772
00:33:06,561 --> 00:33:09,560
In terms of the topics, to give you
a sense of the course's trajectory--

773
00:33:09,560 --> 00:33:12,365
and some of this might be jargon for
now, but not for long, rest assured.

774
00:33:12,365 --> 00:33:15,010
We'll start today, ultimately,
with looking at one programming

775
00:33:15,010 --> 00:33:16,260
language called Scratch.

776
00:33:16,260 --> 00:33:19,380
We'll transition thereafter
next week to something called C

777
00:33:19,380 --> 00:33:22,140
and then looking at other building
blocks for solving problems,

778
00:33:22,140 --> 00:33:26,230
things called arrays and algorithms,
how we use memory to our advantage

779
00:33:26,230 --> 00:33:28,326
and disadvantage, and
things like data structures,

780
00:33:28,326 --> 00:33:31,200
and then toward the tail end of the
class looking at machine learning

781
00:33:31,200 --> 00:33:33,410
and looking at another
language called Python,

782
00:33:33,410 --> 00:33:37,460
how the web works, how the internet more
generally works, protocols like HTTP,

783
00:33:37,460 --> 00:33:40,950
languages for databases like
SQL, JavaScript for the web,

784
00:33:40,950 --> 00:33:43,000
and ultimately tying
all of those together.

785
00:33:43,000 --> 00:33:44,708
>> And so indeed, at the
end of the day, you

786
00:33:44,708 --> 00:33:50,250
will not learn in this class Scratch
or C or Python or SQL or JavaScript.

787
00:33:50,250 --> 00:33:53,560
You will instead more generally learn
computer science and the foundations

788
00:33:53,560 --> 00:33:55,790
thereof, and you will
learn how to program

789
00:33:55,790 --> 00:33:58,800
in any number of these
languages along the way.

790
00:33:58,800 --> 00:34:00,970
So indeed, one of the goals
of the course in the end

791
00:34:00,970 --> 00:34:04,160
is to take off all of the course's
training wheels by those final weeks

792
00:34:04,160 --> 00:34:06,710
so that after this, you can
return to your own fields--

793
00:34:06,710 --> 00:34:08,510
whether that is or is
not computer science

794
00:34:08,510 --> 00:34:12,290
or engineering, in the natural
sciences, arts, humanities, or beyond--

795
00:34:12,290 --> 00:34:14,720
and bring some of this
course's ideas and this field's

796
00:34:14,720 --> 00:34:17,400
ideas and practical
skills to your own domain

797
00:34:17,400 --> 00:34:19,480
in order to solve problems therein.

798
00:34:19,480 --> 00:34:22,990
>> What we'll be doing here meanwhile
in most Thursdays after today

799
00:34:22,990 --> 00:34:26,160
is with the course's heads leading
what we'll call walkthroughs

800
00:34:26,160 --> 00:34:27,620
of the course's problem sets.

801
00:34:27,620 --> 00:34:29,610
So each week when we
have a problem set, we'll

802
00:34:29,610 --> 00:34:32,900
be walking through in a location
like this the course's challenges,

803
00:34:32,900 --> 00:34:35,582
offering you some tips and
tricks and design techniques.

804
00:34:35,582 --> 00:34:37,540
But if you're not able
to make those in person,

805
00:34:37,540 --> 00:34:39,760
realize those same
resources will be embedded

806
00:34:39,760 --> 00:34:41,590
by one of the course's
teaching assistants

807
00:34:41,590 --> 00:34:43,860
in the problem sets themselves, as well.

808
00:34:43,860 --> 00:34:47,124
>> The problem sets this year, unlike
last year, based on feedback,

809
00:34:47,124 --> 00:34:48,540
will still be released on Fridays.

810
00:34:48,540 --> 00:34:51,310
But rather than being due
the subsequent Friday,

811
00:34:51,310 --> 00:34:55,079
thereby giving you only seven days,
will effectively be due 10 days later.

812
00:34:55,079 --> 00:34:57,620
And indeed, this will mean that
they'll overlap by a weekend.

813
00:34:57,620 --> 00:34:59,369
But we hope this year
especially this will

814
00:34:59,369 --> 00:35:02,320
allow students to better accommodate
ebb and flow in their schedules,

815
00:35:02,320 --> 00:35:05,040
whether it's academics or
extracurriculars or athletics

816
00:35:05,040 --> 00:35:06,280
or midterm season.

817
00:35:06,280 --> 00:35:10,060
You can either front-load or back-load
your week focusing on CS50 based

818
00:35:10,060 --> 00:35:12,400
on your own week's actual course load.

819
00:35:12,400 --> 00:35:14,990
>> The problem sets themselves
will cover a range of languages,

820
00:35:14,990 --> 00:35:17,990
though we'll focus
predominantly early on on C

821
00:35:17,990 --> 00:35:23,040
before we focus thereafter on higher
level, more web-centric languages.

822
00:35:23,040 --> 00:35:25,970
And then a couple of FAQs here--
should you take a class like CS50

823
00:35:25,970 --> 00:35:26,900
as a first-year?

824
00:35:26,900 --> 00:35:27,970
So absolutely.

825
00:35:27,970 --> 00:35:30,470
And indeed, it's not
necessarily something

826
00:35:30,470 --> 00:35:34,440
you should postpone until you've cut
your teeth on other types of classes.

827
00:35:34,440 --> 00:35:36,290
But rather, consider
that for many students,

828
00:35:36,290 --> 00:35:39,040
myself included back in the day,
this is a very unfamiliar field,

829
00:35:39,040 --> 00:35:40,997
especially if you
never did take a AP CSA

830
00:35:40,997 --> 00:35:42,580
or something like that in high school.

831
00:35:42,580 --> 00:35:44,705
But realize that early on,
whether it's this course

832
00:35:44,705 --> 00:35:47,900
or some other introductory course,
now is indeed the best time,

833
00:35:47,900 --> 00:35:52,082
I think, to find some new path or
some new academic interest, as well.

834
00:35:52,082 --> 00:35:55,040
And then taking with other courses--
so one of the key differences here

835
00:35:55,040 --> 00:35:57,906
versus Harvard is that we only
take four courses per semester

836
00:35:57,906 --> 00:35:59,030
at Harvard for some reason.

837
00:35:59,030 --> 00:36:01,494
And you guys actually pull
off some 36 courses in total

838
00:36:01,494 --> 00:36:04,410
over the course of your four years,
which means generally four or five

839
00:36:04,410 --> 00:36:05,040
classes.

840
00:36:05,040 --> 00:36:08,367
And I do think it's quite fair to say
and to disclaim CS50, by design, is

841
00:36:08,367 --> 00:36:10,700
probably not the type of class
that you should typically

842
00:36:10,700 --> 00:36:13,680
take with four other
courses for a total of five

843
00:36:13,680 --> 00:36:16,249
because psets are by
design fairly intensive.

844
00:36:16,249 --> 00:36:18,040
Indeed, I too learned
this back in the day.

845
00:36:18,040 --> 00:36:21,110
I wouldn't describe CS50 and
computer science, programming

846
00:36:21,110 --> 00:36:24,477
as so much hard as it
is just time-consuming.

847
00:36:24,477 --> 00:36:26,560
It's not the kind of thing
where after dinner, you

848
00:36:26,560 --> 00:36:28,310
can go back to your dorm
room, sit down, and start

849
00:36:28,310 --> 00:36:29,710
focusing on the pset
thinking, all right,

850
00:36:29,710 --> 00:36:31,340
I'm gonna bang this out
tonight and then move

851
00:36:31,340 --> 00:36:33,000
on to my next subject the next day.

852
00:36:33,000 --> 00:36:34,450
Sometimes you just hit a wall.

853
00:36:34,450 --> 00:36:35,690
You have bugs in your code.

854
00:36:35,690 --> 00:36:37,900
You don't necessarily know
how to solve some problem.

855
00:36:37,900 --> 00:36:41,670
And one of the key features of
programming for myself to this day

856
00:36:41,670 --> 00:36:44,480
is you just kind of need to take
a step back sometimes, sleep on it

857
00:36:44,480 --> 00:36:47,870
or think on it over the course
of a jog or some other activity,

858
00:36:47,870 --> 00:36:49,240
and then come back to it fresh.

859
00:36:49,240 --> 00:36:50,930
And you just need these windows of time.

860
00:36:50,930 --> 00:36:52,910
>> And indeed, that's why we've
lengthened the amount of time

861
00:36:52,910 --> 00:36:55,710
available for the problem sets
this year and also, per that URL

862
00:36:55,710 --> 00:36:57,950
I put up earlier as to
what's new this semester,

863
00:36:57,950 --> 00:37:00,950
trimmed the problem sets so that
they're fundamentally no less rigorous,

864
00:37:00,950 --> 00:37:03,741
and the takeaways are no less, but
there's a lot less front matter,

865
00:37:03,741 --> 00:37:07,630
a lot less legwork that you need to
do at the front of every problem set,

866
00:37:07,630 --> 00:37:10,680
as you'll see, before you can
actually dive into the meat of it.

867
00:37:10,680 --> 00:37:13,500
So realize that those and other
changes are on the horizon

868
00:37:13,500 --> 00:37:16,490
to better accommodate students,
but ultimately to make sure

869
00:37:16,490 --> 00:37:18,860
that the takeaways are
indeed as high as possible.

870
00:37:18,860 --> 00:37:21,330
So while more work than it
might be in a typical class,

871
00:37:21,330 --> 00:37:25,120
we do hope that the returns for
you and the takeaways for you

872
00:37:25,120 --> 00:37:27,490
and the skills and ideas
with which you exit

873
00:37:27,490 --> 00:37:29,921
are all the more compelling as a result.

874
00:37:29,921 --> 00:37:32,420
And to get you there-- and this
is one of the key takeaways,

875
00:37:32,420 --> 00:37:35,470
as Patrick alluded earlier-- is
the course's support structure.

876
00:37:35,470 --> 00:37:39,720
So not only does CS50 have one of
the largest course staffs on campus.

877
00:37:39,720 --> 00:37:41,750
It also has one of the
most undergraduate.

878
00:37:41,750 --> 00:37:43,700
Indeed, CS50 last year
was the first class

879
00:37:43,700 --> 00:37:45,366
to have an undergraduate teaching staff.

880
00:37:45,366 --> 00:37:48,570
And testament to that success do now
many other courses within Yale CS

881
00:37:48,570 --> 00:37:49,500
have that, as well.

882
00:37:49,500 --> 00:37:52,906
And for students, specifically,
will these TAs and course assistants

883
00:37:52,906 --> 00:37:55,030
be supporting a whole
network of support resources,

884
00:37:55,030 --> 00:37:58,250
among them sections or
recitations, weekly opportunities

885
00:37:58,250 --> 00:38:01,674
to have more intimate discussions
and reviews of material targeted

886
00:38:01,674 --> 00:38:04,590
for different tracks, for students
less comfortable, more comfortable,

887
00:38:04,590 --> 00:38:05,720
or somewhere in between.

888
00:38:05,720 --> 00:38:08,886
These will follow the availability of
the lectures by several days each week

889
00:38:08,886 --> 00:38:09,970
on Mondays and Tuesdays.

890
00:38:09,970 --> 00:38:11,970
And then office hours--
one-on-one opportunities

891
00:38:11,970 --> 00:38:15,490
for help from the course CAs and TAs
will be on Wednesdays and Thursdays

892
00:38:15,490 --> 00:38:18,400
and Sundays at multiple
times, all of which

893
00:38:18,400 --> 00:38:22,180
will be posted on the course's website,
even more than last year, as well.

894
00:38:22,180 --> 00:38:25,501
>> But what's key to CS50, if
not admittedly a bit unusual,

895
00:38:25,501 --> 00:38:27,750
is the course's culture that
we've tried to cultivate,

896
00:38:27,750 --> 00:38:30,672
both in Cambridge for many years
and now most recently in New Haven.

897
00:38:30,672 --> 00:38:33,130
And in fact, coming up this
Saturday, if you haven't heard,

898
00:38:33,130 --> 00:38:36,410
is CS50 Puzzle Day, which has
nothing to do with computer science

899
00:38:36,410 --> 00:38:39,430
but is entirely designed to send
a message that computer science is

900
00:38:39,430 --> 00:38:40,665
about problem solving.

901
00:38:40,665 --> 00:38:43,540
And indeed, if you'd like to partner
with one or two or three friends

902
00:38:43,540 --> 00:38:46,150
and form a team for CS50
Puzzle Day, take a look

903
00:38:46,150 --> 00:38:48,010
at the adverts that are on the way out.

904
00:38:48,010 --> 00:38:51,582
And three hours of pizza and
puzzles and prizes await.

905
00:38:51,582 --> 00:38:53,290
And indeed, for the
first time this year,

906
00:38:53,290 --> 00:38:54,873
it won't be held jointly with Harvard.

907
00:38:54,873 --> 00:38:57,530
It will be here independently at Yale.

908
00:38:57,530 --> 00:38:59,815
So keep an eye out for
those if you haven't.

909
00:38:59,815 --> 00:39:02,440
Most every Friday in the semester
do we try to make a big class

910
00:39:02,440 --> 00:39:06,710
feel small and bring some 50 students
to lunch with the course's staff,

911
00:39:06,710 --> 00:39:08,730
with alumni, friends
from industry to talk

912
00:39:08,730 --> 00:39:12,630
about what life is like after a
class like CS50 and over the summers

913
00:39:12,630 --> 00:39:14,250
and after graduation.

914
00:39:14,250 --> 00:39:16,280
So keep an eye out for
invitations to that.

915
00:39:16,280 --> 00:39:17,988
For the first time
ever this year will we

916
00:39:17,988 --> 00:39:21,420
hold the first ever CS50 coding
contest, an optional opt-in opportunity

917
00:39:21,420 --> 00:39:25,580
mid-semester, after all of us have had
some six or seven weeks of programming

918
00:39:25,580 --> 00:39:30,100
in C under their belts to compete,
if you would so choose-- again

919
00:39:30,100 --> 00:39:32,460
on teams-- trying to
solve as many challenges

920
00:39:32,460 --> 00:39:36,160
as you can in programming with
friends of yours against others.

921
00:39:36,160 --> 00:39:38,850
>> And toward the tail of the semester
will we charter some buses,

922
00:39:38,850 --> 00:39:40,850
actually spend some time
in Cambridge, if you'd

923
00:39:40,850 --> 00:39:43,460
like to join us, for the
so-called CS50 hackathon.

924
00:39:43,460 --> 00:39:44,600
At 7 PM we'll begin.

925
00:39:44,600 --> 00:39:45,970
Around 9 PM, we'll have pizza.

926
00:39:45,970 --> 00:39:47,650
Around 1:00 AM, we'll have burritos.

927
00:39:47,650 --> 00:39:50,550
And anyone still awake on the
bus ride home around 5:00 AM,

928
00:39:50,550 --> 00:39:54,070
we'll stop off for pancakes
at IHOP on the way home--

929
00:39:54,070 --> 00:39:57,540
a 12-hour opportunity to immerse
yourself with classmates and staff

930
00:39:57,540 --> 00:39:59,950
in the course's final project,
which is an opportunity

931
00:39:59,950 --> 00:40:01,870
to go well beyond the
course's problem sets

932
00:40:01,870 --> 00:40:05,950
and design and implement most
anything of interest to you,

933
00:40:05,950 --> 00:40:09,170
that will ultimately be
featured here in Commons.

934
00:40:09,170 --> 00:40:12,510
The first ever CS50 fair was last
year, an end-of-semester exhibition

935
00:40:12,510 --> 00:40:15,460
or celebration of what everyone
in the class had accomplished,

936
00:40:15,460 --> 00:40:19,810
especially those, again, who went from
nothing to something, from zero to 60,

937
00:40:19,810 --> 00:40:22,450
having no prior background
and exhibiting, ultimately,

938
00:40:22,450 --> 00:40:26,960
something for the whole campus and,
if online, the world to see, as well.

939
00:40:26,960 --> 00:40:30,780
>> Now, these here are just a few of the
TAs and CAs that makes CS50 possible.

940
00:40:30,780 --> 00:40:33,034
Allow me to invite any
of those staff members

941
00:40:33,034 --> 00:40:35,700
who are here to come up on stage,
as well as the course's heads,

942
00:40:35,700 --> 00:40:38,785
to offer some words of
inspiration, as well.

943
00:40:38,785 --> 00:41:03,138

944
00:41:03,138 --> 00:41:03,950
>> ANDI: Hi, guys.

945
00:41:03,950 --> 00:41:05,290
Can you guys hear me?

946
00:41:05,290 --> 00:41:10,260
Thanks for joining us on this
lovely, rainy Thursday afternoon.

947
00:41:10,260 --> 00:41:11,010
My name is Andi.

948
00:41:11,010 --> 00:41:12,070
I'm a junior in Berkeley.

949
00:41:12,070 --> 00:41:17,250
And along with Stelios and Summer,
we will be your three head teaching

950
00:41:17,250 --> 00:41:19,277
assistants for this upcoming year.

951
00:41:19,277 --> 00:41:21,110
So, I guess, show of
hands-- how many of you

952
00:41:21,110 --> 00:41:26,680
have no intention of being a CS
major nor really diving deeply

953
00:41:26,680 --> 00:41:29,930
into computer science as a major here?

954
00:41:29,930 --> 00:41:30,520
Awesome.

955
00:41:30,520 --> 00:41:32,730
That's brilliant.

956
00:41:32,730 --> 00:41:35,850
>> So I'm actually a global affairs
and cognitive science major.

957
00:41:35,850 --> 00:41:37,780
I literally came to
Yale with the intention

958
00:41:37,780 --> 00:41:42,199
of never having to look at a
number ever again in my life.

959
00:41:42,199 --> 00:41:44,990
When I came to Yale, this was
something that was never on my radar.

960
00:41:44,990 --> 00:41:46,460
I wanted to learn about poetry.

961
00:41:46,460 --> 00:41:48,500
I wanted to learn about
international affairs.

962
00:41:48,500 --> 00:41:50,510
I wanted to learn about
watercolor drawings.

963
00:41:50,510 --> 00:41:53,360
Yes, we offer a class
on watercolor drawings.

964
00:41:53,360 --> 00:41:57,340
>> But I never really was interested
in anything STEM related.

965
00:41:57,340 --> 00:41:59,620
But then the older I
got, the more I realized

966
00:41:59,620 --> 00:42:03,340
that every field really in some
sense employs computer science,

967
00:42:03,340 --> 00:42:05,700
or if not computer science, computation.

968
00:42:05,700 --> 00:42:08,300
In fact, for my global
affairs capstone project,

969
00:42:08,300 --> 00:42:12,020
we're using data analytics
to analyze terrorist attacks

970
00:42:12,020 --> 00:42:13,680
for Boko Haram in Nigeria.

971
00:42:13,680 --> 00:42:17,510
And so as you can see, regardless
of what major you end up pursuing

972
00:42:17,510 --> 00:42:21,640
or what your interests here at Yale
are, programming and the foundations

973
00:42:21,640 --> 00:42:24,130
of whatever skills are super useful.

974
00:42:24,130 --> 00:42:29,840
And CS50 really is well equipped to
kind of lend a lot of its resources

975
00:42:29,840 --> 00:42:32,060
to you, regardless of
how comfortable you are

976
00:42:32,060 --> 00:42:34,590
or how interested you are
in pursuing the class.

977
00:42:34,590 --> 00:42:37,290
>> Summer's going to talk a little
bit about what you guys are

978
00:42:37,290 --> 00:42:39,635
going to learn about this year.

979
00:42:39,635 --> 00:42:40,510
SUMMER: Hi, everyone.

980
00:42:40,510 --> 00:42:41,320
I'm Summer Wu.

981
00:42:41,320 --> 00:42:43,090
I'm a junior in Morse.

982
00:42:43,090 --> 00:42:48,100
And I actually started out
as a CS50 student myself.

983
00:42:48,100 --> 00:42:51,460
So three years ago, I was on a gap year.

984
00:42:51,460 --> 00:42:53,940
I'd never taken a CS
class in high school,

985
00:42:53,940 --> 00:42:57,800
but I thought that in my free time,
it'd be cool to learn how to code.

986
00:42:57,800 --> 00:43:02,610
So I did a quick Google search,
looked for what was available online,

987
00:43:02,610 --> 00:43:06,990
and saw this video with muppets
and DJs and cool websites.

988
00:43:06,990 --> 00:43:09,450
I was like, I want to
learn how to do that.

989
00:43:09,450 --> 00:43:14,690
>> So I took the course, and I
just fell in love with it.

990
00:43:14,690 --> 00:43:20,410
But I remember being so jealous of the
kids who could attend the hackathon,

991
00:43:20,410 --> 00:43:24,610
attend Puzzle Day, attend office
hours, get help from TAs in person.

992
00:43:24,610 --> 00:43:27,410
And so I never imagined
that I'd get the chance

993
00:43:27,410 --> 00:43:30,640
to be here involved in
the course that first got

994
00:43:30,640 --> 00:43:33,630
me interested in computer
science and is the reason why

995
00:43:33,630 --> 00:43:35,690
I'm a computer science major today.

996
00:43:35,690 --> 00:43:39,290
So I'll warn you, this class
is going to stretch you.

997
00:43:39,290 --> 00:43:41,120
It's going to challenge you.

998
00:43:41,120 --> 00:43:43,910
But it's also going to
teach you how to do things

999
00:43:43,910 --> 00:43:45,506
that you never imagined you could.

1000
00:43:45,506 --> 00:43:49,330

1001
00:43:49,330 --> 00:43:52,160
>> STELIOS: Hi, everyone.

1002
00:43:52,160 --> 00:43:53,510
My name is Stelios.

1003
00:43:53,510 --> 00:43:58,240
I am a junior in Branford
College and a CS major.

1004
00:43:58,240 --> 00:44:01,640
I'm also from Athens, Greece.

1005
00:44:01,640 --> 00:44:03,830
I'm really looking forward
to meeting all of you,

1006
00:44:03,830 --> 00:44:09,010
chatting with you at section, at
office hours, at Friday lunches.

1007
00:44:09,010 --> 00:44:11,250
I'm really excited because
we've put so much effort

1008
00:44:11,250 --> 00:44:14,650
into creating a unique support
structure for all of you

1009
00:44:14,650 --> 00:44:19,170
to make your experience with
the course the best possible.

1010
00:44:19,170 --> 00:44:22,640
And I hope that although most of
you have probably not taken a CS

1011
00:44:22,640 --> 00:44:27,310
course before, I hope that's CS50
for you is what sparks interest

1012
00:44:27,310 --> 00:44:30,260
to further pursue computer
science in the future,

1013
00:44:30,260 --> 00:44:33,270
as it has done with so
many people in the past.

1014
00:44:33,270 --> 00:44:36,740
So thank you for being
here, excited to see you.

1015
00:44:36,740 --> 00:44:38,487
Jason Hirschhorn.

1016
00:44:38,487 --> 00:44:39,820
JASON HIRSCHHORN: Hi, everybody.

1017
00:44:39,820 --> 00:44:41,700
My name is Jason Hirschhorn.

1018
00:44:41,700 --> 00:44:43,050
I live in Silliman.

1019
00:44:43,050 --> 00:44:46,910
And I went to Harvard as an undergrad
and majored in social studies

1020
00:44:46,910 --> 00:44:48,870
and minored in computer science.

1021
00:44:48,870 --> 00:44:53,100
And one of my principal roles here
is to support this wonderful staff

1022
00:44:53,100 --> 00:44:54,630
as they support you all.

1023
00:44:54,630 --> 00:44:56,410
In fact, this is not all of them.

1024
00:44:56,410 --> 00:45:01,230
There are 55 undergraduates and
graduates here to support you all.

1025
00:45:01,230 --> 00:45:04,420
And I daresay one of the best
parts of the course for you

1026
00:45:04,420 --> 00:45:08,080
all is getting to work with
them, getting to know them,

1027
00:45:08,080 --> 00:45:12,389
getting to see them, both in CS50
and outside of CS50 this semester

1028
00:45:12,389 --> 00:45:13,680
and for many semesters to come.

1029
00:45:13,680 --> 00:45:16,770
So hopefully you'll take the
course because hopefully you

1030
00:45:16,770 --> 00:45:21,060
get to interact with the
wonderful staff we have on stage.

1031
00:45:21,060 --> 00:45:24,977
>> SPEAKER: Well, let me finish
by saying it will be fun.

1032
00:45:24,977 --> 00:45:26,810
DAVID MALAN: Well,
thanks to our whole team.

1033
00:45:26,810 --> 00:45:29,610
Allow me to dim the lights and
allow some more of our team,

1034
00:45:29,610 --> 00:45:32,890
both from Cambridge and New Haven,
to say hello as these guys file off.

1035
00:45:32,890 --> 00:45:36,130
And after that will we transition to
the first of our programming engagements

1036
00:45:36,130 --> 00:45:37,584
with this language called Scratch.

1037
00:45:37,584 --> 00:45:38,500
So thanks to the team.

1038
00:45:38,500 --> 00:45:40,770
Let's dim the lights and
hear from a few others.

1039
00:45:40,770 --> 00:45:44,207
>> [APPLAUSE]

1040
00:45:44,207 --> 00:45:45,680

1041
00:45:45,680 --> 00:45:48,140
>> [VIDEO  PLAYBACK]

1042
00:45:48,140 --> 00:45:53,210
>> -The mission of CS50 is to make you
more comfortable with a totally new way

1043
00:45:53,210 --> 00:45:54,947
of thinking, this computational mindset.

1044
00:45:54,947 --> 00:45:56,780
-It made computer science
interesting, which

1045
00:45:56,780 --> 00:46:00,241
is something I didn't really realize
was possible until I took the class.

1046
00:46:00,241 --> 00:46:00,990
-I was like, whoa.

1047
00:46:00,990 --> 00:46:04,120
I'm really translating my thoughts
into a computer right now.

1048
00:46:04,120 --> 00:46:07,970
>> -Even if you don't have any background
in computer science or any experience,

1049
00:46:07,970 --> 00:46:10,140
this is actually the class for you.

1050
00:46:10,140 --> 00:46:12,250
>> -So I definitely want
my students to just

1051
00:46:12,250 --> 00:46:14,350
get excited about computer science.

1052
00:46:14,350 --> 00:46:17,187
Not just programming, but
thinking like a computer scientist

1053
00:46:17,187 --> 00:46:19,270
is really what I want to
try to teach my freshman.

1054
00:46:19,270 --> 00:46:21,560
>> -CS50 is hard and rewarding.

1055
00:46:21,560 --> 00:46:22,592
>> -An experience.

1056
00:46:22,592 --> 00:46:23,456
>> -Extravaganza.

1057
00:46:23,456 --> 00:46:26,350
>> -It's bringing us to the next level.

1058
00:46:26,350 --> 00:46:28,850
>> [MUSIC PLAYING]

1059
00:46:28,850 --> 00:46:32,167
>> -The TFs are, I think, the
lifeblood of the course.

1060
00:46:32,167 --> 00:46:34,560
>> -I'm excited to have
my students I'm helping

1061
00:46:34,560 --> 00:46:38,027
have that aha moment to realize
what they're actually trying

1062
00:46:38,027 --> 00:46:39,610
to do, to figure out how to do a pset.

1063
00:46:39,610 --> 00:46:41,438
>> -CS50's definitely a hard course.

1064
00:46:41,438 --> 00:46:43,960
But unlike any other
course really at Yale,

1065
00:46:43,960 --> 00:46:45,759
it has such a great,
supportive community.

1066
00:46:45,759 --> 00:46:47,550
-You absolutely don't
need to know anything

1067
00:46:47,550 --> 00:46:49,341
about coding to be able
to take the course.

1068
00:46:49,341 --> 00:46:52,270
-It's amazing to watch how far
people come in one semester.

1069
00:46:52,270 --> 00:46:55,610
>> -You weren't alone sitting in
your room learning to code,

1070
00:46:55,610 --> 00:46:57,170
but it was more than just a class.

1071
00:46:57,170 --> 00:46:58,044
It was an experience.

1072
00:46:58,044 --> 00:47:02,500
-The best way to learn concepts and
to process them is by teaching others.

1073
00:47:02,500 --> 00:47:04,033
>> -What is the telephone split?

1074
00:47:04,033 --> 00:47:06,934
>> [MUSIC PLAYING]

1075
00:47:06,934 --> 00:47:08,080
>> -And this is CS50.

1076
00:47:08,080 --> 00:47:10,810
>> [MUSIC PLAYING]

1077
00:47:10,810 --> 00:47:11,720

1078
00:47:11,720 --> 00:47:12,380
>> -This is CS50.

1079
00:47:12,380 --> 00:47:13,446
>> -Got a problem?

1080
00:47:13,446 --> 00:47:14,852
Tear it in half.

1081
00:47:14,852 --> 00:47:15,804
>> [MUSIC PLAYING]

1082
00:47:15,804 --> 00:47:18,184
>> Throw it away.

1083
00:47:18,184 --> 00:47:19,520
>> DAVID MALAN: All right.

1084
00:47:19,520 --> 00:47:23,190
So let's tackle-- in a little
bit, incidentally, it's

1085
00:47:23,190 --> 00:47:25,270
been this tradition for
some reason for 10 years

1086
00:47:25,270 --> 00:47:27,260
to serve cake at the
start and the end of CS50.

1087
00:47:27,260 --> 00:47:30,800
So awaiting you at the end of
today, in addition to syllabi,

1088
00:47:30,800 --> 00:47:34,270
will be some cake as well, and
the course's staff to say hello.

1089
00:47:34,270 --> 00:47:37,110
But now, let's transition to the
first of our languages, where

1090
00:47:37,110 --> 00:47:40,970
we'll spend really just a week and one
problem set on this domain, Scratch.

1091
00:47:40,970 --> 00:47:43,160
And you'll find if you've
programmed before, many

1092
00:47:43,160 --> 00:47:46,280
of the ideas and the
possibilities are familiar to you.

1093
00:47:46,280 --> 00:47:48,160
But you'll find that
it's fun along the way

1094
00:47:48,160 --> 00:47:51,201
to figure out exactly how to translate
some of the ideas you already know

1095
00:47:51,201 --> 00:47:53,852
to this particular environment
to really impress your family

1096
00:47:53,852 --> 00:47:57,060
and friends with your work, which can
go online, if you so choose, afterward.

1097
00:47:57,060 --> 00:47:58,851
>> And if you have no
prior experience and are

1098
00:47:58,851 --> 00:48:01,540
among the majority of
students less comfortable,

1099
00:48:01,540 --> 00:48:05,320
realize that many of the ideas we
just explored with reality-- things

1100
00:48:05,320 --> 00:48:07,970
like phone books and attendance
and so forth-- translate

1101
00:48:07,970 --> 00:48:12,280
fairly nicely to a computer,
but not if you use,

1102
00:48:12,280 --> 00:48:13,710
initially, a language like this.

1103
00:48:13,710 --> 00:48:16,290
So this is a program written
in a language called C.

1104
00:48:16,290 --> 00:48:18,590
And we'll spend quite a bit
of time in C, ultimately.

1105
00:48:18,590 --> 00:48:22,560
But odds are, this will look a bit
cryptic to you at first glance.

1106
00:48:22,560 --> 00:48:25,760
In fact, there's a lot of weird
syntax, parentheses, angle brackets,

1107
00:48:25,760 --> 00:48:27,422
curly braces, quotes, and semicolons.

1108
00:48:27,422 --> 00:48:29,880
And indeed, if you dive into
programming for the first time

1109
00:48:29,880 --> 00:48:33,420
looking at and trying to create stuff
like this, honestly, you get so mired

1110
00:48:33,420 --> 00:48:36,500
so often in just stupid
minutia that has nothing

1111
00:48:36,500 --> 00:48:38,240
intellectually interesting about it.

1112
00:48:38,240 --> 00:48:40,980
>> But imagine if you could create
this same program-- which,

1113
00:48:40,980 --> 00:48:46,200
as you might kind of infer, probably
prints "Hello, world" somehow or other.

1114
00:48:46,200 --> 00:48:49,850
We can distill that same idea into
just two puzzle pieces, if you will.

1115
00:48:49,850 --> 00:48:52,850
Indeed, Scratch is interesting
because it's this graphical language.

1116
00:48:52,850 --> 00:48:56,110
You can drag and drop these
puzzle pieces that only interlock

1117
00:48:56,110 --> 00:48:57,940
if it makes logical sense to do so.

1118
00:48:57,940 --> 00:48:59,830
And so in Scratch,
we'll soon see, this is

1119
00:48:59,830 --> 00:49:03,460
how you would implement that same
program, with just two puzzle pieces

1120
00:49:03,460 --> 00:49:05,420
that pretty much do what they say.

1121
00:49:05,420 --> 00:49:08,870
>> But we'll see in just a moment that some
of the building blocks that we alluded

1122
00:49:08,870 --> 00:49:12,140
to earlier and a few more are all that
ultimately are going to constitute

1123
00:49:12,140 --> 00:49:13,460
some of our earliest programs.

1124
00:49:13,460 --> 00:49:15,460
We're going to have things
like functions-- just

1125
00:49:15,460 --> 00:49:18,034
actions that do something,
like say hello, world.

1126
00:49:18,034 --> 00:49:20,200
We're going to have loops,
things that induce cycles

1127
00:49:20,200 --> 00:49:23,210
again and again, just like we
did a moment ago with searching

1128
00:49:23,210 --> 00:49:24,110
for Mike Smith.

1129
00:49:24,110 --> 00:49:27,190
Variables, like in algebra, if you
have x or y, that can store a number.

1130
00:49:27,190 --> 00:49:29,940
Well, in a program, you can actually
store more than just numbers.

1131
00:49:29,940 --> 00:49:34,290
You can store words and sentences
and graphics and other things still.

1132
00:49:34,290 --> 00:49:37,530
Boolean expressions, just
questions-- yes or no, true or false.

1133
00:49:37,530 --> 00:49:40,680
Conditions, making decisions
based on those yes/no answers.

1134
00:49:40,680 --> 00:49:43,890
>> And then fancier things like
array and threads and events

1135
00:49:43,890 --> 00:49:46,100
and any number of other
features, but all of which

1136
00:49:46,100 --> 00:49:48,670
map very nicely to very
friendly blocks like this.

1137
00:49:48,670 --> 00:49:52,736
This is going to be a function, a
purple puzzle piece that just says

1138
00:49:52,736 --> 00:49:54,717
what its name is-- in this case, say.

1139
00:49:54,717 --> 00:49:56,550
And then often, there's
a white box that you

1140
00:49:56,550 --> 00:49:58,550
can type in or drag some value into.

1141
00:49:58,550 --> 00:50:01,370
And that's what's generally
called an argument or a parameter.

1142
00:50:01,370 --> 00:50:04,110
It's a way of altering the
default behavior of a puzzle

1143
00:50:04,110 --> 00:50:07,530
piece or a function so that it does
something custom for you like saying,

1144
00:50:07,530 --> 00:50:12,570
hello, world or hello, Andy or hello,
Jason or some other sentence instead.

1145
00:50:12,570 --> 00:50:15,830
>> If you want to say that a
lot-- literally forever--

1146
00:50:15,830 --> 00:50:18,142
you can take another
puzzle piece called forever

1147
00:50:18,142 --> 00:50:20,100
and just sandwiched the
two together like this.

1148
00:50:20,100 --> 00:50:24,090
And that loop, as the picture suggests,
means just say hello, world forever,

1149
00:50:24,090 --> 00:50:25,810
again and again and again.

1150
00:50:25,810 --> 00:50:28,840
Or, if you only want to do it a
finite number of times, like 50 times,

1151
00:50:28,840 --> 00:50:32,050
there's going to be another puzzle
piece for that-- repeat 50 times.

1152
00:50:32,050 --> 00:50:34,190
>> Meanwhile, if you want
to have a variable

1153
00:50:34,190 --> 00:50:36,090
in this language we're
about to play with,

1154
00:50:36,090 --> 00:50:37,880
you can use a orange block like this.

1155
00:50:37,880 --> 00:50:40,750
And this variable I arbitrarily
called i for integer.

1156
00:50:40,750 --> 00:50:42,290
And I just set it equal to 0.

1157
00:50:42,290 --> 00:50:44,780
And so maybe i, in this
case-- this variable--

1158
00:50:44,780 --> 00:50:46,630
represents someone's score in a game.

1159
00:50:46,630 --> 00:50:49,680
You start at zero, and every time you
make a goal or something like that,

1160
00:50:49,680 --> 00:50:51,270
you get one additional point.

1161
00:50:51,270 --> 00:50:52,830
>> You can ask questions in Scratch.

1162
00:50:52,830 --> 00:50:55,390
If we drag and drop puzzle
pieces in a moment like this,

1163
00:50:55,390 --> 00:50:58,470
you can ask questions like,
well, is i less than 50?

1164
00:50:58,470 --> 00:51:00,060
Maybe you need 50 points to win.

1165
00:51:00,060 --> 00:51:01,893
And so this would be
the question you'd ask.

1166
00:51:01,893 --> 00:51:04,030
Or, more generally, you
could say is x less than y,

1167
00:51:04,030 --> 00:51:05,780
where there's two variables involved?

1168
00:51:05,780 --> 00:51:08,320
Now, this one is a lot
bigger at first glance,

1169
00:51:08,320 --> 00:51:11,470
but really not all that more complex.

1170
00:51:11,470 --> 00:51:15,390
>> This is just a combination
of conditions and variables

1171
00:51:15,390 --> 00:51:19,370
and Boolean expressions to ask
three questions-- is x less than y?

1172
00:51:19,370 --> 00:51:20,660
If so, say so.

1173
00:51:20,660 --> 00:51:22,170
Say, x is less than y.

1174
00:51:22,170 --> 00:51:26,571
Else, if x is greater than
y, else x must be equal to y.

1175
00:51:26,571 --> 00:51:29,070
And whereas with Mike Smith,
there were four scenarios, here

1176
00:51:29,070 --> 00:51:32,270
in the world of numbers, x is either
less than, greater than, or equal to.

1177
00:51:32,270 --> 00:51:34,531
All we have are three forks in the road.

1178
00:51:34,531 --> 00:51:36,530
And then there's fancier
puzzle pieces like this

1179
00:51:36,530 --> 00:51:39,613
for things like arrays, where we're
going to be able to store information.

1180
00:51:39,613 --> 00:51:43,590
We're going to see blocks that allow
us to implement multiple threads,

1181
00:51:43,590 --> 00:51:46,620
another feature we'll use, and
then also something called events.

1182
00:51:46,620 --> 00:51:49,390
But before we get to that
point and create even,

1183
00:51:49,390 --> 00:51:52,000
ultimately, our own custom
puzzle pieces, let's

1184
00:51:52,000 --> 00:51:53,950
actually open the program itself.

1185
00:51:53,950 --> 00:51:54,860
>> So this is Scratch.

1186
00:51:54,860 --> 00:51:57,490
It's available at scratch.mit.edu.

1187
00:51:57,490 --> 00:51:59,650
And you're welcome to play
now or later, as well.

1188
00:51:59,650 --> 00:52:01,330
This happens to be the offline version.

1189
00:52:01,330 --> 00:52:03,129
For people who don't
necessarily have great internet,

1190
00:52:03,129 --> 00:52:04,962
you can download the
same software, as well.

1191
00:52:04,962 --> 00:52:07,460
And there's really only three
components to this software.

1192
00:52:07,460 --> 00:52:10,860
On the top left-hand corner of
the screen is the sort of stage

1193
00:52:10,860 --> 00:52:13,800
that Scratch, who by default
looks like a cat, lives inside.

1194
00:52:13,800 --> 00:52:16,799
He can move up, down, left, and right
and do any number of other things,

1195
00:52:16,799 --> 00:52:20,520
and can look any number of ways based
on the costumes that you assign to him.

1196
00:52:20,520 --> 00:52:22,980
But this is what we'll call a
sprite, a sort of character.

1197
00:52:22,980 --> 00:52:25,429
And you can have multiple
characters, as we'll soon see.

1198
00:52:25,429 --> 00:52:30,060
>> In the middle now are all these puzzle
pieces and these categories or pallets

1199
00:52:30,060 --> 00:52:30,900
thereof.

1200
00:52:30,900 --> 00:52:32,429
So right now, I clicked on Motion.

1201
00:52:32,429 --> 00:52:35,900
And so I'm seeing all of the
motion-related puzzle pieces or blocks,

1202
00:52:35,900 --> 00:52:37,950
so functions that have
to do with going up,

1203
00:52:37,950 --> 00:52:39,950
down, left, or right or
some other operation.

1204
00:52:39,950 --> 00:52:43,790
But if I clicked on Looks, you
could see things like the say block

1205
00:52:43,790 --> 00:52:45,084
that we saw just a moment ago.

1206
00:52:45,084 --> 00:52:48,250
And if I click on Control, you'll see
things like the repeat and the forever

1207
00:52:48,250 --> 00:52:50,410
and the if block that
we saw a moment ago.

1208
00:52:50,410 --> 00:52:51,670
>> And so you'll find
that we'll just scratch

1209
00:52:51,670 --> 00:52:53,750
the surface of some of the
puzzle pieces together,

1210
00:52:53,750 --> 00:52:55,833
but it's all fairly intuitive
and point and click.

1211
00:52:55,833 --> 00:52:58,219
Indeed, Scratch was designed
for younger students

1212
00:52:58,219 --> 00:53:00,730
to help give them an outlet
for creative thinking.

1213
00:53:00,730 --> 00:53:03,040
And yet wonderfully, it's
a wonderful stepping stone

1214
00:53:03,040 --> 00:53:07,020
to exactly the ideas we're going to
explore in C and Python and JavaScript,

1215
00:53:07,020 --> 00:53:07,719
as well.

1216
00:53:07,719 --> 00:53:11,960
>> On the right-hand side, finally, here
is this, the so-called scripts area.

1217
00:53:11,960 --> 00:53:15,750
And this is just the blank slate with
which you begin to write a program.

1218
00:53:15,750 --> 00:53:16,897
And I'll exactly that.

1219
00:53:16,897 --> 00:53:19,980
Now, I happen to know where things are
because I've done this a few times.

1220
00:53:19,980 --> 00:53:22,710
But I know that under
the Events category,

1221
00:53:22,710 --> 00:53:25,520
there's this block here--
when green flag clicked.

1222
00:53:25,520 --> 00:53:28,790
And notice if I zoom out and
back in over here on the stage,

1223
00:53:28,790 --> 00:53:31,190
Scratch lives within this
little rectangular world,

1224
00:53:31,190 --> 00:53:33,920
atop which is a green
flag and a red stop sign.

1225
00:53:33,920 --> 00:53:36,180
So go and stop, respectively.

1226
00:53:36,180 --> 00:53:38,710
>> And so what do I want to do
when that green flag is clicked?

1227
00:53:38,710 --> 00:53:40,900
Well, let me go to that Looks category.

1228
00:53:40,900 --> 00:53:43,357
And let me go ahead
and drag and drop this.

1229
00:53:43,357 --> 00:53:45,940
And notice as soon as it gets
close, they're sort of magnetic.

1230
00:53:45,940 --> 00:53:48,489
So if I now let go, it snaps
together nice and cleanly.

1231
00:53:48,489 --> 00:53:51,030
And I'm going to go ahead and
say something like hello, world

1232
00:53:51,030 --> 00:53:52,190
for two seconds.

1233
00:53:52,190 --> 00:53:56,730
Let me zoom out and click now the
green flag, and say, hello, world.

1234
00:53:56,730 --> 00:53:57,230
All right.

1235
00:53:57,230 --> 00:53:58,460
So that's all fine and good.

1236
00:53:58,460 --> 00:53:59,900
Not all that exciting.

1237
00:53:59,900 --> 00:54:01,340
Let's make it a little cuter.

1238
00:54:01,340 --> 00:54:03,494
And I know that in
advance, Scratch happens

1239
00:54:03,494 --> 00:54:05,160
to come with some cute things like this.

1240
00:54:05,160 --> 00:54:07,720
So play sound meow until done.

1241
00:54:07,720 --> 00:54:09,531
So let's do this.

1242
00:54:09,531 --> 00:54:10,380
>> [MEOW]

1243
00:54:10,380 --> 00:54:11,819
>> Aw, that's adorable.

1244
00:54:11,819 --> 00:54:12,860
And if I click it again--

1245
00:54:12,860 --> 00:54:14,920
>> [MEOW]

1246
00:54:14,920 --> 00:54:15,460
>> And again.

1247
00:54:15,460 --> 00:54:16,740
>> [MEOW]

1248
00:54:16,740 --> 00:54:18,760
>> But I keep having to reanimate Scratch.

1249
00:54:18,760 --> 00:54:20,010
But I can do better than this.

1250
00:54:20,010 --> 00:54:23,060
Why don't I just drag three of these.

1251
00:54:23,060 --> 00:54:24,821
And now it's three times as adorable.

1252
00:54:24,821 --> 00:54:26,264
>> [MEOWING]

1253
00:54:26,264 --> 00:54:28,190
>> OK, actually, it's a little creepy.

1254
00:54:28,190 --> 00:54:31,020
So we need something in between there.

1255
00:54:31,020 --> 00:54:33,870
If I go to Control, it looks like
there's actually a wait block.

1256
00:54:33,870 --> 00:54:36,994
And so notice if I hover over there--
and let me make this a little bigger.

1257
00:54:36,994 --> 00:54:38,780
If I hover, it's going
to snap into place.

1258
00:54:38,780 --> 00:54:41,660
So wait one second, wait one second.

1259
00:54:41,660 --> 00:54:43,170
Let's hit green flag again.

1260
00:54:43,170 --> 00:54:46,330
>> [MEOWING]

1261
00:54:46,330 --> 00:54:47,695

1262
00:54:47,695 --> 00:54:50,660
>> OK, a little more natural,
but not very efficient.

1263
00:54:50,660 --> 00:54:54,300
So this is correct if my program's
goal was meow three times.

1264
00:54:54,300 --> 00:54:56,410
But it's not very well-designed.

1265
00:54:56,410 --> 00:54:57,740
I kind of cut some corners.

1266
00:54:57,740 --> 00:54:58,730
I got a little lazy.

1267
00:54:58,730 --> 00:55:05,620
What feels like-- what do I seem
to have done poorly, would you say?

1268
00:55:05,620 --> 00:55:07,197
Yeah?

1269
00:55:07,197 --> 00:55:08,030
Yeah, in the middle.

1270
00:55:08,030 --> 00:55:11,208

1271
00:55:11,208 --> 00:55:13,083
AUDIENCE: Used more
memory than you needed to

1272
00:55:13,083 --> 00:55:15,005
because you're using
so many different line.

1273
00:55:15,005 --> 00:55:16,380
DAVID MALAN: Yeah, so more lines.

1274
00:55:16,380 --> 00:55:19,190
And it wouldn't necessarily be memory,
though it could be seen as that way.

1275
00:55:19,190 --> 00:55:20,898
But it's definitely--
there's redundancy.

1276
00:55:20,898 --> 00:55:23,440
And I literally kind of dragged
and dropped the same things.

1277
00:55:23,440 --> 00:55:26,564
And if you kind of extrapolate-- if
it's not obvious here-- well, how would

1278
00:55:26,564 --> 00:55:27,910
I meow 30 times?

1279
00:55:27,910 --> 00:55:31,450
I would drag and drop, like,
30 more pairs of puzzle pieces.

1280
00:55:31,450 --> 00:55:32,890
And surely, there's a better way.

1281
00:55:32,890 --> 00:55:34,056
And we've seen a better way.

1282
00:55:34,056 --> 00:55:36,870
What intuitively would
be the better way?

1283
00:55:36,870 --> 00:55:37,890
Yeah, just use a loop.

1284
00:55:37,890 --> 00:55:38,810
No copy and paste.

1285
00:55:38,810 --> 00:55:40,790
And indeed, anytime this
semester if you start

1286
00:55:40,790 --> 00:55:43,890
finding yourself dragging and dropping,
or really copying and pasting,

1287
00:55:43,890 --> 00:55:47,050
dangerous habit to get into because
this is just not very maintainable.

1288
00:55:47,050 --> 00:55:49,740
For instance, if I want to change
the sound to something else,

1289
00:55:49,740 --> 00:55:52,826
I have to change it now in three
locations instead of just one.

1290
00:55:52,826 --> 00:55:54,575
Because indeed, if I
break this away-- I'm

1291
00:55:54,575 --> 00:55:56,170
just going to decouple it like that.

1292
00:55:56,170 --> 00:56:01,900
Let me grab a repeat block, and
then click three, type three,

1293
00:56:01,900 --> 00:56:04,015
throw some of these
away by just letting go.

1294
00:56:04,015 --> 00:56:05,890
And then notice it
doesn't look like it fits,

1295
00:56:05,890 --> 00:56:08,139
but magnetically, it's going
to not only snap in place

1296
00:56:08,139 --> 00:56:09,510
but grow to fit the shape.

1297
00:56:09,510 --> 00:56:10,310
So that's good.

1298
00:56:10,310 --> 00:56:11,530
And now if I click play.

1299
00:56:11,530 --> 00:56:13,870
>> [MEOWING]

1300
00:56:13,870 --> 00:56:14,370

1301
00:56:14,370 --> 00:56:15,620
Very nice.

1302
00:56:15,620 --> 00:56:16,126
All right.

1303
00:56:16,126 --> 00:56:18,500
And now it's very easy to
change, too, because I can just

1304
00:56:18,500 --> 00:56:20,670
change one number in one place.

1305
00:56:20,670 --> 00:56:22,500
But this, too, is not
all that interesting.

1306
00:56:22,500 --> 00:56:25,350
Let's actually have
Scratch not meow, but move.

1307
00:56:25,350 --> 00:56:32,680
Let me go to Motion and move 10 steps
inside of-- whoops, let me fix this.

1308
00:56:32,680 --> 00:56:37,460
Let me have it move 10 steps--
actually, let's not do repeat.

1309
00:56:37,460 --> 00:56:40,670
Let me grab a control block,
and do the following forever.

1310
00:56:40,670 --> 00:56:42,720
Forever, move 10 steps.

1311
00:56:42,720 --> 00:56:45,070
And click Play.

1312
00:56:45,070 --> 00:56:45,600
>> OK.

1313
00:56:45,600 --> 00:56:46,740
So thankfully, he stops.

1314
00:56:46,740 --> 00:56:49,710
Otherwise, kids would get very upset
when they sort of lose their cat.

1315
00:56:49,710 --> 00:56:52,720
But at least I can drag
him back into the screen.

1316
00:56:52,720 --> 00:56:56,177
But this is not all that
great of a game or animation.

1317
00:56:56,177 --> 00:56:58,260
It would be nice if maybe
he bounced off the edge.

1318
00:56:58,260 --> 00:57:00,020
So what do we do?

1319
00:57:00,020 --> 00:57:04,302
What construct do we need to have
Scratch decide to bounce, do you think,

1320
00:57:04,302 --> 00:57:06,010
even if you've never
seen Scratch before?

1321
00:57:06,010 --> 00:57:06,593
Yeah, in back.

1322
00:57:06,593 --> 00:57:08,359
AUDIENCE: You need an
if block or if-then.

1323
00:57:08,359 --> 00:57:10,650
DAVID MALAN: Yeah, so some
kind of if block or if-then.

1324
00:57:10,650 --> 00:57:12,275
So actually, we have one of these here.

1325
00:57:12,275 --> 00:57:14,500
So if-- so let me get
rid of the movement.

1326
00:57:14,500 --> 00:57:16,070
Let me zoom in so it's bigger.

1327
00:57:16,070 --> 00:57:16,890
So how about this.

1328
00:57:16,890 --> 00:57:21,920
Forever, if Sensing--
we've not seen this before.

1329
00:57:21,920 --> 00:57:23,160
I need a Boolean expression.

1330
00:57:23,160 --> 00:57:25,970
And it turns out if touching what?

1331
00:57:25,970 --> 00:57:29,030
If touching the edge,
what do I want to do?

1332
00:57:29,030 --> 00:57:32,030
Well, if I go back to Motion,
turns out, oh, I can turn around.

1333
00:57:32,030 --> 00:57:33,350
Let me drag this in here.

1334
00:57:33,350 --> 00:57:36,430
Why don't I go ahead and
turn around 180 degrees?

1335
00:57:36,430 --> 00:57:38,190
>> And now, let me just move at the end.

1336
00:57:38,190 --> 00:57:40,500
I could put the movement at
the beginning or the end.

1337
00:57:40,500 --> 00:57:44,452
But logically, every time I move, I
want to check, am I touching the edge?

1338
00:57:44,452 --> 00:57:45,410
Am I touching the edge?

1339
00:57:45,410 --> 00:57:46,490
Am I touching the edge?

1340
00:57:46,490 --> 00:57:49,200
So that logically I turn around if so.

1341
00:57:49,200 --> 00:57:51,190
So let's hit play.

1342
00:57:51,190 --> 00:57:52,030
>> OK.

1343
00:57:52,030 --> 00:57:53,910
So it's slightly buggy, so to speak.

1344
00:57:53,910 --> 00:57:56,560
And a bug is just a mistake
in a computer program.

1345
00:57:56,560 --> 00:57:57,800
But at least it's working.

1346
00:57:57,800 --> 00:57:59,420
And in fact, I can go in here.

1347
00:57:59,420 --> 00:58:03,130
And let me make it not 10 steps at a
time, but this is all animation is.

1348
00:58:03,130 --> 00:58:05,310
This is all a cartoon
or even a movie is.

1349
00:58:05,310 --> 00:58:07,110
Let me move 20 steps at a time.

1350
00:58:07,110 --> 00:58:11,200
So 20 times as many things are happening
once, or twice as many, in this case.

1351
00:58:11,200 --> 00:58:12,600
And he's moving faster.

1352
00:58:12,600 --> 00:58:15,150
Let me change to 30.

1353
00:58:15,150 --> 00:58:17,450
100.

1354
00:58:17,450 --> 00:58:19,406
1,000.

1355
00:58:19,406 --> 00:58:20,530
And it's going really fast.

1356
00:58:20,530 --> 00:58:22,190
And this is-- yeah, OK.

1357
00:58:22,190 --> 00:58:24,130
>> So now we're just messing with it.

1358
00:58:24,130 --> 00:58:24,954
OK, so buggy.

1359
00:58:24,954 --> 00:58:26,620
But we can drag him out of the way here.

1360
00:58:26,620 --> 00:58:28,286
But we can make more fun with this, too.

1361
00:58:28,286 --> 00:58:30,710
How about this-- he's upside down.

1362
00:58:30,710 --> 00:58:32,800
But it turns out Scratch--
and there is actually,

1363
00:58:32,800 --> 00:58:35,510
I have to disclaim, no academic
value to what I'm about to do.

1364
00:58:35,510 --> 00:58:40,200
But if I open up the microphone, let's
stop him and do something like this.

1365
00:58:40,200 --> 00:58:41,960
Ouch!

1366
00:58:41,960 --> 00:58:42,884
>> [LAUGH]

1367
00:58:42,884 --> 00:58:45,200

1368
00:58:45,200 --> 00:58:45,990
>> That was adorable.

1369
00:58:45,990 --> 00:58:46,740
Thank you.

1370
00:58:46,740 --> 00:58:50,850
Now, this is what my voice
looks like when I yell ouch.

1371
00:58:50,850 --> 00:58:52,630
I don't think we caught your laughter.

1372
00:58:52,630 --> 00:58:53,280
That's OK.

1373
00:58:53,280 --> 00:58:56,550
Let me save this as "ouch."

1374
00:58:56,550 --> 00:58:57,915
Let's save this as "ouch".

1375
00:58:57,915 --> 00:58:59,410
And now we'll go back to Scripts.

1376
00:58:59,410 --> 00:59:01,660
And now I need-- let's see, Sound.

1377
00:59:01,660 --> 00:59:03,160
Oh, play sound ouch.

1378
00:59:03,160 --> 00:59:08,180
So if I'm touching the edge, let me
first play ouch, and then turn around.

1379
00:59:08,180 --> 00:59:10,191
And now let's put him in the middle.

1380
00:59:10,191 --> 00:59:14,754
>> [SAYING "OUCH"]

1381
00:59:14,754 --> 00:59:15,740
>> Twice as fast.

1382
00:59:15,740 --> 00:59:20,660

1383
00:59:20,660 --> 00:59:21,830
>> OK.

1384
00:59:21,830 --> 00:59:23,780
But it's literally
doing what I'm saying.

1385
00:59:23,780 --> 00:59:27,580
So it is in fact correct, it's
just a little annoying quickly.

1386
00:59:27,580 --> 00:59:30,330
So let's add something
more interesting to this.

1387
00:59:30,330 --> 00:59:32,950
Let me actually open up
one that I made in advance,

1388
00:59:32,950 --> 00:59:38,560
aptly called Pet the
Cat, that does this.

1389
00:59:38,560 --> 00:59:40,340
Here's the script up here.

1390
00:59:40,340 --> 00:59:44,510
What is this going to
do in English terms?

1391
00:59:44,510 --> 00:59:45,820
What's this designed to do?

1392
00:59:45,820 --> 00:59:47,900
Yeah, let's go some-- yeah?

1393
00:59:47,900 --> 00:59:49,665
>> AUDIENCE: When you
pet the cat, it meows.

1394
00:59:49,665 --> 00:59:52,290
DAVID MALAN: Yeah, so when you
pet the cat, it's going to meow.

1395
00:59:52,290 --> 00:59:55,452
So in other words, there's now
a forever loop still, combined

1396
00:59:55,452 --> 00:59:57,660
with a condition, combined
with a Boolean expression,

1397
00:59:57,660 --> 01:00:00,140
combined with a couple
of functions, the effect

1398
01:00:00,140 --> 01:00:02,610
of which, once I play
this program, is nothing

1399
01:00:02,610 --> 01:00:06,820
happens until I move the cursor
closer and closer and closer and--

1400
01:00:06,820 --> 01:00:08,029
>> [MEOW]

1401
01:00:08,029 --> 01:00:09,320
Then it's like petting the cat.

1402
01:00:09,320 --> 01:00:11,210
[MEOW]

1403
01:00:11,210 --> 01:00:14,270
Only once you actually
move the cursor over him.

1404
01:00:14,270 --> 01:00:19,964
Now, I also whipped up don't pet
the cat, which does this instead.

1405
01:00:19,964 --> 01:00:21,350
>> [MEOWING]

1406
01:00:21,350 --> 01:00:23,485
>> So he's just constantly meowing.

1407
01:00:23,485 --> 01:00:24,455
>> [MEOWING]

1408
01:00:24,455 --> 01:00:27,705
>> But if I get too close--

1409
01:00:27,705 --> 01:00:29,645
>> [MEOWING]

1410
01:00:29,645 --> 01:00:32,080
>> [ROAR]

1411
01:00:32,080 --> 01:00:33,590
>> So how does this work?

1412
01:00:33,590 --> 01:00:35,880
Now I just have a
two-way fork in the road.

1413
01:00:35,880 --> 01:00:38,930
If touching mouse pointer,
then play the lion sound.

1414
01:00:38,930 --> 01:00:41,950
Else just play the meow sound,
and then wait three seconds so

1415
01:00:41,950 --> 01:00:43,880
that it's kind of doing
it very tranquilly.

1416
01:00:43,880 --> 01:00:44,380
All right.

1417
01:00:44,380 --> 01:00:47,290
So that's combining
some more ideas still.

1418
01:00:47,290 --> 01:00:50,870
Let's take a look at this example
I whipped up called threads.

1419
01:00:50,870 --> 01:00:54,020
And this one is fundamentally
different in that it leverages

1420
01:00:54,020 --> 01:00:56,070
a feature of many
programming language called

1421
01:00:56,070 --> 01:00:59,970
threads, the ability of a program to
literally do two things simultaneously.

1422
01:00:59,970 --> 01:01:03,600
Indeed, these days if you're using
Google Docs or Microsoft Word,

1423
01:01:03,600 --> 01:01:09,620
and your document's constantly being
spell-checked even as you type-- or you

1424
01:01:09,620 --> 01:01:11,580
hit Command-P or Control-P
and print something,

1425
01:01:11,580 --> 01:01:13,380
it's printing while you continue typing.

1426
01:01:13,380 --> 01:01:16,680
Programs today can indeed do multiple
things at once, just like in Scratch

1427
01:01:16,680 --> 01:01:17,180
here.

1428
01:01:17,180 --> 01:01:20,400
>> So here, I have two sprites
now, a bird and a cat.

1429
01:01:20,400 --> 01:01:22,780
And if I click on each of
those characters one at a time,

1430
01:01:22,780 --> 01:01:25,590
I see right now the bird's
scripts at top right.

1431
01:01:25,590 --> 01:01:27,270
Now I see the cat's.

1432
01:01:27,270 --> 01:01:28,540
Bird's, cat's.

1433
01:01:28,540 --> 01:01:30,270
So each of them have their own script.

1434
01:01:30,270 --> 01:01:32,700
But notice, what puzzle piece
do they both begin with?

1435
01:01:32,700 --> 01:01:34,260
When green flag clicked.

1436
01:01:34,260 --> 01:01:36,890
And bird, when green flag clicked.

1437
01:01:36,890 --> 01:01:40,870
So when I click the green flag,
both of those scripts or programs

1438
01:01:40,870 --> 01:01:42,759
are going to run in parallel.

1439
01:01:42,759 --> 01:01:45,800
And you'll notice that the bird is
just mindlessly bouncing off the edge.

1440
01:01:45,800 --> 01:01:50,890
The cat clearly has been programmed
with a strategic advantage.

1441
01:01:50,890 --> 01:01:52,704
And--

1442
01:01:52,704 --> 01:01:53,490
>> [ROAR]

1443
01:01:53,490 --> 01:01:53,821

1444
01:01:53,821 --> 01:01:54,320
All right.

1445
01:01:54,320 --> 01:01:57,340
So the cat caught the bird in this case.

1446
01:01:57,340 --> 01:01:58,390
Why is that?

1447
01:01:58,390 --> 01:02:03,310
Well, notice first we just have
the bird just mindlessly going

1448
01:02:03,310 --> 01:02:05,120
to this initial location,
and then forever,

1449
01:02:05,120 --> 01:02:06,774
if not touching the cat, just move.

1450
01:02:06,774 --> 01:02:08,190
And if you're on the edge, bounce.

1451
01:02:08,190 --> 01:02:08,870
And just move.

1452
01:02:08,870 --> 01:02:10,286
And if you're on the edge, bounce.

1453
01:02:10,286 --> 01:02:12,840
But the cat, meanwhile,
has some additional logic

1454
01:02:12,840 --> 01:02:18,250
that says this-- first, just so
that this isn't completely biased

1455
01:02:18,250 --> 01:02:21,191
against the bird, notice that I've
used a green puzzle piece there

1456
01:02:21,191 --> 01:02:22,690
that actually picks a random number.

1457
01:02:22,690 --> 01:02:26,179
A feature of many languages is to give
you random or pseudorandom numbers.

1458
01:02:26,179 --> 01:02:29,220
So in this case, the cat initially
chooses a random number between, like,

1459
01:02:29,220 --> 01:02:31,727
90 degrees and 180
degrees, essentially, so

1460
01:02:31,727 --> 01:02:33,310
that there's a little bit of variance.

1461
01:02:33,310 --> 01:02:37,000
And then forever, if touching
the bird, play the lion sound.

1462
01:02:37,000 --> 01:02:38,754
Otherwise, just point toward the bird.

1463
01:02:38,754 --> 01:02:39,670
Point toward the bird.

1464
01:02:39,670 --> 01:02:44,199
Point toward the bird, which is a
puzzle piece unto itself in this case.

1465
01:02:44,199 --> 01:02:45,740
Well, we can do one other thing here.

1466
01:02:45,740 --> 01:02:49,820
Let me open up the events program here.

1467
01:02:49,820 --> 01:02:53,520
And here we again have two sprites,
which look like these two puppets here.

1468
01:02:53,520 --> 01:02:55,710
And what's interesting here is this.

1469
01:02:55,710 --> 01:02:59,140
The orange guy has this
set of puzzle pieces here.

1470
01:02:59,140 --> 01:03:01,700
Forever do the following--
if the space bar is pressed,

1471
01:03:01,700 --> 01:03:05,430
then say, Marco, and
then broadcast an event.

1472
01:03:05,430 --> 01:03:09,460
And meanwhile, the blue guy here has
this-- when you receive the event,

1473
01:03:09,460 --> 01:03:10,540
say Polo.

1474
01:03:10,540 --> 01:03:12,850
So it turns out in Scratch
and in other languages,

1475
01:03:12,850 --> 01:03:15,830
there are ways for two programs
or two scripts, in this case,

1476
01:03:15,830 --> 01:03:20,310
to intercommunicate so that when I
hit the space bar, he says Marco.

1477
01:03:20,310 --> 01:03:23,539
And the other one hears that, so to
speak, and says Polo in response.

1478
01:03:23,539 --> 01:03:26,080
So you can write programs that
actually interact in this way.

1479
01:03:26,080 --> 01:03:28,630
And if I do this one instead,
I can even add variables,

1480
01:03:28,630 --> 01:03:31,287
just using one sprite in this case.

1481
01:03:31,287 --> 01:03:32,578
This one's especially annoying.

1482
01:03:32,578 --> 01:03:36,174
>> [SEAL BARKING]

1483
01:03:36,174 --> 01:03:37,100

1484
01:03:37,100 --> 01:03:42,490
>> Now, notice on the right we've got
some additional logic over here.

1485
01:03:42,490 --> 01:03:45,146
How do I stop this seal from barking?

1486
01:03:45,146 --> 01:03:48,320
>> [SEAL BARKING]

1487
01:03:48,320 --> 01:03:52,830

1488
01:03:52,830 --> 01:03:55,630
>> It looks like on the right-hand
side is what's playing the sound.

1489
01:03:55,630 --> 01:03:58,500
But it's only playing a
sound if what is true?

1490
01:03:58,500 --> 01:04:03,390
If a variable-- orange
block-- muted is zero.

1491
01:04:03,390 --> 01:04:07,916
How do I change muted to be 1,
meaning true, make this muted?

1492
01:04:07,916 --> 01:04:12,455
Apparently, the other script, I can
hit the space bar, and now he stops.

1493
01:04:12,455 --> 01:04:15,080
So we can have this intercommunication
across scripts, as well,

1494
01:04:15,080 --> 01:04:18,349
by just sharing a variable
across the two like this.

1495
01:04:18,349 --> 01:04:19,890
Now, this isn't all that interesting.

1496
01:04:19,890 --> 01:04:24,360
Let's go ahead and do this and combine
a lot of these ideas with this program

1497
01:04:24,360 --> 01:04:25,940
here.

1498
01:04:25,940 --> 01:04:28,100
Before we do that, though,
how about one volunteer?

1499
01:04:28,100 --> 01:04:30,558
Let me take the pressure off
of me because I don't actually

1500
01:04:30,558 --> 01:04:31,280
play this game.

1501
01:04:31,280 --> 01:04:33,030
Let's have someone we
haven't seen before.

1502
01:04:33,030 --> 01:04:36,420
You have to be comfortable coming
up on stage here, on camera.

1503
01:04:36,420 --> 01:04:37,430
OK, come on up.

1504
01:04:37,430 --> 01:04:38,130
Very brave.

1505
01:04:38,130 --> 01:04:39,052
What's your name?

1506
01:04:39,052 --> 01:04:39,962
>> IDRIS: Idris.

1507
01:04:39,962 --> 01:04:40,753
DAVID MALAN: Sorry?

1508
01:04:40,753 --> 01:04:41,196
IDRIS: Idris.

1509
01:04:41,196 --> 01:04:42,670
DAVID MALAN: Idris, nice to meet you.

1510
01:04:42,670 --> 01:04:43,170
Come on up.

1511
01:04:43,170 --> 01:04:45,630
And now, on your own mobile
phone, do you play Pokemon GO?

1512
01:04:45,630 --> 01:04:46,567
>> IDRIS: No.

1513
01:04:46,567 --> 01:04:47,400
DAVID MALAN: Really?

1514
01:04:47,400 --> 01:04:48,104
IDRIS: Yeah.

1515
01:04:48,104 --> 01:04:48,770
DAVID MALAN: OK.

1516
01:04:48,770 --> 01:04:49,020
All right.

1517
01:04:49,020 --> 01:04:49,978
Well, nice to meet you.

1518
01:04:49,978 --> 01:04:50,820
Come on over.

1519
01:04:50,820 --> 01:04:51,950
I don't either.

1520
01:04:51,950 --> 01:04:55,380
So we'll figure out together how to
play this, which someone actually

1521
01:04:55,380 --> 01:05:00,140
went and implemented in Scratch
by changing the cat to essentially

1522
01:05:00,140 --> 01:05:02,260
different characters all together.

1523
01:05:02,260 --> 01:05:04,870
And if I fullscreen
this here, we're going

1524
01:05:04,870 --> 01:05:06,860
to see the following game together.

1525
01:05:06,860 --> 01:05:09,560
Still loading, still loading.

1526
01:05:09,560 --> 01:05:10,270
Come on.

1527
01:05:10,270 --> 01:05:12,590
Let me do this.

1528
01:05:12,590 --> 01:05:14,520
Come on.

1529
01:05:14,520 --> 01:05:17,200
This game is so big that it crashed.

1530
01:05:17,200 --> 01:05:18,980
Stand by.

1531
01:05:18,980 --> 01:05:22,390
Try this once more.

1532
01:05:22,390 --> 01:05:23,710
Come on.

1533
01:05:23,710 --> 01:05:24,211
All right.

1534
01:05:24,211 --> 01:05:26,840

1535
01:05:26,840 --> 01:05:27,430
>> There we go.

1536
01:05:27,430 --> 01:05:27,929
OK.

1537
01:05:27,929 --> 01:05:29,330
Green flag.

1538
01:05:29,330 --> 01:05:30,190
So here we go.

1539
01:05:30,190 --> 01:05:30,950
>> [MUSIC PLAYING]

1540
01:05:30,950 --> 01:05:32,510
>> Choose the middle level here.

1541
01:05:32,510 --> 01:05:34,200
Click the blue guy there.

1542
01:05:34,200 --> 01:05:34,700
All right.

1543
01:05:34,700 --> 01:05:37,860
And you can use the arrow
keys-- up, down, left, right.

1544
01:05:37,860 --> 01:05:42,150
Now, let's consider as we do this--
and then go after the character there.

1545
01:05:42,150 --> 01:05:42,650
Yep.

1546
01:05:42,650 --> 01:05:45,786
And now click him with the mouse.

1547
01:05:45,786 --> 01:05:46,693
Oh, yeah.

1548
01:05:46,693 --> 01:05:47,193
Move.

1549
01:05:47,193 --> 01:05:47,662
Where's the arrow?

1550
01:05:47,662 --> 01:05:48,131
Here you go.

1551
01:05:48,131 --> 01:05:48,600
So click on there.

1552
01:05:48,600 --> 01:05:48,840
Yeah.

1553
01:05:48,840 --> 01:05:49,340
All right.

1554
01:05:49,340 --> 01:05:56,008
So now, I'm told you have a Poke ball,
that if click it, it will do that.

1555
01:05:56,008 --> 01:05:57,932
Very good.

1556
01:05:57,932 --> 01:06:00,390
In practicing for today, I
found this version of the game's

1557
01:06:00,390 --> 01:06:01,760
actually not very hard.

1558
01:06:01,760 --> 01:06:07,650
So if you want to go again here,
walk down to this Poke ball.

1559
01:06:07,650 --> 01:06:08,780
And then go take a right.

1560
01:06:08,780 --> 01:06:11,200
Try clicking on it.

1561
01:06:11,200 --> 01:06:13,120
Oh, actually, that's
the store, apparently.

1562
01:06:13,120 --> 01:06:14,170
OK so close that.

1563
01:06:14,170 --> 01:06:15,150
Never done that before.

1564
01:06:15,150 --> 01:06:16,566
Maybe go up to this thing up here.

1565
01:06:16,566 --> 01:06:17,460
Oh, there you go.

1566
01:06:17,460 --> 01:06:18,910
Wait, there's one over there.

1567
01:06:18,910 --> 01:06:20,412
Oh, there's another.

1568
01:06:20,412 --> 01:06:21,285
OK.

1569
01:06:21,285 --> 01:06:21,785
Down.

1570
01:06:21,785 --> 01:06:23,210
Yeah, click.

1571
01:06:23,210 --> 01:06:24,635
>> OK, that's very cute.

1572
01:06:24,635 --> 01:06:27,485

1573
01:06:27,485 --> 01:06:28,810
OK, very well done.

1574
01:06:28,810 --> 01:06:30,010
This game is not very hard.

1575
01:06:30,010 --> 01:06:30,510
OK.

1576
01:06:30,510 --> 01:06:32,150
Congratulations.

1577
01:06:32,150 --> 01:06:34,720
Here, we have a CS50
stress ball for you.

1578
01:06:34,720 --> 01:06:37,680
But consider for just a moment what
some of the takeaways are there.

1579
01:06:37,680 --> 01:06:39,263
Easier than the real game, apparently.

1580
01:06:39,263 --> 01:06:41,809
But all we have going
on here is a character

1581
01:06:41,809 --> 01:06:44,100
that probably has some kind
of loop associated with it.

1582
01:06:44,100 --> 01:06:44,590
It's not a cat.

1583
01:06:44,590 --> 01:06:45,860
It's this character instead.

1584
01:06:45,860 --> 01:06:49,052
And that loop is just constantly
saying, if up arrow pressed,

1585
01:06:49,052 --> 01:06:51,260
if down arrow pressed, if
left arrow pressed or right

1586
01:06:51,260 --> 01:06:53,960
arrow pressed, move up
or down or left or right.

1587
01:06:53,960 --> 01:06:57,870
Or if there's another puzzle piece there
that says when touching another sprite,

1588
01:06:57,870 --> 01:07:01,320
when touching one of the characters
to the Poke ball, if touching,

1589
01:07:01,320 --> 01:07:02,440
then do this.

1590
01:07:02,440 --> 01:07:04,800
>> So all of the ideas we've
been using thus far really

1591
01:07:04,800 --> 01:07:09,130
can just be applied in this particular
context to play this game, as well.

1592
01:07:09,130 --> 01:07:13,580
Let me go ahead and pull
up one other here, in fact.

1593
01:07:13,580 --> 01:07:17,060
Let me go ahead and pull
up, let's say, this.

1594
01:07:17,060 --> 01:07:18,540
This is something we remixed.

1595
01:07:18,540 --> 01:07:20,329
Made by one of our
students in Cambridge,

1596
01:07:20,329 --> 01:07:23,370
and then I went through and changed
pretty much every instance of Harvard

1597
01:07:23,370 --> 01:07:25,580
to Yale this time.

1598
01:07:25,580 --> 01:07:27,870
Would someone like to
compete against the Ivies

1599
01:07:27,870 --> 01:07:31,650
here in another accumulation
of all of these ideas?

1600
01:07:31,650 --> 01:07:32,662
Come on down, yes.

1601
01:07:32,662 --> 01:07:33,370
What's your name?

1602
01:07:33,370 --> 01:07:34,190
>> DINA: Dina.

1603
01:07:34,190 --> 01:07:35,015
>> DAVID MALAN: Adina?

1604
01:07:35,015 --> 01:07:35,380
>> DINA: Dina.

1605
01:07:35,380 --> 01:07:36,410
>> DAVID MALAN: Dina, come on down.

1606
01:07:36,410 --> 01:07:37,320
All right, Dina.

1607
01:07:37,320 --> 01:07:40,780
So this game gets harder and
harder, because in this game,

1608
01:07:40,780 --> 01:07:43,970
there's variables being used as well
that are constantly keeping track

1609
01:07:43,970 --> 01:07:45,930
of what level you are in the game.

1610
01:07:45,930 --> 01:07:47,120
So nice to meet you.

1611
01:07:47,120 --> 01:07:48,250
Come around here.

1612
01:07:48,250 --> 01:07:51,460
And so the goal here is to sort
of make your way through a maze

1613
01:07:51,460 --> 01:07:52,710
that this student implemented.

1614
01:07:52,710 --> 01:07:55,334
>> And just to set the stage, each
of these pictures on the screen

1615
01:07:55,334 --> 01:07:57,222
is its own sprite, its own character.

1616
01:07:57,222 --> 01:07:59,680
So these were by default cats,
but the student changed them

1617
01:07:59,680 --> 01:08:01,480
to the various Ivies logos here.

1618
01:08:01,480 --> 01:08:04,370
And then you'll see that just
by using conditions and loops

1619
01:08:04,370 --> 01:08:06,848
and functions and more, you get this.

1620
01:08:06,848 --> 01:08:07,478
>> [MUSIC PLAYING]

1621
01:08:07,478 --> 01:08:09,228
[MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1622
01:08:09,228 --> 01:08:13,060

1623
01:08:13,060 --> 01:08:14,018
>> Yeah, OK.

1624
01:08:14,018 --> 01:08:17,371

1625
01:08:17,371 --> 01:08:20,110
Yeah, keep going.

1626
01:08:20,110 --> 01:08:21,180
First level's very easy.

1627
01:08:21,180 --> 01:08:22,554
You've just got to go over there.

1628
01:08:22,554 --> 01:08:25,644
But again, consider, this is just a
loop listening for the arrow keys--

1629
01:08:25,644 --> 01:08:26,560
up, down, left, right.

1630
01:08:26,560 --> 01:08:28,320
And now a sensing block.

1631
01:08:28,320 --> 01:08:29,345
Very nice.

1632
01:08:29,345 --> 01:08:32,600
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1633
01:08:32,600 --> 01:08:40,340

1634
01:08:40,340 --> 01:08:41,288
>> Very nice.

1635
01:08:41,288 --> 01:08:44,550
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1636
01:08:44,550 --> 01:08:55,540

1637
01:08:55,540 --> 01:08:56,430
>> Very nice.

1638
01:08:56,430 --> 01:08:57,975
Pretty easy, Crimson.

1639
01:08:57,975 --> 01:08:59,060
All right.

1640
01:08:59,060 --> 01:09:00,250
Levels-- uh-oh.

1641
01:09:00,250 --> 01:09:02,380
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1642
01:09:02,380 --> 01:09:04,980

1643
01:09:04,980 --> 01:09:07,150
>> And again, in these
three Harvard crests,

1644
01:09:07,150 --> 01:09:10,334
you just have logic
saying if on edge, bounce.

1645
01:09:10,334 --> 01:09:13,238
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1646
01:09:13,238 --> 01:09:14,690

1647
01:09:14,690 --> 01:09:17,651
>> OK, what you're doing is
more interesting than why.

1648
01:09:17,651 --> 01:09:18,150
Very nice.

1649
01:09:18,150 --> 01:09:18,810
Very nice.

1650
01:09:18,810 --> 01:09:20,270
Uh-oh.

1651
01:09:20,270 --> 01:09:23,350
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1652
01:09:23,350 --> 01:09:26,005

1653
01:09:26,005 --> 01:09:27,630
I think you have to sacrifice yourself.

1654
01:09:27,630 --> 01:09:30,372
[MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1655
01:09:30,372 --> 01:09:30,872
Quick!

1656
01:09:30,872 --> 01:09:34,848

1657
01:09:34,848 --> 01:09:38,824
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1658
01:09:38,824 --> 01:09:44,288

1659
01:09:44,288 --> 01:09:44,788
Nice.

1660
01:09:44,788 --> 01:09:52,240

1661
01:09:52,240 --> 01:09:52,740
That's OK.

1662
01:09:52,740 --> 01:09:53,770
You'll get it.

1663
01:09:53,770 --> 01:09:56,304
Yes, yes!

1664
01:09:56,304 --> 01:09:58,256
Very nice.

1665
01:09:58,256 --> 01:10:01,184
>> [CHEERING]

1666
01:10:01,184 --> 01:10:02,160

1667
01:10:02,160 --> 01:10:05,576
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1668
01:10:05,576 --> 01:10:09,480

1669
01:10:09,480 --> 01:10:10,940
>> Nice!

1670
01:10:10,940 --> 01:10:14,370
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1671
01:10:14,370 --> 01:10:39,050

1672
01:10:39,050 --> 01:10:39,550
Got it.

1673
01:10:39,550 --> 01:10:41,510
Come on!

1674
01:10:41,510 --> 01:10:43,470
Second to last level.

1675
01:10:43,470 --> 01:10:46,900
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1676
01:10:46,900 --> 01:10:49,588

1677
01:10:49,588 --> 01:10:50,580
>> All right.

1678
01:10:50,580 --> 01:10:54,052
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1679
01:10:54,052 --> 01:11:00,992

1680
01:11:00,992 --> 01:11:01,492
Yes.

1681
01:11:01,492 --> 01:11:06,230

1682
01:11:06,230 --> 01:11:08,020
Good use of variables here.

1683
01:11:08,020 --> 01:11:09,770
[MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1684
01:11:09,770 --> 01:11:10,770
Yes.

1685
01:11:10,770 --> 01:11:14,743
[MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1686
01:11:14,743 --> 01:11:16,707

1687
01:11:16,707 --> 01:11:17,689
Nice.

1688
01:11:17,689 --> 01:11:21,126
[MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1689
01:11:21,126 --> 01:11:23,581

1690
01:11:23,581 --> 01:11:24,570
It's OK.

1691
01:11:24,570 --> 01:11:25,770
We got to get to the end.

1692
01:11:25,770 --> 01:11:26,270
There.

1693
01:11:26,270 --> 01:11:27,010
Oh!

1694
01:11:27,010 --> 01:11:29,980
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1695
01:11:29,980 --> 01:11:32,160

1696
01:11:32,160 --> 01:11:34,866
>> Might run late today, but
it's gonna be worth it.

1697
01:11:34,866 --> 01:11:38,200
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1698
01:11:38,200 --> 01:11:39,640

1699
01:11:39,640 --> 01:11:42,520
>> You can do it!

1700
01:11:42,520 --> 01:11:43,480
Yeah!

1701
01:11:43,480 --> 01:11:46,278
>> [CHEERING]

1702
01:11:46,278 --> 01:11:47,230

1703
01:11:47,230 --> 01:11:50,086
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1704
01:11:50,086 --> 01:11:51,470
>> This one's really hard.

1705
01:11:51,470 --> 01:11:54,070
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1706
01:11:54,070 --> 01:11:57,458

1707
01:11:57,458 --> 01:11:59,000
>> We'll give you two more lives.

1708
01:11:59,000 --> 01:11:59,968
Can you do it?

1709
01:11:59,968 --> 01:12:03,356
>> [MUSIC - MC HAMMER, "U CAN'T TOUCH
 THIS"]

1710
01:12:03,356 --> 01:12:06,744

1711
01:12:06,744 --> 01:12:07,244
>> All right.

1712
01:12:07,244 --> 01:12:09,160
How about a big round
of applause nonetheless.

1713
01:12:09,160 --> 01:12:10,800
You got to the second to last level.

1714
01:12:10,800 --> 01:12:11,723
Thank you.

1715
01:12:11,723 --> 01:12:14,964
>> [APPLAUSE]

1716
01:12:14,964 --> 01:12:15,890

1717
01:12:15,890 --> 01:12:19,600
>> So this is only to say how much you
can do with these kinds of things.

1718
01:12:19,600 --> 01:12:21,892
And realize, too, that when
puzzle pieces don't exist--

1719
01:12:21,892 --> 01:12:24,891
and indeed, this is going to be one
of the powers with the first problem

1720
01:12:24,891 --> 01:12:27,080
sets and beyond-- is to
actually create your own.

1721
01:12:27,080 --> 01:12:28,860
And this is just a snippet
of one of the examples

1722
01:12:28,860 --> 01:12:30,230
you'll be able to play
with online, where

1723
01:12:30,230 --> 01:12:33,380
if you don't have built into Scratch
something like a cough puzzle piece,

1724
01:12:33,380 --> 01:12:35,470
you can actually make it yourself.

1725
01:12:35,470 --> 01:12:37,387
>> And so all of this and more awaits.

1726
01:12:37,387 --> 01:12:39,470
And just to paint a final
picture of indeed what's

1727
01:12:39,470 --> 01:12:43,120
ahead in store for the class for you,
based on some pictures from classmates

1728
01:12:43,120 --> 01:12:46,970
past, allow me to dim the lights
one last time and show you CS50.

1729
01:12:46,970 --> 01:12:49,580

1730
01:12:49,580 --> 01:12:53,038
>> [MUSIC PLAYING]

1731
01:12:53,038 --> 01:14:38,210

1732
01:14:38,210 --> 01:14:38,860
>> All right.

1733
01:14:38,860 --> 01:14:41,540
That's it for CS50.

1734
01:14:41,540 --> 01:14:44,964
Cake is now served.

1735
01:14:44,964 --> 01:14:50,610

1736
01:14:50,610 --> 01:14:53,960
>> [MUSIC PLAYING]

1737
01:14:53,960 --> 01:15:07,624