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[ROCK MUSIC]

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[MUSIC - "HAPPY TOGETHER" THE TURTLES]

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DAVID MALAN: Today, we begin our exploration of the

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fundamentals of computer science and our introduction,

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now, to the art of programming, of which that is

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just one example.

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But in the same vein of problem solving, know that

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CS50's traditional Puzzle Day is tomorrow.

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If you've not yet RSVPed for this event, you can go to

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cs50.net/rsvp.

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You can bring teams of two or three or four.

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You will be challenged with problems like the one you saw

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up there in the beginning.

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All new problems this year if you partook last year, and you

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will have a chance then to win some fabulous prizes.

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Among them, a Wii and some gift certificates and like,

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all while hanging out with CS50 students and classmates

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and pizza and Facebook.

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So more on that tomorrow if you would like.

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>> This then is CS50, for those of you joining us for the

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first time.

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And know that this course is particularly determined to get

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you through this course to its final end, at which point you

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will have not only an understanding of these

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fundamentals of computer science, but you will have

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this very practical skill set that you can then take back to

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your own department, whether it's engineering sciences,

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applied mathematics, the physical sciences, natural

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science, or the like.

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Indeed, what's so exciting about computer science these

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days is that it is just so applicable to

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all sorts of fields.

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And even though today, we will only scratch the surface of a

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very traditional programming language known as C, we'll

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instead look at something called Scratch, something with

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which that cookie love story was implemented by one of your

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predecessors in years past, to tell stories, to create games,

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to create interactive art, and to actually start to wrap our

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minds around some of the fundamental constructs that

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underlie programming but in a context, as you'll see, that's

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much less scary, that's much less arcane, than you will see

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before long.

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And realize, too, for those of you just joining us this

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semester, the phrases on which we ended Wednesday's lecture,

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it is not so important where you end up relative to your

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classmates in this class, but where you, by semester's end

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in week 11, end up relative to yourself right this very day.

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>> So without further ado, this is also worth noting that

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those less comfortable, those somewhere in between, are

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also, by design of this course, not at any disadvantage

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vis-a-vis those more comfortable

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coming into this class.

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As you'll see over the weeks to come, we have different

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tracks for disparate back background, sections for those

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less comfortable, more comfortable, those somewhere

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in between.

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As you'll see this weekend and next week, two versions of

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most problem sets in both standard and hacker edition so

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that you all can self-select down the path with which you

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are most comfortable.

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>> So today, we dive into this programming

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language called Scratch.

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It's a graphical programming language, and yet it has the

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same features of some of the higher level languages we'll

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explore later in the semester, among them C and

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JavaScript and PHP.

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But it's going to allow us to avoid some of the distractions

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early on of syntax, things like semicolons, parentheses,

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and other arcane details that, at first glance, are just not

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at all intellectually interesting and tend to get in

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the way from some fundamental understanding.

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In terms of now the support structure ahead, realized this

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tool, CS50 Discuss, which will be linked on the course's

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website later today, is the online discussion forum in

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which you'll be able to ask questions of each other and

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staff, and it's also a tool, as we'll see next week,

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that we'll use quite integratedly in

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office hours as well.

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Indeed, office hours begin on Monday, and frankly, the first

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week of office hours is fairly low key.

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>> I don't think you'll find Scratch all that inaccessible.

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It's rather self-explanatory, as we'll see, and so what

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we'll use it for today is to extract some of the

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fundamental ideas that will, then, persist throughout the

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rest of the semester.

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But starting Monday, at 8:00 PM through 11:00 PM will be

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office hours in Annenberg over brain break.

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Sectioning two will begin this evening, so sometime this

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weekend, go to cs50.net/section, and you'll

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be asked a number of questions.

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Among them, how would you describe your

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comfort level as of today?

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There's no hard, fast rule as to who's in which bucket.

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It's just the sort of thing that you probably know if you

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are among those the less comfortable or otherwise.

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>> And now, walkthroughs.

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The problem set specification for this week, both standard

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and hacker edition alike, is online at cs50.net as of now.

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And you'll see that the first of these editions, the

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standard edition each week, is accompanied by something we

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call a code walkthrough, a session led by one of the

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course's teaching fellows to guide you through, provide

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tips on, and get you down a particular path when it comes

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to starting these problem sets.

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So with each of these problem sets, if you're ever wanting

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for yourself where do I begin, you begin with these

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walkthroughs.

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And in fact, allow me to introduce Zamyla Chan, this

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year's teaching fellow who will be leading each and every

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one of these walkthroughs.

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Zamyla?

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>> [APPLAUSE]

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ZAMYLA CHAN: Oh, hi, everyone.

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My name is Zamyla.

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I'm a junior studying engineering in Winthrop House.

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But I try and fit in as many computer science courses as I

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can, which is why I'm really excited to be leading this

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year's walkthroughs.

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Walkthroughs, for me, were an essential part of my CS50

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experience.

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During the walkthrough, for every p-set, we'll go through

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the problem set together, look over the problems, kind of

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divide them up into manageable bites.

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I'll give you tips, techniques, for getting

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through and getting started.

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I hope to see you all at the walkthroughs.

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If you can't make it in person, then

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please do tune in online.

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>> DAVID MALAN: Excellent.

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Thank you, Zamyla.

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So realize that walkthroughs are, indeed, on a Friday

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afternoon, but this is deliberately by design so that

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even if you'd rather not attend class on a Friday

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afternoon, the videos will be up all the more quickly over

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the weekend.

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So realize that cs50.net will be all of Zamyla's

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walkthroughs.

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And that there is today's date and time, 3:00 PM,

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Harberd Hall, 104.

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And some of the teaching fellows have also prepared

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some remarks for you in absentia to give you a bit of

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perspective as to what their experience coming into and

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going out of CS50 was like.

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So if we could dim the lights for just a moment, I give you

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some of CS50's staff.

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>> JACKSON STEINKAMP: I took CS50 last year as a freshman in the

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fall, and it absolutely blew my mind.

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I had never taken any programming classes before,

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and never came in with any computer science

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experience at all.

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And just, I heard the buzz about the class and

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decided to take it.

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>> JULIA MITELMAN: It was really fun, really engaging.

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I still, to this day--

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I just recently saw I have the floppy disk that David gave us

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on our first day of lecture.

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I had hung it on my wall.

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This is probably a little nerdy, but I hung in on my

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wall during the class as a reminder of how cool it was.

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>> TRAVIS DOWNS: It's so embarrassing.

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>> JACKSON STEINKAMP: Computer science is something you

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should try even if you're not one for

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the traditional sciences.

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It's its own experience, and CS50 will make sure you're

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supported well through it with its veritable army of TFs.

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>> ALI NAHM: I took it as a freshman, and so I made a lot

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of new friends.

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I also got introduced to this entire concentration and

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entire school of engineering, and so I highly recommend it

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and welcome you to our CS50 family.

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>> YANIV YACOBY: CS50 just teaches you how to use tools

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that are widely accessible.

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You just need a laptop, you need a web browser, and you

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need to learn to write some code, and you can really build

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neat things.

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>> KAREN XIAO: It's just so cool to be able to make something

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and have people use it and have people see it, and that's

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what I really love about it.

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>> TIM MCLAUGHLIN: --a sense of community, I think, in this

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course more than any other course I've taken so far.

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You're not just taking another--

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you're not filling another requirement.

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You're not just going to lectures and going to section.

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But you're doing tons of things that are all about

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programming and all about technology, but it doesn't

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really feel like a class most of the time.

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>> TRAVIS DOWNS: And on the first day, they handed out cake, and

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I was instantly sold.

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>> JACOB PRITT: Free candy and pizza.

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>> ZAMYLA CHAN: --regardless of your interest level, I think

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that CS50, you'll have fun, and you'll be intellectually

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stimulated.

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>> MARK GROZEN-SMITH: It's always a party in class, and it's a

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party every night working on your p-set.

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>> JACKSON STEINKAMP: Each time you finish a problem set, you

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will feel like you've finished a project.

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>> MELISSA NIU: It was freshman year, and I was done shopping.

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I had my four classes ready, and I was in Annenberg.

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And I bump into a friend, and he says, hey, I'm shopping

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this class called CS50, and you should come with me.

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Ended up taking it that fall, and after that class, I

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thought maybe I'm going to minor in CS.

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But here I am, three years later, still studying computer

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science and actually doing it as a major, and I loved every

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moment of it.

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>> ROB BOWDEN: I have no idea.

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>> ALI NAHM: Let's see.

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>> MELISSA NIU: Craziness.

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>> SPEAKER 1: I feel like I'd be very cliche.

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I'd just say, like, awesome.

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>> YANIV YACOBY: Accessibility.

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>> JACOB PRITT: Free candy and pizza.

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>> TRAVIS DOWNS: Can't turn down a class that hands out cake.

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>> TIM MCLAUGHLIN: Energetic.

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>> VIPUL SHEKHAWAT: Essential.

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>> ROB BOWDEN: Let me think of an answer.

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I think I got something.

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Oh, god.

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Yeah, my name's Rob Bowden, and this is CS50.

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>> [APPLAUSE]

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DAVID MALAN: All right, so let's start to paint a picture

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of the direction in which we can go, and let's introduce

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this concept here known as pseudocode.

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So pseudocode is not a programming

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language unto itself.

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It's nothing technical per se, but it's just sort of a

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general way of expressing yourself fairly precisely,

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fairly algorithmically, fairly procedurally, but without

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having to worry about what language you're expressing

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yourself in.

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It's some model of English and programming languages with

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which you happen to be familiar, so we can start

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writing this sort of thing as we go.

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And in fact, Joseph, could I borrow you up on stage to be

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scribe here?

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I've gone ahead here in advance and forgotten to put

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on some socks today, and this'll be among our more

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ridiculous examples.

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>> Now, I need you over here.

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I'll do the socks part.

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So here we have a little scratch pad.

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This is literally just TextEdit in a Mac.

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We're not actually going to write a runnable program, but

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we're just going to start sketching out pseudocode based

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on some of the counsel you provide to me here.

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So here is my pile of socks at home.

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I have no socks on when I wake up in the morning, and we now

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need to write a program, an algorithm of sorts, with which

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to get these socks on my feet.

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And along the way, let's see if we trip over, or encounter,

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some of the ideas that you're going to have to start

252
00:11:32,860 --> 00:11:35,470
thinking about much more seriously when programming

253
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lest your programs don't behave quite as intended.

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>> So I sit down here.

255
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I've got my pile of socks.

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What's the first thing a reasonable human being would

257
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do when the goal is to put on a pair of socks?

258
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Someone give me one step, and only one step.

259
00:11:49,190 --> 00:11:49,796
Yeah?

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00:11:49,796 --> 00:11:50,690
AUDIENCE: Bend down.

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00:11:50,690 --> 00:11:54,290
DAVID MALAN: Bend down, Okay.

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00:11:54,290 --> 00:11:55,540
Step two.

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00:11:59,975 --> 00:12:00,645
Step two.

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00:12:00,645 --> 00:12:01,840
AUDIENCE: Pick up your sock.

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00:12:01,840 --> 00:12:03,220
DAVID MALAN: Pick up your sock.

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Okay, so slight ambiguity here, and this is one of the first

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stumbling blocks that we're supposed to deliberately

268
00:12:09,380 --> 00:12:10,290
encounter here.

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It's a little ambiguous, so pick up your sock.

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Fine, I'll take this one, but a computer, realize in just a

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bit, is not going to have that sort of human instinct to just

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pick the nearest one.

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We're going to have to start, before long, expressing

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ourselves more precisely.

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>> All right, so step two is pick up your sock.

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We'll take it.

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Step three.

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In the back.

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AUDIENCE: Find a matching pair.

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DAVID MALAN: Find matching pair.

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Okay, so this is good.

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I had to choose this sock.

283
00:12:35,650 --> 00:12:39,200
So the goal is to find a matching sock, now, but what

284
00:12:39,200 --> 00:12:39,990
does that mean?

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A reasonable human being, much like on Wednesday when I just

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00:12:42,130 --> 00:12:44,700
knew how to find Mike Smith in a phone book, just kind of

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went with their instinct.

288
00:12:45,640 --> 00:12:49,220
But here, it's obviously this sock here, but a computer's

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not going to be so instinctive.

290
00:12:51,350 --> 00:12:53,830
A computer is going to have a collection of bits, as we

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discussed on Wednesday, and those are

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organized somehow in memory.

293
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But the point is that a computer has only the ability

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to look at things one at a time, and in fact, even we

295
00:13:03,390 --> 00:13:05,980
humans-- even though it feels like I glanced down and a split

296
00:13:05,980 --> 00:13:08,900
second later I know where the sock is, my brain and my eyes

297
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presumably did a quick skim of those socks, and then latched

298
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on to the one in question.

299
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>> So if we be all the more deliberate now as a computer,

300
00:13:15,780 --> 00:13:17,780
how do I find this matching pair?

301
00:13:17,780 --> 00:13:18,890
Well, we have to iterate.

302
00:13:18,890 --> 00:13:21,440
We have to perhaps loop over this mess of socks on the

303
00:13:21,440 --> 00:13:25,760
floor whereby I say something like FOR EACH sock, pick it

304
00:13:25,760 --> 00:13:30,195
up, AND IF the same shape and size as the other one, THEN

305
00:13:30,195 --> 00:13:32,150
dot, dot, dot, we'll continue the story.

306
00:13:32,150 --> 00:13:34,430
So for each sock, so I pick up this one.

307
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I check is this equal to this one.

308
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It's not, so I put aside side.

309
00:13:38,250 --> 00:13:39,220
Then I iterate again.

310
00:13:39,220 --> 00:13:40,340
Is this one equal?

311
00:13:40,340 --> 00:13:42,010
No, it's not, so I put it aside.

312
00:13:42,010 --> 00:13:42,710
Is this one?

313
00:13:42,710 --> 00:13:43,200
No.

314
00:13:43,200 --> 00:13:43,670
This one?

315
00:13:43,670 --> 00:13:44,520
No, and so forth.

316
00:13:44,520 --> 00:13:48,730
And then finally, hopefully, I will encounter this sock here.

317
00:13:48,730 --> 00:13:51,350
So if it's the same shape, size, take it.

318
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>> And now, what would be our next step here?

319
00:13:53,980 --> 00:13:54,402
Yeah?

320
00:13:54,402 --> 00:13:56,090
AUDIENCE: Identify right from left.

321
00:13:56,090 --> 00:13:58,980
DAVID MALAN: Okay, identify right and left, so

322
00:13:58,980 --> 00:14:01,870
fortunately, that kind of works.

323
00:14:01,870 --> 00:14:04,020
A little symmetric, or I've just worn them that way.

324
00:14:04,020 --> 00:14:07,020
All right, so I've identified the right.

325
00:14:07,020 --> 00:14:09,980
And now, before we proceed, let me point out what Joseph's

326
00:14:09,980 --> 00:14:12,890
been doing here as sort of a versed programmer.

327
00:14:12,890 --> 00:14:15,800
So again, there's no one way of doing this, but beyond just

328
00:14:15,800 --> 00:14:18,480
numbering the lines, Joseph has already started to do this

329
00:14:18,480 --> 00:14:19,460
sort of indentation.

330
00:14:19,460 --> 00:14:21,950
Indeed, this is a very common convention in programming,

331
00:14:21,950 --> 00:14:26,110
whereby when you do something iterative, looping style, as

332
00:14:26,110 --> 00:14:28,480
we're implying with the English phrase "FOR EACH

333
00:14:28,480 --> 00:14:31,090
sock," the convention in pseudocode and, as we'll see,

334
00:14:31,090 --> 00:14:33,410
normal programming languages, is to just indent.

335
00:14:33,410 --> 00:14:36,240
Hit the space bar a few times, hit the Tab key or the like,

336
00:14:36,240 --> 00:14:40,660
so that nested underneath "FOR EACH sock" is the chunk of

337
00:14:40,660 --> 00:14:43,430
stuff that you need to do as a result of that loop.

338
00:14:43,430 --> 00:14:45,940
So that's all that's conveying semantically.

339
00:14:45,940 --> 00:14:48,620
Now meanwhile, the "If it's the same shape and size," the

340
00:14:48,620 --> 00:14:51,590
fact that "Take it" is indented further just means

341
00:14:51,590 --> 00:14:54,370
that's the only thing you should do if that condition,

342
00:14:54,370 --> 00:14:59,770
if that branch, that fork in the road, is in fact true.

343
00:14:59,770 --> 00:15:02,130
>> So now here, we're on step four, identify right and left.

344
00:15:02,130 --> 00:15:03,410
I've identified right.

345
00:15:03,410 --> 00:15:05,150
Give me step five.

346
00:15:05,150 --> 00:15:07,880
And technically, we could really call

347
00:15:07,880 --> 00:15:09,510
the FOR EACH thing--

348
00:15:09,510 --> 00:15:11,450
you should probably number all the lines if

349
00:15:11,450 --> 00:15:12,190
we're going to do this.

350
00:15:12,190 --> 00:15:14,006
JOSEPH: [INAUDIBLE]

351
00:15:14,006 --> 00:15:14,610
DAVID MALAN: [INAUDIBLE]?

352
00:15:14,610 --> 00:15:16,070
Okay, fine, all right, we'll do it your way.

353
00:15:16,070 --> 00:15:19,590
All right, so step five, how do we do the right sock?

354
00:15:19,590 --> 00:15:22,310
How do we proceed next, here?

355
00:15:22,310 --> 00:15:22,730
Yes?

356
00:15:22,730 --> 00:15:24,300
AUDIENCE: Lift up right leg.

357
00:15:24,300 --> 00:15:27,100
DAVID MALAN: Lift up right leg, Okay.

358
00:15:27,100 --> 00:15:29,330
Step six?

359
00:15:29,330 --> 00:15:30,728
Quickly.

360
00:15:30,728 --> 00:15:31,202
Yeah.

361
00:15:31,202 --> 00:15:33,100
AUDIENCE: Find an open end of the sock.

362
00:15:33,100 --> 00:15:34,440
DAVID MALAN: Okay, find the open end of the sock.

363
00:15:34,440 --> 00:15:34,840
So good.

364
00:15:34,840 --> 00:15:37,180
So here, honestly, a very common instinct would just be

365
00:15:37,180 --> 00:15:39,880
put on right sock, but that too is fairly ambiguous.

366
00:15:39,880 --> 00:15:42,750
Unless the computer or human knows exactly what that means,

367
00:15:42,750 --> 00:15:45,160
it's not going to be to execute that, so here, I've

368
00:15:45,160 --> 00:15:46,520
found the opening of the stock.

369
00:15:46,520 --> 00:15:48,222
Step seven?

370
00:15:48,222 --> 00:15:49,700
AUDIENCE: Touch your toes.

371
00:15:49,700 --> 00:15:51,070
DAVID MALAN: Touch toes.

372
00:15:51,070 --> 00:15:53,546
Okay, so now we go--

373
00:15:53,546 --> 00:15:56,480
All right, I'm going to take some liberties here.

374
00:15:56,480 --> 00:15:57,110
Thanks.

375
00:15:57,110 --> 00:15:59,210
Step eight?

376
00:15:59,210 --> 00:16:00,060
Put on sock.

377
00:16:00,060 --> 00:16:02,060
Okay, so now I, think we're close enough to sort of take

378
00:16:02,060 --> 00:16:03,480
this one at home.

379
00:16:03,480 --> 00:16:05,980
All right, so this goes up.

380
00:16:05,980 --> 00:16:07,950
I'll take some liberties with what it means, actually.

381
00:16:07,950 --> 00:16:08,920
Put the sock on.

382
00:16:08,920 --> 00:16:10,853
All right, now step nine?

383
00:16:10,853 --> 00:16:11,740
AUDIENCE: Put foot down.

384
00:16:11,740 --> 00:16:12,510
DAVID MALAN: Thank you.

385
00:16:12,510 --> 00:16:15,610
Step nine, put foot down, and now, we can repeat.

386
00:16:15,610 --> 00:16:19,540
So presumably, we can now go into step 10 and say identify

387
00:16:19,540 --> 00:16:21,960
left sock, but that's presumably already done.

388
00:16:21,960 --> 00:16:24,100
>> And so then, I can sort of repeat these steps.

389
00:16:24,100 --> 00:16:25,260
But this sort of begs the question.

390
00:16:25,260 --> 00:16:28,100
Before, the last time we wanted to repeat something, we

391
00:16:28,100 --> 00:16:30,730
did it iteratively, again and again, a FOR

392
00:16:30,730 --> 00:16:32,660
EACH loop so to speak.

393
00:16:32,660 --> 00:16:36,420
Would it make sense to use a loop in order to handle both

394
00:16:36,420 --> 00:16:38,470
the left and the right sock?

395
00:16:38,470 --> 00:16:40,460
Because it feels like these operations are pretty much

396
00:16:40,460 --> 00:16:43,090
identical except for the fact that one starts here and one

397
00:16:43,090 --> 00:16:44,850
starts here?

398
00:16:44,850 --> 00:16:47,400
Do we loop, or do we just keep writing steps

399
00:16:47,400 --> 00:16:48,745
10 and 11 and 12?

400
00:16:48,745 --> 00:16:50,540
AUDIENCE: Loop.

401
00:16:50,540 --> 00:16:52,240
>> DAVID MALAN: Okay, so loop.

402
00:16:52,240 --> 00:16:54,730
I actually might have said just keep going.

403
00:16:54,730 --> 00:16:55,720
So why is this the case?

404
00:16:55,720 --> 00:16:57,940
Well, this is actually the first of our non-obvious

405
00:16:57,940 --> 00:17:01,210
design decisions, and in fact, one of the metrics with which

406
00:17:01,210 --> 00:17:03,990
we'll begin to evaluate, for ourselves and for you, the

407
00:17:03,990 --> 00:17:06,780
quality of a program is just how well designed it is.

408
00:17:06,780 --> 00:17:09,619
Have you done the minimal amount of work necessary to

409
00:17:09,619 --> 00:17:12,480
get the job done the most quickly, either in terms of

410
00:17:12,480 --> 00:17:15,430
your time or in terms of the computer's running time?

411
00:17:15,430 --> 00:17:17,589
How many operations does it takes to execute?

412
00:17:17,589 --> 00:17:20,180
So arguably, this is an opportunity for a loop because

413
00:17:20,180 --> 00:17:22,829
as soon as I start copying and pasting, as Joseph effectively

414
00:17:22,829 --> 00:17:25,010
would start doing in a moment, you're kind of wasting your

415
00:17:25,010 --> 00:17:27,140
time, and you're being doubly expressive.

416
00:17:27,140 --> 00:17:30,260
>> But at the same time, these are really just two special

417
00:17:30,260 --> 00:17:31,890
cases, left and right.

418
00:17:31,890 --> 00:17:34,930
And whereas before, I might have 10 or 20 or more socks in

419
00:17:34,930 --> 00:17:37,960
a pile, it definitely makes sense not to have 50 lines of

420
00:17:37,960 --> 00:17:40,180
code saying check this sock, then the next one,

421
00:17:40,180 --> 00:17:41,300
then the next one.

422
00:17:41,300 --> 00:17:43,450
Here, it's a little less obvious, and I would proposed

423
00:17:43,450 --> 00:17:44,390
that we could go either way.

424
00:17:44,390 --> 00:17:46,720
We could either have that loop, although it only loops

425
00:17:46,720 --> 00:17:50,620
twice, or we could simply copy and paste just a little bit

426
00:17:50,620 --> 00:17:53,270
here in order to get the job done.

427
00:17:53,270 --> 00:17:55,210
But this program is buggy, so to speak.

428
00:17:55,210 --> 00:17:59,120
It might have some mistakes, errors, or corner cases, so to

429
00:17:59,120 --> 00:18:00,860
speak, that we didn't really anticipate.

430
00:18:00,860 --> 00:18:03,400
Nothing went wrong this time, but what could have gone wrong

431
00:18:03,400 --> 00:18:06,100
while executing this program?

432
00:18:06,100 --> 00:18:07,040
AUDIENCE: You don't have any socks.

433
00:18:07,040 --> 00:18:08,980
>> DAVID MALAN: So there could be no socks there whatsoever.

434
00:18:08,980 --> 00:18:10,380
So let's consider that corner case.

435
00:18:10,380 --> 00:18:14,670
So if we could scroll back up to step one, so step one was

436
00:18:14,670 --> 00:18:16,540
bend down, so that checks out.

437
00:18:16,540 --> 00:18:21,250
Step two, pick up your sock, but then find matching sock,

438
00:18:21,250 --> 00:18:22,420
identify right.

439
00:18:22,420 --> 00:18:25,050
We kind of made a whole bunch of assumptions, and this is,

440
00:18:25,050 --> 00:18:27,720
frankly, why program sometimes crash.

441
00:18:27,720 --> 00:18:30,240
If you, the programmer, have made certain assumptions, like

442
00:18:30,240 --> 00:18:33,000
surely there's going to be socks or surely there's going

443
00:18:33,000 --> 00:18:35,310
to be memory left in the computer, surely there's going

444
00:18:35,310 --> 00:18:37,420
to be disk space left on the hard drive--

445
00:18:37,420 --> 00:18:39,040
Well, if you make these assumptions, and that's not,

446
00:18:39,040 --> 00:18:41,880
in fact, reality, who knows sometimes what the computer's

447
00:18:41,880 --> 00:18:42,520
going to do?

448
00:18:42,520 --> 00:18:44,450
And sometimes, when you get the spinning beach ball or the

449
00:18:44,450 --> 00:18:47,750
frozen Windows or the like, that's precisely because some

450
00:18:47,750 --> 00:18:50,980
programmer did not anticipate those so-called corner cases.

451
00:18:50,980 --> 00:18:53,050
What else could have gone wrong in this program?

452
00:18:53,050 --> 00:18:53,330
Yeah?

453
00:18:53,330 --> 00:18:55,950
>> AUDIENCE: You don't have a right leg and a left leg.

454
00:18:55,950 --> 00:18:58,920
DAVID MALAN: Okay, might not have both a right leg and a

455
00:18:58,920 --> 00:19:01,940
left leg, and so this program might not be universally

456
00:19:01,940 --> 00:19:02,590
applicable.

457
00:19:02,590 --> 00:19:03,658
Others?

458
00:19:03,658 --> 00:19:05,650
AUDIENCE: You might have picked up an orphan sock.

459
00:19:05,650 --> 00:19:07,970
DAVID MALAN: I might have picked up an orphan sock, so a

460
00:19:07,970 --> 00:19:11,250
non-matching sock that just has no siblings because I've

461
00:19:11,250 --> 00:19:13,650
lost it, it's torn, it's in the wash still, or the like.

462
00:19:13,650 --> 00:19:15,550
So that, too, hasn't really been handled.

463
00:19:15,550 --> 00:19:15,790
Yeah?

464
00:19:15,790 --> 00:19:17,510
AUDIENCE: You might already have socks on.

465
00:19:17,510 --> 00:19:19,050
DAVID MALAN: I might already have socks on.

466
00:19:19,050 --> 00:19:20,080
I didn't actually check.

467
00:19:20,080 --> 00:19:23,350
IF you don't have socks on, THEN proceed to do

468
00:19:23,350 --> 00:19:24,330
line one and two.

469
00:19:24,330 --> 00:19:24,980
And that could happen.

470
00:19:24,980 --> 00:19:26,880
You fall asleep with your socks on and the like, so

471
00:19:26,880 --> 00:19:28,860
that, too, a very reasonable corner case.

472
00:19:28,860 --> 00:19:29,630
And maybe one other?

473
00:19:29,630 --> 00:19:31,310
AUDIENCE: The sock is inside out.

474
00:19:31,310 --> 00:19:34,000
DAVID MALAN: So the sock is inside out, so we did no error

475
00:19:34,000 --> 00:19:35,330
checking, in short.

476
00:19:35,330 --> 00:19:38,550
We didn't check if the state of the world is as we expect.

477
00:19:38,550 --> 00:19:41,310
We didn't check if we actually found what we're looking for.

478
00:19:41,310 --> 00:19:43,310
>> And even though this is sort of a ridiculous example

479
00:19:43,310 --> 00:19:46,110
involving socks, at the end of the day, this is exactly the

480
00:19:46,110 --> 00:19:48,740
sort of mindset you need to have while writing programs,

481
00:19:48,740 --> 00:19:51,740
even in Scratch as well as in C, in JavaScript, in PHP,

482
00:19:51,740 --> 00:19:53,920
because otherwise, your programs will exhibit the

483
00:19:53,920 --> 00:19:56,980
equivalent of that spinning beach ball or just yield

484
00:19:56,980 --> 00:19:58,210
inaccurate results.

485
00:19:58,210 --> 00:20:00,250
So many thanks here to our scribe Joseph.

486
00:20:00,250 --> 00:20:05,060
[APPLAUSE]

487
00:20:05,060 --> 00:20:06,850
>> DAVID MALAN: All right, so what is, in

488
00:20:06,850 --> 00:20:08,230
fact, a computer program?

489
00:20:08,230 --> 00:20:09,670
Well, let's take a quick glance at a

490
00:20:09,670 --> 00:20:11,100
representative one here.

491
00:20:11,100 --> 00:20:15,260
So this is a program written in a language called C. C is

492
00:20:15,260 --> 00:20:17,830
fairly old these days, but many newer languages are built

493
00:20:17,830 --> 00:20:18,460
on top of it.

494
00:20:18,460 --> 00:20:21,180
Indeed, PHP, one of the web-centric languages we'll

495
00:20:21,180 --> 00:20:24,050
use toward term's end, itself has what's called an

496
00:20:24,050 --> 00:20:27,550
interpreter, a program that's written in C, but more on that

497
00:20:27,550 --> 00:20:28,690
in many weeks from now.

498
00:20:28,690 --> 00:20:30,840
But this program, and this is what it means to write a

499
00:20:30,840 --> 00:20:32,880
program, albeit a very simple one.

500
00:20:32,880 --> 00:20:35,540
We have some fairly cryptic syntax here, but you can

501
00:20:35,540 --> 00:20:38,340
probably guess, even if you've never programmed before, what

502
00:20:38,340 --> 00:20:39,750
this program does.

503
00:20:39,750 --> 00:20:43,640
Indeed, I don't know what printf is, but print certainly

504
00:20:43,640 --> 00:20:45,700
conjures up the idea of printing something out.

505
00:20:45,700 --> 00:20:48,590
And so yes, this program is ultimately going to print out

506
00:20:48,590 --> 00:20:50,250
the words "Hello, World."

507
00:20:50,250 --> 00:20:53,390
>> Now, whether you have a Mac or a PC or a Linux computer, odds

508
00:20:53,390 --> 00:20:55,550
are, at least if you downloaded some freely

509
00:20:55,550 --> 00:20:58,000
available software, you could have been writing programs on

510
00:20:58,000 --> 00:21:00,310
your own laptop for quite some time now.

511
00:21:00,310 --> 00:21:03,040
On Mac OS, for instance, there's this program called

512
00:21:03,040 --> 00:21:05,340
Terminal that comes with a Mac, that's usually in your

513
00:21:05,340 --> 00:21:07,550
Utilities folder, and it generally opens a black and

514
00:21:07,550 --> 00:21:09,820
white or a white and black window at which you have a

515
00:21:09,820 --> 00:21:11,410
prompt at which you can type commands.

516
00:21:11,410 --> 00:21:13,060
So this is actually reminiscent of what computers

517
00:21:13,060 --> 00:21:15,100
used to be before graphical user

518
00:21:15,100 --> 00:21:16,710
interfaces, GUIs, came along.

519
00:21:16,710 --> 00:21:19,200
Now, in Windows, you have a similar mechanism in the form

520
00:21:19,200 --> 00:21:20,130
of the command prompt.

521
00:21:20,130 --> 00:21:23,430
>> But what I'm going to do here is open up, let's say,

522
00:21:23,430 --> 00:21:26,580
TextEdit again, so the same program we were using for

523
00:21:26,580 --> 00:21:28,870
pseudocode a moment ago, and I'm going to go ahead and

524
00:21:28,870 --> 00:21:30,080
write my first program.

525
00:21:30,080 --> 00:21:34,030
Include stdio.h, whatever that means, int main

526
00:21:34,030 --> 00:21:36,880
void, whatever that means, and then in the middle here,

527
00:21:36,880 --> 00:21:40,560
printf("hello, world."). And then close quote,

528
00:21:40,560 --> 00:21:42,340
close paren, semicolon.

529
00:21:42,340 --> 00:21:44,460
Now I'm going to go ahead and just hit Command-S. I'm going

530
00:21:44,460 --> 00:21:47,650
to go ahead and save this as hello.c, so the convention in

531
00:21:47,650 --> 00:21:50,530
the world of C programming is name the file dot c.

532
00:21:50,530 --> 00:21:52,880
I'm going to just put in John Harvard's Home directory,

533
00:21:52,880 --> 00:21:56,370
here, click Save, and now I'm going to go over to this

534
00:21:56,370 --> 00:21:59,110
terminal window, which again is this black and white prompt

535
00:21:59,110 --> 00:22:00,860
where I can execute commands.

536
00:22:00,860 --> 00:22:04,550
>> I can run programs by typing their name, not by

537
00:22:04,550 --> 00:22:06,710
double-clicking icons in the usual sense.

538
00:22:06,710 --> 00:22:10,590
But the thing is about C is that a language like C first

539
00:22:10,590 --> 00:22:13,150
comes in this form, something called source code.

540
00:22:13,150 --> 00:22:15,400
Something that looks a little like English but is

541
00:22:15,400 --> 00:22:17,980
definitely less like English than Joseph's pseudocode a

542
00:22:17,980 --> 00:22:18,930
moment ago.

543
00:22:18,930 --> 00:22:20,560
It's a little more arcane.

544
00:22:20,560 --> 00:22:22,830
It seems to follow some patterns or rules.

545
00:22:22,830 --> 00:22:25,300
The fact that I have curly braces, semicolons, quotes,

546
00:22:25,300 --> 00:22:28,240
angle braces, feels like a computer came up with this

547
00:22:28,240 --> 00:22:29,250
sort of language.

548
00:22:29,250 --> 00:22:32,010
But if I go, now, to this terminal window, I can run a

549
00:22:32,010 --> 00:22:35,250
command that's going to convert that source code is

550
00:22:35,250 --> 00:22:36,610
something called object code.

551
00:22:36,610 --> 00:22:39,690
That's going to convert English-like syntax to zeros

552
00:22:39,690 --> 00:22:41,760
and ones, the same sorts of zeros and ones we

553
00:22:41,760 --> 00:22:42,820
talked about Wednesday.

554
00:22:42,820 --> 00:22:44,810
Now, I'm going to run a command called Clang.

555
00:22:44,810 --> 00:22:47,750
More on this in the weeks to come, but it's a program with

556
00:22:47,750 --> 00:22:52,330
which I can convert hello.c into a whole bunch

557
00:22:52,330 --> 00:22:53,640
of zeros and ones.

558
00:22:53,640 --> 00:22:54,950
>> Now, I've run this command.

559
00:22:54,950 --> 00:22:57,940
I've run Clang, and then I said run yourself on this file

560
00:22:57,940 --> 00:23:00,630
called hello.c, which I created a moment ago, and

561
00:23:00,630 --> 00:23:02,090
nothing seems to happen.

562
00:23:02,090 --> 00:23:05,550
But indeed, if I poked around my home directory, I would see

563
00:23:05,550 --> 00:23:09,620
that this stupidly named program a.out now exists.

564
00:23:09,620 --> 00:23:12,250
This is just the default name for a program when writing in

565
00:23:12,250 --> 00:23:15,140
C. We can override this eventually, but a.out is the

566
00:23:15,140 --> 00:23:18,340
name of the program I just converted into zeros and ones.

567
00:23:18,340 --> 00:23:21,490
And now that it's zeros and ones, my Mac, in this case, or

568
00:23:21,490 --> 00:23:24,670
your Windows PC, can understand those bits, those

569
00:23:24,670 --> 00:23:25,410
zeros and ones.

570
00:23:25,410 --> 00:23:29,130
And so when I hit Enter, I see "hello, world!"

571
00:23:29,130 --> 00:23:31,270
>> But it's a bit buggy.

572
00:23:31,270 --> 00:23:34,110
I didn't quite say "hello, world!:air:- jharvard." Air is

573
00:23:34,110 --> 00:23:35,020
the name of my computer.

574
00:23:35,020 --> 00:23:37,730
Jharvard's the name of the account, so what did I clearly

575
00:23:37,730 --> 00:23:41,070
omit from the program?

576
00:23:41,070 --> 00:23:42,480
Some kind of line break.

577
00:23:42,480 --> 00:23:45,150
I didn't hit the equivalent of Enter or the carriage return,

578
00:23:45,150 --> 00:23:47,760
and this is, again, testament to the fact that computers can

579
00:23:47,760 --> 00:23:49,800
only do what you tell them to do.

580
00:23:49,800 --> 00:23:52,400
And the fact that I didn't tell the computer move the

581
00:23:52,400 --> 00:23:53,860
cursor to the next line--

582
00:23:53,860 --> 00:23:55,200
well, it's certainly not going to just do it

583
00:23:55,200 --> 00:23:56,750
presumptuously for me.

584
00:23:56,750 --> 00:24:00,860
So if I go back to my program, and I say \n--

585
00:24:00,860 --> 00:24:03,180
So \n, as we'll soon see, is the way of

586
00:24:03,180 --> 00:24:06,910
representing weird things like new line characters, things

587
00:24:06,910 --> 00:24:08,840
that would otherwise be the result of

588
00:24:08,840 --> 00:24:09,940
hitting the Enter key.

589
00:24:09,940 --> 00:24:11,730
But for now, just know that hitting the Enter key would

590
00:24:11,730 --> 00:24:14,170
just make our code look odd, so the world decided, you know

591
00:24:14,170 --> 00:24:16,580
what, to keep things prettier, to keep it on one line, let's

592
00:24:16,580 --> 00:24:19,070
just say \n represents a new line.

593
00:24:19,070 --> 00:24:22,260
Let me resave my file, go back to the terminal window, and

594
00:24:22,260 --> 00:24:25,290
re-run a.out, Enter.

595
00:24:25,290 --> 00:24:28,374
Still buggy, but why?

596
00:24:28,374 --> 00:24:29,750
>> AUDIENCE: [INAUDIBLE]

597
00:24:29,750 --> 00:24:31,540
DAVID MALAN: Yeah, so I need to recompile it.

598
00:24:31,540 --> 00:24:34,610
So to compile a program just means convert it from source

599
00:24:34,610 --> 00:24:37,660
code to object code, source code to zeros and ones.

600
00:24:37,660 --> 00:24:40,830
Now, the mere fact that I hit Save in this TextEdit has no

601
00:24:40,830 --> 00:24:43,450
bearing on those zeros and ones because I first need to

602
00:24:43,450 --> 00:24:48,260
tell Clang hey, I've changed those lines of code, the

603
00:24:48,260 --> 00:24:49,040
source code.

604
00:24:49,040 --> 00:24:51,010
You need to regenerate a.out.

605
00:24:51,010 --> 00:24:52,990
Nothing appears to have happened, but in a computer,

606
00:24:52,990 --> 00:24:55,820
at a command line, so to speak, when nothing happens,

607
00:24:55,820 --> 00:24:57,510
that usually means all is well.

608
00:24:57,510 --> 00:24:59,040
When something does happen, it means

609
00:24:59,040 --> 00:25:00,420
you messed up, generally.

610
00:25:00,420 --> 00:25:04,020
>> So let's now go to a.out, and indeed now, I have "hello,

611
00:25:04,020 --> 00:25:04,440
world."

612
00:25:04,440 --> 00:25:05,990
And now, what about these zeros and ones?

613
00:25:05,990 --> 00:25:07,520
Where, in fact, are those?

614
00:25:07,520 --> 00:25:10,710
Well, I can't really just kind of poke around very

615
00:25:10,710 --> 00:25:11,210
effectively.

616
00:25:11,210 --> 00:25:12,650
Let me open up TextEdit.

617
00:25:12,650 --> 00:25:14,060
Here's a.out.

618
00:25:14,060 --> 00:25:17,390
Let me go ahead and open this, and this is apparently what my

619
00:25:17,390 --> 00:25:19,080
program looks like.

620
00:25:19,080 --> 00:25:21,570
So I've opened, not hello.c, but a.out.

621
00:25:21,570 --> 00:25:23,960
But this is actually not what my program really is.

622
00:25:23,960 --> 00:25:26,450
Clearly, this is some kind of alphabetical characters.

623
00:25:26,450 --> 00:25:28,670
I see no zeros and ones, but this is

624
00:25:28,670 --> 00:25:30,780
because a.out is a program.

625
00:25:30,780 --> 00:25:33,540
Zeros and ones-- but TextEdit, as the name suggests, it's

626
00:25:33,540 --> 00:25:37,130
just like Notepad on Windows, is just a text editor, so it's

627
00:25:37,130 --> 00:25:39,730
confusing all of those zeros and ones as

628
00:25:39,730 --> 00:25:41,870
though they were, what?

629
00:25:41,870 --> 00:25:43,380
ASCII characters.

630
00:25:43,380 --> 00:25:45,020
>> So recall on Wednesday, we just came up with this

631
00:25:45,020 --> 00:25:48,740
arbitrary mapping of numbers, or bits, to letters of the

632
00:25:48,740 --> 00:25:51,690
alphabet and punctuation symbols and the like.

633
00:25:51,690 --> 00:25:54,920
So TextEdit, that it's a text editor is misinterpreting

634
00:25:54,920 --> 00:25:57,270
those patterns of zeros and ones that are supposed to be

635
00:25:57,270 --> 00:25:59,990
printing words, like "hello, world." It's displaying them

636
00:25:59,990 --> 00:26:02,150
as ASCII, and that's why it looks a little messy.

637
00:26:02,150 --> 00:26:04,910
Now, there are some hints of correctness in here.

638
00:26:04,910 --> 00:26:08,090
Notice if I highlight, there is a hint of actually "hello,

639
00:26:08,090 --> 00:26:10,810
world," so somewhere in that program is

640
00:26:10,810 --> 00:26:12,120
the sentence I wrote.

641
00:26:12,120 --> 00:26:14,610
But let's go ahead and now see with a different program.

642
00:26:14,610 --> 00:26:17,420
This is not one we'll use that often, but it also comes with

643
00:26:17,420 --> 00:26:19,970
a Mac and will be inside of the CS50 appliance.

644
00:26:19,970 --> 00:26:23,940
Let me go ahead and open with a program called XXD.

645
00:26:23,940 --> 00:26:26,530
>> Back in the day, most programs were named fairly cryptically,

646
00:26:26,530 --> 00:26:28,200
and so the trend continues.

647
00:26:28,200 --> 00:26:31,160
But -b means spit this program out as binary.

648
00:26:31,160 --> 00:26:32,330
Don't run it.

649
00:26:32,330 --> 00:26:35,770
Display it to me as zeros and ones, and this is the C

650
00:26:35,770 --> 00:26:37,190
program we just wrote.

651
00:26:37,190 --> 00:26:39,920
Now, I, as a human here, I honestly have no idea what

652
00:26:39,920 --> 00:26:42,230
these various patterns of zeros and ones represent.

653
00:26:42,230 --> 00:26:45,320
Back in the day, I, with my punch cards or the like, would

654
00:26:45,320 --> 00:26:48,080
actually have to look up what these various patterns of

655
00:26:48,080 --> 00:26:51,560
01111000, actually represent.

656
00:26:51,560 --> 00:26:54,390
Or worse, I would need to do the punching or the creation

657
00:26:54,390 --> 00:26:56,130
of these patterns of zeros and ones.

658
00:26:56,130 --> 00:27:00,410
But for now, take on faith that a CPU, Intel inside, so

659
00:27:00,410 --> 00:27:03,440
to speak, inside of all of our computers these days, knows

660
00:27:03,440 --> 00:27:05,100
how to interpret these zeros and ones.

661
00:27:05,100 --> 00:27:07,010
And some zeros and ones mean print.

662
00:27:07,010 --> 00:27:09,040
Some zeros and ones mean play a sound.

663
00:27:09,040 --> 00:27:13,630
Some zeroes and ones mean take user input from a keyboard.

664
00:27:13,630 --> 00:27:15,960
There's all sorts of different patterns, but we thankfully,

665
00:27:15,960 --> 00:27:19,680
as humans, only generally need to worry about programming at

666
00:27:19,680 --> 00:27:21,270
this fairly higher level.

667
00:27:21,270 --> 00:27:24,320
And in other CS classes can you delve down deeper and look

668
00:27:24,320 --> 00:27:26,820
at things like those zeros and ones, or yet

669
00:27:26,820 --> 00:27:28,000
other things still.

670
00:27:28,000 --> 00:27:29,210
>> So now let's convert this.

671
00:27:29,210 --> 00:27:32,550
Let's move very quickly away from C and move to something a

672
00:27:32,550 --> 00:27:36,390
little more comforting, a little more exciting, in that

673
00:27:36,390 --> 00:27:39,340
we can get back our animations and sounds and the like that

674
00:27:39,340 --> 00:27:40,940
clearly have escaped us in this

675
00:27:40,940 --> 00:27:42,480
fairly primitive interface.

676
00:27:42,480 --> 00:27:46,430
So this same program in C can be represented now in this

677
00:27:46,430 --> 00:27:50,060
programming language called Scratch as follows.

678
00:27:50,060 --> 00:27:54,230
This is the equivalent of this hello world program written in

679
00:27:54,230 --> 00:27:57,370
this puzzle piece style language called Scratch.

680
00:27:57,370 --> 00:28:00,600
So let me go ahead and open up this very program.

681
00:28:00,600 --> 00:28:01,850
It's again called Scratch.

682
00:28:01,850 --> 00:28:03,880
It's freely available, and this is the same thing we

683
00:28:03,880 --> 00:28:05,120
started today on.

684
00:28:05,120 --> 00:28:07,790
>> So this here is Scratch, and it's broken up into a few

685
00:28:07,790 --> 00:28:09,700
different pieces.

686
00:28:09,700 --> 00:28:12,690
On the very top right, we have the so-called stage, and

687
00:28:12,690 --> 00:28:13,670
indeed, that's where the cookies

688
00:28:13,670 --> 00:28:15,440
performed just a bit ago.

689
00:28:15,440 --> 00:28:19,660
And on that stage are things called sprites, characters, or

690
00:28:19,660 --> 00:28:21,030
objects, or entities.

691
00:28:21,030 --> 00:28:23,320
It doesn't really matter how you think of them, but they

692
00:28:23,320 --> 00:28:26,740
are programmable, movable things, and in this case, this

693
00:28:26,740 --> 00:28:30,730
program that our student wrote has a couple of gingerbread

694
00:28:30,730 --> 00:28:32,990
cookies, a couple of circular cookies, a whole bunch of

695
00:28:32,990 --> 00:28:35,430
hearts, a whole bunch of eyeglasses.

696
00:28:38,170 --> 00:28:42,420
Because of this, he or she is able to program each of those

697
00:28:42,420 --> 00:28:44,720
individual characters separately.

698
00:28:44,720 --> 00:28:46,480
>> Now, what does it mean to program these characters?

699
00:28:46,480 --> 00:28:49,440
Well, let me go ahead and click on this left hand cookie

700
00:28:49,440 --> 00:28:51,830
and scroll over to the top left here.

701
00:28:51,830 --> 00:28:53,670
In the top left of my screen now is the

702
00:28:53,670 --> 00:28:55,280
so-called scripts area.

703
00:28:55,280 --> 00:28:58,790
This is sort of a blank slate, initially, onto which I can

704
00:28:58,790 --> 00:29:01,540
drag and drop puzzle pieces that, frankly, do

705
00:29:01,540 --> 00:29:03,020
exactly what they say.

706
00:29:03,020 --> 00:29:05,820
At the very top of this stack of puzzle pieces is the word

707
00:29:05,820 --> 00:29:09,340
When Green Flag Clicked, and if you didn't notice before,

708
00:29:09,340 --> 00:29:12,300
the way I started that cookie song was clicking, literally,

709
00:29:12,300 --> 00:29:13,320
a green flag.

710
00:29:13,320 --> 00:29:16,590
So that puzzle piece at top left there means when the

711
00:29:16,590 --> 00:29:19,780
human clicks the green flag, proceeded to do

712
00:29:19,780 --> 00:29:21,110
the following things.

713
00:29:21,110 --> 00:29:23,330
>> Now, what did they cookie proceed to do?

714
00:29:23,330 --> 00:29:25,810
I don't really know how to interpret this yet, but the

715
00:29:25,810 --> 00:29:29,220
cookie apparently set its groove to zero, then it waited

716
00:29:29,220 --> 00:29:31,740
three seconds, then it changed its group to one, then it

717
00:29:31,740 --> 00:29:33,590
waited a second, then it changed its

718
00:29:33,590 --> 00:29:35,100
groove back to one.

719
00:29:35,100 --> 00:29:37,373
And then this actually looks like a bit of a bug, shouldn't

720
00:29:37,373 --> 00:29:39,440
have to change its groove again and again unless it's

721
00:29:39,440 --> 00:29:43,380
being changed elsewhere, but this series of steps is what's

722
00:29:43,380 --> 00:29:46,800
dictating the behavior of this particular cookie.

723
00:29:46,800 --> 00:29:49,180
So let's actually scroll back and not look at something

724
00:29:49,180 --> 00:29:50,430
quite so complex yet.

725
00:29:50,430 --> 00:29:52,260
Let me go ahead and go to File, New,

726
00:29:52,260 --> 00:29:53,560
and get a clean slate.

727
00:29:53,560 --> 00:29:56,590
>> So now, I indeed have an empty script area, an empty stage,

728
00:29:56,590 --> 00:29:59,380
with our default sprite Scratch, and at the top left

729
00:29:59,380 --> 00:30:01,670
on my screen do I have the pallet of all of their

730
00:30:01,670 --> 00:30:03,400
available puzzle pieces.

731
00:30:03,400 --> 00:30:06,300
And we won't go through nearly that many of these things

732
00:30:06,300 --> 00:30:09,470
today because, again, most are self-explanatory, but we will

733
00:30:09,470 --> 00:30:12,370
try to categorize them and point out the similarities

734
00:30:12,370 --> 00:30:15,380
with these future languages to which we will dive.

735
00:30:15,380 --> 00:30:19,180
And at top left here is the first When Green Flag Clicked,

736
00:30:19,180 --> 00:30:22,170
so let me drag this over here, zoom out a bit.

737
00:30:22,170 --> 00:30:25,780
And if I click the green flag, nothing really happens because

738
00:30:25,780 --> 00:30:29,360
I haven't attached any logic, any statement so to speak, to

739
00:30:29,360 --> 00:30:31,040
that green flag, so let me go up to the

740
00:30:31,040 --> 00:30:32,500
categories over here.

741
00:30:32,500 --> 00:30:34,840
I'm currently in the Control category.

742
00:30:34,840 --> 00:30:37,630
I'm instead going to go down to the Looks category, and

743
00:30:37,630 --> 00:30:39,970
there's a whole bunch of things here that say Say,

744
00:30:39,970 --> 00:30:43,110
Think, Change Color, Switch Costume.

745
00:30:43,110 --> 00:30:44,860
>> So you can do silly things with costumes and

746
00:30:44,860 --> 00:30:45,870
sounds and the like.

747
00:30:45,870 --> 00:30:49,970
Let me go ahead and just say Say, and now notice as I drag

748
00:30:49,970 --> 00:30:52,940
and drop this puzzle piece, it's going to want to latch

749
00:30:52,940 --> 00:30:55,010
into the corresponding shape.

750
00:30:55,010 --> 00:30:58,020
So when I go ahead and let go of my mouse, they lock

751
00:30:58,020 --> 00:31:01,770
together, and now if I go over here and click the green flag,

752
00:31:01,770 --> 00:31:05,910
the cat does in fact say hello because that's what is inside

753
00:31:05,910 --> 00:31:06,980
of this white box.

754
00:31:06,980 --> 00:31:08,900
We'll soon see that this white box is what's called an

755
00:31:08,900 --> 00:31:10,250
argument, or a parameter.

756
00:31:10,250 --> 00:31:12,560
It's a way of changing the behavior of, in this case, a

757
00:31:12,560 --> 00:31:15,070
puzzle piece, but if I want to say exactly what I said

758
00:31:15,070 --> 00:31:18,610
before, say hello, world, I can now go back over here,

759
00:31:18,610 --> 00:31:21,990
click play, and "hello, world" is what's said.

760
00:31:21,990 --> 00:31:24,340
>> So we are literally programming now.

761
00:31:24,340 --> 00:31:27,540
It's not all that compelling of a program, but at least

762
00:31:27,540 --> 00:31:30,140
it's a little more compelling than something that looks, at

763
00:31:30,140 --> 00:31:31,300
first glance, like this.

764
00:31:31,300 --> 00:31:34,190
And we can very quickly get all the more expressive

765
00:31:34,190 --> 00:31:36,600
because in Scratch, like in other languages, there's all

766
00:31:36,600 --> 00:31:40,010
sorts of statements, not just Say or printing something, but

767
00:31:40,010 --> 00:31:42,650
you can do things like waiting, as we just saw with

768
00:31:42,650 --> 00:31:44,240
the cookie, some number of seconds.

769
00:31:44,240 --> 00:31:46,440
You can play sounds in the environment of Scratch just

770
00:31:46,440 --> 00:31:49,050
like you can in a normal computer program play sound.

771
00:31:49,050 --> 00:31:52,440
You can check what are called Boolean expressions.

772
00:31:52,440 --> 00:31:55,630
So now, let's start to add to our toolkit some terminology

773
00:31:55,630 --> 00:31:58,840
that actually relates to the example that Joseph and I did

774
00:31:58,840 --> 00:32:00,080
here with the socks.

775
00:32:00,080 --> 00:32:03,040
So statements are just statements of fact.

776
00:32:03,040 --> 00:32:04,160
Do this.

777
00:32:04,160 --> 00:32:07,370
A directive for the sprite, or me the human, to do something.

778
00:32:07,370 --> 00:32:10,420
A Boolean expression is something that has a value, a

779
00:32:10,420 --> 00:32:14,580
so-called truth value, that's either a zero or one, false or

780
00:32:14,580 --> 00:32:17,890
true, off or on, no or yes.

781
00:32:17,890 --> 00:32:19,730
Doesn't really matter how you think of this, but it's a

782
00:32:19,730 --> 00:32:20,700
binary state.

783
00:32:20,700 --> 00:32:22,610
As Nate discussed in Wednesday's video, two

784
00:32:22,610 --> 00:32:23,570
different things.

785
00:32:23,570 --> 00:32:25,740
>> So in Scratch, Boolean expressions happen to look

786
00:32:25,740 --> 00:32:28,910
like these blue objects here, and in this case, the question

787
00:32:28,910 --> 00:32:30,960
mark implies that you're asking a question.

788
00:32:30,960 --> 00:32:34,250
Is the cat, or the sprite, touching the mouse pointer?

789
00:32:34,250 --> 00:32:36,580
So this is just one example of a Scratch block that's going

790
00:32:36,580 --> 00:32:39,950
to allow us to check yes or no, is the mouse touching the

791
00:32:39,950 --> 00:32:40,790
sprite on the screen?

792
00:32:40,790 --> 00:32:42,840
And this can be useful if you actually want to do things

793
00:32:42,840 --> 00:32:43,820
with your mouse.

794
00:32:43,820 --> 00:32:46,470
In addition to Boolean expressions, we have things

795
00:32:46,470 --> 00:32:49,110
like is the mouse down, so you can detect that kind of

796
00:32:49,110 --> 00:32:50,200
question as well.

797
00:32:50,200 --> 00:32:52,590
We can do mathematics if you actually want, and there's

798
00:32:52,590 --> 00:32:55,170
actually more compelling uses for this than just pure of

799
00:32:55,170 --> 00:32:56,400
arithmetic, as we'll see.

800
00:32:56,400 --> 00:32:59,690
Pseudo randomness and making your program appear to think

801
00:32:59,690 --> 00:33:03,490
or behave differently based on some seemingly random values,

802
00:33:03,490 --> 00:33:06,610
and then we have things like Boolean expressions like AND.

803
00:33:06,610 --> 00:33:08,830
So if you actually want to check two values, we'll see in

804
00:33:08,830 --> 00:33:11,110
Scratch that we can actually test if this is

805
00:33:11,110 --> 00:33:13,190
true and this is true.

806
00:33:13,190 --> 00:33:15,300
For instance, in the case of my socks, I could've at the

807
00:33:15,300 --> 00:33:19,045
very end asked the question if left sock is on and right sock

808
00:33:19,045 --> 00:33:20,610
is on, quit.

809
00:33:20,610 --> 00:33:22,710
You're all done for the day, so that would be an

810
00:33:22,710 --> 00:33:24,300
opportunity for that.

811
00:33:24,300 --> 00:33:27,740
>> So let's go ahead and try to piece some of these together

812
00:33:27,740 --> 00:33:31,570
and go into a couple of examples more

813
00:33:31,570 --> 00:33:32,610
compelling than this one.

814
00:33:32,610 --> 00:33:35,770
So let me go ahead here and open up some of the examples

815
00:33:35,770 --> 00:33:38,350
that will always be on the course's website as well, and

816
00:33:38,350 --> 00:33:39,870
open up hello2.

817
00:33:39,870 --> 00:33:42,910
So in hello2 here, we have a program that's doing a few

818
00:33:42,910 --> 00:33:46,290
things, but it's not doing it as effectively as we might.

819
00:33:46,290 --> 00:33:48,900
So here it says "hello, world" for one second and then waits

820
00:33:48,900 --> 00:33:49,450
for a second.

821
00:33:49,450 --> 00:33:51,720
And then does it again, and then does it again.

822
00:33:51,720 --> 00:33:55,530
So if I click the green flag, Scratch says "Hello, world.

823
00:33:55,530 --> 00:33:57,160
Hello, world.

824
00:33:57,160 --> 00:34:00,080
Hello, world." And this is obviously candidate now for

825
00:34:00,080 --> 00:34:00,860
improvement.

826
00:34:00,860 --> 00:34:03,070
>> What's the marginal improvement, hopefully, we can

827
00:34:03,070 --> 00:34:07,610
now make if Scratch supports the concept?

828
00:34:07,610 --> 00:34:08,310
Some kind of loop.

829
00:34:08,310 --> 00:34:10,960
Some kind of repetition, now, would be nice, so let me

830
00:34:10,960 --> 00:34:11,920
actually try that.

831
00:34:11,920 --> 00:34:13,870
Let me actually go and move this.

832
00:34:13,870 --> 00:34:16,449
So notice can detach blocks as easily as you can add to

833
00:34:16,449 --> 00:34:17,420
attach them.

834
00:34:17,420 --> 00:34:20,730
Let me go under Control, scroll down here, and indeed,

835
00:34:20,730 --> 00:34:24,000
there's this puzzle piece here, Repeat and Forever and

836
00:34:24,000 --> 00:34:24,870
Forever If.

837
00:34:24,870 --> 00:34:28,500
So there's a number of ways of expressing looping constructs

838
00:34:28,500 --> 00:34:29,340
in Scratch.

839
00:34:29,340 --> 00:34:31,730
The one I probably want here is not Forever because I only

840
00:34:31,730 --> 00:34:34,639
want this to happen three times, but probably Repeat.

841
00:34:34,639 --> 00:34:38,449
So let me drag Repeat over here, drag and drop it, and

842
00:34:38,449 --> 00:34:42,530
now instead of saying "hello, world" three separate times,

843
00:34:42,530 --> 00:34:44,230
let me drag this puzzle piece in here.

844
00:34:44,230 --> 00:34:46,610
And even though it doesn't seem to fit, the program is smart

845
00:34:46,610 --> 00:34:48,920
enough to realize it will grow to fill, so it's the

846
00:34:48,920 --> 00:34:51,270
shapes that matter and not the absolute size.

847
00:34:51,270 --> 00:34:54,030
Let me change the repetition to three, and now let me go

848
00:34:54,030 --> 00:34:56,560
ahead and drag Wait One Second in there as well.

849
00:34:56,560 --> 00:34:59,220
It's going to snap in as well, and so now I'm going to drag

850
00:34:59,220 --> 00:35:01,370
these guys over here and just throw them away because I

851
00:35:01,370 --> 00:35:02,710
don't need them anymore.

852
00:35:02,710 --> 00:35:05,920
Let me zoom out and click the green flag now, and we have

853
00:35:05,920 --> 00:35:09,600
the same program but, as I predicted before, better

854
00:35:09,600 --> 00:35:12,200
designed because you can imagine how bad this program

855
00:35:12,200 --> 00:35:14,410
would get, certainly aesthetically, if you had to

856
00:35:14,410 --> 00:35:16,640
start copying and pasting, copying and pasting, or

857
00:35:16,640 --> 00:35:20,020
dragging and dropping the same darn things again and again.

858
00:35:20,020 --> 00:35:22,260
>> Now, simply saying stuff on the screen, printing to the

859
00:35:22,260 --> 00:35:25,350
screen, really all not that exciting, so let's open a

860
00:35:25,350 --> 00:35:26,580
third variance here.

861
00:35:26,580 --> 00:35:29,810
And now, as you'll see, this'll quickly get annoying--

862
00:35:29,810 --> 00:35:30,850
[MEOW]

863
00:35:30,850 --> 00:35:31,940
DAVID MALAN: --but it's also kind of cute.

864
00:35:31,940 --> 00:35:35,244
[MEOW]

865
00:35:35,244 --> 00:35:37,460
DAVID MALAN: Okay, so better, and we can certainly use that

866
00:35:37,460 --> 00:35:41,060
same transition of chunking this up into a looping

867
00:35:41,060 --> 00:35:43,520
structure, but let's make it more interesting still.

868
00:35:43,520 --> 00:35:47,480
>> Let me go ahead and open up a fourth variance here, where I

869
00:35:47,480 --> 00:35:50,020
take things one step further.

870
00:35:50,020 --> 00:35:54,450
So according to this, silly though this is at first

871
00:35:54,450 --> 00:35:58,530
glance, what is this program going to do?

872
00:35:58,530 --> 00:35:59,790
It's going to meow once.

873
00:35:59,790 --> 00:36:00,160
Why?

874
00:36:00,160 --> 00:36:03,340
Well, one is, as far as I know, always less than two.

875
00:36:03,340 --> 00:36:04,890
There's no notion of randomness here.

876
00:36:04,890 --> 00:36:07,240
I've literally hard-coded one and two, but this is an

877
00:36:07,240 --> 00:36:10,370
example now of actually using a Boolean expression.

878
00:36:10,370 --> 00:36:13,650
Much like as Joseph did in his pseudocode, the indentation IF

879
00:36:13,650 --> 00:36:17,170
you find matching socks THEN do the following, here we have

880
00:36:17,170 --> 00:36:19,750
an expression IF one is less than two THEN--

881
00:36:19,750 --> 00:36:21,820
and in fact we even have a little bit of indentation,

882
00:36:21,820 --> 00:36:24,480
where the purple is slightly indented to the right--

883
00:36:24,480 --> 00:36:26,600
THEN you're going to play the sound meow.

884
00:36:26,600 --> 00:36:30,060
Now, in this case, that one is always less than two, so this

885
00:36:30,060 --> 00:36:31,970
is kind of a waste of a condition.

886
00:36:31,970 --> 00:36:34,890
But we'll be able, as we'll see, to plug other things into

887
00:36:34,890 --> 00:36:37,420
these placeholders where one and two now are.

888
00:36:37,420 --> 00:36:41,340
So let's now advance to example five of these several

889
00:36:41,340 --> 00:36:45,850
hellos and look at what this program's going to do.

890
00:36:45,850 --> 00:36:48,710
>> So now, in an English sentence, how does this

891
00:36:48,710 --> 00:36:51,460
program behave?

892
00:36:51,460 --> 00:36:52,950
AUDIENCE: Meows half the time.

893
00:36:52,950 --> 00:36:55,380
DAVID MALAN: Meow's half of the time, so this is a way of

894
00:36:55,380 --> 00:36:56,700
conveying a very simple idea.

895
00:36:56,700 --> 00:36:59,400
Even though we happen to be using some inequalities here

896
00:36:59,400 --> 00:37:01,860
in some numbers, this is really just a programmatical

897
00:37:01,860 --> 00:37:09,490
way, a precise way, of saying if the coin comes up heads, go

898
00:37:09,490 --> 00:37:10,270
ahead and meow.

899
00:37:10,270 --> 00:37:13,540
Or conversely, if the coin comes up tails, don't meow.

900
00:37:13,540 --> 00:37:15,110
And in this case, how do we express that?

901
00:37:15,110 --> 00:37:17,950
We'll pick a random number from 1 to 10, and if that

902
00:37:17,950 --> 00:37:21,110
number is less than 6, go ahead and meow.

903
00:37:21,110 --> 00:37:22,260
And how did this get in here?

904
00:37:22,260 --> 00:37:24,620
Well again, notice just the dragging and dropping and

905
00:37:24,620 --> 00:37:26,190
things latches into place.

906
00:37:26,190 --> 00:37:28,020
>> So now let's see if this randomness works.

907
00:37:28,020 --> 00:37:29,845
Let me go ahead and click the green flag.

908
00:37:29,845 --> 00:37:31,120
[MEOW]

909
00:37:31,120 --> 00:37:32,000
DAVID MALAN: Okay.

910
00:37:32,000 --> 00:37:33,430
[MEOW]

911
00:37:33,430 --> 00:37:35,260
DAVID MALAN: Okay.

912
00:37:35,260 --> 00:37:38,510
Okay, good, so we got heads, heads, tails effectively.

913
00:37:38,510 --> 00:37:39,295
Tails.

914
00:37:39,295 --> 00:37:39,590
[MEOW]

915
00:37:39,590 --> 00:37:41,829
DAVID MALAN: Heads.

916
00:37:41,829 --> 00:37:43,220
[MEOW]

917
00:37:43,220 --> 00:37:44,040
DAVID MALAN: Excellent.

918
00:37:44,040 --> 00:37:46,220
It's always awkward when just statistically you get a bad

919
00:37:46,220 --> 00:37:48,510
run, and it's all heads, and the program actually doesn't

920
00:37:48,510 --> 00:37:49,230
work as you'd hope.

921
00:37:49,230 --> 00:37:51,210
But this time, it worked, and we seem to have, if we did

922
00:37:51,210 --> 00:37:53,830
this an infinite number of times, 50% odds.

923
00:37:53,830 --> 00:37:56,950
Now again, not all that interesting, just making cats

924
00:37:56,950 --> 00:37:59,840
meow, so let's see if we can't advance this a bit further

925
00:37:59,840 --> 00:38:01,820
here in version six.

926
00:38:01,820 --> 00:38:05,620
So now, we have really annoying version--

927
00:38:05,620 --> 00:38:08,215
[MEOWS EVERY FEW SECONDS]

928
00:38:08,215 --> 00:38:10,640
DAVID MALAN: --and this is what's known, general, as an

929
00:38:10,640 --> 00:38:11,670
infinite loop.

930
00:38:11,670 --> 00:38:14,190
>> So infinite loop in this case feels bad.

931
00:38:14,190 --> 00:38:16,970
It's definitely going to start sounding bad, and yet infinite

932
00:38:16,970 --> 00:38:18,740
loops aren't always bad.

933
00:38:18,740 --> 00:38:20,910
Can you think of context in computer programs where you'd

934
00:38:20,910 --> 00:38:22,910
actually want an infinite loop?

935
00:38:22,910 --> 00:38:23,691
Yeah?

936
00:38:23,691 --> 00:38:26,520
AUDIENCE: When you want to check a condition.

937
00:38:26,520 --> 00:38:28,240
DAVID MALAN: Okay, when you want to keep checking a condition?

938
00:38:28,240 --> 00:38:29,190
Like what?

939
00:38:29,190 --> 00:38:30,440
AUDIENCE: [INAUDIBLE]

940
00:38:36,060 --> 00:38:38,170
DAVID MALAN: Okay, good, so if you had some program, some

941
00:38:38,170 --> 00:38:40,440
kind of home automation thing, where you want to constantly

942
00:38:40,440 --> 00:38:41,980
monitor is something the case.

943
00:38:41,980 --> 00:38:42,850
Are the lights on?

944
00:38:42,850 --> 00:38:45,020
Are the lights on because maybe you have a timer, and

945
00:38:45,020 --> 00:38:47,360
you want them to go off, you might need to do something

946
00:38:47,360 --> 00:38:48,290
again and again.

947
00:38:48,290 --> 00:38:50,540
And in fact, speaking of timers, any of you who have

948
00:38:50,540 --> 00:38:53,100
clocks on your computer or digital watches, that's an

949
00:38:53,100 --> 00:38:54,210
infinite loop.

950
00:38:54,210 --> 00:38:56,980
It continues to update the time because it's constantly

951
00:38:56,980 --> 00:38:59,750
checking and checking and checking has the time changed,

952
00:38:59,750 --> 00:39:01,000
and if so, oh, my god.

953
00:39:03,670 --> 00:39:04,740
The clock's finally changed.

954
00:39:04,740 --> 00:39:07,020
It needs display that value to you.

955
00:39:07,020 --> 00:39:10,880
So whereas most of the time infinite loops are a mistake,

956
00:39:10,880 --> 00:39:13,950
or at least a poor design decision, sometimes they do

957
00:39:13,950 --> 00:39:14,785
have their value.

958
00:39:14,785 --> 00:39:18,170
>> Well, let's advance further here to hello7.

959
00:39:18,170 --> 00:39:20,410
So now the program will get a little more interactive.

960
00:39:20,410 --> 00:39:23,120
Let me zoom in here, and again, this is what's nice

961
00:39:23,120 --> 00:39:23,640
about Scratch.

962
00:39:23,640 --> 00:39:26,760
And we'll use it to be clear, Scratch, just today and into

963
00:39:26,760 --> 00:39:27,780
next week's problem set.

964
00:39:27,780 --> 00:39:31,920
But on Monday, we dive into C. In this program here, it does,

965
00:39:31,920 --> 00:39:33,190
forever, the following.

966
00:39:33,190 --> 00:39:35,890
IF touching the mouse pointer-- now, who's the

967
00:39:35,890 --> 00:39:36,420
context here?

968
00:39:36,420 --> 00:39:39,670
Well, and notice that who's selected down here at bottom

969
00:39:39,670 --> 00:39:43,060
right is the cat, sprite one, so these scripts, this

970
00:39:43,060 --> 00:39:45,720
program, applies to him specifically.

971
00:39:45,720 --> 00:39:48,950
So if that cat is touching the mouse pointer, then it's going

972
00:39:48,950 --> 00:39:51,350
to play this down and wait two seconds, and

973
00:39:51,350 --> 00:39:53,030
then repeat ad nauseum.

974
00:39:53,030 --> 00:39:55,360
>> So let's go ahead and hit play.

975
00:39:55,360 --> 00:39:57,580
Nothing happens, but if I want to pet the

976
00:39:57,580 --> 00:39:59,310
cat now, I can simply.

977
00:39:59,310 --> 00:40:01,130
[MEOW]

978
00:40:01,130 --> 00:40:02,244
DAVID MALAN: Adorable.

979
00:40:02,244 --> 00:40:05,956
[MEOW]

980
00:40:05,956 --> 00:40:08,750
DAVID MALAN: Okay, less annoying, but also gets dull,

981
00:40:08,750 --> 00:40:11,480
so let's move on and see if we can't inject

982
00:40:11,480 --> 00:40:12,460
a little more logic.

983
00:40:12,460 --> 00:40:14,300
That was example seven.

984
00:40:14,300 --> 00:40:16,090
>> Here in example eight, we're going to

985
00:40:16,090 --> 00:40:18,670
introduce an ELSE condition.

986
00:40:18,670 --> 00:40:21,440
So much like a literal fork in the road, in which you can go

987
00:40:21,440 --> 00:40:24,260
left or you can go right, a condition in a programming

988
00:40:24,260 --> 00:40:28,120
language like Scratch, or as we'll see C, can allow you to

989
00:40:28,120 --> 00:40:32,230
go in one direction or another via an IF ELSE construct.

990
00:40:32,230 --> 00:40:35,160
So quite literally, IF touching mouse pointer, this

991
00:40:35,160 --> 00:40:38,680
will play some sound, ELSE it's going to play this other

992
00:40:38,680 --> 00:40:39,800
sound, meow.

993
00:40:39,800 --> 00:40:42,510
Now, if you can infer from the name of these sounds, you can

994
00:40:42,510 --> 00:40:45,310
probably guess what this program's meant to conjure up

995
00:40:45,310 --> 00:40:46,250
the idea of.

996
00:40:46,250 --> 00:40:48,742
This cat is meowing happily.

997
00:40:48,742 --> 00:40:49,230
[MEOW]

998
00:40:49,230 --> 00:40:51,410
DAVID MALAN: Happily, but doesn't

999
00:40:51,410 --> 00:40:52,926
quite like to be touched.

1000
00:40:52,926 --> 00:40:54,550
[ROARS]

1001
00:40:54,550 --> 00:40:57,500
DAVID MALAN: So now we have a cat who will yell at you.

1002
00:40:57,500 --> 00:41:00,570
>> All right, well, one last example with cats here, and

1003
00:41:00,570 --> 00:41:02,920
let's open version nine of this here.

1004
00:41:02,920 --> 00:41:06,190
So now, we have the next most annoying sound that I could

1005
00:41:06,190 --> 00:41:11,630
find, so we have a walrus or sea lion here who's going to

1006
00:41:11,630 --> 00:41:13,222
do the following.

1007
00:41:13,222 --> 00:41:20,870
[SEAL BARK]

1008
00:41:20,870 --> 00:41:23,310
DAVID MALAN: Okay, so this will go on until you figure out how

1009
00:41:23,310 --> 00:41:24,640
this program works.

1010
00:41:24,640 --> 00:41:28,560
So this time, this animal has two scripts, and what's

1011
00:41:28,560 --> 00:41:31,050
interesting here is that these scripts are going to execute

1012
00:41:31,050 --> 00:41:32,260
in parallel.

1013
00:41:32,260 --> 00:41:35,340
So because they both start with one green flag clicked,

1014
00:41:35,340 --> 00:41:38,080
it's like going like this, and both programs start running it

1015
00:41:38,080 --> 00:41:41,370
once even if they're looping forever.

1016
00:41:41,370 --> 00:41:44,420
So in the top script, I have some logic.

1017
00:41:44,420 --> 00:41:49,275
What features does that provide up there?

1018
00:41:49,275 --> 00:41:50,740
[SEAL BARK]

1019
00:41:50,740 --> 00:41:51,670
AUDIENCE: [INAUDIBLE]

1020
00:41:51,670 --> 00:41:53,295
DAVID MALAN: If it's what?

1021
00:41:53,295 --> 00:41:53,760
[SEAL BARK]

1022
00:41:53,760 --> 00:41:55,852
AUDIENCE: If muted to zero, it's going to keep playing the

1023
00:41:55,852 --> 00:41:58,410
sound. [SEAL BARK]

1024
00:41:58,410 --> 00:41:58,940
DAVID MALAN: Okay, good.

1025
00:41:58,940 --> 00:42:01,960
So IF muted, whatever this is, this orange thing is zero,

1026
00:42:01,960 --> 00:42:05,370
THEN play the sea lion sound and think "Oh,

1027
00:42:05,370 --> 00:42:06,770
hi," for two seconds.

1028
00:42:06,770 --> 00:42:09,380
Now, I don't know what muted is, but zero conjures up the

1029
00:42:09,380 --> 00:42:11,230
idea of false or off.

1030
00:42:11,230 --> 00:42:14,490
So if muted is false, so if not muted,

1031
00:42:14,490 --> 00:42:15,660
keep playing the sound.

1032
00:42:15,660 --> 00:42:17,350
All right, well, how do we disable this thing?

1033
00:42:17,350 --> 00:42:19,310
>> Well, let's look at the second script down there.

1034
00:42:19,310 --> 00:42:22,130
The second script says set muted to zero.

1035
00:42:22,130 --> 00:42:25,180
Notice it's also orange, so what Scratch does is it colors

1036
00:42:25,180 --> 00:42:27,400
blocks in the same shade if it's sort

1037
00:42:27,400 --> 00:42:28,850
of logically related.

1038
00:42:28,850 --> 00:42:32,850
So just as muted up top was orange, so is muted down here

1039
00:42:32,850 --> 00:42:34,060
mentioned in orange block.

1040
00:42:34,060 --> 00:42:36,170
But this is a variable assignments, so just like in

1041
00:42:36,170 --> 00:42:39,170
algebra, you have x and y and z, in programming you have

1042
00:42:39,170 --> 00:42:41,440
variables, but they're generally--

1043
00:42:41,440 --> 00:42:42,580
let's pause for a moment and figure how

1044
00:42:42,580 --> 00:42:44,280
to stop this barking.

1045
00:42:44,280 --> 00:42:45,480
How do I do this?

1046
00:42:45,480 --> 00:42:46,480
[SEAL BARK]

1047
00:42:46,480 --> 00:42:47,920
DAVID MALAN: Okay.

1048
00:42:47,920 --> 00:42:49,490
It stopped. Okay.

1049
00:42:49,490 --> 00:42:52,230
>> So just as in algebra you have variables x, y, and z, but in

1050
00:42:52,230 --> 00:42:55,260
programming, having variables like x, y, and z is generally

1051
00:42:55,260 --> 00:42:57,330
frowned upon because they're not at all expressive.

1052
00:42:57,330 --> 00:43:00,070
They have no semantic meaning whatsoever, so in most

1053
00:43:00,070 --> 00:43:02,110
programming languages, variables can have full

1054
00:43:02,110 --> 00:43:05,270
fledged names or words or phrases, like muted, to say

1055
00:43:05,270 --> 00:43:06,030
what they do.

1056
00:43:06,030 --> 00:43:09,380
So this second script also was listening forever, and it said

1057
00:43:09,380 --> 00:43:12,100
if the key, the keyboard key Space, is

1058
00:43:12,100 --> 00:43:13,370
pressed, question mark.

1059
00:43:13,370 --> 00:43:16,300
So there's a condition with a Boolean expression that's

1060
00:43:16,300 --> 00:43:19,010
going to answer a question either truthfully or false,

1061
00:43:19,010 --> 00:43:22,880
then I have inside of it IF the space key is pressed AND

1062
00:43:22,880 --> 00:43:28,020
IF muted is zero, set muted to one, ELSE set muted to zero.

1063
00:43:28,020 --> 00:43:30,510
So this other fork in the road, and notice how I've

1064
00:43:30,510 --> 00:43:33,500
nested the two IF conditions, is a way of checking is the

1065
00:43:33,500 --> 00:43:36,610
Space Bar pressed because if so, I either want to go this

1066
00:43:36,610 --> 00:43:38,000
way or that way.

1067
00:43:38,000 --> 00:43:40,210
And how do I invert the value of muted?

1068
00:43:40,210 --> 00:43:41,410
I have to check is it zero?

1069
00:43:41,410 --> 00:43:45,890
If so, make it one, else make it zero to therefore toggle

1070
00:43:45,890 --> 00:43:47,670
its two states.

1071
00:43:47,670 --> 00:43:50,030
>> All right, so we have then some of these fundamental

1072
00:43:50,030 --> 00:43:50,540
constructs.

1073
00:43:50,540 --> 00:43:52,810
We have Boolean expressions, and realize, too, these are

1074
00:43:52,810 --> 00:43:54,050
not all that unfamiliar.

1075
00:43:54,050 --> 00:43:56,730
In fact, here's a quick screenshot of Harvard course's CS50

1076
00:43:56,730 --> 00:43:59,740
shopping tool, and any website out there that has checkboxes

1077
00:43:59,740 --> 00:44:02,420
and drop downs have really, all this time, been using

1078
00:44:02,420 --> 00:44:03,430
Boolean expressions.

1079
00:44:03,430 --> 00:44:05,760
In this case here, if you click the checkbox next to

1080
00:44:05,760 --> 00:44:08,620
course greater than or equal to 4.5, or the same next to

1081
00:44:08,620 --> 00:44:11,200
faculty, you're specifying a Boolean expression.

1082
00:44:11,200 --> 00:44:14,780
Show me courses for which that expression is true.

1083
00:44:14,780 --> 00:44:17,080
Or to the right, doesn't conflict with courses I'm

1084
00:44:17,080 --> 00:44:20,440
taking, if that is checked, then yes, you want to check

1085
00:44:20,440 --> 00:44:22,890
that condition, else you want to ignore it.

1086
00:44:22,890 --> 00:44:25,170
So Boolean expressions are sort of all around, but when

1087
00:44:25,170 --> 00:44:28,080
we put them in conditions, whether IF conditions, IF

1088
00:44:28,080 --> 00:44:30,480
ELSEs, or we can even simulate deeper

1089
00:44:30,480 --> 00:44:32,830
levels, IF ELSE IF ELSE--

1090
00:44:32,830 --> 00:44:34,430
so that's sort of a ternary state.

1091
00:44:34,430 --> 00:44:36,920
You can go this way or this way or this way.

1092
00:44:36,920 --> 00:44:39,680
We can keep nesting things to go in different directions.

1093
00:44:39,680 --> 00:44:41,800
>> So Scratch has these loops, like Forever.

1094
00:44:41,800 --> 00:44:44,750
It has these features like Repeat 10, some

1095
00:44:44,750 --> 00:44:46,260
finite number of times.

1096
00:44:46,260 --> 00:44:49,040
We have the ability now to set variables, so in this case

1097
00:44:49,040 --> 00:44:51,810
I've declared, for instance, a variable called socks.

1098
00:44:51,810 --> 00:44:54,340
I've initialized to 0, and that's yet another direction

1099
00:44:54,340 --> 00:44:56,940
we could have taken up here with Joseph, whereby maybe I

1100
00:44:56,940 --> 00:45:00,150
just keep track of how many socks I have on and terminate

1101
00:45:00,150 --> 00:45:03,490
the program when that variable's value is 2.

1102
00:45:03,490 --> 00:45:05,660
That would be another way of sort of generalizing that

1103
00:45:05,660 --> 00:45:08,820
problem and doing something again and again.

1104
00:45:08,820 --> 00:45:12,080
Well, let's go ahead and now introduce a couple new things.

1105
00:45:12,080 --> 00:45:14,100
So those of you with prior programing experience will

1106
00:45:14,100 --> 00:45:17,120
know that a lot of languages have arrays, or vectors or

1107
00:45:17,120 --> 00:45:19,970
lists, and indeed, Scratch has something like this, too.

1108
00:45:19,970 --> 00:45:22,510
So let's see if we can't take things to the next level here.

1109
00:45:22,510 --> 00:45:24,920
If I have the ability, now, with these puzzle pieces to

1110
00:45:24,920 --> 00:45:29,080
add something, like a word or number, to a variable, I can

1111
00:45:29,080 --> 00:45:30,550
start to accumulate things.

1112
00:45:30,550 --> 00:45:33,630
And this is actually pretty apropos for things like games,

1113
00:45:33,630 --> 00:45:35,810
role-playing games where you're kind of walking around

1114
00:45:35,810 --> 00:45:38,600
some fantasy world collecting things, picking things up,

1115
00:45:38,600 --> 00:45:39,900
earning points, or the like.

1116
00:45:39,900 --> 00:45:42,310
You might want to keep track of some kind of inventory, and

1117
00:45:42,310 --> 00:45:45,180
indeed, that's what one of our former students here did with

1118
00:45:45,180 --> 00:45:47,750
something called Fruitcraft RPG.

1119
00:45:47,750 --> 00:45:51,820
>> So let me go ahead and open up this thing here, and in

1120
00:45:51,820 --> 00:45:56,260
Fruitcraft, we have this world up at top.

1121
00:45:56,260 --> 00:45:58,820
So let me go ahead and click the green flag.

1122
00:45:58,820 --> 00:46:01,040
Notice at top left is some kind of inventory.

1123
00:46:01,040 --> 00:46:04,110
That's implemented in Scratch as what we'll call an array or

1124
00:46:04,110 --> 00:46:06,890
a list, and now we have this little animation.

1125
00:46:06,890 --> 00:46:10,440
So just as we started earlier with this cookie love story,

1126
00:46:10,440 --> 00:46:13,320
and then we advanced to cats and sea lions, now we can have

1127
00:46:13,320 --> 00:46:15,100
things that are even more interactive.

1128
00:46:15,100 --> 00:46:18,250
And this little blue guy, I can start to move around his

1129
00:46:18,250 --> 00:46:19,500
little home here.

1130
00:46:19,500 --> 00:46:22,570
So it looks like he's got an exit down here, so I'm using

1131
00:46:22,570 --> 00:46:25,260
the arrows keys, up, down, left right.

1132
00:46:25,260 --> 00:46:28,370
And now I'm outside, so let's what I've got here.

1133
00:46:28,370 --> 00:46:30,920
Looks like an orange, and indeed, as soon as I touch the

1134
00:46:30,920 --> 00:46:33,110
orange, it gets plopped into my inventory.

1135
00:46:33,110 --> 00:46:35,640
If I go over here to the cherries, now I have something

1136
00:46:35,640 --> 00:46:36,760
else in my inventory.

1137
00:46:36,760 --> 00:46:39,100
And this is all nice and cute, but think about how, now, this

1138
00:46:39,100 --> 00:46:39,840
is implemented.

1139
00:46:39,840 --> 00:46:42,490
Well, we have this notion of a list, and that's apparently a

1140
00:46:42,490 --> 00:46:45,470
puzzle piece that you just say what you want add to it, add

1141
00:46:45,470 --> 00:46:46,950
orange, add cherries.

1142
00:46:46,950 --> 00:46:48,540
Now, what is this little blue guy doing?

1143
00:46:48,540 --> 00:46:49,810
Well, he's a sprite.

1144
00:46:49,810 --> 00:46:52,190
And presumably, the orange and the cherries--

1145
00:46:52,190 --> 00:46:54,140
they themselves were separate sprites.

1146
00:46:54,140 --> 00:46:56,980
And using conditions in Boolean expressions, the

1147
00:46:56,980 --> 00:47:00,530
student was probably able to express IF blue guy is

1148
00:47:00,530 --> 00:47:04,590
touching cherries THEN add the word cherries to his

1149
00:47:04,590 --> 00:47:07,890
inventory, and then also hide the cherry sprite.

1150
00:47:07,890 --> 00:47:10,600
So underneath the hood, there probably still is a cherry

1151
00:47:10,600 --> 00:47:11,350
sprite there.

1152
00:47:11,350 --> 00:47:14,160
We've just told it to become effectively invisible.

1153
00:47:14,160 --> 00:47:16,340
>> Now, if I keep walking over here, we can also do this

1154
00:47:16,340 --> 00:47:19,150
proximal thing, where I can go and read the sign.

1155
00:47:19,150 --> 00:47:23,170
So if blue guy touching sign, we can have this Say block

1156
00:47:23,170 --> 00:47:26,455
just like the cat spoke to us in words, hello, earlier, "Got

1157
00:47:26,455 --> 00:47:26,880
some fruit?

1158
00:47:26,880 --> 00:47:29,290
Bring it to the fruit place." All right, so now, apparently,

1159
00:47:29,290 --> 00:47:30,550
I have my directions.

1160
00:47:30,550 --> 00:47:33,050
I can go over here to the fruit place, line myself up

1161
00:47:33,050 --> 00:47:33,740
with the door.

1162
00:47:33,740 --> 00:47:35,390
Now, I'm in here.

1163
00:47:35,390 --> 00:47:38,120
I can go up to the man at the counter.

1164
00:47:38,120 --> 00:47:40,330
He detects that I'm close to him, so it doesn't have to be

1165
00:47:40,330 --> 00:47:46,780
quite identically touching, and I have won the game.

1166
00:47:46,780 --> 00:47:49,890
So there we have Fruitcraft RPG.

1167
00:47:49,890 --> 00:47:52,200
>> So we can do things even more advanced than this.

1168
00:47:52,200 --> 00:47:53,200
We can add sounds.

1169
00:47:53,200 --> 00:47:54,610
We can add pseudo randomness.

1170
00:47:54,610 --> 00:47:56,260
We can add complexity.

1171
00:47:56,260 --> 00:47:58,490
Let me go ahead here, and rather than do this myself,

1172
00:47:58,490 --> 00:48:01,240
let me show you one of the more sophisticated submissions

1173
00:48:01,240 --> 00:48:06,640
we got last year from a certain someone named Blake.

1174
00:48:06,640 --> 00:48:08,910
Can we have one volunteer who is comfortable appearing on

1175
00:48:08,910 --> 00:48:12,600
camera and is up for playing a game?

1176
00:48:12,600 --> 00:48:13,450
How about right there?

1177
00:48:13,450 --> 00:48:15,380
Come on up.

1178
00:48:15,380 --> 00:48:19,680
All right, so the game that you have just unknowingly

1179
00:48:19,680 --> 00:48:20,780
volunteered to play--

1180
00:48:20,780 --> 00:48:25,910
[APPLAUSE]

1181
00:48:25,910 --> 00:48:28,610
DAVID MALAN: --is something from yesteryear called

1182
00:48:28,610 --> 00:48:30,100
"Frogger." What is your name?

1183
00:48:30,100 --> 00:48:30,730
RENDA: Renda.

1184
00:48:30,730 --> 00:48:31,050
DAVID MALAN: Redna?

1185
00:48:31,050 --> 00:48:32,530
RENDA: It's like Brenda with a b.

1186
00:48:32,530 --> 00:48:33,190
DAVID MALAN: Okay, Renda.

1187
00:48:33,190 --> 00:48:33,590
David.

1188
00:48:33,590 --> 00:48:34,580
Nice to meet you.

1189
00:48:34,580 --> 00:48:38,690
>> So here in "Frogger," and if we could raise the volume just

1190
00:48:38,690 --> 00:48:39,780
a little bit, you are this little

1191
00:48:39,780 --> 00:48:41,010
green frog on the bottom.

1192
00:48:41,010 --> 00:48:43,940
You can use left, right, up and down, and your goal is to

1193
00:48:43,940 --> 00:48:46,510
cross the street, cross the river, and touch the lily pads

1194
00:48:46,510 --> 00:48:47,760
at the top.

1195
00:48:52,090 --> 00:48:53,440
Aw.

1196
00:48:53,440 --> 00:48:54,630
One more t--

1197
00:48:54,630 --> 00:48:56,400
redo, all right?

1198
00:48:56,400 --> 00:48:58,890
Let's hit stop.

1199
00:48:58,890 --> 00:49:00,140
No one saw that.

1200
00:49:10,470 --> 00:49:12,930
[APPLAUSE]

1201
00:49:17,023 --> 00:49:17,850
[APPLAUSE]

1202
00:49:17,850 --> 00:49:19,341
DAVID MALAN: Yeah.

1203
00:49:19,341 --> 00:49:20,832
Very well done.

1204
00:49:20,832 --> 00:49:22,082
Excellent, thank you.

1205
00:49:27,300 --> 00:49:29,430
>> So that there was Frogger. Now that you know what you've

1206
00:49:29,430 --> 00:49:32,170
gotten yourselves into, one more volunteer for a different

1207
00:49:32,170 --> 00:49:34,910
game submitted by another student.

1208
00:49:34,910 --> 00:49:35,560
You want to come on up?

1209
00:49:35,560 --> 00:49:36,576
What's your name?

1210
00:49:36,576 --> 00:49:37,000
RICHARD: Richard.

1211
00:49:37,000 --> 00:49:37,420
DAVID MALAN: Richard.

1212
00:49:37,420 --> 00:49:39,732
All right, Richard, come on up.

1213
00:49:39,732 --> 00:49:42,187
[APPLAUSE]

1214
00:49:45,140 --> 00:49:47,030
DAVID MALAN: You have something that'll sound

1215
00:49:47,030 --> 00:49:54,180
familiar soon, so here are your instructions.

1216
00:49:54,180 --> 00:49:56,580
So in a moment, some puzzle pieces are going to scroll up

1217
00:49:56,580 --> 00:49:58,150
from the screen that look either left

1218
00:49:58,150 --> 00:49:59,450
or right up or down.

1219
00:49:59,450 --> 00:50:01,230
You're going to have to hit the arrow keys in such a way

1220
00:50:01,230 --> 00:50:04,540
that it corresponds to those puzzle pieces lining up with

1221
00:50:04,540 --> 00:50:05,720
the placeholders at top.

1222
00:50:05,720 --> 00:50:07,500
So when you see a left arrow, and it lines up with the left

1223
00:50:07,500 --> 00:50:09,260
arrow, hit the left arrow.

1224
00:50:09,260 --> 00:50:10,866
You may begin.

1225
00:50:10,866 --> 00:50:13,674
[MUSIC - "STRONGER" KAYNE WEST]

1226
00:51:21,222 --> 00:51:23,470
DAVID MALAN: All right, big round of applause for Richard.

1227
00:51:23,470 --> 00:51:26,082
[APPLAUSE]

1228
00:51:32,426 --> 00:51:33,720
DAVID MALAN: Very well done.

1229
00:51:33,720 --> 00:51:34,110
Thank you.

1230
00:51:34,110 --> 00:51:37,104
[APPLAUSE]

1231
00:51:41,600 --> 00:51:44,330
>> DAVID MALAN: So fun and, seemingly, sort of

1232
00:51:44,330 --> 00:51:46,590
intimidating as it might be to implement something as

1233
00:51:46,590 --> 00:51:49,550
seemingly sophisticated as this, realize that the student

1234
00:51:49,550 --> 00:51:51,450
didn't set out and just write this all at once.

1235
00:51:51,450 --> 00:51:54,200
Rather, you can break down a problem as seemingly complex

1236
00:51:54,200 --> 00:51:56,840
as this into much smaller pieces, and this, too, is

1237
00:51:56,840 --> 00:51:57,560
going to be a theme.

1238
00:51:57,560 --> 00:52:00,490
The worst thing you can do in writing a program in most any

1239
00:52:00,490 --> 00:52:03,480
language is to sit down, get really excited, write the

1240
00:52:03,480 --> 00:52:07,200
whole damn thing, and then just hope that it works by the

1241
00:52:07,200 --> 00:52:08,510
time you're finished writing.

1242
00:52:08,510 --> 00:52:10,920
Rather, the process of programming should generally

1243
00:52:10,920 --> 00:52:13,440
be very deliberate, very iterative, whereby you just

1244
00:52:13,440 --> 00:52:16,955
set very small steps for yourselves, bite-size pieces

1245
00:52:16,955 --> 00:52:20,070
do you want to bite off, and so that you have these sanity

1246
00:52:20,070 --> 00:52:21,740
checks, little milestones you can meet.

1247
00:52:21,740 --> 00:52:24,170
And then you build on top of those to create more

1248
00:52:24,170 --> 00:52:25,400
sophisticated things still.

1249
00:52:25,400 --> 00:52:26,820
>> So for instance, how could we go about

1250
00:52:26,820 --> 00:52:28,380
implementing a game like this?

1251
00:52:28,380 --> 00:52:30,160
Well, frankly, I would certainly start by just

1252
00:52:30,160 --> 00:52:32,260
supporting one key at a time.

1253
00:52:32,260 --> 00:52:34,840
Let's just implement support for the left arrow.

1254
00:52:34,840 --> 00:52:37,610
So the student had to somehow create in Photoshop, or in

1255
00:52:37,610 --> 00:52:40,340
Scratch itself using the little graphical editor, an

1256
00:52:40,340 --> 00:52:42,680
arrow key that looks like the one at top left there, just

1257
00:52:42,680 --> 00:52:43,910
the gray placeholder.

1258
00:52:43,910 --> 00:52:46,610
Then the student had to figure out what x, y coordinate to

1259
00:52:46,610 --> 00:52:48,380
put it, where to put it in the window.

1260
00:52:48,380 --> 00:52:50,730
0, 0 is up here, so you have to figure out the number of

1261
00:52:50,730 --> 00:52:53,690
pixels, or dots, to offset that arrow from

1262
00:52:53,690 --> 00:52:54,800
the top of the screen.

1263
00:52:54,800 --> 00:52:57,040
And then once that's in place, your program doesn't actually

1264
00:52:57,040 --> 00:53:00,380
do anything yet, so you then need a second sprite, for

1265
00:53:00,380 --> 00:53:04,470
instance a green arrow that's also pointing left, and you

1266
00:53:04,470 --> 00:53:06,710
then need to start writing some scripts for it.

1267
00:53:06,710 --> 00:53:09,310
>> And you notice, perhaps, that these things started coming at

1268
00:53:09,310 --> 00:53:11,470
different speeds, and the colors were in different

1269
00:53:11,470 --> 00:53:13,750
locations, and that's because the student used a bit of

1270
00:53:13,750 --> 00:53:14,990
pseudo randomness.

1271
00:53:14,990 --> 00:53:17,440
And by pseudo randomness, I just mean pick a number

1272
00:53:17,440 --> 00:53:20,230
between something and something because you can

1273
00:53:20,230 --> 00:53:22,890
start to map things, like if the number is between one and

1274
00:53:22,890 --> 00:53:24,940
five, well, let's make the thing green.

1275
00:53:24,940 --> 00:53:28,290
If it's between 6 and 10, let's make the puzzle piece

1276
00:53:28,290 --> 00:53:29,730
red instead.

1277
00:53:29,730 --> 00:53:31,840
So long as you have a way of generating some kind of

1278
00:53:31,840 --> 00:53:34,730
randomness, you can then make decisions based on that

1279
00:53:34,730 --> 00:53:35,260
randomness.

1280
00:53:35,260 --> 00:53:37,490
And I keep saying pseudo random because there's a

1281
00:53:37,490 --> 00:53:38,390
little dirty secret.

1282
00:53:38,390 --> 00:53:41,130
Computers can't come up with random numbers.

1283
00:53:41,130 --> 00:53:42,800
They can only do what they're told because

1284
00:53:42,800 --> 00:53:44,110
they're man-made devices.

1285
00:53:44,110 --> 00:53:45,880
They can't just guess a number like we humans

1286
00:53:45,880 --> 00:53:46,960
feel like we can.

1287
00:53:46,960 --> 00:53:50,970
A computer has to do something mathematical to conjure up the

1288
00:53:50,970 --> 00:53:53,970
illusion of mathematical number, sometimes using the

1289
00:53:53,970 --> 00:53:57,420
current day of time as an input to figuring out what

1290
00:53:57,420 --> 00:53:59,570
number to return, but more on that another time.

1291
00:53:59,570 --> 00:54:02,060
For now, just know that we can generate pseudo randomness.

1292
00:54:02,060 --> 00:54:04,610
>> So once I have the ability for the left arrow to start

1293
00:54:04,610 --> 00:54:07,990
appearing at different times and at different speeds, then

1294
00:54:07,990 --> 00:54:10,230
I can go back and add some of these IF conditions.

1295
00:54:10,230 --> 00:54:14,950
IF this sprite is touching the other one, AND the left arrow

1296
00:54:14,950 --> 00:54:16,330
key has been pressed--

1297
00:54:16,330 --> 00:54:17,900
so three conditions in that case.

1298
00:54:17,900 --> 00:54:20,750
I can use that AND block perhaps, in that case--

1299
00:54:20,750 --> 00:54:23,210
THEN I want to go ahead and increment the score.

1300
00:54:23,210 --> 00:54:25,130
And at top, we have a score, we have Awesome,

1301
00:54:25,130 --> 00:54:26,510
Cool, Good, and Boo.

1302
00:54:26,510 --> 00:54:29,810
So there's apparently five variables that this student

1303
00:54:29,810 --> 00:54:32,220
used to keep track of these various metrics.

1304
00:54:32,220 --> 00:54:34,180
So in short, the end result is amazing.

1305
00:54:34,180 --> 00:54:37,320
It's fun, it's fun to play, it's engaging, but this is not

1306
00:54:37,320 --> 00:54:38,380
where the student began.

1307
00:54:38,380 --> 00:54:42,210
He or she started at a much smaller set of steps.

1308
00:54:42,210 --> 00:54:44,470
>> So what are some other building blocks that we can

1309
00:54:44,470 --> 00:54:45,760
weave into these programs?

1310
00:54:45,760 --> 00:54:48,750
Well, there's this other concept in most languages,

1311
00:54:48,750 --> 00:54:50,730
Scratch among them, known as threads.

1312
00:54:50,730 --> 00:54:54,660
So a computer can actually not really do multiple things at a

1313
00:54:54,660 --> 00:54:56,730
time, at least not usually.

1314
00:54:56,730 --> 00:54:59,930
Rather, a computer generally has just one CPU, and even

1315
00:54:59,930 --> 00:55:02,230
though computers are super fast and can, therefore,

1316
00:55:02,230 --> 00:55:04,740
create the illusion of doing multiple things at once--

1317
00:55:04,740 --> 00:55:06,180
checking your mail, getting an instant

1318
00:55:06,180 --> 00:55:07,770
message, printing a document--

1319
00:55:07,770 --> 00:55:11,530
really, a computer is just jumping from printing to IMing

1320
00:55:11,530 --> 00:55:15,160
to emailing, back and back and back and forth so fast that we

1321
00:55:15,160 --> 00:55:18,170
slow witted humans just don't realize that it's actually

1322
00:55:18,170 --> 00:55:21,240
running those programs a little bit at a time.

1323
00:55:21,240 --> 00:55:23,960
Now, this a bit of a white lie these days because, nowadays,

1324
00:55:23,960 --> 00:55:26,740
many our computers are what are called multi-core, so you

1325
00:55:26,740 --> 00:55:29,760
have one CPU but multiple cores, which is kind of like

1326
00:55:29,760 --> 00:55:31,760
having multiple CPUs.

1327
00:55:31,760 --> 00:55:35,020
>> And so sometimes, computers can truly do multiple things

1328
00:55:35,020 --> 00:55:38,500
at a time, but generally within a program, programs

1329
00:55:38,500 --> 00:55:40,200
rely on these things called threads.

1330
00:55:40,200 --> 00:55:43,160
So a thread is sort of like a miniature program that can

1331
00:55:43,160 --> 00:55:46,540
exist alongside another miniature program and can run

1332
00:55:46,540 --> 00:55:51,250
in parallel, or at least can run under the illusion that

1333
00:55:51,250 --> 00:55:52,990
they're running simultaneously.

1334
00:55:52,990 --> 00:55:55,060
So Scratch supports these things called threads.

1335
00:55:55,060 --> 00:55:57,580
You can have multiple scripts executing at once, just as we

1336
00:55:57,580 --> 00:56:00,220
did with the sea lion, and this allows us to actually

1337
00:56:00,220 --> 00:56:03,620
then have interactions among these sprites.

1338
00:56:03,620 --> 00:56:09,910
Let me go ahead here and pull up, let's say, threads, and

1339
00:56:09,910 --> 00:56:11,870
play this as follows.

1340
00:56:11,870 --> 00:56:13,950
We have two sprites, each of whom we'll see

1341
00:56:13,950 --> 00:56:15,720
has just one script.

1342
00:56:15,720 --> 00:56:18,000
And you notice there seems to be some intelligence in the

1343
00:56:18,000 --> 00:56:21,335
cat in this one because he's getting closer--

1344
00:56:21,335 --> 00:56:21,730
[ROARS]

1345
00:56:21,730 --> 00:56:24,540
DAVID MALAN: --and closer to the little bird.

1346
00:56:24,540 --> 00:56:26,570
>> So how is the bird operating?

1347
00:56:26,570 --> 00:56:28,200
Well, let's take a look at the bird first.

1348
00:56:28,200 --> 00:56:31,220
The bird script said when the green flag is clicked, go to x

1349
00:56:31,220 --> 00:56:34,020
equals negative 115 and y equals 150.

1350
00:56:34,020 --> 00:56:37,010
So I just figured out a random location where I wanted the

1351
00:56:37,010 --> 00:56:39,580
bird to start, and I just plopped him there by default.

1352
00:56:39,580 --> 00:56:42,800
Then Forever IF not touching cat, so this is a different

1353
00:56:42,800 --> 00:56:45,080
kind of looping construct, but same idea, do this again and

1354
00:56:45,080 --> 00:56:45,780
again and again.

1355
00:56:45,780 --> 00:56:49,410
So long as you're not touching the cat, move three steps, and

1356
00:56:49,410 --> 00:56:51,680
if you're on the edge, bounce, where it's sort of a

1357
00:56:51,680 --> 00:56:53,320
reflection in the billiard sense.

1358
00:56:53,320 --> 00:56:56,140
So that's how the bird is moving around kind of

1359
00:56:56,140 --> 00:56:58,890
seemingly randomly, but it's just because it's bouncing off

1360
00:56:58,890 --> 00:57:00,170
the walls in this case.

1361
00:57:00,170 --> 00:57:03,170
>> Now the cat, meanwhile, is kind of cheating.

1362
00:57:03,170 --> 00:57:05,520
The cat, when the green flag is clicked, yes, starts in

1363
00:57:05,520 --> 00:57:07,390
some location, a random location--

1364
00:57:07,390 --> 00:57:09,590
at least in part as per the pick

1365
00:57:09,590 --> 00:57:11,430
random green block there--

1366
00:57:11,430 --> 00:57:14,660
and then Forever IF touching the bird, play the lion sound,

1367
00:57:14,660 --> 00:57:15,790
and then stop script.

1368
00:57:15,790 --> 00:57:18,560
So when I said terminate, or exit before, there's a puzzle

1369
00:57:18,560 --> 00:57:20,600
piece in Scratch that will just kill the program at that

1370
00:57:20,600 --> 00:57:22,900
point because it's kind of logically done.

1371
00:57:22,900 --> 00:57:25,030
But otherwise, here, notice what's going to happen.

1372
00:57:25,030 --> 00:57:28,470
Point toward the bird and move one step.

1373
00:57:28,470 --> 00:57:31,390
So this point toward bird is kind of an advantage the cat

1374
00:57:31,390 --> 00:57:34,640
has in that it's homing in on the moving bird, and we can

1375
00:57:34,640 --> 00:57:37,370
now make this program all the more interesting.

1376
00:57:37,370 --> 00:57:41,520
Instead of moving one step at a time per CPU cycle, per

1377
00:57:41,520 --> 00:57:44,280
strike toll of the bell, so to speak, let me go ahead and

1378
00:57:44,280 --> 00:57:47,350
move, let's say, five steps at a time as the cat.

1379
00:57:47,350 --> 00:57:51,120
Click run, and now he really finds him quickly.

1380
00:57:51,120 --> 00:57:55,620
If we double this further to 10 steps, it kind of goes

1381
00:57:55,620 --> 00:57:56,370
right for him.

1382
00:57:56,370 --> 00:57:57,750
Now, we can give the bird,

1383
00:57:57,750 --> 00:57:59,540
perhaps, a bit of an advantage.

1384
00:57:59,540 --> 00:58:02,370
Let's go to the bird and say instead of moving three steps,

1385
00:58:02,370 --> 00:58:07,140
let's move him 30 steps.

1386
00:58:07,140 --> 00:58:08,700
But he still got caught in the end.

1387
00:58:08,700 --> 00:58:10,430
>> So here, we have two threads.

1388
00:58:10,430 --> 00:58:13,230
It's incarcerated in Scratch with two scripts and two

1389
00:58:13,230 --> 00:58:16,150
sprites, but the idea in other languages is that you can

1390
00:58:16,150 --> 00:58:19,220
write, essentially, too many programs like this and have

1391
00:58:19,220 --> 00:58:22,950
them run truly, or imaginarily, in parallel.

1392
00:58:22,950 --> 00:58:25,280
>> Now, there's also this concept in programming known as

1393
00:58:25,280 --> 00:58:28,010
events, and this is something we won't see in C, the

1394
00:58:28,010 --> 00:58:30,500
language known as C. We will see it towards semester's end

1395
00:58:30,500 --> 00:58:33,240
in web programming, when we introduce JavaScript and the

1396
00:58:33,240 --> 00:58:36,970
notion of building web pages that are dynamic and interact

1397
00:58:36,970 --> 00:58:37,710
with users.

1398
00:58:37,710 --> 00:58:40,430
So in this case, we have a very simple example of two

1399
00:58:40,430 --> 00:58:43,860
sprites, boy and girl, each of whom have their respective

1400
00:58:43,860 --> 00:58:48,040
threads, but somehow these two are inter-communicating by way

1401
00:58:48,040 --> 00:58:49,490
of something called events.

1402
00:58:49,490 --> 00:58:52,750
So let me go ahead here and zoom in on the boy's script,

1403
00:58:52,750 --> 00:58:54,050
which looks like this.

1404
00:58:54,050 --> 00:58:57,510
When green flag clicked, forever do the following.

1405
00:58:57,510 --> 00:59:01,510
If the key Space, or the Space Bar, is pressed, say

1406
00:59:01,510 --> 00:59:03,900
Marco for two seconds, that's purely aesthetic on the

1407
00:59:03,900 --> 00:59:06,860
screen, a little speech bubble, but then broadcast.

1408
00:59:06,860 --> 00:59:09,240
>> So broadcast is another Scratch piece that's

1409
00:59:09,240 --> 00:59:12,050
representative of a class of functionality in programming

1410
00:59:12,050 --> 00:59:15,080
that allows different programs, different threads,

1411
00:59:15,080 --> 00:59:17,120
to inter-communicate, to somehow send

1412
00:59:17,120 --> 00:59:18,520
messages, one to another.

1413
00:59:18,520 --> 00:59:21,930
Passing a piece of paper in class is sort of the low tech

1414
00:59:21,930 --> 00:59:22,700
equivalent.

1415
00:59:22,700 --> 00:59:24,300
So broadcast event.

1416
00:59:24,300 --> 00:59:26,210
I can send this message, and the word event

1417
00:59:26,210 --> 00:59:27,270
is completely arbitrary.

1418
00:59:27,270 --> 00:59:30,000
Scratch sometimes has these drop downs, so I just came up

1419
00:59:30,000 --> 00:59:33,230
with a random word like event because now, what the boy does

1420
00:59:33,230 --> 00:59:36,600
when I press the key is he broadcasts this event.

1421
00:59:36,600 --> 00:59:39,270
And if I look now at the girl's script, her script is

1422
00:59:39,270 --> 00:59:42,690
super simple because all she needs to do is not act when

1423
00:59:42,690 --> 00:59:44,100
the green flag is clicked.

1424
00:59:44,100 --> 00:59:47,910
She is designed to action when she receives quote, unquote,

1425
00:59:47,910 --> 00:59:51,210
"event", and at that point, she's listening therefore for

1426
00:59:51,210 --> 00:59:52,960
the so-called event again and again.

1427
00:59:52,960 --> 00:59:55,790
As soon as she receives that event, she's going to shout

1428
00:59:55,790 --> 00:59:57,680
Polo for two seconds.

1429
00:59:57,680 --> 01:00:01,090
>> And so you can perhaps infer from this exactly what the

1430
01:00:01,090 --> 01:00:03,190
next result is going to be.

1431
01:00:03,190 --> 01:00:04,910
Let me click the green flag.

1432
01:00:04,910 --> 01:00:06,610
Nothing happens because I need to do what?

1433
01:00:06,610 --> 01:00:07,310
AUDIENCE: Space Bar.

1434
01:00:07,310 --> 01:00:08,630
DAVID MALAN: Space Bar.

1435
01:00:08,630 --> 01:00:11,990
Boy says Marco, girl says Polo.

1436
01:00:11,990 --> 01:00:13,670
But that's not hard coded per se.

1437
01:00:13,670 --> 01:00:16,740
That's inter-communication between scripts, so now we

1438
01:00:16,740 --> 01:00:19,240
have the ability to make even more complex programs where

1439
01:00:19,240 --> 01:00:22,830
these two are somehow inter-communicating.

1440
01:00:22,830 --> 01:00:25,260
>> So in what directions can we take this?

1441
01:00:25,260 --> 01:00:28,260
Well, in problem set 0, really, the objective is to

1442
01:00:28,260 --> 01:00:29,830
have fun with Scratch.

1443
01:00:29,830 --> 01:00:32,440
For the hacker edition, you'll instead have fun with a more

1444
01:00:32,440 --> 01:00:35,850
sophisticated version of Scratch called BYOB, Build

1445
01:00:35,850 --> 01:00:38,040
Your Own Blocks, but the idea is the same.

1446
01:00:38,040 --> 01:00:38,780
You'll be able--

1447
01:00:38,780 --> 01:00:39,710
Yes, that was deliberate.

1448
01:00:39,710 --> 01:00:42,450
It came from Berkeley.

1449
01:00:42,450 --> 01:00:45,200
BYOB is the hacker edition version of this, but both

1450
01:00:45,200 --> 01:00:48,180
demographics, standard edition and hacker edition alike, the

1451
01:00:48,180 --> 01:00:50,940
goal at hand for the coming week is really just to dive in

1452
01:00:50,940 --> 01:00:53,050
deep, get your hands dirty with programming, and make

1453
01:00:53,050 --> 01:00:55,240
something interesting, make something interactive, make

1454
01:00:55,240 --> 01:00:58,020
something artistic, make something fun to actually

1455
01:00:58,020 --> 01:01:01,020
demonstrate, so that by week's end, you'll have a project,

1456
01:01:01,020 --> 01:01:03,770
not only for your first CS50 pset, but you'll have a

1457
01:01:03,770 --> 01:01:05,620
little something that you can show off in Annenberg to

1458
01:01:05,620 --> 01:01:08,830
friends or even family by uploading it to MIT's website.

1459
01:01:08,830 --> 01:01:11,760
And so as I said on Wednesday, we expect 90% of the class,

1460
01:01:11,760 --> 01:01:13,970
generally, to do the standard editions.

1461
01:01:13,970 --> 01:01:16,660
Realize that there's also this outlet for those of you who

1462
01:01:16,660 --> 01:01:18,990
might otherwise find yourself a bit bored with the basics

1463
01:01:18,990 --> 01:01:21,650
and really want to dive in and craft a vision you already

1464
01:01:21,650 --> 01:01:25,380
have with prior background in this more friendly

1465
01:01:25,380 --> 01:01:26,240
environment.

1466
01:01:26,240 --> 01:01:29,030
>> So let me pull up one other example that one of our former

1467
01:01:29,030 --> 01:01:33,830
students here did and tell a little something through song.

1468
01:01:33,830 --> 01:01:37,430
That similarly, as this plays, think about how you go about

1469
01:01:37,430 --> 01:01:40,520
implementing this program using precisely these same

1470
01:01:40,520 --> 01:01:44,180
building blocks, a little bit of pseudo randomness, and a

1471
01:01:44,180 --> 01:01:46,010
bit of familiar song.

1472
01:01:46,010 --> 01:01:47,910
If we could raise the volume just a little bit?

1473
01:01:47,910 --> 01:01:49,160
[MUSIC - "IT'S RAINING MEN" THE WEATHER GIRLS]

1474
01:03:02,261 --> 01:03:03,930
>> DAVID MALAN: That's it for CS50.

1475
01:03:03,930 --> 01:03:05,860
We will see you on Monday.

1476
01:03:05,860 --> 01:03:21,152
[APPLAUSE]