Placeholder Image

字幕列表 影片播放

  • >> [MUSIC PLAYING]

  • >> -Alright!

  • >> -This is CS50.

  • >> -This is CS50.

  • >> -This is CS50.

  • [MUSIC -- IGGY AZALEA, "FANCY"]

  • >> -My favorite memory from CS50 was when I went to Puzzle Day.

  • >> -Probably just the time spent working on problem sets

  • with my friends and people who wold eventually become my blockmates.

  • >> -My best memory from CS50 is the Hackathon.

  • -The CS50 Hackathon.

  • >> -Hackathon.

  • >> -Hackathon.

  • -Hackathon.

  • -The Hackathon

  • -Rob Bowden.

  • Just everything about him.

  • >> [MUSIC -- IGGY AZALEA, "FANCY"]

  • >> -My favorite memory is when I was on stage and I played the prestigious role

  • of a node [? in the Linked ?] [? List. ?]

  • >> -When we all got free Dropbox space and David was like, look under your seats.

  • And it was like, space for everyone!

  • >> [MUSIC -- IGGY AZALEA, "FANCY"]

  • >> -My advice for any incoming student would

  • be to really work on P-sets with friends.

  • >> -Office hours is very much your friend.

  • >> -Make the most of your experience and meet as many people as you can.

  • >> -Don't be afraid to ask for help.

  • -Start the P-sets early in the week.

  • -I think the biggest thing is to take advantage of all the resources

  • that CS50 has.

  • >> -Go to office hours early in the week.

  • >> -Definitely watch the shorts.

  • >> -Don't procrastinate on your P-sets.

  • -Make sure you find a great group of people to work on P-sets with.

  • You can have a lot of fun and get work done together.

  • >> -Don't be afraid to push yourself.

  • Go for the hacker edition sometimes.

  • >> -Write things on paper before you ever touch your computer.

  • >> -CS50 is really great at providing ways to get help.

  • >> -My one piece of advice is sleep.

  • Has anybody said that?

  • Sleep, for sure.

  • It's easy not to do, but you've got to do it, I think.

  • >> -I would say really be mentally prepared because you're going to love it.

  • [MUSIC -- IGGY AZALEA, "FANCY"]

  • >> -This is CS50.

  • >> -This is CS50.

  • >> -This is CS50.

  • [MUSIC -- IGGY AZALEA, "FANCY"]

  • >> -This is CS50.

  • >> [APPLAUSE]

  • >> DAVID J. MALAN: So this is CS50 and this is the end of Week 0.

  • And that was just some of CS50's staff who

  • await you not only in sections and office hours, but,

  • also, this coming weekend at CS50 Puzzle Day.

  • Which, again, is not all about programming.

  • Indeed, it's expected that you won't have to program anything,

  • but rather solve problems using wits and friends alongside you.

  • >> We will be joined by some of our friends at Facebook--

  • if you register here-- who for the past several years,

  • have actually been writing these challenges with us.

  • And so, they will be the ones ultimately running Puzzle Day.

  • And so, you will be challenged with precisely the kinds of things

  • and problems that folks at Facebook like to think about.

  • So that is tomorrow.

  • Register at cs50.harvard.edu/register.

  • >> Now a word on a couple of staff in particular.

  • This here is Ansel Duff, who is actually one

  • of the co-authors of these binary bulbs that we saw on Wednesday,

  • in addition to CS50's own Dan Bradley.

  • Ansel Duff was also a former freshman advisee of mine 3 years ago

  • and he actually even built this lectern.

  • He's gone on to do engineering sciences and more.

  • Now, his picture here is actually Ansel 3 years ago at the CS50 Hackathon

  • when he borrowed one of our balloons, stuck it to his laptop,

  • and, for the next 12 some odd hours, focused on his final project,

  • taking breaks only to open bags of candy at the Hackathon.

  • >> But he went on more recently to spend this past summer with us,

  • since CS50 for its staff, and now students this semester,

  • has its own 3D printer.

  • And in a nutshell a 3D printers is a device that looks quite like this.

  • You fill it with a plastic spool that is melted down by the device

  • and you build things literally from nothing.

  • Much like an inkjet printer, you start spitting out little dots of plastic

  • that form together to form whole objects.

  • And so Ansel for instance, earlier this summer, has an iPhone 5

  • and decided he really wanted to prop it up on his desk.

  • But he didn't want to go out and buy something

  • from the Apple store or the like, so he sat down and started drawing something.

  • He took a few measurements as to how thick

  • and how wide his iPhone was, he drew this image here,

  • he decided that he wanted to have a 75 degree tilt

  • as it was staring at him on his desk there.

  • He then turned this, using software, into a 3D CAD model

  • that looked a little something like this.

  • And then he proceeded, ultimately, to actually create it.

  • So in fact, if any of you here, perhaps in a row that I can throw to, have

  • an-- there we have folks with iPhone 5 , and here we have two more.

  • >> Now, not to be outdone, CS50's own Cheng Gong also set out this summer

  • to build quite a few things and, in fact, for reasons that are still

  • unclear, has been slowly printing an army of elephants

  • with articulating arms and trunks.

  • A couple of which are actually here if anyone would now like-- an elephant.

  • All right, . but what Cheng also did for us is he very kindly set up a camera

  • because that elephant, believe it or not,

  • takes some two and a half hours to print.

  • Even the iPhone stand took an hour and a half to print.

  • And what Cheng went ahead and did was set up a nice camera in front

  • of this 3D printer, filmed for an hour and a half as Ansel's design printed.

  • We overlaid some sexy music to it in order

  • to give you this to look at how 3D printing works.

  • And even though this is actually in plastic,

  • realize that if this is an area of interest to you academically,

  • there are folks, among them Jennifer Lewis here

  • at the School of Engineering, who are actually

  • working on 3D printing of plastic objects.

  • But even, increasingly, biological materials to solve

  • physiological problems for humans.

  • But here is a little something from CS50.

  • >> [LOUD MECHANICAL NOISES]

  • DAVID J. MALAN: It doesn't sound anything like that in reality,

  • but it's much cooler to watch it at that speed, and with that sound.

  • >> Now, on Wednesday, how did we first get here?

  • We started talking about computer science and we asked what it was.

  • And it's about a number of things, and there's so many different directions

  • in which you can head after a course like CS50.

  • In fact, if you picked up one of those unofficial guides

  • to CS outside, the booklet that we've provided, whether you're

  • thinking of taking just CS50, or maybe doing a secondary,

  • or maybe even concentrating in CS, do flip through that.

  • And you'll see a diagram toward the end that

  • shows you the many different directions in CS that you can go off in.

  • >> But for today, we'll focus, again, on really one of the fundamental views,

  • perhaps, where you have inputs to problems,

  • you have outputs from problems, and you have

  • algorithms with which to create those outputs from those inputs.

  • And one such example, was of course, this phone book here.

  • And we used as an example to go through an algorithm that was correct.

  • And then another one was correct, but a little faster.

  • And then another one that was a little more dramatic, but fundamentally

  • faster.

  • >> Right, this phone book we claimed had about 1,000 pages.

  • And how many times did I have to tear the phone book in half

  • to find someone like Mike Smith, maximally, in 1,000 page book?

  • So, 10 give or take.

  • And so once I tore this thing in half, or simply, more maturely,

  • divided in half, it's only 10 pages out of 1,000.

  • And if you extrapolate, a little unrealistically for a phone book,

  • but if this phone book had some 4 billion pages in it, so completely

  • unwieldy physically, how many times do you divide a 4 billion

  • page phone book in half?

  • So it's actually 32, give or take.

  • And so 32 times only, out of 4 billion pages, can

  • you find someone like Mike Smith.

  • And that's efficiency.

  • That's a good algorithm, daresay.

  • >> But then we moved from that to try to formalize it.

  • And I proposed this pseudocode code.

  • Pseudocode code is not anything formal.

  • It's not something you memorize.

  • It's just something you express fairly intuitively using English,

  • or any language really, that conveys your ideas succinctly.

  • But what's key about pseudocode code is that you

  • try to anticipate all of the possible cases that might happen.

  • And indeed, in this pseudocode code, there were really three cases

  • every time I divided the phone book.

  • Mike might be to the left.

  • Mike might be to the right.

  • Or he might be right on the page I'm on.

  • Or a fourth corner case, so to speak.

  • A bad scenario might be one which-- what is happening?

  • Mike's just not in the phone book at all.

  • >> And when programs crash-- when Mac and PC software that you guys run

  • on your computers sometimes hangs or quits unexpectedly,

  • that generally means that some programmer, some human like you soon,

  • just screwed up and made some mistake.

  • Maybe didn't anticipate that maybe there is no Mike Smith in the phone book.

  • And if you don't actually write code to handle situations like that,

  • generally unpredictable things can happen.

  • Your machine can freeze.

  • It can reboot.

  • The program can quit.

  • And so all of these stupidities that you may

  • have encountered in your actual life just using computers,

  • will increasingly be just explained away by this intuition

  • and this understanding of what is actually going on underneath the hood.

  • >> Now let's try to take a look at a more general problem.

  • Rather than take attendance in a place like

  • this, which would be quite slow to do one, two, three, four.

  • Or maybe two, four, six, eight.

  • Let's focus, instead, on how we might formalize

  • the algorithm of the process by which we could take attendance.

  • And along the way, let's start to apply some nomenclature

  • that we'll use today when we actually start programming in a language.

  • So I give you now, a four minute video that we put together with our friends

  • from TED, the organization.

  • Whereby we supplied a script and they brought their animators to bear,

  • and actually created a 2D animation of what an algorithm is.

  • If we could dim the lights.

  • >> [MUSIC PLAYING]

  • NARRATOR: What's an algorithm?

  • In computer science, an algorithm is a set

  • of instructions resolving some problem step-by-step.

  • Typically, algorithms are executed by computers,

  • but we humans have algorithms as well.

  • For instance, how would you go about counting

  • the number of people in a room?

  • Well, if you're like me, you'd probably point at each person one at a time

  • and count up from zero.

  • One, two, three, four, and so forth.

  • Well, that's an algorithm.

  • In fact, let's try to express it a bit more formally in pseudocode code.

  • English-like syntax that resembles a programming language.

  • >> Let n equal 0.

  • For each person in room, set n equal to n plus 1.

  • How to interpret the pseudocode?

  • Well line one declares, so to speak, a variable

  • called n and initializes its value to 0 This just

  • means that at the beginning of our algorithm,

  • the thing with which we're counting has a value of 0.

  • After all, before we start counting we haven't counted anything yet.

  • Calling this variable n is just a convention.

  • I could have called it most anything.

  • Now line two demarks the start of a loop,

  • a sequence of steps that will repeat some number of times.

  • So in our example, the step we're taking is counting people in the room.

  • Beneath line two is line three which describes

  • exactly how we'll go about counting.

  • The indentation implies that it's line three that will repeat.

  • So with the pseudocode code is saying is that after starting at 0

  • for each person in the room we'll increase n by 1

  • Now is this algorithm correct?

  • Well let's bang on it a bit.

  • >> Does it work if there are two people in the room?

  • Let's see.

  • In line one we initialize n to 0.

  • For each of these two people, we then increment n by 1.

  • So in the first trip through the loop, we update n from 0 to 1.

  • On the second trip through that same loop, we update n from 1 to 2.

  • And so, by this algorithm's end, n is 2, which

  • indeed matches the number of people in the room.

  • So far, so good.

  • >> How about a corner case though?

  • Suppose that there are 0 people in the room-- besides me, who's

  • doing counting.

  • In line one, we again initialize n to 0.

  • This time though, line three doesn't execute at all

  • since there isn't a person in the room.

  • And so n remains 0, which indeed matches the number of people in the room.

  • Pretty simple, right?

  • But counting people one at a time is pretty inefficient, too, no?

  • Surely we can do better.

  • Why not count two people at a time, instead of counting one, two, three,

  • four, five, six, seven, eight, and so forth.

  • Why not count two, four, six, eight, and so on?

  • It even sounds faster.

  • And it surely is.

  • >> Let's express this optimization in pseudocode code.

  • Let n equal 0.

  • For each pair of people in room, set n equal to n plus 2.

  • Pretty simple change, right?

  • Rather than count people one at a time, we instead count them two at a time.

  • This algorithm's, thus, twice as fast as the last.

  • But is it correct?

  • Let's see.

  • Does it work if there are two people in the room?

  • In line one, we initialize n to 0.

  • For that one pair of people, we then increment n by 2.

  • And so by this algorithm's end n is 2, which

  • indeed matches the number of people in the room.

  • >> Suppose next that there are zero people in the room.

  • In line one we initialize n to 0.

  • As before, line three doesn't execute it all

  • since there aren't any pairs of people in the room, and so n remains 0.

  • Which indeed matches the number of people in the room.

  • But what if there are three people in the room?

  • How does this algorithm fare?

  • Let's see, in line one, we initialize n to 0.

  • For a pair of those people, we then increment n by 2.

  • But then what?

  • There isn't another full pair of people in the room,

  • so line two no longer applies.

  • And so by this algorithm's end, n is still 2 which isn't correct.

  • Indeed this algorithm's said to be buggy because it has a mistake.

  • >> Let's redress with some new pseudocode code.

  • Let n equal 0.

  • For each pair of people in room, set N equal to n plus 2.

  • If one person remains unpaired, set N equal to n plus 1.

  • To solve this particular problem, we've introduced in line four a condition,

  • otherwise known as a branch, that only executes

  • if there's one person we could not pair with another.

  • And so now, whether there's one, or three,

  • or any odd number of people in the room, this algorithm will now count them.

  • Can we do even better?

  • Well, we could count in threes, or fours, or even fives and tens,

  • but beyond that, it's going to get a little bit difficult to point.

  • >> At the end of the day, whether executed by computers or humans,

  • algorithms are just a set of instructions

  • with which to solve problems.

  • These were just three.

  • What problem would you solve with an algorithm?

  • >> DAVID J. MALAN: So deliberately, a very simple program,

  • a very simple algorithm, for achieving something

  • very simple, counting the number of people in the room.

  • >> But let's tease apart some of the representative

  • features here that are actually going to be useful even when

  • implementing the most complex of software.

  • So for instance, in this first line, we have what we call the variable,

  • and from algebra, you're generally familiar using x and y

  • and z sometimes, and so forth.

  • But in programming, variables are still, at the end of the day,

  • very similar to that.

  • But it's perhaps simpler to think of a variable as just a container.

  • And, in fact, it's some number of bits implemented somehow in your hard disk

  • or in your computer's memory, but more on that in the future.

  • It's just a container.

  • And if you say something like let n equal 0,

  • well that's like calling this glass bowl here n, just an arbitrary name,

  • and putting nothing in it initially.

  • So the value of this bowl right now is zero.

  • And of course if you perceive in a subsequent line,

  • to actually increment some line of code, as in this third line here,

  • by 1, that's like saying what's the current value of n, it's 0, plus 1,

  • put something like a ping pong ball in here.

  • Now the value of this variable is quite simply 1.

  • And you could very quickly extrapolate, but now it's 2, now it's 3, and so on.

  • So that's all a variable is.

  • It's a piece of storage to actually store some data.

  • For now it's a ping pong ball.

  • There it's a number.

  • But it could be words in a dictionary, like the spell checker

  • I alluded to on Wednesday for one of last year's problem sets.

  • >> Now another key idea, that similarly is pretty intuitive I would claim,

  • is that of a loop.

  • And the loop in the process of counting everyone

  • is, of course, doing the same thing again and again-- either one

  • at a time or two at a time.

  • And you can express this in English, or pseudocode code, in any number of ways,

  • but using this preposition "for" is a very common way of doing that.

  • For each person in the room, do this.

  • Again and again.

  • And the fact that it's indented, line three,

  • just means that what you're supposed to do

  • is the stuff that's indented below the line two itself.

  • Just a human convention, but a common one

  • as we'll see in actual higher level programming languages.

  • >> Now little more interesting is when you get in a corner case.

  • For instance, a corner case was when there

  • were three people, or five, or seven, or any odd number of people in the room,

  • because doing that by twos brakes eventually because your going

  • to miss someone, either at the very beginning or the very end

  • depending on how you do it.

  • And so, now, I have this branch, or condition, if one person remains,

  • then go ahead and handle that one lonesome person who didn't get

  • paired with someone else.

  • So that's what we would call a condition, or a branch.

  • >> Now pseudocode code more generally can be

  • written to solve any number of problems.

  • And what I thought we'd do here is take a moment

  • to invite shall we say CS50's own Rob Bowden on stage

  • to be joined by two volunteers, who have no idea what awaits.

  • A hand went down as soon as I said that.

  • How about you on the end here, come on up.

  • And how about from farther away, how about way in the back.

  • Back row, come on up with your hands up.

  • Alright, and what's your name?

  • >> ANITA: Anita.

  • >> DAVID J. MALAN: Anita.

  • Okay, nice to meet you.

  • Let me introduce you to Rob Bowden.

  • This is Anita.

  • And what is your name?

  • >> KIERSTEN: Kiersten

  • >> DAVID J. MALAN: Kiersten.

  • Kiersten, come on up and meet Rob Bowden and Anita.

  • Nice to meet you.

  • KIERSTEN: Nice to meet you.

  • DAVID J. MALAN: Alright, Rob.

  • ROB BOWDEN: Nice to meet you.

  • DAVID J. MALAN: Anita.

  • KIERSTEN: Hi Anita.

  • DAVID J. MALAN: And your several hundred classmates.

  • So, now let me go ahead and pull up just a simple program here

  • on Mac OS that'll let me actually jot some notes down.

  • And if you guys want to each take a position at one of those schools there,

  • let me go ahead and starts a list of pseudocode code, if you will.

  • And what I want to do here, ultimately is type for you

  • some instructions that our audience members are actually

  • going to recite for us.

  • Let me go ahead and just change this to a numbered list

  • to match what we were doing up there.

  • And what I'm going to do with your help, is write a program

  • in pseudocode, with which these guys are going

  • to implement a peanut butter and jelly sandwich.

  • So it's perhaps apropos to show something some of you

  • might have seen on the internet for just a brief annoying moment.

  • >> [MUSIC BUCKEWHEAT BOYS, "PEANUT BUTTER JELLY TIME"]

  • DAVID J. MALAN: OK.

  • That's enough of that.

  • So here meanwhile, I have a pair of Google Glasses which

  • we'll put on CS50's own Rob Bowden to see the world through his eyes.

  • And we'll do our best in post production to actually weave

  • the footage of what Rob is seeing now, into this actual lecture

  • video with our two volunteers beside him.

  • So what I'm going to do is, I'll be the typist.

  • We have the goal here of actually writing a program

  • with which to make, ultimately, a peanut butter and jelly sandwich,

  • but these three are going to behave as though they are computers.

  • And computers, at the end of the day, are actually pretty dumb devices.

  • They're super fast, but they can only do, literally, what they are told.

  • You can't just say make a peanut butter and jelly sandwich.

  • You have to program them to do that.

  • You have to tell them with precision what to do,

  • less things go horribly and, hopefully, amusingly awry.

  • >> So with that said, we need one call-out from the audience

  • for what should step one be, if the goal here

  • is to make a peanut butter and jelly sandwich.

  • Yes?

  • >> AUDIENCE: [INAUDIBLE] the bag of bread.

  • DAVID J. MALAN: Open the bag of bread.

  • So if the three contestants would like to proceed to do that literally.

  • Open the bag of bread.

  • >> [AUDIENCE LAUGHING]

  • DAVID J. MALAN: So let's work on that.

  • All right.

  • So step two, how-- let's take this further.

  • Yeah, in the front.

  • >> AUDIENCE: [INAUDIBLE] the bread.

  • >> DAVID J. MALAN: What's that?

  • >> AUDIENCE:Remove the bread.

  • DAVID J. MALAN: Remove the bread.

  • Similarly succinct.

  • Thank you.

  • >> [APPLAUSE]

  • DAVID J. MALAN: That's it?

  • OK, so step two is going to be remove the bread.

  • Alright, someone want to write us a longer sentence?

  • Someone else?

  • A little more [INAUDIBLE].

  • No, nothing now.

  • Yes?

  • >> AUDIENCE: Place two slices next to each other.

  • >> DAVID J. MALAN: Place two slices next to each other.

  • >> [AUDIENCE LAUGHING]

  • >> DAVID J. MALAN: Place two slices next to each other.

  • Step four.

  • Yes?

  • >> AUDIENCE: Take your hand and set it lightly

  • on top of the peanut butter lid.

  • >> [AUDIENCE LAUGHING]

  • AUDIENCE: [INAUDIBLE] next to the peanut butter.

  • DAVID J. MALAN: What?

  • Say that again.

  • >> AUDIENCE: Unscrew the lid and put it gently next to the peanut butter.

  • >> DAVID J. MALAN: Put it gently next to the peanut butter.

  • OK, progress.

  • Step five.

  • Excellent.

  • Yes?

  • >> Pick up knife.

  • DAVID J. MALAN: Pick up knife.

  • OK, step six.

  • Yeah?

  • >> AUDIENCE: Hold knife by the handle.

  • DAVID J. MALAN: Hold knife by the handle.

  • Hold knife by the handle.

  • Step seven.

  • Yes?

  • >> AUDIENCE: [INAUDIBLE] knife in peanut butter and as little out [INAUDIBLE].

  • >> DAVID J. MALAN: Put knife in-- I heard "put knife in peanut butter

  • and take as little out as possible."

  • By the way, remove the paper first.

  • All right, step nine.

  • Step nine.

  • Step nine.

  • We haven't actually made a sandwich yet.

  • Yes?

  • AUDIENCE: Using knife in peanut butter, apply peanut butter on said bread.

  • >> DAVID J. MALAN: Using knife in peanut butter, apply peanut butter on

  • said bread.

  • >> [AUDIENCE LAUGHING]

  • DAVID J. MALAN: All right step 10.

  • Step 10.

  • Yes?

  • >> AUDIENCE: Taste peanut butter to ensure quality.

  • >> [AUDIENCE LAUGHING]

  • DAVID J. MALAN: Step 11.

  • Step 11.

  • Step 11.

  • Come on.

  • Yeah?

  • Right there.

  • >> AUDIENCE: Carefully pick up jelly.

  • >> DAVID J. MALAN: Carefully pick up jelly.

  • OK, and then another hand was up.

  • Right behind you.

  • Yeah, in blue.

  • >> AUDIENCE: All right, remove lid from [INAUDIBLE], yeah,

  • remove lid from the jelly.

  • >> [AUDIENCE LAUGHING]

  • >> DAVID J. MALAN: From jelly.

  • Ha ha.

  • >> [AUDIENCE LAUGHING]

  • DAVID J. MALAN: And?

  • AUDIENCE: And barely sweep any [INAUDIBLE].

  • [AUDIENCE LAUGHING]

  • AUDIENCE: Of course, before [INAUDIBLE], remove the paper from jelly.

  • DAVID J. MALAN: Remove paper from jelly.

  • Step 14.

  • We're almost there.

  • Yes?

  • >> AUDIENCE: Invert jelly bottle before everything falls out.

  • >> DAVID J. MALAN: Invert jelly bottle before jelly falls out.

  • Step 15.

  • >> AUDIENCE: Replace the cap.

  • >> DAVID J. MALAN: Replace the cap.

  • Step 16.

  • Yeah?

  • >> AUDIENCE: [INAUDIBLE]

  • DAVID J. MALAN: Say that again.

  • AUDIENCE: Take cap off of your jelly.

  • DAVID J. MALAN: Off your jelly.

  • So really-- Oops.

  • Come on.

  • Replace the cap.

  • Put cap-- You said remove cap from jelly.

  • Feel like we're in a bit of a loop.

  • Step 17.

  • Yes?

  • >> AUDIENCE: [INAUDIBLE]

  • DAVID J. MALAN: Say that again.

  • AUDIENCE: [INAUDIBLE]

  • DAVID J. MALAN: Go back to step--

  • AUDIENCE: [INAUDIBLE]

  • DAVID J. MALAN: Remove cap from peanut butter.

  • Yes?

  • >> AUDIENCE: Drop all the jelly on the bread.

  • >> DAVID J. MALAN: Drop all the jelly on the bread.

  • DAVID J. MALAN: We're almost there.

  • Step 19.

  • >> AUDIENCE: Remove excess jelly.

  • >> DAVID J. MALAN: Haha, jelly.

  • >> [APPLAUSE]

  • DAVID J. MALAN: Why don't we-- one more step to take this home.

  • One more step and then we'll serve sandwiches.

  • Yes?

  • >> AUDIENCE: [INAUDIBLE]

  • >> DAVID J. MALAN: While any sandwich remains-- let's indent this-- eat.

  • >> [AUDIENCE LAUGHTER]

  • >> DAVID J. MALAN: All right, thank you to our volunteers here.

  • >> [APPLAUSE}

  • >> DAVID J. MALAN: We have some nice parting gifts for each of you.

  • Your own peanut butter, jelly, and bread to bring back home.

  • Thank you.

  • >> KIERSTEN: Thank you.

  • DAVID J. MALAN: [INAUDIBLE] welcome.

  • [APPLAUSE]

  • DAVID J. MALAN: So, this is, of course, a ridiculous example.

  • Right?

  • But it does kind of reveal how we humans just take clarity for granted.

  • And the fact I've been talking to another human,

  • he or she just knows what you mean.

  • >> Computers are not going to know what you mean,

  • even when using, as we're about to do today,

  • programming something in Scratch, a drag and drop, puzzle piece style language.

  • Even designed for young children, you have

  • to be so explicit and so literal with what you want your program to do.

  • Now ultimately, we're going to be programming

  • not in pseudocode code, English like syntax,

  • but code or, more properly, source code.

  • Source code is just the fancy way for describing code you actually

  • write with a keyboard that's not in English per se.

  • It's in C or Java or C++ or something like that, as we'll soon see.

  • >> And in fact, just to scare a few of you, at first glance,

  • this is a program written in a language called C. But to un-scare a few of you,

  • you will completely understand what's going

  • on come next Monday when it comes to something like this.

  • Frankly, this is an older language.

  • It's fairly arcane, but it's representative

  • of a lot of languages these days that have lots of parentheses and curly

  • braces and quote marks and semicolons.

  • And a lot of this syntactic stuff that is not

  • at all intellectually interesting.

  • Indeed, it's an utter distraction from the very simple ideas

  • that are staring us in the face.

  • This program, as you might just guess, prints to someone's computer screen

  • the words "Hello comma world."

  • That's it.

  • So clearly, there's a lot of stuff that's

  • getting in the way of some obviousness there,

  • but it's going to very quickly slip away and be completely intuitive.

  • >> Indeed, what we're going to do today is distill this fairly complex

  • looking program, which again you'll come to understand quickly, but to something

  • much simpler.

  • Let's just say what we mean.

  • Let's draw a picture of what we mean, by way of these puzzle pieces here.

  • >> So this is a programming language known as Scratch.

  • It was developed by MIT's Media Lab.

  • And what you'll see in problem set zero, which will be released later tonight,

  • we'll have you go to this URL here scratch.mit.edu.

  • And they have a web based interface via which

  • you will write your first program.

  • Or those of you with prior experience, your second programs,

  • but in an environment that's probably a little unfamiliar and that

  • will push you to create something using this very visual environment.

  • >> Now, what I'm going to do here is open up the program itself.

  • It exists not only as a web browser, but also as a downloadable program

  • so that you can actually use it if you don't have internet access.

  • And I'm going to do that in here, in Sanders, just

  • in case the Wi-Fi doesn't cooperate super well.

  • And what I'm going to do is point out a few features of this program.

  • So, to be clear, I have just double clicked the icon on my desktop,

  • or equivalently gone to scratch.mit.edu, and it's pulled up this window.

  • This is a programming environment.

  • It's a piece of software that some of our friends at MIT

  • wrote that let's us and you write programs in a language called Scratch.

  • >> Now this happens to be a cat who's also named Scratch

  • and this is his world in which he lives.

  • This is the stage, so to speak, that rectangle on the top left hand corner.

  • And he doesn't have to look like a cat.

  • You can make him look like anything and you

  • can have many such sprites, or characters, in a program.

  • Meanwhile, over here on the far right, is a big blank slate.

  • And this is where, in a moment, we are going to start programming

  • by dragging and dropping these graphical puzzle

  • pieces that are right here in the middle.

  • And there's way more of them than we'll spend time on here in class

  • because you'll find that they're all fairly intuitive.

  • Again, it's designed for children, but we

  • use it to tease apart some of those fundamental ideas of variables, loops,

  • conditions, and, soon, things like functions and events

  • and threads and other fancy things we'll get to before long to actually create

  • something from Scratch.

  • Pun intended.

  • >> Now, what I'm going to do here is click on not motion, but control.

  • And this is just a categorization of here--

  • and I see a different color set of blocks.

  • But notice a few familiar words.

  • "If" and "else if" and "repeat."

  • And you can probably guess that's reminiscent of the branch,

  • or the conditions we saw, and even the looping construct.

  • So we have similar blocks here.

  • But the most interesting one is this one here.

  • When this green flag is clicked, this, for those with prior programing

  • experience, is equivalent to a main function.

  • But for those unfamiliar, this is the puzzle piece

  • that will kick start our entire program.

  • It literally means when I go, in this program, and click a green flag--

  • which you can see up here in the top left hand corner of the UI,

  • so see the green flag next to the red stop sign?

  • When I click that, my program is going to run.

  • Now, I'm going to do something super simple with Scratch.

  • I'm going to go ahead and go to the looks panel

  • here, where I have a bunch of purple puzzle pieces,

  • and I'm going to go ahead and do something super simple like, say.

  • And then-- notice this text in the white box

  • is editable-- I'm going to say "Hello world," just like we

  • did in that textual version a moment ago.

  • And now if I go and click this green flag, I have now programmed.

  • It's not a particularly interesting program,

  • but I made the computer do something.

  • I started a program and it did what I told it to do.

  • Now, I can continue to drag and drop more and more of these puzzle pieces

  • and they're going to interlock, but let's slap some terminology on here

  • that we'll see recurring throughout the course,

  • and really throughout computer science and programming more generally.

  • >> This "say" block, in purple, let's just start calling a statement.

  • It's like a statement of fact.

  • Do this.

  • So, it's a category of instructions that you

  • might feed a computer as part of a program or an algorithm.

  • And to be clear, you've probably taken for granted

  • that you have programs on your computer.

  • And they're kind of algorithms, but a program is really a bunch of algorithms

  • that some humans wrote.

  • They packaged it up and they sold it so you,

  • or they posted on a website for you to download.

  • So, a program is just a whole bunch of zeros and ones

  • that, somehow, humans created.

  • And those patterns of zeros and ones represent things, ultimately,

  • like "say hello world" or "play this music" or "play this video"

  • or "send an email."

  • But we'll come back in way more detail what

  • a program is when you, yourself, write them.

  • >> Here's another statement-- "Wait for one second."

  • I didn't use this yet, but if I want my program to pause for a moment

  • to do something, I can tell it to do so.

  • Wait one second.

  • Now another one might be "play sound."

  • So, this is unique to Scratch, it has the ability to play sounds.

  • So, a statement I might use is, here, "play sound."

  • Meanwhile, Boolean expression, so this is a fancier word

  • named after just a guy named Mr. Bool, and this is all about a question.

  • True or false-- is the mouse down?

  • A Boolean expression is just some expression in English

  • that is either true or false.

  • Either on or off.

  • Either one or zero.

  • You can think of it in any number of ways,

  • but it's either true or this false, ultimately.

  • So "mouse down question mark," that would be a Boolean expression.

  • And you can think of others, perhaps.

  • For instance, "is the left number less than the right number?"

  • That, too, would be a Boolean expression.

  • "Less than" is a Boolean expression.

  • >> This one, too, "touching mouse pointer."

  • I'm not sure why they called it mouse pointer.

  • It just means, is the cursor, is the arrow on the screen, touching the cat,

  • for instance.

  • Or some other aspect of the screen.

  • And it's a question, again, and that denotes a Boolean expression.

  • Something that you might want to use in a condition.

  • So we'll come to that in just a moment.

  • You can "and" things together.

  • So, if you want to check if this is the case AND that is the case,

  • you can use an "and" block like this.

  • And here's that condition.

  • Notice the shape of the little opening in the top of this yellow puzzle piece,

  • it's reminiscent of the shape that we just saw a moment ago.

  • Each of these Boolean expressions have these pointed edges

  • on the left and right.

  • And that's because MIT folks decided that by visually conveying shapes,

  • you can kind of help people, students and children alike,

  • to kind of fill in the blanks literally.

  • >> Now that puzzle-- that opening is a little small,

  • and as we'll see in the program, in Scratch, it will grow to fit.

  • It will maintain its shape, ultimately.

  • So a condition let's you decide "should I do something or not?"

  • A Boolean expression is the actual question

  • you're using to decide do I go to the left OR do I go to the right

  • when I encounter this so-called fork in the road?

  • You can have two branches.

  • IF something is true, do this, else go that way,

  • or you can just do nothing at all, as this block implied.

  • Similarly, we can nest these things.

  • So if you want to triple fork in the road, either do this or this or that,

  • you can just nest these things together.

  • And it starts to get a little ugly, eventually, for sure,

  • but the logic is still the same.

  • You can literally read this top to bottom

  • and it says what it means-- if this is true, do this, else if else.

  • >> A loop doesn't get simpler in Scratch.

  • Forever do the following.

  • Now you might not think you can do much because there's not

  • much space between the top and the bottom of this puzzle piece opening.

  • But you'll see Scratch is going to grow to fit as many puzzle pieces

  • as you want to cram in there.

  • Another loop might be expressed with repeats.

  • If you know in advance, "I want to do something 10 times,"

  • you can just tell Scratch to do something 10 times.

  • And, meanwhile, we can have variables.

  • So here's an arbitrary one, it's orange in this case,

  • and this is a whirlwind tour.

  • Again, you'll find this very accessible once you start pointing and clicking.

  • I've named my variable n, but I could have named it anything I want,

  • and I'm setting it here, in this arbitrary example, to zero.

  • >> Now seeing a program like hello world is not all that compelling,

  • so let's actually open up something that a former student made.

  • Let me go ahead and open up, for instance, this one here,

  • for which I would love to have a volunteer.

  • All right, how about-- let's go farther.

  • Yes, come on up.

  • What's your name?

  • >> ABBY: Abby.

  • DAVID J. MALAN: Abby, come on up.

  • So have you ever played this game before?

  • ABBY: No.

  • DAVID J. MALAN: All right.

  • David, nice to meet you.

  • Come on over.

  • And what is your programming background, if any.

  • >> ABBY: I've learned some C++.

  • DAVID J. MALAN: You've learned some C++.

  • And what is your game playing background?

  • >> ABBY: Not a lot.

  • DAVID J. MALAN: OK, so we'll take that.

  • So here's how the game is going to work.

  • I'm going to go ahead and click the green flag, which

  • is up here at the top right.

  • Now your predecessor in the class has given you some instructions here.

  • And in just a moment, it says "space to begin."

  • So go ahead and hit the spacebar.

  • >> COMPUTER GAME: Pikachu.

  • DAVID J. MALAN: And the goal is to catch the food, as depicted there

  • on the left.

  • And to [INAUDIBLE]

  • >> [GAME MUSIC PLAYING]

  • >> DAVID J. MALAN: Aww, well, thank you for playing.

  • We have here a little parting gift for you.

  • We have CS50 stress ball, if you'd like to choose.

  • All right, good to meet you.

  • Thank you for coming and challenging.

  • So we have more stress balls, so let's do one more example to motivate.

  • A volunteer?

  • All right, how about right here in front.

  • What's your name?

  • >> PHILLIP: Phillip.

  • >> DAVID J. MALAN: Phillip.

  • Come on up, Phillip.

  • So, Phillip is going to be challenged with another game

  • that one of your predecessors wrote as part of problem set zero,

  • called Ivy's Hardest Game.

  • And we'll see in just a moment what's meant by this.

  • Phillip, nice to meet you.

  • What is your background?

  • PHILLIP: Done a lot of coding.

  • Done a little gaming, too.

  • >> DAVID J. MALAN: OK.

  • Got a lot of gaming, too.

  • And have you played this game before?

  • >> PHILLIP: No

  • DAVID J. MALAN: All right, so here we go.

  • I'm going to go ahead and click the green flag.

  • >> [GAME MUSIC]

  • >> [MUSIC MC HAMMER, "U CAN'T TOUCH THIS"]

  • >> PHILLIP: [INAUDIBLE]

  • DAVID J. MALAN: [INAUDIBLE]

  • PHILLIP: [INAUDIBLE]

  • [LAUGHING]

  • [MUSIC MC HAMMER, "U CAN'T TOUCH THIS"]

  • DAVID J. MALAN: [INAUDIBLE] Plow through it.

  • PHILLIP: [INAUDIBLE]

  • DAVID J. MALAN: Go ahead.

  • [MUSIC MC HAMMER, "U CAN'T TOUCH THIS"]

  • DAVID J. MALAN: All right.

  • Congratulations.

  • >> [APPLAUSE]

  • >> DAVID J. MALAN: We will post that online later so

  • that you can procrastinate with it as well.

  • Princeton comes up next, after that.

  • >> So now let's actually proceed to start from scratch,

  • so to speak, and actually build up until we can tease apart some of these ideas

  • and get to something even more complex by the end.

  • I'm going to go over here and I'm going to go ahead and create a new file.

  • So again, the problem set will walk you through some of these steps.

  • But, all I did was go to the File menu and I

  • said "new," so much like Microsoft Word, or any program like that.

  • >> And let's go ahead now-- and we implemented "Hello world" a moment ago,

  • but let's do something a little cuter.

  • I'm going to go up to events.

  • And I'm going to do "when green flag clicked."

  • And then I'm going to use, shall we say, a branch.

  • So I'm going to use an "if" condition.

  • And notice how as soon as I get close to it, it wants to snap together.

  • So I let go and it snaps together.

  • And now I can do something interesting.

  • If I scroll through here, I'm going to see a whole bunch of blocks.

  • If I go to "data"-- let me zoom in-- there is something about variables.

  • If I go to "motion," you can apparently turn things around.

  • If I go to "operators"-- oh, this is interesting,

  • I can pick a random number.

  • So let me do something with only some probability, just because.

  • I'm going to go ahead and drag this puzzle piece,

  • this is that less than block, so it's just

  • "is this number less than that one?"

  • But I don't want a hard code a number because that be pretty pointless.

  • So I'm going to drag this piece here, and notice how it snaps in,

  • and now let me go ahead and say "if the number that's picked randomly

  • is less than six, do the following."

  • Now why less than six?

  • What probability is this effectively going to give me, just intuitively?

  • About 50%, right?

  • If the number that's guess randomly between 1 and 10 is less than six,

  • clearly it's one, two, three, four, or five.

  • And so that's going to give me a 50% probability of what happening?

  • >> Well let's do something like this, "play sound meow."

  • And notice, again, the puzzle piece grows

  • to fit, so long as the shapes match.

  • That's what's important.

  • Let me go over to Scratch here and click "play."

  • Nothing happens.

  • Is that a bug?

  • No, not necessary.

  • It could just be that a bigger number was chosen.

  • So let's do it again.

  • Nothing.

  • >> [MEOW]

  • DAVID J. MALAN: There it is.

  • [MEOW]

  • DAVID J. MALAN: Again.

  • No.

  • >> [MEOWING]

  • DAVID J. MALAN: So if you've ever played a game, of course,

  • where stuff is happening randomly, like the bad guys are coming or not

  • coming on to the screen, or things are falling or not falling,

  • that's just because something super simple like this is happening.

  • Pick a random number, and if it's less than some value,

  • maybe do this or maybe do that.

  • We can incorporate that into a condition.

  • Let's do something different.

  • >> Let me throw that away.

  • You can get rid of stuff by just dragging it off to the left

  • and letting go.

  • Let me go ahead and do a forever block and very quickly do something annoying.

  • Let me go ahead and say "play sound meow."

  • But I don't want this to be too annoying, so let me grab this block,

  • "wait one second," and notice there's no more room for it.

  • But if you go close enough, it wants to go there.

  • So I let go and it will grow to fill the block.

  • So now, this is a loop.

  • [MEOWING]

  • DAVID J. MALAN: I'm literally doing this forever.

  • Again and again.

  • That is just not natural sounding.

  • Let me go ahead and change this to not one second, to two seconds

  • and hit Enter.

  • And what's nice about Scratch is that you could program interactively.

  • Make a change, maybe hit Enter or Play again, and it will just keep going.

  • Well, now why don't we do something a little cuter?

  • Rather than do everything from scratch, let me go ahead and open up an advance

  • made one, that I pre-baked, called "Pet the Cat."

  • And we'll post all of these files online,

  • and problem set zero will tell you how to access them and open them up.

  • And notice, this is pretty simple.

  • But in advance, I seem to have grabbed a "forever" block, so I've got a loop.

  • And then I've got an "if" condition.

  • I've got a Boolean expression.

  • >> So in English, just glancing at this, especially if you've never

  • programmed before, just intuitively, what is this going to do, this program?

  • >> [AUDIENCE CHATTER]

  • >> DAVID J. MALAN: If you pet the cat, the cat is going to meow, right?

  • Because "forever unchecking" is the mouse pointer touching the cat.

  • So to be clear, and I didn't say this before, all of these puzzle

  • pieces and the script that we have combined them into,

  • so to speak, a script is synonymous with program, really, belongs to this cat.

  • And the reason is-- because notice how this same cat here is highlighted,

  • down here you can create a second cat or dog or really

  • anything you want graphically.

  • You can import something from Photoshop, or the like,

  • and integrate something if you want to make it for a project.

  • And what you'll find, ultimately, is that you

  • can add scripts, add puzzle pieces, to individual characters.

  • So these all belong to this particular cat.

  • So now if I go ahead and play this program, nothing's happening.

  • But it is running.

  • The program is in this forever loop, kind of waiting for what to happen?

  • >> [AUDIENCE CHATTER]

  • >> DAVID J. MALAN: To pet the cat.

  • So if I move my mouse toward him--

  • >> [MEOWING]

  • >> DAVID J. MALAN: So now I'm petting the cat.

  • Meanwhile, we can have a double branch.

  • Let me go ahead and open up don't pet the cat.

  • We'll post this online as well.

  • This is a little more complex, but without saying

  • you can guess perhaps what's going to happen.

  • >> [MEOWING]

  • >> DAVID J. MALAN: Cat is meowing, grand old time.

  • But this program is of course called "Don't Pet the Cat,"

  • so clearly, I want to pet the cat.

  • >> [ROARING]

  • >> DAVID J. MALAN: Don't pet the cat.

  • Now, let's take a look at another example here.

  • This one will be called "Hi, Hi, Hi," for reasons

  • that will be clear, because as soon as I run this thing--

  • >> [SEAL BARKING]

  • DAVID J. MALAN: Very easy to write annoying Scratch programs.

  • [SEAL BARKING]

  • DAVID J. MALAN: Now we won't go into great detail with all of these,

  • but notice there is a couple of new features here.

  • There's a variable, called "muted," and then there's a bunch of conditions.

  • And for now, let me just tease you by saying

  • that if this guy gets too annoying, I can hit the spacebar and he stops.

  • Because the role being played by these two scripts is as follows.

  • One of them is doing the barking every few seconds.

  • The other one is just waiting for me to hit the spacebar.

  • And if I do hit the spacebar, it's going to change

  • the state of this variable, the value of this variable, that's apparently

  • called "muted," I didn't call it something boring like n,

  • I called it an English word, "muted."

  • From 1 to 0 or 0 to 1.

  • From true to false or false to true.

  • And so you can actually see now, that you

  • could have two scripts, two programs running simultaneously,

  • and both of them might be sitting in a loop doing something.

  • Waiting for the human to do something like hitting spacebar.

  • [SEAL BARKING]

  • DAVID J. MALAN: And now he's on his way again.

  • [SEAL BARKING]

  • DAVID J. MALAN: So what else might we do with these examples?

  • Well, let's go ahead and do an example with counting sheep.

  • We'll continue this along the lines here of-- let's open up the sheep example.

  • And notice, much like before, this one, thankfully, is a little less annoying.

  • Once I hit play here-- Oh no, this is annoying.

  • He is going to baa with some probability.

  • Let's play this again.

  • >> [SHEEP BLEETING]

  • DAVID J. MALAN: The only difference is we've changed the costume, so to speak,

  • and we're halving and say what that actual number is.

  • Well let's go ahead and take this one step further.

  • And let me tease you with one other feature here.

  • Let's go ahead and open up a program called Threads.

  • So much like you can have one sprite, having multiple scripts,

  • you can have two sprites, a bird in a cat, each having their own script.

  • And each of them can operate simultaneously.

  • If you've ever heard the word "multithreading"

  • a computer is "multithreaded" means it can do multiple things at once,

  • and, indeed, so can Scratch.

  • >> And now notice the bird is kind of dumb.

  • It's just doing things randomly, bouncing off the screen.

  • Clearly the cat has been programmed to kind of have seeking capabilities

  • to home in on the bird because he's always pointing at him.

  • Until finally, he catches the actual bird.

  • So I won't dwell on the details here, but you can kind of

  • glimpse the same shapes, some "if" conditions, some variables,

  • maybe some loops, are ultimately doing the same thing.

  • >> Now let me go ahead and open up something else altogether,

  • to tease some of you, particularly those more comfortable,

  • what you can actually do with this.

  • I'm going to go ahead and open up Button here.

  • And this is a super simple-seeming program,

  • but watch what happens when I-- Watch what

  • happens when I open this version of it, here in the browser, scratch.mit.edu.

  • And when I click this big red button, notice what happens.

  • So what's going on here?

  • There's actually a lot more complexity going on underneath the hood,

  • even though I've implemented this, or frankly, Dan Bradley

  • has implemented this so simply.

  • He has designed a custom Scratch puzzle piece, known as a Scratch extension,

  • he called it Toggle.

  • >> And what Toggle does is it sends a message over the internet.

  • And that message itself looks a little cryptic.

  • But this, too, you'll understand by term's end.

  • The message he's actually sending looks a little something like this.

  • It's just text.

  • Cryptic.

  • You'll understand it by the time we get the problem set seven.

  • It's just sending a textual message over the internet to a server.

  • And that server is, ultimately, talking to this light bulb, which

  • has one of those fancy light bulbs in it that, itself, is talking

  • to the Wi-Fi in the network here, where a local device down here--

  • and, ultimately, it's actually turning that light on and off.

  • But we can do more interesting things still.

  • Notice what Dan also did for us.

  • He made multiple sprites, each of which has a script waiting for a click.

  • And if I click red, it goes red.

  • If I get blue, it goes blueish.

  • Green, yellow, orange.

  • And all of this is happening by going way out onto the internet,

  • back down here to the light bulb, on and off.

  • >> And if you can believe it-- and this is perhaps one of the most complex Scratch

  • projects someone has implemented-- he did it, in fairness, in a day.

  • And that was the challenge we threw down to Dan when we saw have binary bulbs,

  • we have Scratch, how can we combine them?

  • And, indeed, this is absolutely something anyone in this room

  • could do by semester's end-- Is he reimplemented binary bulbs using

  • the same interface that was on our iPad on Wednesday.

  • So that now, if I actually click the plus over here,

  • we have one, two, three.

  • I can hit the 16 and turn that one on.

  • 128, and so forth.

  • >> Now if that alone has and blown your mind,

  • you could do things far more accessible, not involving any hardware certainly.

  • Just things on the screen itself.

  • And, indeed, what most students end up doing is some kind of game,

  • some piece of artistic work, or some kind of interactive animation.

  • And I will say one of our favorites has been this one.

  • And I thought if we could dim the lights,

  • we'll take a glimpse at this final Scratch project in conclusion.

  • But what you'll should keep in mind, as I can tease you with one last detail,

  • sectioning will begin next week.

  • Office hours will begin next week.

  • Problem set zero will be posted CS50.harvard.edu later today.

  • And you will be greeted in problem set zero

  • to CS50's own Zamyla Chan, who guides most of our walkthroughs.

  • These are embedded videos in the problem sets

  • that will hold your hand through a lot of the initial set up and through many

  • of the possible design decisions.

  • So if you've ever had that emotion when picking up a homework assignment

  • and wondering, where do I begin?

  • She will have that answer for you.

  • And now, a final program from one of your predecessors

  • in answer to the question "What does the fox say?"

  • [MUSIC YLVIS, "WHAT DOES THE FOX SAY?]

  • >> YLVIS: (SINGING) Dog goes woof.

  • Cat goes meow.

  • Bird goes tweet and mouse goes squeak.

  • Cow goes moo.

  • Frog goes croak.

  • And the elephant goes toot.

  • Ducks say quack and fish go blub.

  • And the seal goes ow ow ow, but there's one sound that no one knows,

  • what does the fox say?

  • And

  • >> DAVID J. MALAN: That's it for CS50.

  • We will see you at Puzzle Day and on Monday.

  • >> [APPLAUSE]

  • >> [MUSIC YLVIS, "WHAT DOES THE FOX SAY?"]

  • >> NARRATOR: And now deep thoughts, by Daven Farnham.

  • Today I got hit in the face with a phone book.

  • Confusing, as I'm at home watching online.

>> [MUSIC PLAYING]

字幕與單字

單字即點即查 點擊單字可以查詢單字解釋

B1 中級 美國腔

第0周,繼續 (Week 0, continued)

  • 221 13
    Ssutung Liu 發佈於 2021 年 01 月 14 日
影片單字