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SPEAKER 1: This is CS50.

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So I was where you are now some years ago, and when I was a freshman at the

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time, I wasn't really on this path of doing computers science, of doing

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engineering.

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Indeed, I came into this place as a government concentrator.

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I'd been in high school a kid who liked things like history.

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I liked constitutional law, kind of English and math, so kind of

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well-rounded, but didn't necessarily know things that I hadn't been taught

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in high school.

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And so freshman year, I had this trepidation, whereby even though I

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liked computers, play computer games and the like, I certainly never

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thought of myself as a computer person, a computer scientist, and

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frankly, I thought my friends in high school who were taking computer

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science were a bit of a--geeks.

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>> And yet, when I got here on campus, there was this course, CS50, and at

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the time, it had this reputation of really being something to beware.

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It was a good course.

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It was a fun course.

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But you had to actually get that foot in the door.

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And even I did not cross that threshold freshman year.

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And I went on my way being a government concentrator, going through

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as many of the prerequisites as I could, cross counting things for gen

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ed or core and the like, and then sophomore year, for some reason, I got

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up the nerve to step foot in Science Center B, where CS50.

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A very famous computer scientist by the name of Brian Kernighan was

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teaching here that year, and even then, I was only willing to actually

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fill out my study card by taking this class pass/fail.

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I looked around me, and I figured everyone in this room's got to know

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way more about computer science, about programming, about computers.

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Everyone must be programming in this room since they were 12, but indeed,

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that wasn't the case.

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And so the very last day, the fifth Monday of the semester that I take

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this leap and change from pass/fail to a letter grade, and end up changing my

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concentration that same day to computer science.

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>> Now that's not our objective in this class, to turn you all into computer

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science concentrators, but really, to propose that there's this opportunity

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in this field and in other fields with which you might be quite unfamiliar,

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given that high schools typically follow a fairly standard curricular

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path, but to venture in, in CS50, into new waters.

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And if you are sitting here today thinking you don't actually belong, so

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do most of the people to your right and to your left.

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Indeed, last year, 76% of the students in this class had no prior experience.

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So contrary to what you might think, most of the people sitting around you

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today do not, in fact, have any prior experience.

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18% have taken one CS class, and 6% have taken two or more.

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Meanwhile, we ask our students every year to describe themselves in terms

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of comfort level, and this is--

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there's no one definition of this.

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You just kind of know it, if you're not very comfortable in CS50.

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And last year, we had 55% in this green pie slice here, self-describing

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as less comfortable students who frankly we had no idea why'd they

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shop the course on that first day, but the same 55% remained with us until

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term's end, as did 35%, who were somewhere in between those more

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comfortable and those less comfortable.

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>> So what is computer science?

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Well, in high school, and really, more generally out there these days,

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there's this perception or this misconception that computer science is

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programming.

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And that's absolutely one aspect of computer science, but programming,

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whatever the language is, is really just a tool that computer scientists

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use to solve problems, either in the domain of computer science or,

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increasingly these days, in the physical sciences, the natural

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sciences, in medicine, in the humanities, to analyze

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large sets of data.

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Anywhere now there is computer and data, there's an opportunity to apply

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lessons learned in a class like CS50.

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>> So let's solve a problem that a computer scientist might go about

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solving and try to put some jargon, put some conceptual framework, around

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what might otherwise be some fairly abstract ideas.

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So this is a telephone.

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You don't see these things too often.

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The college still seems to have these in the houses and dormitories, but

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back in the day, when you wanted to use a phone like this, there was no

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electronic address book in your cell phone.

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Rather, you pulled up something known as a phone book, and these phone books

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had about 1,000 pages typically.

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They were sorted from A to Z, and you simply had to find the right page to

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find the person you're looking for in order to find their name and their

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telephone number.

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Now how do you go about looking up someone in this book?

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>> Suppose my goal is to give my friend Mike Smith a call.

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Well, how do I go about finding Mike Smith?

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Well, a very reasonable approach, if naive and inefficient, would be start

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here, and start flipping to page four, to page five, to page six, and to sort

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of linearly, along a straight line, go through this phone book, and even

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though it's going to be incredibly tedious, if Mike Smith is in this

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book, I'm eventually going to reach him when I finally flip to the S

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section of this book.

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Now, of course, you don't need to be a computer scientist to know that this

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is a stupid way of solving this problem.

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What would a typical human being do?

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Well done.

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So you would flip to the middle, right?

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So you'd flip roughly to the middle, look here, and I seem to find myself

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in the M section.

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Okay, so M is clearly not what I'm looking for.

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Mike's to the right, so to speak, of this section, and as some of you have

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seen before, we can literally now proceed to tear this

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problem in a half.

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>> You really shouldn't be that impressed.

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Tearing it down the seam is actually not that hard.

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The real people do it this way, but down the seam, we

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now have two problems.

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Each of which is half as big, and we can literally throw that half of the

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problem away.

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Now we're left not with 1,000 pages, but say, 500.

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So now what do I do?

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Well, a typical human will go roughly in the middle again, and I find myself at

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the R section, so not quite there.

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So again, I can tear this problem in half.

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Thank you.

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So now I only have some 250 pages, and I can do this again, and again, and

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again, and go from 125 down to roughly 60, to 30, to 15, and so forth.

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And finally, I'll get whittled down to one of the S pages, on which, if he's

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in the phone book, Mike Smith should be.

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>> Now that's an obviously fairly reasonable algorithm, and it's a

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one-time use algorithm in this case, but what can we sort of

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take away from that?

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Well, the first approach, correct, if naive, though, it was, can be

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described by this straight line.

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So if on the x-axis, here we say, this is the size of the problem.

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So as the x-axis goes to the right, the problem gets bigger.

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What does it mean to be bigger?

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In the context of this problem, more pages in the phone book.

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There's more something we can quantify.

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On the y-axis, time to solve.

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So as the axis goes up, we presume it presumably takes more time.

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So that first approach of linearly searching from page one to page 1,000

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is a linear procedure, a linear algorithm or process, and we can

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describe it by this straight line.

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If I add one more page to the phone book, it's going to, in the worst

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case, take me one more page flip to find Mike Smith.

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If I add 100 pages, 100 more flips, or units of time.

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>> Now I can be a little clever with this.

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I don't need to really turn it one page at a time.

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I can do things like two at a time or four at a time, but even that's not

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all that fundamentally better.

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Even if it's two at a time, yeah, that kind of moves this line down a bit,

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and it means that it takes less time, given the same number of pages, but

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it's not fundamentally better.

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But what do we just do, and what did all of you do instinctively?

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You actually achieved a little something like this, logarithmic time,

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whereby the problem can grow and grow and grow, but the cost of solving that

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problem, the time required to solve that problem, does not

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grow nearly as fast.

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This would be a logarithmic curve, log of n, where n is just in the size of

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the problem, the number of pages in this phone book.

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And what does this mean in real terms?

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Well, if we have, like, 500 people in this room right now--

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or rather, if we have--

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mixing metaphor--

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didn't do that example yet this year-- so if we have 500 pages in the phone

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book, and we double it to 1,000, in this more intelligent model of

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flipping to the middle, how many more page-tearers does it take to go from

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500 pages to 1,000?

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Well, just one additional page tear.

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If you handed me a 2,000 page phone book, no big deal.

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I'd just tear it one additional time.

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>> So in short, the size of the problem can grow much faster than the cost of

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actually solving it.

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Now this is just one such algorithm.

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There are others we can solve in this same way.

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And so why don't we do this-- if you would humor me, albeit awkwardly here

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in Sanders, go ahead, everyone, if you could, and stand up in place.

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As you see on the screen here, this is an algorithm, a process, a computer

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program, if you will, to be executed by humans, that has just three steps.

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We're already on step one, you've stood up.

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And now think to yourself the number one.

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That is your current number.

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Everyone here is number one.

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Step two, pair up with someone standing and your numbers together,

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and then adopt this sum as your new number.

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One of you should sit down, then repeat.

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>> At this point, fewer and fewer people should be standing.

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This is where it gets more awkward.

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Someone here, here.

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The worst part is, you also have to very verbally do arithmetic in front

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of hundreds of Harvard undergrads.

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Okay, bit of a bug here.

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Okay, what's your number?

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Nine.

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Okay.

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What's your number?

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179.

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Okay, good.

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So 188.

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So you guys can sit down.

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What's your number?

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118.

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Some smart undergrad start doing the math.

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Okay, 118, 188.

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What else we got?

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71.

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79.

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Okay.

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And?

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47, which, teaching staff, that gives us how many?

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705 is the answer.

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And that's, in fact, exactly correct.

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>> No, we were actually a little bit off there, but how

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should this have worked?

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What should have just happened?

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So on every iteration of this algorithm, we started with some number

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of people standing, and that was the total number, n, at first.

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Then half of you sat down, and we went to n over two.

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Then half of you sat down, we went to n over four, n over eight, over 16,

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and so forth, until, even though it kind of disintegrated there at the

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end, in theory, had everyone paired off in balcony, and mezzanine, and

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orchestra here, we would have had just one lone person standing, with a total

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value, in this case, of 705.

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Now what does that mean, though, for the running time?

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Well, think about if I, as the human, had done this manually.

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I would have started fairly naively but correctly with one, two, three,

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four, five, six, seven, eight, and so forth; takes quite some time.

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So I can do better, right?

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In grade school, you don't just count in ones.

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You count in twos.

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So two, four, six, eight, ten, twelve.

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And that gets much faster, but now fundamentally, by leveraging the

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collective intelligence of everyone in this room, we can achieve a curve much

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more like this, whereby now, the number of the people in this room

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could double, another 700 people walk into this room for 1,400 people, but

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it would only take us one more iteration of this algorithm to solve.

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>> And so increasingly, these days, when we have these huge data sets in

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Facebook, and Google, and the like, it's solving problems with this bit of

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insight, this bit of cleverness, that's allowing us increasingly to do much,

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much more powerful things with computers today.

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If you like these kinds of things, you might have seen on Facebook CS50's

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own Puzzle Day, coming up this Saturday.

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If you would like to participate in something like this, whereby you and

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two, or three, or four teams of four would like to solve some puzzles such

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as this one, you stand a chance to win some fabulous prizes, among which is a

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Wii, and some gift cards, and some other Facebook swag this Saturday,

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noon to 3:00 PM.

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Go to cs50.net/rsvp for such.

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And this slide is online if you'd like to play around.

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The puzzles this year shall be new.

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>> You may notice in the classroom, too, all the more cameras this year.

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So not only will the course be filmed in the usual way, CS50 may also be

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taking part in a documentary on higher education that's looking at the

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transformative experience that a student can have these days in an

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undergraduate course of study.

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So toward these end, not only we will be filming for that, we'll be filming

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as well for, increasingly, our online audience, as well as, on occasion,

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this audience here.

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So we welcome to the class this year our Harvard extension school students,

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graduate school of design, education, the business school, the Kennedy

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School, the law school, as well as a number of students from Belmont,

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Lexington, Newton, and Watertown High Schools.

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Welcome to you all.

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In addition, this year, as you may have heard, Harvard, and MIT, and

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Berkeley now have entered into a collaborative partnership on an

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initiative called EdX, which is an initiative to open up education to all

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the more people online and, fundamentally, start doing research on

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a much broader scale as to how people learn.

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And so CS50 will be the college's first course participating in that

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initiative, as well, which means you will have access to all the more

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tools, all the more curricular content, all the more video content as

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a result, as well as of yesterday morning, the 53,019 people who have

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registered to take CS50 along with you this year on the Internet.

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So what this means, in particular, is that the teaching staff and I have

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spent quite a bit of time this summer preparing for the fall, both on campus

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and off, so that we can begin to build up a corpus of interesting, of

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compelling, of engaging educational content, that focuses in particular on

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more intimate conveyances of fairly complex material.

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>> So in addition to the course's lectures, and sections, and things

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called walkthroughs, which we'll revisit in just a bit, we'll also have

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these shorts this year that allow you to engage with the course from a

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different angle all together.

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So let's use this as an opportunity to take a quick peek at one that

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discusses this notion of binary.

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>> So in computer science there are things called algorithms, two of which

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we just took a look at-- these procedures for solving problems.

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But at the end of the day, you need to represent information somehow, and you

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need to represent it in a way that a computer can understand.

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And even if you don't really understand computers, and you're in

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that 76% right now, you probably have some vague sense that computers

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somehow deal in zeros and ones, the binary system, so to speak.

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Now why is that the case?

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Well, turns out when computers first came about, if you needed to represent

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information, you could do it with electricity, and though this is a bit

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of an oversimplification, a very easy way of recording information is either

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by turning that electricity on, a one in binary, so to speak, or turning

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that electricity off.

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>> So Barry, if you wouldn't mind, could we dim the lights

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fully for just a moment?

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This here is a very gratuitous binary zero.

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All right, if we turn the lights back up, now Sanders Theatre is

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representing the binary value of one.

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Unfortunately, with just one bit, with just one set of lights, we can only

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represent two numbers in the world, zero and one, and it'd be nice if

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computers could count a bit higher than that, but indeed, they can.

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So let me pull up on screen here our friend, Nate Hardison, who will give

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us a quick look over the course of just a few minutes at

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this notion of binary.

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>> NATE HARDISON: Back when you learned how to read and write numbers, you

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learned about the digits zero to nine.

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To write whole numbers larger than nine, you learn that all you had to do

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was use some combination of these digits, as in 52 and 437.

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So this way of writing numbers has a name, decimal notation.

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Why decimal?

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Well, the Latin root of decimal, deca, means 10, and when you have 10 digits

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in your notation system, 10 becomes a rather special number.

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Let's look at the number 437 written in decimal notation to understand why.

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>> We can first break up 437 into 400 plus 30 plus 7.

306
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We can take it apart even more, so that we've got four times 100 plus 3

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times 10 plus 7 times 1.

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remember learning about the ones place, the 10s place, the 100s

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place, and so on?

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This is exactly where that comes from.

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And finally, we can see how we've got a bunch of powers of

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10 embedded in here.

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We've got 4 times 10 to the 2 plus 3 times 10 to the 1 plus 7

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times 10 to the zero.

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So now you see why 10 is a special number in decimal notation.

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In fact, we've got a name for it.

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It's called the base, since it's the base of the exponent in

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our arithmetic here.

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>> Decimal notation is not the only way to represent numbers.

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In fact, even if we get rid of the digits two through nine, we can still

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represent all of the numbers that we could with decimal.

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So now that we have two digits, zero and one, two is our special number,

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the base of our notation system.

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The name of this notation system is called binary, since the

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prefix bi means two.

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So instead, now, of having a ones place, and 10s place, and so on, we

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now have a ones place, a twos place, a fours place, and so on, going up by

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powers of two.

329
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So let's see this by doing some counting.

330
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So zero is still zero, and one is still one.

331
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However, now that we've got a twos place instead of a 10s place, 10

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represents the number two.

333
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To get three, we add one to that, and get 11.

334
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Four, since there's now a fours place, is represented by 100.

335
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Five is 101.

336
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Six is 110.

337
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Seven is 111.

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Eight, again, has its own place.

339
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So it's 1000.

340
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And I think you get the point.

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>> SPEAKER 1: So this is to say what computers do and what binary is is

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actually not that dissimilar from what we've been taking for granted for some

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years, right?

344
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Back in grade school, you learned to count precisely in the

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fashion that Nate proposed, but you probably haven't really thought about

346
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it since, the fact that there is this ones place, 10s place, and 100s

347
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place, and that's pretty arbitrary.

348
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And indeed, computers simply use this different base.

349
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But at the end of the day, to actually physically represent this notion of a

350
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zero and one, you obviously don't just turn the lights on and off

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necessarily.

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You need to do it on a much finer grain scale.

353
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And by finer grain, you might remember this silly little toy from childhood,

354
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Woolly Willy and these little magnetic particles.

355
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>> So magnetic particles are something that you can align in a couple of

356
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different directions, perhaps north-south or south-north.

357
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And so a lot of physical incarnations of technology these days that use

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binary, that use zeros and ones, simply have magnetism on the inside

359
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that aligns things up-down or down-up, thereby representing a zero or a one,

360
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respectively.

361
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So indeed, let's move away from the abstract here, and look at the inside

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of what's a more traditional computer hard drive.

363
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This one happens to be a bit larger on screen, in that it's from a desktop

364
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computer, but laptops today still have this same technology, but is gradually

365
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being replaced by more sophisticated things that have actually no moving

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parts-- the inside, then, of a hard drive.

367
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>> SPEAKER 2: The hard drive is where your PC stores most of

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its permanent data.

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To do that, the data travels from RAM along with software signals that tell

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the hard drive how to store that data.

371
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The hard drive circuits translate those signals into voltage

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fluctuations.

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These, in turn, control the hard drive's moving parts, some of the few

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moving parts left in the modern computer.

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Some of the signals control a motor, which schools metal coated platters.

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Your data is actually stored on these platters.

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Other signals move the read-write heads to read or

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write data on the platters.

379
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This machinery is so precise that a human hair couldn't even pass between

380
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the heads of spinning platters, yet it all works at terrific speeds.

381
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>> SPEAKER 1: So we now zoom in on what's actually happening on top of these

382
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platters in terms of the magnetism, we have this second of two looks.

383
00:22:19,620 --> 00:22:22,940
>> SPEAKER 3: Let's look at what we just saw in slow motion.

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When a brief pulse of electricity is sent to the read-write head, it flips

385
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on a tiny electromagnetic for a fraction of a second.

386
00:22:30,390 --> 00:22:34,280
The magnet creates a field, which changes the polarity of a tiny, tiny

387
00:22:34,280 --> 00:22:37,995
portion of the metal particles which coat each platter's surface.

388
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A pattern series of these tiny charged up areas on the disk represents a

389
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single bit of data in a binary number system used by computers.

390
00:22:45,820 --> 00:22:49,430
Now if the current is sent one way through the read-write head, the area

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is polarized in one direction.

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If the current is sent in the opposite direction, the

393
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polarization is reversed.

394
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How do you get data off the hard disk?

395
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Just reverse the process.

396
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So it's the particles on the disk that get the current and the read-write

397
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head moving.

398
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Put together millions of these magnetized segments, and

399
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you've got a file.

400
00:23:07,440 --> 00:23:12,240
Now the pieces of a single file may be scattered all over a drive's platters,

401
00:23:12,240 --> 00:23:15,270
kind of like the mess of papers on your desk.

402
00:23:15,270 --> 00:23:18,800
So a special extra file keeps track of where everything is.

403
00:23:18,800 --> 00:23:21,640
Don't you wish you had something like that?

404
00:23:21,640 --> 00:23:23,340
>> SPEAKER 1: Indeed.

405
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So we have this ability to represent this information, numbers

406
00:23:27,560 --> 00:23:28,440
at a very low level.

407
00:23:28,440 --> 00:23:32,730
We have a physical way of representing that same thing, but we can't really

408
00:23:32,730 --> 00:23:36,440
do all that much of interest yet, other than perhaps some arithmetic and

409
00:23:36,440 --> 00:23:36,940
mathematics.

410
00:23:36,940 --> 00:23:40,830
We have no way of representing thus far things like alphabetical letters,

411
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so that we humans can communicate, using these same devices.

412
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But thankfully, there exists encodings, patterns of zeros and ones

413
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that represent higher level constructs like A, and B, and C, and entire

414
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sentences, and paragraphs, and the like.

415
00:23:54,880 --> 00:24:00,300
And so ASCII, which is an acronym that refers to this coding system whereby

416
00:24:00,300 --> 00:24:04,350
one number represents a given letter-- for instance, the number that we know

417
00:24:04,350 --> 00:24:09,120
as decimal value 65 is known as the capital letter A to computers.

418
00:24:09,120 --> 00:24:13,670
The decimal value of 97 in computers is known as a lowercase a.

419
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And what does that really mean?

420
00:24:15,210 --> 00:24:19,510
Well, even though Nate, a moment ago, only counted up from zero to eight, if

421
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we were to continue counting up to 65, or further, to 97, the pattern of

422
00:24:24,910 --> 00:24:28,440
zeros and ones that he would have drawn on the screen would be exactly

423
00:24:28,440 --> 00:24:33,420
what a computer uses to represent the letter A in all caps, or the letter a

424
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in lowercase.

425
00:24:34,610 --> 00:24:36,690
And indeed, there's a whole scheme to this.

426
00:24:36,690 --> 00:24:41,570
This is, at first glance, overwhelming chart of encodings, but if you focus

427
00:24:41,570 --> 00:24:46,210
just on the right half here, notice in this middle column, we have this

428
00:24:46,210 --> 00:24:49,010
notion of numbers followed by letters.

429
00:24:49,010 --> 00:24:53,830
And at top, we have 32, and the character char, to which 32, the

430
00:24:53,830 --> 00:24:57,360
integer, now refers, is apparently the space bar character.

431
00:24:57,360 --> 00:25:00,720
When you get the space bar character on your laptop, what you're really

432
00:25:00,720 --> 00:25:04,160
sending is a number, a pattern of zeros and ones, a flow of electricity,

433
00:25:04,160 --> 00:25:07,770
if you will, representing those zeros and ones, that the computer then

434
00:25:07,770 --> 00:25:10,330
interprets as a space character on the screen.

435
00:25:10,330 --> 00:25:12,210
An exclamation point is 33.

436
00:25:12,210 --> 00:25:13,345
Double quotes is 34.

437
00:25:13,345 --> 00:25:18,945
And if we scroll down here over to the right, we see that 65 is indeed A, and

438
00:25:18,945 --> 00:25:21,290
97 is indeed lowercase a.

439
00:25:21,290 --> 00:25:23,910
>> So now that we have this encoding scheme, we can start to

440
00:25:23,910 --> 00:25:25,080
spell things out.

441
00:25:25,080 --> 00:25:30,170
Indeed, computers typically express themselves in standard units, not

442
00:25:30,170 --> 00:25:33,020
using an individual bit, which again, is not all that useful to just

443
00:25:33,020 --> 00:25:36,600
represent zero or one, lights on or off, but rather, using

444
00:25:36,600 --> 00:25:37,530
sequences of bits.

445
00:25:37,530 --> 00:25:40,100
And the most common unit of measure, as you probably know, or at least

446
00:25:40,100 --> 00:25:41,810
inferred, is a byte.

447
00:25:41,810 --> 00:25:45,960
A byte is just eight bits, eight zeroes or ones in a row.

448
00:25:45,960 --> 00:25:48,090
So we can start spelling things out.

449
00:25:48,090 --> 00:25:52,580
And so if we could, why not try this a little bit collectively here?

450
00:25:52,580 --> 00:25:55,670
Are there eight people in this room who would be willing

451
00:25:55,670 --> 00:25:56,870
to come up on stage?

452
00:25:56,870 --> 00:26:00,880
You have to be comfortable appearing on camera, but you don't really need

453
00:26:00,880 --> 00:26:03,690
to know otherwise what's going on just yet.

454
00:26:03,690 --> 00:26:07,240
I see one person being volunteered over here.

455
00:26:07,240 --> 00:26:18,380
Two, three, four, five, six, seven, and how about eight?

456
00:26:18,380 --> 00:26:18,920
Come on up.

457
00:26:18,920 --> 00:26:22,550
>> So you are about to represent a byte of people.

458
00:26:22,550 --> 00:26:30,190
Let me have you been the 128s place, you the 64s place, you the 32s place.

459
00:26:30,190 --> 00:26:32,720
But we're going to very rapidly have to reverse this.

460
00:26:32,720 --> 00:26:35,170
So let me meet you all over there.

461
00:26:35,170 --> 00:26:39,840
And you should be in the 128s place all the way over, here much like the

462
00:26:39,840 --> 00:26:43,080
hundreds place or the thousands place would be farther to the left, we want

463
00:26:43,080 --> 00:26:45,740
the biggest placeholder to be here on the left as well.

464
00:26:45,740 --> 00:26:55,230
We have 64s, 32, 16s, 8s, 4s, 2s, and 1s--

465
00:26:55,230 --> 00:26:56,030
excellent.

466
00:26:56,030 --> 00:26:58,100
So now we have--

467
00:26:58,100 --> 00:27:00,190
okay, you can help me.

468
00:27:00,190 --> 00:27:01,890
So now we have-- what's your name?

469
00:27:01,890 --> 00:27:02,560
JOANNE: Joanne.

470
00:27:02,560 --> 00:27:06,240
SPEAKER 1: So Joanne and I are now going to advise these guys on how we

471
00:27:06,240 --> 00:27:07,850
can go about spelling something out.

472
00:27:07,850 --> 00:27:10,640
So on the backs of their sheets of paper, they have a little cheat sheet

473
00:27:10,640 --> 00:27:14,720
that's going to tell them whether they were representing a zero or a one.

474
00:27:14,720 --> 00:27:18,000
And why don't, for simplicity, we'll represent zero by just

475
00:27:18,000 --> 00:27:19,580
standing there awkwardly.

476
00:27:19,580 --> 00:27:20,360
Very good.

477
00:27:20,360 --> 00:27:24,200
Or a one by raising your hand, representing a one.

478
00:27:24,200 --> 00:27:28,380
And let's see if we can't spell out a four character phrase here.

479
00:27:28,380 --> 00:27:33,870
>> So go ahead now, volunteers, and execute round one by raising your hand

480
00:27:33,870 --> 00:27:38,940
if you're a one, or keeping it down if you're a zero.

481
00:27:38,940 --> 00:27:45,550
So now that we have these three hands up, what number, everyone else, are

482
00:27:45,550 --> 00:27:47,800
they actually representing?

483
00:27:47,800 --> 00:27:48,760
Okay, 67.

484
00:27:48,760 --> 00:27:49,040
Why?

485
00:27:49,040 --> 00:27:49,970
Well, quick sanity check.

486
00:27:49,970 --> 00:27:51,910
64s place, because it's a one.

487
00:27:51,910 --> 00:27:58,870
That's like one times 64, plus 1 times two, so that's 66, plus 1 times 1.

488
00:27:58,870 --> 00:28:00,610
That's plus 1, so 67.

489
00:28:00,610 --> 00:28:03,740
So now these guys are collectively representing 67, which apparently

490
00:28:03,740 --> 00:28:06,240
represents what here in ASCII?

491
00:28:06,240 --> 00:28:06,510
Okay,

492
00:28:06,510 --> 00:28:07,650
so a C. All right.

493
00:28:07,650 --> 00:28:09,530
So now let's proceed to round two.

494
00:28:09,530 --> 00:28:12,735
Everyone starting with their hands down, and then round two.

495
00:28:12,735 --> 00:28:15,260
Actually, there's not much of a role here, I suppose, but we'll pretend.

496
00:28:15,260 --> 00:28:19,820
So round two, raise or lower your hands.

497
00:28:19,820 --> 00:28:20,530
>> All right.

498
00:28:20,530 --> 00:28:23,230
Audience, what are we now?

499
00:28:23,230 --> 00:28:24,860
Expressing is 83.

500
00:28:24,860 --> 00:28:26,050
So you can do the mathematics.

501
00:28:26,050 --> 00:28:29,810
But for anyone whose hand is up, you add in the number that they represent.

502
00:28:29,810 --> 00:28:31,610
So now we have 83.

503
00:28:31,610 --> 00:28:36,510
Let's expand the cheat sheet a little bit, and we now have--

504
00:28:36,510 --> 00:28:38,890
this might be obvious where we're going here, but

505
00:28:38,890 --> 00:28:42,220
nonetheless, round three.

506
00:28:42,220 --> 00:28:45,870
Okay, round three's good to go down there, so round three, what number are

507
00:28:45,870 --> 00:28:47,120
these guys now representing?

508
00:28:50,090 --> 00:28:56,690
Okay, I heard 53, which now represents--interesting.

509
00:28:56,690 --> 00:29:00,770
Now why this sort of counter-intuitive result, right?

510
00:29:00,770 --> 00:29:02,910
If we want to represent five--

511
00:29:02,910 --> 00:29:03,900
we all probably know where this is going--

512
00:29:03,900 --> 00:29:07,360
why don't I just raise the fours place and the ones place?

513
00:29:07,360 --> 00:29:10,060
>> Well, we realize that there's a difference, fundamentally, between how

514
00:29:10,060 --> 00:29:11,760
a computer interprets these bits.

515
00:29:11,760 --> 00:29:15,470
If you're trying to represent the number five, then absolutely, we just

516
00:29:15,470 --> 00:29:18,330
raise your hand number four and raise hand number one.

517
00:29:18,330 --> 00:29:20,010
But we're not representing numbers here.

518
00:29:20,010 --> 00:29:23,690
The context here on stage is that we're representing characters or

519
00:29:23,690 --> 00:29:28,110
chars, and in this context, the computer has to realize that oh, this

520
00:29:28,110 --> 00:29:30,720
pattern of bits is not a number alone.

521
00:29:30,720 --> 00:29:33,210
It's actually representing a higher level concept.

522
00:29:33,210 --> 00:29:35,010
In this case, an alphabetical letter.

523
00:29:35,010 --> 00:29:38,820
So the fact that it is now representing the number five with the

524
00:29:38,820 --> 00:29:44,290
value 53 is because in ASCII, the thing we aesthetically see as the

525
00:29:44,290 --> 00:29:48,200
number five itself needs a pattern of bits-- because why?

526
00:29:48,200 --> 00:29:51,310
Well, the world just decided to use the lower numbers, zero, one, two,

527
00:29:51,310 --> 00:29:54,130
three for what looked to be fairly cryptic things.

528
00:29:54,130 --> 00:29:56,830
And indeed, these are the characters that aren't on a keyboard.

529
00:29:56,830 --> 00:30:00,330
Special expressions that you need in a computer to do interesting things, but

530
00:30:00,330 --> 00:30:02,010
humans never actually type them.

531
00:30:02,010 --> 00:30:04,220
So 53, indeed, represents five.

532
00:30:04,220 --> 00:30:10,460
Now just as a final sanity check, what number should they

533
00:30:10,460 --> 00:30:12,880
represent in just a moment?

534
00:30:12,880 --> 00:30:14,060
Okay, so 48.

535
00:30:14,060 --> 00:30:15,720
And indeed, go ahead, round four.

536
00:30:18,650 --> 00:30:22,680
16 plus 32 is indeed 48.

537
00:30:22,680 --> 00:30:26,510
And so a big round of applause, if we could, for our eight volunteers here.

538
00:30:31,410 --> 00:30:32,660
Very well done.

539
00:30:35,290 --> 00:30:36,490
Any direction is fine.

540
00:30:36,490 --> 00:30:41,050
>> So we now have a way not only of thinking about how to represent data,

541
00:30:41,050 --> 00:30:44,120
and actually representing it physically, but also doing higher

542
00:30:44,120 --> 00:30:45,710
level things on top of.

543
00:30:45,710 --> 00:30:48,430
And indeed, this is going to be a theme throughout computer science of

544
00:30:48,430 --> 00:30:52,410
building more and more interesting and complex things on top of

545
00:30:52,410 --> 00:30:54,030
fairly simple ideas--

546
00:30:54,030 --> 00:30:56,170
in this case, just zeros and ones.

547
00:30:56,170 --> 00:31:00,120
In terms of why this is useful, well, even though in a course like this,

548
00:31:00,120 --> 00:31:03,810
we'll focus on fundamentals, and on programming, and on solving of

549
00:31:03,810 --> 00:31:07,050
problems, you can go off in computer science in any number of directions.

550
00:31:07,050 --> 00:31:10,950
In this case here, this is a chart that you have at the back page of your

551
00:31:10,950 --> 00:31:14,580
unofficial guide to CS at Harvard, one of today's two printouts.

552
00:31:14,580 --> 00:31:17,450
This suggests the many different directions in which you can go after a

553
00:31:17,450 --> 00:31:18,400
course like this--

554
00:31:18,400 --> 00:31:21,530
learning about artificial intelligence, about graphics, about

555
00:31:21,530 --> 00:31:24,370
machine learning, about language itself.

556
00:31:24,370 --> 00:31:26,410
Realize, too, that there are yet other paths.

557
00:31:26,410 --> 00:31:28,670
There are more mathematical paths in computer science.

558
00:31:28,670 --> 00:31:31,740
If you're not even able to take something like CS50 this fall, there's

559
00:31:31,740 --> 00:31:35,070
introductory courses in the spring, Computer Science 1, for instance, is

560
00:31:35,070 --> 00:31:38,050
yet another on-ramp to this new world.

561
00:31:38,050 --> 00:31:41,600
>> Now as an aside, in the interest of solving problems related to courses,

562
00:31:41,600 --> 00:31:46,050
realize CS50 set out some time ago to try to solve one of these problems, a

563
00:31:46,050 --> 00:31:49,640
problem known as my.harvard, which many of you might be using to actually

564
00:31:49,640 --> 00:31:54,040
shop for courses, but if not, check out a tool like this, as well as other

565
00:31:54,040 --> 00:31:57,800
descendants that some of our past students and staff have created.

566
00:31:57,800 --> 00:32:00,770
But in Harvard Courses, which is a web-based tool, something that you

567
00:32:00,770 --> 00:32:05,510
will be capable of designing and deploying yourselves, as well as yet

568
00:32:05,510 --> 00:32:09,210
other things as well by the end of the semester, realize that this builds

569
00:32:09,210 --> 00:32:10,590
upon an open data set.

570
00:32:10,590 --> 00:32:13,490
In this case, of course, catalog, and allows students in this case to

571
00:32:13,490 --> 00:32:16,320
explore a fairly complex data set.

572
00:32:16,320 --> 00:32:19,495
>> We dug up last night a few statistics based on the few thousand folks who

573
00:32:19,495 --> 00:32:21,530
have been using this over the past few days.

574
00:32:21,530 --> 00:32:25,260
If you've been curious to know how many courses your friends actually

575
00:32:25,260 --> 00:32:29,720
tend to shop, well, today this data suggests that 7.6 is the average

576
00:32:29,720 --> 00:32:32,610
number of courses on someone's shopping list.

577
00:32:32,610 --> 00:32:36,350
And now I'll give you also the statistic of the most number of

578
00:32:36,350 --> 00:32:39,550
courses on someone's shopping list-- and we all probably know

579
00:32:39,550 --> 00:32:40,800
someone like this.

580
00:32:43,820 --> 00:32:46,740
201 is this year's record.

581
00:32:46,740 --> 00:32:52,320
Now some of our former students and staff actually put together a clip to

582
00:32:52,320 --> 00:32:55,560
paint a picture for you of what this path of computer science

583
00:32:55,560 --> 00:32:57,360
and CS50 itself is.

584
00:32:57,360 --> 00:33:02,060
Let me go ahead and pull up, thanks to Mr. Harvahd here, a video produced by

585
00:33:02,060 --> 00:33:06,825
some of your predecessors, if we could keep the lights up for this.

586
00:33:10,980 --> 00:35:39,126
[MUSIC PLAYING]

587
00:35:39,126 --> 00:35:42,630
>> SPEAKER 1: I had no idea that was going to happen.

588
00:35:42,630 --> 00:35:46,170
So a more serious look at what lies ahead.

589
00:35:46,170 --> 00:35:49,060
So in terms of the expectations of this course, you're indeed expected to

590
00:35:49,060 --> 00:35:52,817
attend or watch the course's lecture, submit eight problem sets, take two

591
00:35:52,817 --> 00:35:54,720
quizzes, submit a final project.

592
00:35:54,720 --> 00:35:58,640
In terms of grades, realize that my comments at the opening about

593
00:35:58,640 --> 00:36:02,380
pass-fail, something that we very much take to heart in CS50--there is not

594
00:36:02,380 --> 00:36:05,110
nearly enough of a culture at Harvard of trying

595
00:36:05,110 --> 00:36:07,080
something and risking failure.

596
00:36:07,080 --> 00:36:09,870
Indeed, we had numbers of students, and myself in particular, who was

597
00:36:09,870 --> 00:36:15,310
worried about hurting your GPA, or getting a B in something like CS50,

598
00:36:15,310 --> 00:36:19,222
and the opportunity to take a course like this and other gateway courses at

599
00:36:19,222 --> 00:36:23,950
the introductory level pass/fail is a very underutilized opportunity at this

600
00:36:23,950 --> 00:36:24,740
college in general.

601
00:36:24,740 --> 00:36:29,750
And so please know, even I enrolled in this course initially for pass/fail

602
00:36:29,750 --> 00:36:32,840
credit alone, and even though I did switch at the end of the day, it was

603
00:36:32,840 --> 00:36:36,170
those five initial weeks, up to the fifth Monday of the semester, which is

604
00:36:36,170 --> 00:36:39,700
the cutoff, that allowed me to actually put foot into these new

605
00:36:39,700 --> 00:36:43,660
waters, and actually try something very unfamiliar and very uncomfortable

606
00:36:43,660 --> 00:36:45,540
for me at the time.

607
00:36:45,540 --> 00:36:49,440
>> So in terms now of what role the various angles by which you can

608
00:36:49,440 --> 00:36:51,530
approach this course serve--

609
00:36:51,530 --> 00:36:53,080
so lectures. It's up to you.

610
00:36:53,080 --> 00:36:56,550
If you engage with us in person at this venue, indeed, we know

611
00:36:56,550 --> 00:37:00,920
statistically that roughly 40% of you will kind of come and go over the

612
00:37:00,920 --> 00:37:05,140
course of the semester, and 10% of you we will never see again after today.

613
00:37:05,140 --> 00:37:07,540
And that's perfectly fine, to be honest.

614
00:37:07,540 --> 00:37:10,470
One of the defining characteristics of CS50 is that there are these

615
00:37:10,470 --> 00:37:14,220
innumerable resources, some of which we'll rattle through in just a moment,

616
00:37:14,220 --> 00:37:17,890
including lectures and sections, and things called walkthroughs, and office

617
00:37:17,890 --> 00:37:20,850
hours, and the like, and it's more resources than the typical student

618
00:37:20,850 --> 00:37:24,400
should have to or could physically take advantage of, but that's because

619
00:37:24,400 --> 00:37:27,510
of the disparate learning styles that any student body manifests.

620
00:37:27,510 --> 00:37:31,590
And so in lectures, the primary role as I see it is not to verbally push

621
00:37:31,590 --> 00:37:35,360
out fairly complex material and to necessarily deliver all of the

622
00:37:35,360 --> 00:37:39,040
intricacies of the fundamentals that we'll explore this semester, but

623
00:37:39,040 --> 00:37:42,430
rather, to do things like we've been doing thus far already, these examples

624
00:37:42,430 --> 00:37:46,480
involving humans on stage trying to paint a mental picture, and also

625
00:37:46,480 --> 00:37:49,430
create, dare I say, some of these memorable moments, so that even as you

626
00:37:49,430 --> 00:37:53,280
struggle with certain topics, you have these memories, like oh, even though

627
00:37:53,280 --> 00:37:56,740
that was fairly abstractly, the math, I got lost with carrying a one, like,

628
00:37:56,740 --> 00:38:00,260
it really, at the end of the day, is not all that dissimilar to something I

629
00:38:00,260 --> 00:38:01,060
already know.

630
00:38:01,060 --> 00:38:04,560
And so the role that lectures will serve either in person here in Sanders

631
00:38:04,560 --> 00:38:09,040
or online on video is really to set the stage mentally for you each week

632
00:38:09,040 --> 00:38:12,370
for the various concepts and problems that we'll be diving into.

633
00:38:12,370 --> 00:38:16,800
>> In terms of the high level concepts, most of these words might flow over

634
00:38:16,800 --> 00:38:17,670
your head for the moment.

635
00:38:17,670 --> 00:38:18,680
And that's fine.

636
00:38:18,680 --> 00:38:21,210
Those of you who will come into the course more comfortable will know of

637
00:38:21,210 --> 00:38:24,870
some of these topics, but typically for that 10% of the class for which

638
00:38:24,870 --> 00:38:28,600
they have much more background, taking AP computer science, programming since

639
00:38:28,600 --> 00:38:31,850
they were 12, realize that there will be opportunities in sections and

640
00:38:31,850 --> 00:38:36,030
problem sets to go all the more depth into various topics, filling in

641
00:38:36,030 --> 00:38:39,590
whatever gaps you might have from your high school or prior background.

642
00:38:39,590 --> 00:38:43,440
In terms of the languages, realize that what language we use in CS50 is

643
00:38:43,440 --> 00:38:45,360
largely irrelevant at the end of the day.

644
00:38:45,360 --> 00:38:49,500
We happen to use primarily a language called C. Toward the end of the

645
00:38:49,500 --> 00:38:53,260
semester, we introduce web-centric languages, like PHP and JavaScript.

646
00:38:53,260 --> 00:38:56,800
But we and others could teach a course like this in most any modern

647
00:38:56,800 --> 00:38:57,740
high-level language--

648
00:38:57,740 --> 00:39:01,710
Python, and Ruby, and others are quite popular these days, because realize,

649
00:39:01,710 --> 00:39:04,350
at the end of the day, you're not learning in this C. You're not

650
00:39:04,350 --> 00:39:06,310
learning PHP or JavaScript.

651
00:39:06,310 --> 00:39:10,190
You're learning how to solve problems, whether web-based, computer-based, or

652
00:39:10,190 --> 00:39:14,810
data-oriented itself, using these simply as tools.

653
00:39:14,810 --> 00:39:17,910
>> Now in terms of the logistics, you'll use something eventually called the

654
00:39:17,910 --> 00:39:18,810
CS50 Appliance.

655
00:39:18,810 --> 00:39:22,150
Does not matter if you have a Mac, a PC, a Linux computer, or the like,

656
00:39:22,150 --> 00:39:25,740
you'll have freely available software starting next week with which to use

657
00:39:25,740 --> 00:39:28,890
the CS50 Appliance, a virtual environment that you'll use on your

658
00:39:28,890 --> 00:39:32,720
own computer, so that you and all of your classmates have a uniform Linux

659
00:39:32,720 --> 00:39:34,220
desktop in this case.

660
00:39:34,220 --> 00:39:37,010
It's the problem sets, though, in which you'll really get your hands

661
00:39:37,010 --> 00:39:37,910
dirty in the course.

662
00:39:37,910 --> 00:39:40,820
And at the end of the day, it's the problem sets, I think, that really

663
00:39:40,820 --> 00:39:43,510
define a student's experience in this course.

664
00:39:43,510 --> 00:39:46,660
Realize that many of the problem sets will be released in two editions--

665
00:39:46,660 --> 00:39:50,140
a standard edition that we expect and encourage 90% of the

666
00:39:50,140 --> 00:39:51,690
class to dive into.

667
00:39:51,690 --> 00:39:55,255
But we also released some problem sets in so-called hacker editions, and you

668
00:39:55,255 --> 00:39:58,110
know it's the hacker edition, because on every page with a watermark, it

669
00:39:58,110 --> 00:39:59,970
says hacker edition on it.

670
00:39:59,970 --> 00:40:03,390
And that's for this demographic of you who have AP computer science with 10

671
00:40:03,390 --> 00:40:06,500
years of programming under your belt, and are looking to fill in those gaps,

672
00:40:06,500 --> 00:40:10,170
and to have more formal, rather than self-taught training perhaps, realize

673
00:40:10,170 --> 00:40:13,420
that there is a very substantial demographic in the class that has

674
00:40:13,420 --> 00:40:15,230
precisely that same goal.

675
00:40:15,230 --> 00:40:16,460
You'll have five late days.

676
00:40:16,460 --> 00:40:19,380
Problem sets are generally due on Thursdays, but you can extend five of

677
00:40:19,380 --> 00:40:22,900
those deadlines using these things called late days, and we'll also drop

678
00:40:22,900 --> 00:40:25,520
your lowest score at the end of the semester, per the

679
00:40:25,520 --> 00:40:27,540
particulars in the syllabus.

680
00:40:27,540 --> 00:40:31,630
>> But another defining characteristic of CS50 over the years has become office

681
00:40:31,630 --> 00:40:36,060
hours, an opportunity that you saw visually in photos a bit ago in which

682
00:40:36,060 --> 00:40:39,090
we gather previously in the house dining halls, prior to that in the

683
00:40:39,090 --> 00:40:42,120
basement of the science center, and this year in Annenberg Hall, four

684
00:40:42,120 --> 00:40:45,670
nights a week from 8:00 PM to 11:00 PM, where you'll have this very much

685
00:40:45,670 --> 00:40:49,810
shared experience of working on, struggling through certain problems,

686
00:40:49,810 --> 00:40:52,520
but with a substantial support structure in place.

687
00:40:52,520 --> 00:40:55,780
Indeed, the way this will work is you'll arrive at Annenberg if you have

688
00:40:55,780 --> 00:40:56,950
some question during the week.

689
00:40:56,950 --> 00:41:00,190
You'll bring your laptop, you'll sit down, grab some food, and you'll log

690
00:41:00,190 --> 00:41:04,100
in to CS50 Discuss, a web-based utility that the teaching staff has

691
00:41:04,100 --> 00:41:07,860
developed that will allow you to post questions and see followups in a

692
00:41:07,860 --> 00:41:12,590
typical discussion forum sense, using labels and the like, and auto-complete

693
00:41:12,590 --> 00:41:13,790
to search the data.

694
00:41:13,790 --> 00:41:17,950
But you'll also be able to, during the hours of office hours, have your

695
00:41:17,950 --> 00:41:20,490
questions escalated to actual human beings.

696
00:41:20,490 --> 00:41:24,000
Indeed, the goal ultimately is so that one, we begin to build up over the

697
00:41:24,000 --> 00:41:27,740
course of the semester a corpus of hopefully really useful information,

698
00:41:27,740 --> 00:41:31,770
common answers to common questions, so that you yourself can solve problems

699
00:41:31,770 --> 00:41:35,980
and get unstuck as quickly as possible, while having the teaching

700
00:41:35,980 --> 00:41:40,320
staff--usually 20% to 30% of the teaching fellows and course assistants

701
00:41:40,320 --> 00:41:41,780
on staff at once.

702
00:41:41,780 --> 00:41:45,150
>> We will have what's called the CS50 Greeter in Annenberg, and when we

703
00:41:45,150 --> 00:41:46,610
determine that, you know what?

704
00:41:46,610 --> 00:41:50,900
This question, we really answer effectively online.

705
00:41:50,900 --> 00:41:51,930
We need to see your computer.

706
00:41:51,930 --> 00:41:53,470
We want to talk to you one on one.

707
00:41:53,470 --> 00:41:56,060
You're really struggling, and you therefore want to talk one on one

708
00:41:56,060 --> 00:41:59,160
alongside someone, you'll be dispatched to the CS50 Greeter, a

709
00:41:59,160 --> 00:42:02,700
teaching fellow holding, literally, an iPod that has students' names on one

710
00:42:02,700 --> 00:42:04,880
side, teaching staff's names on the other.

711
00:42:04,880 --> 00:42:07,900
We will collect your name, followed by the name of a teaching staff, and your

712
00:42:07,900 --> 00:42:12,020
computer screen will start blinking, saying please go see Alice or please

713
00:42:12,020 --> 00:42:14,110
go see Bob at the staff table.

714
00:42:14,110 --> 00:42:17,240
And so in this way, we'll be able to dispatch things as efficiently as

715
00:42:17,240 --> 00:42:22,030
possible, as well as guide you toward solutions all the more readily.

716
00:42:22,030 --> 00:42:25,900
In sections, these will be opportunities for more intimate hands-

717
00:42:25,900 --> 00:42:30,080
on opportunities with one of the teaching fellows and 12 to 16 or so of

718
00:42:30,080 --> 00:42:34,020
your classmates, in which each week, we'll have problems in the problem set

719
00:42:34,020 --> 00:42:36,750
that ask a number of conceptual questions, and a number of bite-sized

720
00:42:36,750 --> 00:42:39,180
programming questions that you could figure out on your own, and you could

721
00:42:39,180 --> 00:42:42,140
work on your own, but in the context of section, we'll work through

722
00:42:42,140 --> 00:42:45,230
collectively some of those problems and go where the

723
00:42:45,230 --> 00:42:46,700
conversation takes us.

724
00:42:46,700 --> 00:42:51,340
>> In addition, in section will you have opportunities to review submissions of

725
00:42:51,340 --> 00:42:54,470
homework that you've made, your classmates, sometimes anonymized,

726
00:42:54,470 --> 00:42:56,780
always by an opt-in, if you would like to share the

727
00:42:56,780 --> 00:42:57,930
work that you've submitted.

728
00:42:57,930 --> 00:43:00,890
So it'll really be a two-directional conversation, and opportunity to

729
00:43:00,890 --> 00:43:05,050
review your own work in a much more dynamic sense, rather than simply

730
00:43:05,050 --> 00:43:08,620
looking at a PDF or printout and thinking about it for a few seconds

731
00:43:08,620 --> 00:43:10,760
and not necessarily absorbing the feedback that the

732
00:43:10,760 --> 00:43:12,270
teaching staff have provided.

733
00:43:12,270 --> 00:43:14,670
And you'll use a tool here called CS50 Spaces.

734
00:43:14,670 --> 00:43:18,165
For those unfamiliar, this is the language known as C at top left, and

735
00:43:18,165 --> 00:43:21,360
you'll get to know this over time, but this is a web-based utility that we'll

736
00:43:21,360 --> 00:43:25,350
use in section that will allow you and your 15 or so classmates to log in with

737
00:43:25,350 --> 00:43:27,120
your teaching fellow at the front of the room.

738
00:43:27,120 --> 00:43:28,890
You'll be able to write code in this window.

739
00:43:28,890 --> 00:43:31,800
You'll be able to chat electronically if you're not actually at section at

740
00:43:31,800 --> 00:43:35,300
that particular moment, and your teaching fellow, when it comes time to

741
00:43:35,300 --> 00:43:41,295
discuss Alice or Bob's solution in class, the teaching fellow can click a

742
00:43:41,295 --> 00:43:45,210
button, and voila, project onto the screen whatever that student has been

743
00:43:45,210 --> 00:43:47,120
working on at that particular point in time.

744
00:43:47,120 --> 00:43:50,400
So for those of you have friends who have taken CS50 in the past, realize

745
00:43:50,400 --> 00:43:53,790
that sections have been significantly rebooted this year to be all the more

746
00:43:53,790 --> 00:43:57,700
active, all the more dynamic, and really, a two-way conversation between

747
00:43:57,700 --> 00:44:00,300
teaching staff and students.

748
00:44:00,300 --> 00:44:01,160
>> And walkthroughs.

749
00:44:01,160 --> 00:44:04,520
So for these problem sets, we also offer not only the specification

750
00:44:04,520 --> 00:44:08,110
itself, which is generally a fairly detailed PDF, but also things known

751
00:44:08,110 --> 00:44:12,070
as walkthroughs, whereby one member of the teaching staff will lead a weekly

752
00:44:12,070 --> 00:44:15,460
session that literally walks you through the problem set, giving you

753
00:44:15,460 --> 00:44:18,700
hints, and advice, and starting points, and is meant to ask the very

754
00:44:18,700 --> 00:44:21,370
frequently asked question: where do I begin?

755
00:44:21,370 --> 00:44:25,550
Well, you begin either by diving into the spec on its own, or by attending

756
00:44:25,550 --> 00:44:27,230
or watching these walkthroughs.

757
00:44:27,230 --> 00:44:29,550
The first walkthrough, in fact, will be this Friday.

758
00:44:29,550 --> 00:44:32,120
They'll be on Fridays, not so much because we think it'll be a popular

759
00:44:32,120 --> 00:44:34,940
time, but because we can then film them very early in the week and get

760
00:44:34,940 --> 00:44:38,410
them online by the weekend so that you have as many days as possible to

761
00:44:38,410 --> 00:44:40,530
actually engage in that content as well.

762
00:44:40,530 --> 00:44:43,050
But more on that in lecture this Friday.

763
00:44:43,050 --> 00:44:46,930
>> Now in terms of the support structure, the most significant statistic is

764
00:44:46,930 --> 00:44:50,600
perhaps the 108 teaching fellows and course assistants that this course

765
00:44:50,600 --> 00:44:51,870
currently has.

766
00:44:51,870 --> 00:44:54,490
If some of you who don't have conflicting classes would like to join

767
00:44:54,490 --> 00:44:58,900
me up here on stage, it is these guys who will ultimately really define your

768
00:44:58,900 --> 00:45:00,150
experience in the course.

769
00:45:08,890 --> 00:45:11,870
I had a lot of teaching fellows teaching me classes in the day, and I

770
00:45:11,870 --> 00:45:15,650
remember very few of those, frankly, but to date, I still remember among

771
00:45:15,650 --> 00:45:19,990
those few my CS50 TF, who really helped me answer questions, who really

772
00:45:19,990 --> 00:45:22,830
helped me when I was struggling, and really was a partner in this

773
00:45:22,830 --> 00:45:25,770
experience of learning a very new world.

774
00:45:25,770 --> 00:45:29,100
In a little bit, all of these guys will join you outside for cake, which

775
00:45:29,100 --> 00:45:32,560
is a tradition of CS50, in the transept of Memorial Hall, but let me

776
00:45:32,560 --> 00:45:36,980
first introduce you to Nate Hardison again, Rob Bowden, and Tommy

777
00:45:36,980 --> 00:45:39,370
McWilliams, this year's course heads.

778
00:45:39,370 --> 00:45:41,385
If you guys would join me here in the middle.

779
00:45:47,380 --> 00:45:50,045
They have all prepared some inspirational remarks.

780
00:45:53,212 --> 00:45:55,775
>> TOMMY MCWILLIAMS: I didn't prepare anything inspirational.

781
00:45:55,775 --> 00:45:57,170
But my name is Tommy.

782
00:45:57,170 --> 00:45:58,370
I'm a senior in Mather.

783
00:45:58,370 --> 00:46:01,620
I'm studying computer science, really excited to be on the heads team and

784
00:46:01,620 --> 00:46:03,720
going through the CS50 journey with you.

785
00:46:03,720 --> 00:46:06,750
What I really love about CS50 is how it really teaches you to think about

786
00:46:06,750 --> 00:46:08,180
problems in a new way.

787
00:46:08,180 --> 00:46:11,170
This is really a skill that's going to be invaluable, no matter what field

788
00:46:11,170 --> 00:46:12,090
you go into.

789
00:46:12,090 --> 00:46:14,800
And not only that, but we offer more free candy than any

790
00:46:14,800 --> 00:46:16,050
other course on campus.

791
00:46:22,730 --> 00:46:24,350
So I'm just really looking forward to seeing what

792
00:46:24,350 --> 00:46:25,645
everyone builds this semester.

793
00:46:25,645 --> 00:46:28,860
And if anyone has any questions now or throughout the semester, definitely

794
00:46:28,860 --> 00:46:32,976
feel free to reach out to me, and I'd be happy to help.

795
00:46:32,976 --> 00:46:33,920
>> ROB BOWDEN: Hi.

796
00:46:33,920 --> 00:46:34,670
I'm Rob Bowden.

797
00:46:34,670 --> 00:46:35,920
I'm a senior in Kirkland.

798
00:46:40,030 --> 00:46:41,890
Yeah, that's right.

799
00:46:41,890 --> 00:46:44,070
We're all really excited for this next semester.

800
00:46:44,070 --> 00:46:45,325
We hope you're all excited.

801
00:46:49,850 --> 00:46:51,580
I wasn't expecting that.

802
00:46:51,580 --> 00:46:52,530
Yeah.

803
00:46:52,530 --> 00:46:56,030
So we put so much effort into making this semester really great, and as

804
00:46:56,030 --> 00:46:58,830
long as you're willing to put in the effort, there's so much you can get

805
00:46:58,830 --> 00:47:00,080
out of this course.

806
00:47:06,810 --> 00:47:10,470
You can get a lot of fun out of this course.

807
00:47:10,470 --> 00:47:13,510
We wouldn't have a staff of 108 if you couldn't get a lot of fun out of it,

808
00:47:13,510 --> 00:47:17,450
so just try to be involved, and you won't regret it.

809
00:47:24,815 --> 00:47:26,020
>> NATE HARDISON: Hi, guys.

810
00:47:26,020 --> 00:47:26,930
I'm Nate.

811
00:47:26,930 --> 00:47:29,700
I'm the preceptor for the course.

812
00:47:29,700 --> 00:47:31,200
I'm really excited to be here as well.

813
00:47:31,200 --> 00:47:33,100
This is my first year here.

814
00:47:33,100 --> 00:47:36,520
I hope you all take this course, and enjoy it as much as I've

815
00:47:36,520 --> 00:47:37,960
enjoyed it so far.

816
00:47:37,960 --> 00:47:41,520
And if you ever want to learn how to come to nine or ten in binary, come

817
00:47:41,520 --> 00:47:43,351
talk to me.

818
00:47:43,351 --> 00:47:46,480
>> SPEAKER 1: So at the risk of leaving these guys here on stage a bit

819
00:47:46,480 --> 00:47:50,330
awkwardly, let's whirl through just a few of the things that await before we

820
00:47:50,330 --> 00:47:51,500
adjourn for cake.

821
00:47:51,500 --> 00:47:52,980
What is it that lies ahead?

822
00:47:52,980 --> 00:47:56,410
Well, if we take a quick look back at last year, in problem set zero, your

823
00:47:56,410 --> 00:47:59,960
predecessors dove into a programming language called Scratch, a graphical

824
00:47:59,960 --> 00:48:03,160
programming language you'll use in the first days of the course, starting

825
00:48:03,160 --> 00:48:07,080
this Friday, to learn some concepts unfamiliar to some of you, but realize

826
00:48:07,080 --> 00:48:10,330
there will be an advanced aspect of this for those of you with prior

827
00:48:10,330 --> 00:48:11,070
background.

828
00:48:11,070 --> 00:48:14,082
>> In last year's problem set two, students dove into the world of

829
00:48:14,082 --> 00:48:17,740
cryptography, the art of enciphering or scrambling information,

830
00:48:17,740 --> 00:48:21,310
implementing programs that encrypted data, and in the hacker edition last

831
00:48:21,310 --> 00:48:26,710
year did students proceed to crack or decode the passwords in a typical

832
00:48:26,710 --> 00:48:31,190
computer's .xc password file by coming up with algorithms and heuristics for

833
00:48:31,190 --> 00:48:34,980
brute force figuring out what someone's password on a computer

834
00:48:34,980 --> 00:48:35,770
system was.

835
00:48:35,770 --> 00:48:39,820
Last year or two, in problem set four did students

836
00:48:39,820 --> 00:48:41,420
implement the game of Sudoku.

837
00:48:41,420 --> 00:48:45,180
And the hacker edition that year did students not just implement how to

838
00:48:45,180 --> 00:48:49,360
play the game, but actually a solver, whereby the computer can provide you,

839
00:48:49,360 --> 00:48:52,260
the human, with hints, by more rapidly than you solving

840
00:48:52,260 --> 00:48:53,550
that particular problem.

841
00:48:53,550 --> 00:48:57,420
>> In problem set five, we did forensics, this art of recovering information

842
00:48:57,420 --> 00:49:00,160
that was accidentally or very deliberately

843
00:49:00,160 --> 00:49:01,450
deleted from the computer.

844
00:49:01,450 --> 00:49:04,530
Last year, the teaching staff and I strolled around campus taking

845
00:49:04,530 --> 00:49:07,780
photographs of people, places, and things and then accidentally

846
00:49:07,780 --> 00:49:10,900
formatted the media card on our camera that had all those photos.

847
00:49:10,900 --> 00:49:15,920
But no problem, we made a forensics image of this media card, handed it out

848
00:49:15,920 --> 00:49:19,410
to all students in the class, and challenged them to write programs that

849
00:49:19,410 --> 00:49:22,240
recovered all of the JPEGs from that card.

850
00:49:22,240 --> 00:49:24,720
And this is actually one of our favorite problem sets, and I dug up an

851
00:49:24,720 --> 00:49:27,750
old email from one of your predecessors which was great fun to

852
00:49:27,750 --> 00:49:29,940
read sometime ago.

853
00:49:29,940 --> 00:49:31,270
>> This is from Matt.

854
00:49:31,270 --> 00:49:34,970
Dear David, yesterday my sister accidentally formatted her camera's SD

855
00:49:34,970 --> 00:49:37,770
card and lost a year's worth of memorable photos.

856
00:49:37,770 --> 00:49:40,830
She unfortunately isn't the best at backing up her data.

857
00:49:40,830 --> 00:49:43,780
But this situation reminded me of p-set five, so I thought I would try to

858
00:49:43,780 --> 00:49:47,620
run her SD card through the recover program that I wrote all the way back

859
00:49:47,620 --> 00:49:48,180
in October.

860
00:49:48,180 --> 00:49:51,660
So after four hours of figuring out how to create a raw image from the

861
00:49:51,660 --> 00:49:53,350
formatted SD card--

862
00:49:53,350 --> 00:49:56,050
Google prove to be pretty helpful in this regard, until I ironically

863
00:49:56,050 --> 00:50:00,140
happened to come across your instructions on the Internet--

864
00:50:00,140 --> 00:50:03,080
after tinkering around with some of the command line arguments, I managed

865
00:50:03,080 --> 00:50:06,020
to create the forensic image, and after installing and configuring the

866
00:50:06,020 --> 00:50:10,600
CS50 Appliance, I managed to run the forensic image through my program and

867
00:50:10,600 --> 00:50:16,065
recover all 1,027 of my sister's photographs, writes Matt.

868
00:50:25,210 --> 00:50:28,600
>> In last year's problem set six, we gave the students a dictionary of

869
00:50:28,600 --> 00:50:33,350
150,000 English words and challenged them to write a spell checker that

870
00:50:33,350 --> 00:50:36,870
answered queries of the form, is this word spelled correctly or incorrectly

871
00:50:36,870 --> 00:50:38,150
as fast as possible.

872
00:50:38,150 --> 00:50:41,900
And on opt-in basis were students allowed to then challenge classmates

873
00:50:41,900 --> 00:50:45,220
by posting their results, the amount of RAM that they use, the number of

874
00:50:45,220 --> 00:50:49,190
CPU cycles or seconds that they used, so that students were then ranked on

875
00:50:49,190 --> 00:50:53,010
the course's website-- again, purely optional aspect of it, but great fun

876
00:50:53,010 --> 00:50:56,970
in that very often would a student get to position number 10 or so on the big

877
00:50:56,970 --> 00:51:00,370
board, on the website, go off to dinner and then come back, and realize

878
00:51:00,370 --> 00:51:04,340
his roommate had just edged in front of him or her on the big board,

879
00:51:04,340 --> 00:51:07,620
thereby pouring another two or three hours just to one

880
00:51:07,620 --> 00:51:09,300
up his or her roommate.

881
00:51:09,300 --> 00:51:11,730
>> So we look forward to something similar this year as well.

882
00:51:11,730 --> 00:51:15,400
In problem set 7 did we steer in the direction of web programming, actually

883
00:51:15,400 --> 00:51:19,320
solving problems in the ever-increasingly common environment

884
00:51:19,320 --> 00:51:20,300
of a web browser.

885
00:51:20,300 --> 00:51:23,670
Know then decreasingly do we download software on Macs and PCs, but

886
00:51:23,670 --> 00:51:25,790
increasingly, do we do it all within the web.

887
00:51:25,790 --> 00:51:30,140
>> And indeed, last year, some 88% of students' final projects in the course

888
00:51:30,140 --> 00:51:31,790
were web-based.

889
00:51:31,790 --> 00:51:34,670
And those, too, are skills that you will derive from this class by

890
00:51:34,670 --> 00:51:38,410
course's end, because what awaits at course's end is the CS50 Fair, this

891
00:51:38,410 --> 00:51:42,100
exhibition that's based on the idea of a science fair, but in this version of

892
00:51:42,100 --> 00:51:45,370
a fair do all students in the class bring their laptops, and their

893
00:51:45,370 --> 00:51:49,720
friends, and family, and others to Northwest Science, a large building on

894
00:51:49,720 --> 00:51:53,420
campus, set up their laptop, get some food, get some popcorn and drink, and

895
00:51:53,420 --> 00:51:57,180
then exhibit their final projects for all those in attendance, who last year

896
00:51:57,180 --> 00:52:01,510
numbered some 2,500 attendees from across campus, and expressions like

897
00:52:01,510 --> 00:52:05,510
this, and like this were not uncommon at the fair.

898
00:52:05,510 --> 00:52:09,660
>> Leading up to the fair is the CS50 Hackathon, an opportunity to hop on a

899
00:52:09,660 --> 00:52:13,610
Harvard shuttle, head down the street to Microsoft at 8:00 PM, and not go home

900
00:52:13,610 --> 00:52:14,810
until 7:00 AM.

901
00:52:14,810 --> 00:52:19,040
We serve first dinner at 8:00 PM, second dinner at 1:00 AM, and for those still

902
00:52:19,040 --> 00:52:22,540
standing at 5:00 AM do we treat to pancakes at IHOP.

903
00:52:22,540 --> 00:52:26,580
And the Hackathon is an opportunity, as pictured here, to dive into your

904
00:52:26,580 --> 00:52:30,470
final projects, whether working on your own or with friends in a

905
00:52:30,470 --> 00:52:33,770
collaborative environment, where the entire teaching staff is working well

906
00:52:33,770 --> 00:52:37,560
into the night with an ample supply of Hong Kong Chinese food.

907
00:52:37,560 --> 00:52:43,070
At 5:00 AM will such images as these be quite common this year as well.

908
00:52:43,070 --> 00:52:48,170
>> So as we adjourn in a moment for cake, keep in mind that 76% percent of the

909
00:52:48,170 --> 00:52:51,720
people in this room have no prior experience, and as per the syllabus,

910
00:52:51,720 --> 00:52:55,300
what ultimately matters in this course is not so much where you end up

911
00:52:55,300 --> 00:52:59,540
relative to your classmates, but where you, in week 11, end up relative to

912
00:52:59,540 --> 00:53:01,540
yourself in week 0.

913
00:53:01,540 --> 00:53:03,690
This is CS50.