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[MUSIC PLAYING]

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DAVID J. MALAN: This is CS50.

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So I was where you are now some years ago.

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And when I was a freshman at the time, I wasn't really on

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this path of doing computer science, of doing 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

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liked things like history.

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I liked constitutional law, kind of English and math.

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It's like kind of well-rounded, but didn't

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necessarily know things that I hadn't been

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taught in high school.

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And so freshman year, I had this trepidation whereby even

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though I liked computers, played computer games and the

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like, I certainly never thought of myself as a

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computer person, a computer scientist.

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And frankly, I thought my friends in high school who

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were taking computer science were bit of a geeks.

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And yet, when I got here on campus, there was

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this course, CS50.

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>> And at the time, it had this reputation of really being

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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

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through as many of the prerequisites as I could,

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cross-counting things for gen ed or core and the like.

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And then sophomore year, for some reason, I got up the

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nerve to step foot in Science Center B, where CS50 was.

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A very famous computer scientist by the name of Brian

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Kernighan was teaching here that year.

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And even then, I was only willing to actually fill out

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my study card by taking this class pass/fail.

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I looked around me and I figured everyone in this one's

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gotta know way more about computer science, about

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programming, about computers.

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Everyone one must be programming, in this room,

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since they were 12.

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But, indeed, that wasn't the case.

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And so the very last day, the fifth Monday of the semester,

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did I take this leap and change from pass/fail to a

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letter grade and ended up changing my concentration that

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same day to computer science.

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>> Now, that's not our objective in this class, to turn you all

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into computer science concentrators, but really to

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propose that there's an opportunity in this field and

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in other fields with which you might be quite unfamiliar

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given that high schools typically follow a fairly

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standard curricular path, but to venture in, in CS50, into

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new waters.

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And if you are sitting here today thinking you don't

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actually belong, so do most of the people to your right and

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to your left.

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Indeed, last year, 76% of the students in this class had no

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prior experience.

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So contrary to what you might think, most of the people

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sitting around here today do not, in fact, have any prior

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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

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themselves in terms of comfort level.

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And there's no one definition of this.

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You just kind of know it if you're not very

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comfortable in CS50.

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And last year, we had 55% in this green pie slice here

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self-describing as less comfortable, students who

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frankly had no idea why they'd even shopped the course on

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that first day.

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But the same 55% remained with us until term's end, as did

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35% who were somewhere in between those more comfortable

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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

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there these days, there's this perception or this

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misconception that computer science is programming.

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And that's absolutely one aspect of computer science.

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But programming, whatever the language is, is really just a

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tool that computer scientists use to solve problems, either

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in the domain of computer science or increasingly these

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days in the physical sciences, the natural sciences, in

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medicine, in humanities, to analyze large sets of data.

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Anywhere now there is computer and data, there's an

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opportunity to apply lessons learned in a class like CS50.

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>> So let's solve a problem that a computer scientist might go

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about solving and try to put some jargon, put some

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conceptual framework, around what might otherwise be some

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fairly abstract idea.

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So this is a telephone.

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You don't see these things too often, though the college

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still seems to have these in the houses and dormitories.

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But back in the day when you wanted to use a phone like

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this, there was no electronic address book

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in your cell phone.

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Rather, you pulled up something

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known as a phone book.

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And these phone books had about 1,000 pages, typically.

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They were sorted from A to Z. And you simply had to find the

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right page to find the person you're looking for in order to

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find their name and their telephone number.

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Now how do you go about looking up

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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

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inefficient, would be start here and start flipping to

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page 4 to page 5 to page 6, and to sort of linearly, along

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a straight line, go through this phone book.

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And even though it's gonna be incredibly tedious, if Mike

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Smith is in this book, I'm eventually gonna reach him

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when I finally flip to the S section of this book.

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Now of course, you don't need to be a computer scientist to

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know that this 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

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to find myself in the M section.

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OK, so M is clearly not what I'm looking for.

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And Mike's to the right, so to speak, of this section.

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And as some of you have seen before, we can literally now

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proceed to tear this problem in half.

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>> [APPLAUSE]

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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.

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But, down the seam, we now have two problems, each of

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which is half as big.

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And we can literally throw that half of the 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.

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And I find myself an the R section.

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So not quite there.

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So again, I can tear this problem in half.

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[APPLAUSE]

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Thank you.

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So now I only have some 250 pages.

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And I can do this again and again and again and go from

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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

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which, if he's in the phone book, Mike Smith should be.

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>> Now, that's an obviously fairly reasonable algorithm,

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and it's a one-time-use algorithm in this case.

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But what can we sort of take away from that?

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Well, the first approach, correct if naive though it

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was, can be described by this straight line.

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So if on the x-axis here we say this is the size of the

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problem, so as the x-axis goes to the right, the

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problem gets bigger.

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What does it mean to be bigger in the

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context of this problem?

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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, it presumably takes more time.

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So that first approach of linearly searching from page 1

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to dot dot dot page 1,000 is a linear procedure, a linear

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algorithm or process.

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And we can 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

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the worst case, take me one more page flip

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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 2 at a time or 4 at a time.

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But even that's not all that fundamentally better.

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Even if it's 2 at a time, yeah, that kind of moves this

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line down a bit, and it means that it takes less time given

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the same number of pages.

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But it's not fundamentally better.

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But what did we just do, and what did all of you do

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instinctively?

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You actually achieved a little something like this,

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logarithmic time, whereby the problem can grow and grow and

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grow but the cost of solving that problem, the time

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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

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just the size of the problem, the number of pages in this

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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, didn't do that example yet this year--

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so if we have 500 pages in the phone book and we double it to

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1,000, in this more intelligent model of flipping

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to the middle, how many more page tears does it take to go

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from 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 just tear it one additional time.

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>> So in short, the size of the problem can grow much faster

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than the cost of 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 the same way.

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And so why don't we do this?

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If you would humor me, albeit awkwardly here in Sanders, go

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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

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process, a computer program if you will, to be executed by

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humans that has just 3 steps.

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We're already on step 1.

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You've stood up.

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And now think to yourself the number 1.

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That is your current number.

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Everyone here is number 1.

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Step 2, pair off with someone standing, add your numbers

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together, and then adopt the sum as your new number.

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One of you should sit down, then repeat.

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>> SPEAKER 1: 205.

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DAVID J. MALAN: What's that?

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SPEAKER 1: 205.

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DAVID J. MALAN: OK.

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SPEAKER 2: He has the other ones.

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DAVID J. MALAN: 205?

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SPEAKER 3: Yeah.

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DAVID J. MALAN: OK.

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3.

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SPEAKER 4: 400.

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SPEAKER 5: 5.

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700.

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DAVID J. MALAN: All right.

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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.

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Here.

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The worst part is you also have to very verbally do

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arithmetic in front of hundreds of Harvard

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undergrads.

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OK.

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Bit of a bug here.

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Okay.

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What's your number?

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SPEAKER 6: Nine.

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DAVID J. MALAN: What's that?

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SPEAKER 6: Nine.

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DAVID J. MALAN: Nine.

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Okay.

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What's your number?

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SPEAKER 7: 179.

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DAVID J. MALAN: 179?

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Okay.

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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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SPEAKER 8: 118.

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DAVID J. MALAN: 118.

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Some smart undergrad start doing the math.

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Okay.

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118, 188.

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What else do we got?

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SPEAKER 9: 71.

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DAVID J. MALAN: 71.

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00:16:13,884 --> 00:16:14,310
SPEAKER 10: 79.

259
00:16:14,310 --> 00:16:15,510
DAVID J. MALAN: 79.

260
00:16:15,510 --> 00:16:17,100
Okay.

261
00:16:17,100 --> 00:16:18,080
SPEAKER 11: 47.

262
00:16:18,080 --> 00:16:19,100
DAVID J. MALAN: 47.

263
00:16:19,100 --> 00:16:21,100
Which, teaching staff, that gives us how many?

264
00:16:24,100 --> 00:16:27,690
705 is the answer.

265
00:16:27,690 --> 00:16:29,905
And that's, in fact, exactly correct.

266
00:16:35,060 --> 00:16:36,970
No, we were actually a little bit off there.

267
00:16:36,970 --> 00:16:40,435
>> But how should this have worked?

268
00:16:40,435 --> 00:16:41,940
What should have just happened?

269
00:16:41,940 --> 00:16:45,560
So, on every iteration of this algorithm, we started with

270
00:16:45,560 --> 00:16:48,000
some number of people standing, and that was the

271
00:16:48,000 --> 00:16:49,350
total number n at first.

272
00:16:49,350 --> 00:16:51,750
Then half of you sat down, and we went to n over 2.

273
00:16:51,750 --> 00:16:52,830
Then half of you sat down.

274
00:16:52,830 --> 00:16:56,310
We went to n over 4, n over 8, n over 16, and so forth,

275
00:16:56,310 --> 00:16:59,080
until, even though it kind of disintegrated there at the

276
00:16:59,080 --> 00:17:02,000
end, in theory, had everyone paired off in balcony and

277
00:17:02,000 --> 00:17:05,780
mezzanine and orchestra here, we would have had just one lone

278
00:17:05,780 --> 00:17:11,790
person standing with a total value, in this case, of 705.

279
00:17:11,790 --> 00:17:14,460
Now, what does that mean, though, for the running time?

280
00:17:14,460 --> 00:17:16,829
Well think about if I as the human had done this manually.

281
00:17:16,829 --> 00:17:19,819
I would have started fairly naively but correctly with 1,

282
00:17:19,819 --> 00:17:23,740
2, 3, 4, 5, 6, 7, 8, and so forth.

283
00:17:23,740 --> 00:17:24,920
Takes quite some time.

284
00:17:24,920 --> 00:17:25,980
So I can do better, right?

285
00:17:25,980 --> 00:17:27,690
In grade school, you don't just count in ones.

286
00:17:27,690 --> 00:17:28,109
You count in twos.

287
00:17:28,109 --> 00:17:30,050
So 2, 4, 6, 8, 10, 12.

288
00:17:30,050 --> 00:17:31,630
And that gets much faster.

289
00:17:31,630 --> 00:17:34,620
But now fundamentally, by leveraging the collective

290
00:17:34,620 --> 00:17:37,070
intelligence of everyone in this room, we can achieve a

291
00:17:37,070 --> 00:17:40,660
curb much more like this, whereby now the number of the

292
00:17:40,660 --> 00:17:42,450
people in this room could double.

293
00:17:42,450 --> 00:17:45,940
Another 700 people walk into this room for 1,400 people,

294
00:17:45,940 --> 00:17:49,680
but it would only take us one more iteration of this

295
00:17:49,680 --> 00:17:50,880
algorithm to solve.

296
00:17:50,880 --> 00:17:53,170
>> And so, increasingly these days, when we have these huge

297
00:17:53,170 --> 00:17:55,860
data sets in Facebook and Google and the like, it's

298
00:17:55,860 --> 00:17:58,710
solving problems with a bit of insight, this bit of

299
00:17:58,710 --> 00:18:02,280
cleverness, that's allowing us increasingly to do much, much

300
00:18:02,280 --> 00:18:05,150
more powerful things with computers today.

301
00:18:05,150 --> 00:18:07,340
If you like these kinds of things, you might have seen on

302
00:18:07,340 --> 00:18:10,650
Facebook CS50's own Puzzle Day coming up this Saturday.

303
00:18:10,650 --> 00:18:12,580
If you would like to participate in something like

304
00:18:12,580 --> 00:18:17,010
this whereby you, in 2 or 3 or 4 teams of 4, would like to

305
00:18:17,010 --> 00:18:20,820
solve some puzzles such as this one, you stand a chance

306
00:18:20,820 --> 00:18:23,510
to win some fabulous prizes, among which is a Wii and some

307
00:18:23,510 --> 00:18:25,900
gift cards or some other Facebook swag.

308
00:18:25,900 --> 00:18:28,660
This Saturday, noon to 3:00 PM, go to

309
00:18:28,660 --> 00:18:33,440
cs50.net/rsvp for such.

310
00:18:33,440 --> 00:18:35,940
And this slide is online if you'd like to play around.

311
00:18:35,940 --> 00:18:38,010
The problems this year shall be new.

312
00:18:38,010 --> 00:18:40,210
>> You may notice in the classroom, too, all the more

313
00:18:40,210 --> 00:18:41,210
cameras this year.

314
00:18:41,210 --> 00:18:45,180
So not only will the course be filmed in the usual way, CS50

315
00:18:45,180 --> 00:18:47,760
may also be taking part in a documentary on higher

316
00:18:47,760 --> 00:18:50,370
education that's looking at the transformative experience

317
00:18:50,370 --> 00:18:53,000
that a student can have these days in an undergraduate

318
00:18:53,000 --> 00:18:53,870
course of study.

319
00:18:53,870 --> 00:18:56,990
So toward these, then, not only will we be filming for

320
00:18:56,990 --> 00:19:00,170
that, we will be filming as well for increasingly our

321
00:19:00,170 --> 00:19:03,670
online audience, as well as on occasion this audience here.

322
00:19:03,670 --> 00:19:05,880
So we welcome to the class this year our Harvard

323
00:19:05,880 --> 00:19:08,190
Extension School students, Graduate School of Design,

324
00:19:08,190 --> 00:19:10,470
Education, the business school, the Kennedy School,

325
00:19:10,470 --> 00:19:12,970
the law school, as well as a number of students from

326
00:19:12,970 --> 00:19:16,140
Belmont, Lexington, Newton, and Watertown high schools.

327
00:19:16,140 --> 00:19:17,120
Welcome to you all.

328
00:19:17,120 --> 00:19:20,070
In addition this year, you may have heard, Harvard and MIT,

329
00:19:20,070 --> 00:19:22,500
and Berkeley now, have entered into a collaborative

330
00:19:22,500 --> 00:19:26,020
partnership, an initiative called edX, which is an

331
00:19:26,020 --> 00:19:28,650
initiative to open up education to all the more

332
00:19:28,650 --> 00:19:31,210
people online and fundamentally start doing

333
00:19:31,210 --> 00:19:34,400
research on a much broader scale as to how people learn.

334
00:19:34,400 --> 00:19:36,920
And so CS50 will be the college's first course

335
00:19:36,920 --> 00:19:38,790
participating in that initiative as well.

336
00:19:38,790 --> 00:19:41,320
Which means you will have access to all the more tools,

337
00:19:41,320 --> 00:19:44,020
all the more curricular content, all the more video

338
00:19:44,020 --> 00:19:48,190
content as a result, as well, as of yesterday morning, the

339
00:19:48,190 --> 00:19:52,210
53,019 people who have registered to take CS50 along

340
00:19:52,210 --> 00:19:55,290
with you this year on the Internet.

341
00:19:55,290 --> 00:19:56,530
So without--

342
00:19:56,530 --> 00:20:03,800
[APPLAUSE]

343
00:20:03,800 --> 00:20:05,680
So what this means, in particular, is that the

344
00:20:05,680 --> 00:20:07,770
teaching staff and I have spent quite a bit of time this

345
00:20:07,770 --> 00:20:10,600
summer preparing for the fall, both on campus and off, so

346
00:20:10,600 --> 00:20:13,580
that we can begin to build up a corpus of interesting, of

347
00:20:13,580 --> 00:20:16,130
compelling, of engaging educational content that

348
00:20:16,130 --> 00:20:22,490
focuses, in particular, on more intimate conveyances of

349
00:20:22,490 --> 00:20:23,960
fairly complex material.

350
00:20:23,960 --> 00:20:26,210
>> So in addition to the course's lectures and sections and

351
00:20:26,210 --> 00:20:28,300
things called walkthroughs, which we'll revisit in just a

352
00:20:28,300 --> 00:20:31,620
bit, we'll also have these shorts this year that allow

353
00:20:31,620 --> 00:20:33,570
you to engage with the course from a different angle

354
00:20:33,570 --> 00:20:34,520
altogether.

355
00:20:34,520 --> 00:20:36,540
So let's use this as an opportunity to take a quick

356
00:20:36,540 --> 00:20:38,905
peek at one that discusses this notion of binaries.

357
00:20:38,905 --> 00:20:42,220
>> So in computer science, there are things called algorithms--

358
00:20:42,220 --> 00:20:44,610
two of which we just took a look at-- these procedures for

359
00:20:44,610 --> 00:20:45,690
solving problems.

360
00:20:45,690 --> 00:20:46,770
But at the end of the day, you need to

361
00:20:46,770 --> 00:20:48,730
represent information somehow.

362
00:20:48,730 --> 00:20:51,640
And you need to represent it in a way that a computer can

363
00:20:51,640 --> 00:20:52,400
understand.

364
00:20:52,400 --> 00:20:54,940
And even if you don't really understand computers and

365
00:20:54,940 --> 00:20:58,150
you're in that 76% right now, you probably have some vague

366
00:20:58,150 --> 00:21:01,860
sense that computers somehow deal in 0s and 1s, the binary

367
00:21:01,860 --> 00:21:03,160
system, so to speak.

368
00:21:03,160 --> 00:21:04,480
Now why is that the case?

369
00:21:04,480 --> 00:21:07,060
Well, it turns out when computers first came about, if

370
00:21:07,060 --> 00:21:09,400
you needed to represent information, you could do it

371
00:21:09,400 --> 00:21:10,600
with electricity.

372
00:21:10,600 --> 00:21:13,460
And though this is a bit of an oversimplification, a very

373
00:21:13,460 --> 00:21:16,190
easy way of recording information is either by

374
00:21:16,190 --> 00:21:18,000
turning that electricity on--

375
00:21:18,000 --> 00:21:20,460
a 1 in binary, so to speak-- or turning

376
00:21:20,460 --> 00:21:22,100
that electricity off.

377
00:21:22,100 --> 00:21:24,080
>> So, if Barry, if you wouldn't mind, could we dim the lights

378
00:21:24,080 --> 00:21:26,000
fully for just a moment?

379
00:21:26,000 --> 00:21:29,730
This here is a very gratuitous binary 0.

380
00:21:29,730 --> 00:21:33,830
If we turn the lights back up, now Sanders Theatre is

381
00:21:33,830 --> 00:21:35,860
representing the binary value of 1.

382
00:21:35,860 --> 00:21:38,710
Unfortunately, with just one bit, with just one set of

383
00:21:38,710 --> 00:21:41,070
lights, we can only represent two numbers in the

384
00:21:41,070 --> 00:21:42,690
world, 0 and 1.

385
00:21:42,690 --> 00:21:44,470
And it'd be nice if computers could count a

386
00:21:44,470 --> 00:21:45,340
bit higher than that.

387
00:21:45,340 --> 00:21:46,370
But indeed they can.

388
00:21:46,370 --> 00:21:49,230
So let me pull up on screen here our friend Nate Hardison

389
00:21:49,230 --> 00:21:52,000
who will give us a quick look over the course of just a few

390
00:21:52,000 --> 00:21:54,680
minutes at this notion of binary.

391
00:21:54,680 --> 00:22:01,870
>> [VIDEO PLAYBACK]

392
00:22:01,870 --> 00:22:03,450
NATE HARDISON: Back when you learned how to read and write

393
00:22:03,450 --> 00:22:08,450
numbers, you learned about the digits 0 to 9.

394
00:22:08,450 --> 00:22:10,840
To write whole numbers larger than 9, you learned that all

395
00:22:10,840 --> 00:22:13,830
you had to do was use some combination of these digits,

396
00:22:13,830 --> 00:22:21,000
as in 52 and 437.

397
00:22:21,000 --> 00:22:23,220
So, this way of writing numbers has a

398
00:22:23,220 --> 00:22:25,620
name, decimal notation.

399
00:22:25,620 --> 00:22:26,690
Why decimal?

400
00:22:26,690 --> 00:22:30,340
Well, the Latin root of a decimal, decem, means 10.

401
00:22:30,340 --> 00:22:33,190
And when you have 10 digits in your notation system, 10

402
00:22:33,190 --> 00:22:35,290
becomes a rather special number.

403
00:22:35,290 --> 00:22:38,950
Let's look at the number 437 written in decimal notation to

404
00:22:38,950 --> 00:22:40,340
understand why.

405
00:22:40,340 --> 00:22:50,200
>> We can first break up 437 into 400 plus 30 plus 7.

406
00:22:50,200 --> 00:22:56,050
We can take it apart even more so that we've got 4 times 100

407
00:22:56,050 --> 00:23:02,990
plus 3 times 10 plus 7 times 1.

408
00:23:02,990 --> 00:23:05,970
Remember learning about the ones place, the tens place,

409
00:23:05,970 --> 00:23:08,050
the hundreds place, and so on?

410
00:23:08,050 --> 00:23:10,990
This is exactly where that comes from.

411
00:23:10,990 --> 00:23:14,150
And finally, we can see we've got a bunch of powers of 10

412
00:23:14,150 --> 00:23:15,770
embedded in here.

413
00:23:15,770 --> 00:23:25,330
We've got 4 times 10 to the 2 plus 3 times 10 to the 1 plus

414
00:23:25,330 --> 00:23:29,580
7 times 10 to the 0.

415
00:23:29,580 --> 00:23:31,330
So now you see why 10 is a special

416
00:23:31,330 --> 00:23:33,240
number of decimal notation.

417
00:23:33,240 --> 00:23:35,240
In fact, we've got a name for it.

418
00:23:35,240 --> 00:23:38,170
It's called the base since it's the base of the exponent

419
00:23:38,170 --> 00:23:39,960
in our arithmetic here.

420
00:23:39,960 --> 00:23:43,190
>> Decimal notation is not the only way to represent numbers.

421
00:23:43,190 --> 00:23:48,020
In fact, even if we get rid of the digits 2 through 9, we can

422
00:23:48,020 --> 00:23:49,610
still represent all of the numbers that

423
00:23:49,610 --> 00:23:51,620
we could with decimal.

424
00:23:51,620 --> 00:23:56,220
So now we have two digits, 0 and 1, 2 is our special

425
00:23:56,220 --> 00:23:59,070
number, the base of our notation system.

426
00:23:59,070 --> 00:24:02,340
The name of this notation system is called binary since

427
00:24:02,340 --> 00:24:04,890
the prefix "bi" means 2.

428
00:24:04,890 --> 00:24:08,130
So instead now of having a ones place and tens place and

429
00:24:08,130 --> 00:24:11,950
so on, we now have a ones place, a twos place, a fours

430
00:24:11,950 --> 00:24:15,420
place, and so on, going up by powers of 2.

431
00:24:15,420 --> 00:24:17,320
So let's see this by doing some counting.

432
00:24:17,320 --> 00:24:22,790
So, 0 is still 0, and 1 is still 1.

433
00:24:22,790 --> 00:24:25,700
However, now that we've got a twos place instead of the tens

434
00:24:25,700 --> 00:24:31,070
place, 10 represents the number 2.

435
00:24:31,070 --> 00:24:36,240
To get 3, we add one to that and get 11.

436
00:24:36,240 --> 00:24:38,310
4, since there's now a fours place, is

437
00:24:38,310 --> 00:24:42,520
represented by 100.

438
00:24:42,520 --> 00:24:47,280
Five is 101.

439
00:24:47,280 --> 00:24:50,610
6 is 110.

440
00:24:50,610 --> 00:24:54,470
7 is 111.

441
00:24:54,470 --> 00:24:56,330
8, again, has its own place.

442
00:24:56,330 --> 00:25:00,650
So it's 1000.

443
00:25:00,650 --> 00:25:01,750
And I think you get the point.

444
00:25:01,750 --> 00:25:03,510
[END VIDEO PLAYBACK]

445
00:25:03,510 --> 00:25:06,900
>> DAVID J. MALAN: So, this is to say, what computers do and

446
00:25:06,900 --> 00:25:09,650
what binary is is actually not that dissimilar from what

447
00:25:09,650 --> 00:25:12,260
we've been taking for granted for some years, right?

448
00:25:12,260 --> 00:25:14,330
You reckon grade school, you learned to count in precisely

449
00:25:14,330 --> 00:25:16,310
the fashion that Nate proposed.

450
00:25:16,310 --> 00:25:18,250
But you probably haven't really thought about it since,

451
00:25:18,250 --> 00:25:19,990
the fact that there is this ones place, tens place, and

452
00:25:19,990 --> 00:25:20,490
hundreds place.

453
00:25:20,490 --> 00:25:21,185
And that's pretty arbitrary.

454
00:25:21,185 --> 00:25:24,090
And indeed, computers simply use this different base.

455
00:25:24,090 --> 00:25:27,430
But at the end of the day, to actually physically represent

456
00:25:27,430 --> 00:25:30,100
this notion of a 0 and 1, you obviously don't just turn the

457
00:25:30,100 --> 00:25:31,630
lights on and off necessarily.

458
00:25:31,630 --> 00:25:33,960
You need to do it on a much finer-grained scale.

459
00:25:33,960 --> 00:25:36,240
And by finer-grained, you might remember this silly

460
00:25:36,240 --> 00:25:38,990
little toy from childhood, Woolly Willy and these little

461
00:25:38,990 --> 00:25:40,360
magnetic particles.

462
00:25:40,360 --> 00:25:43,830
>> So magnetic particles are something that you can align

463
00:25:43,830 --> 00:25:45,720
in a couple of different directions, perhaps

464
00:25:45,720 --> 00:25:47,780
north-south or south-north.

465
00:25:47,780 --> 00:25:50,340
And so a lot of physical incarnations of technology

466
00:25:50,340 --> 00:25:53,410
these days that use binary, that use 0s and 1s, simply

467
00:25:53,410 --> 00:25:58,190
have magnetism on the inside that aligns things, up-down or

468
00:25:58,190 --> 00:26:01,970
down-up, with thereby representing a 0 or a 1,

469
00:26:01,970 --> 00:26:02,750
respectively.

470
00:26:02,750 --> 00:26:05,360
So indeed, let's move away from the abstract here and

471
00:26:05,360 --> 00:26:07,580
look at the inside of what's a more traditional

472
00:26:07,580 --> 00:26:08,640
computer hard drive.

473
00:26:08,640 --> 00:26:10,980
This one happens to be a bit larger on screen in that it's

474
00:26:10,980 --> 00:26:12,420
from a desktop computer.

475
00:26:12,420 --> 00:26:15,670
But laptops today still have the same technology, but is

476
00:26:15,670 --> 00:26:18,190
gradually being replaced by more sophisticated things that

477
00:26:18,190 --> 00:26:20,490
have actually no moving parts.

478
00:26:20,490 --> 00:26:22,440
The inside, then, of a hard drive.

479
00:26:22,440 --> 00:26:23,610
>> [VIDEO PLAYBACK]

480
00:26:23,610 --> 00:26:26,320
SPEAKER 12: The hard drive is where your PC stores most of

481
00:26:26,320 --> 00:26:27,870
its permanent data.

482
00:26:27,870 --> 00:26:31,600
To do that, the data travels from RAM along with software

483
00:26:31,600 --> 00:26:35,290
signals that tell the hard drive how to store that data.

484
00:26:35,290 --> 00:26:38,000
The hard drive circuits translate those signals into

485
00:26:38,000 --> 00:26:40,130
voltage fluctuations.

486
00:26:40,130 --> 00:26:43,700
These in turn control the hard drive's moving parts, some of

487
00:26:43,700 --> 00:26:46,750
the few moving parts left in the modern computer.

488
00:26:46,750 --> 00:26:49,460
Some of the signals control a motor which spins

489
00:26:49,460 --> 00:26:51,300
metal-coated platters.

490
00:26:51,300 --> 00:26:54,670
Your data is actually stored on these platters.

491
00:26:54,670 --> 00:26:57,810
Other signals move the read-write head to read or

492
00:26:57,810 --> 00:26:59,900
write data on the platters.

493
00:26:59,900 --> 00:27:03,520
This machine is so precise that a human hair couldn't

494
00:27:03,520 --> 00:27:06,470
even pass between the heads and spinning platters.

495
00:27:06,470 --> 00:27:08,780
Yet it all works at terrific speeds.

496
00:27:08,780 --> 00:27:09,520
[END VIDEO PLAYBACK]

497
00:27:09,520 --> 00:27:11,410
>> DAVID J. MALAN: So, if we now zoom in on what's actually

498
00:27:11,410 --> 00:27:14,300
happening on top of these platters in terms of the

499
00:27:14,300 --> 00:27:17,470
magnetism, we have this second of two looks.

500
00:27:17,470 --> 00:27:17,755
>> [VIDEO PLAYBACK]

501
00:27:17,755 --> 00:27:20,820
SPEAKER 13: Let's look at what we just saw in slow-motion.

502
00:27:20,820 --> 00:27:23,300
When a brief pulse of electricity is sent to the

503
00:27:23,300 --> 00:27:26,520
read-write head, it flips on a tiny electromagnet for a

504
00:27:26,520 --> 00:27:28,110
fraction of a second.

505
00:27:28,110 --> 00:27:31,320
The magnet creates a field which changes the polarity of

506
00:27:31,320 --> 00:27:34,280
a tiny, tiny portion of the metal particles which coat

507
00:27:34,280 --> 00:27:35,790
each platter's surface.

508
00:27:35,790 --> 00:27:39,090
A pattern series of these tiny charged up areas on the disk

509
00:27:39,090 --> 00:27:41,650
represents a single bit of data in the binary number

510
00:27:41,650 --> 00:27:43,680
system used by computers.

511
00:27:43,680 --> 00:27:45,977
Now, if the current is sent one way through the read-write

512
00:27:45,977 --> 00:27:49,030
head, the area is polarized in one direction.

513
00:27:49,030 --> 00:27:51,110
If the current is set in the opposite direction, the

514
00:27:51,110 --> 00:27:53,070
polarization is reversed.

515
00:27:53,070 --> 00:27:55,540
How do you get data off the hard disk?

516
00:27:55,540 --> 00:27:57,270
Just reverse the process.

517
00:27:57,270 --> 00:27:59,670
So it's the particles on the disk that get the current in

518
00:27:59,670 --> 00:28:01,190
the read-write head moving.

519
00:28:01,190 --> 00:28:04,090
Put together millions of these magnetized segments and you've

520
00:28:04,090 --> 00:28:05,300
got a file.

521
00:28:05,300 --> 00:28:08,600
Now, the pieces of a single file may be scattered all over

522
00:28:08,600 --> 00:28:11,140
a drive's platters, kind of like the mess of

523
00:28:11,140 --> 00:28:13,140
papers on your desk.

524
00:28:13,140 --> 00:28:16,680
So a special extra file keeps track of where everything is.

525
00:28:16,680 --> 00:28:18,560
Don't you wish you had something like that?

526
00:28:18,560 --> 00:28:19,990
[END VIDEO PLAYBACK]

527
00:28:19,990 --> 00:28:21,200
>> DAVID J. MALAN: Indeed.

528
00:28:21,200 --> 00:28:25,420
So, we have this ability to represent information, numbers

529
00:28:25,420 --> 00:28:26,310
at a very low level.

530
00:28:26,310 --> 00:28:29,920
We have a physical way of representing that same thing.

531
00:28:29,920 --> 00:28:32,710
But we can't really do all that much of interest yet

532
00:28:32,710 --> 00:28:34,580
other than perhaps some arithmetic and mathematics.

533
00:28:34,580 --> 00:28:37,660
We have no way of representing thus far things like

534
00:28:37,660 --> 00:28:40,310
alphabetical letters so that we humans can communicate

535
00:28:40,310 --> 00:28:41,970
using these same devices.

536
00:28:41,970 --> 00:28:45,710
But thankfully there exists encodings, patterns of 0s and

537
00:28:45,710 --> 00:28:49,240
1s, that represent higher level constructs like a and b

538
00:28:49,240 --> 00:28:52,740
and c and entire sentences and paragraphs and the like.

539
00:28:52,740 --> 00:28:56,070
And so ASCII, which is an acronym that refers to this

540
00:28:56,070 --> 00:29:00,410
coding system whereby a number represents a given letter.

541
00:29:00,410 --> 00:29:04,580
>> For instance, the number that we know as decimal value 65 is

542
00:29:04,580 --> 00:29:06,980
known as the capital letter A to computers.

543
00:29:06,980 --> 00:29:10,700
The decimal value of 97 in computers is known as a

544
00:29:10,700 --> 00:29:11,980
lowercase a.

545
00:29:11,980 --> 00:29:13,070
And what does that really mean?

546
00:29:13,070 --> 00:29:15,520
Well, even though Nate a moment ago only counted up

547
00:29:15,520 --> 00:29:20,460
from 0 to 8, if we were to continue counting up to 65 or

548
00:29:20,460 --> 00:29:24,180
further to 97, the pattern of 0s and 1s that he would have

549
00:29:24,180 --> 00:29:27,810
drawn on the screen would be exactly what a computer uses

550
00:29:27,810 --> 00:29:30,300
to represent the letter A in all caps or

551
00:29:30,300 --> 00:29:32,470
the letter a in lowercase.

552
00:29:32,470 --> 00:29:35,460
And indeed, there's a whole scheme to this.

553
00:29:35,460 --> 00:29:38,170
This is a, at first glance, overwhelming chart of

554
00:29:38,170 --> 00:29:41,150
encodings, but if you focus just on the right half here,

555
00:29:41,150 --> 00:29:45,820
notice in this middle column we have this notion of numbers

556
00:29:45,820 --> 00:29:46,860
followed by letters.

557
00:29:46,860 --> 00:29:48,640
And at top we have 32.

558
00:29:48,640 --> 00:29:53,270
And the character, char, to which 32, the integer, refers

559
00:29:53,270 --> 00:29:55,220
is apparently the Space Bar character.

560
00:29:55,220 --> 00:29:57,350
When you hit the Space Bar character on your laptop,

561
00:29:57,350 --> 00:30:00,110
well, what you're really sending is a number, a pattern

562
00:30:00,110 --> 00:30:02,620
of 0s and 1s, a flow of electricity if you will,

563
00:30:02,620 --> 00:30:05,630
representing those 0s and 1s that the computer then

564
00:30:05,630 --> 00:30:08,190
interprets as a space character on the screen.

565
00:30:08,190 --> 00:30:10,070
An exclamation point is 33.

566
00:30:10,070 --> 00:30:11,210
Double quotes is 34.

567
00:30:11,210 --> 00:30:14,890
And if we scroll down here over to the right, we see that

568
00:30:14,890 --> 00:30:18,840
65 is indeed A, and 97 is indeed lowercase a.

569
00:30:18,840 --> 00:30:21,690
>> And so now that we have this encoding scheme, we can start

570
00:30:21,690 --> 00:30:22,920
to spell things out.

571
00:30:22,920 --> 00:30:27,180
Indeed, computers typically express themselves in standard

572
00:30:27,180 --> 00:30:29,980
units, not using an individual bit, which again is not all

573
00:30:29,980 --> 00:30:33,630
that useful to just represent 0 or 1, lights on or off, but

574
00:30:33,630 --> 00:30:35,390
rather using sequences of bits.

575
00:30:35,390 --> 00:30:37,610
And the most common unit of measure, as you probably know

576
00:30:37,610 --> 00:30:39,370
and or at least inferred, is a byte.

577
00:30:39,370 --> 00:30:43,820
A byte is just eight bits, eight 0s or 1s in a row.

578
00:30:43,820 --> 00:30:45,950
So we can start spelling things out.

579
00:30:45,950 --> 00:30:48,990
And so, if we could, why not try this a little bit

580
00:30:48,990 --> 00:30:50,440
collectively here.

581
00:30:50,440 --> 00:30:53,630
Are there eight people in this room who would be willing to

582
00:30:53,630 --> 00:30:54,660
come up on stage?

583
00:30:54,660 --> 00:30:57,790
You have to be comfortable appearing on camera, but you

584
00:30:57,790 --> 00:30:59,860
don't really need to know, otherwise, what's

585
00:30:59,860 --> 00:31:01,550
going on just yet.

586
00:31:01,550 --> 00:31:05,080
I see one person being volunteered over here.

587
00:31:05,080 --> 00:31:16,220
Two, three, four, five, six, seven, and how about eight.

588
00:31:16,220 --> 00:31:16,800
Come on up.

589
00:31:16,800 --> 00:31:20,390
>> So you are about to represent a byte of people.

590
00:31:20,390 --> 00:31:26,750
Let me have you be the 128's place, you the 64's place, you

591
00:31:26,750 --> 00:31:28,050
the 32's place.

592
00:31:28,050 --> 00:31:30,530
But we're gonna very rapidly have to reverse this.

593
00:31:30,530 --> 00:31:33,030
So let me meet you all over there.

594
00:31:33,030 --> 00:31:37,230
And you should be in the 128's place all the way over here.

595
00:31:37,230 --> 00:31:39,500
Much like the hundreds place and the thousands place would

596
00:31:39,500 --> 00:31:42,050
be farther to the left, we want the biggest placeholder

597
00:31:42,050 --> 00:31:43,600
to be here on the left as well.

598
00:31:43,600 --> 00:31:53,090
We have 64s's 32, 16's, 8's, 4's, 2's, and 1's.

599
00:31:53,090 --> 00:31:53,900
Excellent.

600
00:31:53,900 --> 00:31:56,090
So now we have--

601
00:31:56,090 --> 00:31:58,050
OK, you can help me.

602
00:31:58,050 --> 00:32:00,085
So now we have-- what's your name?

603
00:32:00,085 --> 00:32:00,420
JOANNE: Joanne.

604
00:32:00,420 --> 00:32:00,830
DAVID J. MALAN: Joanne.

605
00:32:00,830 --> 00:32:04,100
So Joanne and I are now going to advise these guys on how we

606
00:32:04,100 --> 00:32:05,860
can go about spelling something out.

607
00:32:05,860 --> 00:32:07,730
So on the backs of their sheets of paper, they have a

608
00:32:07,730 --> 00:32:10,160
little cheat sheet that's going to tell them whether

609
00:32:10,160 --> 00:32:12,590
they were representing a 0 or a 1.

610
00:32:12,590 --> 00:32:15,870
And why don't for simplicity, we'll represent 0 by just

611
00:32:15,870 --> 00:32:17,440
standing there awkwardly.

612
00:32:17,440 --> 00:32:18,220
Very good.

613
00:32:18,220 --> 00:32:22,060
Or a 1 by raising your hand, representing a 1.

614
00:32:22,060 --> 00:32:25,430
And let's see if we can't spell out a four character

615
00:32:25,430 --> 00:32:26,250
phrase here.

616
00:32:26,250 --> 00:32:30,930
>> So, go ahead now, volunteers, and execute round one by

617
00:32:30,930 --> 00:32:33,390
raising your hand if you're a 1 or keeping it

618
00:32:33,390 --> 00:32:36,820
down if you're a 0.

619
00:32:36,820 --> 00:32:42,280
So, now that we have these three hands up, what number,

620
00:32:42,280 --> 00:32:45,670
everyone else, are they actually representing?

621
00:32:45,670 --> 00:32:45,950
OK.

622
00:32:45,950 --> 00:32:46,290
67.

623
00:32:46,290 --> 00:32:46,910
Why?

624
00:32:46,910 --> 00:32:47,830
Well, quick sanity check.

625
00:32:47,830 --> 00:32:53,200
64's place, because it's a 1, that's like 1 times 64 plus 1

626
00:32:53,200 --> 00:32:56,565
times 2, so that's 66 plus 1 times 1.

627
00:32:56,565 --> 00:32:58,470
That's plus 1, so 67.

628
00:32:58,470 --> 00:33:01,020
So now these guys are collectively representing 67

629
00:33:01,020 --> 00:33:04,110
which apparently represents what here in ASCII?

630
00:33:04,110 --> 00:33:04,380
OK.

631
00:33:04,380 --> 00:33:05,140
So a c.

632
00:33:05,140 --> 00:33:05,510
All right.

633
00:33:05,510 --> 00:33:07,400
So now let's proceed to round two.

634
00:33:07,400 --> 00:33:08,940
Everyone starting with their hands down.

635
00:33:08,940 --> 00:33:10,590
And in round two--

636
00:33:10,590 --> 00:33:12,410
actually there's not much of a role here, I suppose, but

637
00:33:12,410 --> 00:33:13,130
we'll pretend.

638
00:33:13,130 --> 00:33:17,690
So round two, raise or lower your hands.

639
00:33:17,690 --> 00:33:18,380
>> All right.

640
00:33:18,380 --> 00:33:22,700
Audience, what are we now expressing is 83.

641
00:33:22,700 --> 00:33:23,910
So you could do the mathematics.

642
00:33:23,910 --> 00:33:27,000
But for anyone whose hand is up, you add in the number that

643
00:33:27,000 --> 00:33:27,670
they represent.

644
00:33:27,670 --> 00:33:29,470
So now we have 83.

645
00:33:29,470 --> 00:33:32,140
Let's expand the cheat sheet a little bit, and we now have--?

646
00:33:32,140 --> 00:33:34,340
[INAUDIBLE]

647
00:33:34,340 --> 00:33:35,370
DAVID J. MALAN: OK.

648
00:33:35,370 --> 00:33:36,740
This might be obvious where we're going here, but

649
00:33:36,740 --> 00:33:40,080
nonetheless, round three.

650
00:33:40,080 --> 00:33:40,455
OK.

651
00:33:40,455 --> 00:33:41,730
Round three's good to go down there.

652
00:33:41,730 --> 00:33:44,450
So round three, what number are these guys now

653
00:33:44,450 --> 00:33:45,700
representing?

654
00:33:47,940 --> 00:33:48,200
OK.

655
00:33:48,200 --> 00:33:51,770
I heard 53, which now represents?

656
00:33:51,770 --> 00:33:54,550
Interesting.

657
00:33:54,550 --> 00:33:58,630
Now why this sort of counter intuitive result, right?

658
00:33:58,630 --> 00:34:00,890
If we want to represent 5-- we all probably know

659
00:34:00,890 --> 00:34:01,770
where this is going--

660
00:34:01,770 --> 00:34:05,230
why don't I just raise the 4's place and the 1's place?

661
00:34:05,230 --> 00:34:07,340
>> Well, realize that there's a difference, fundamentally,

662
00:34:07,340 --> 00:34:09,630
between how a computer interprets these bits.

663
00:34:09,630 --> 00:34:13,030
If you're trying to represent the number 5, then absolutely,

664
00:34:13,030 --> 00:34:16,199
we just raise hand number 4 and raise hand number 1.

665
00:34:16,199 --> 00:34:17,880
But we're not representing numbers here.

666
00:34:17,880 --> 00:34:20,710
The context here on stage is that we're representing

667
00:34:20,710 --> 00:34:22,300
characters, or chars.

668
00:34:22,300 --> 00:34:25,139
And in this context the computer has to realize that,

669
00:34:25,139 --> 00:34:28,780
oh, this pattern of bits is not a number alone, it's

670
00:34:28,780 --> 00:34:31,620
actually representing a higher level concept, in this case an

671
00:34:31,620 --> 00:34:32,840
alphabetical letter.

672
00:34:32,840 --> 00:34:36,600
So the fact that it is now representing the number 5 with

673
00:34:36,600 --> 00:34:40,550
the value of 53 is because in ASCII the thing we

674
00:34:40,550 --> 00:34:44,280
aesthetically see as the number 5 itself needs a

675
00:34:44,280 --> 00:34:45,290
pattern of bits.

676
00:34:45,290 --> 00:34:46,080
Because why?

677
00:34:46,080 --> 00:34:48,780
Well, the world just decided to use the lower numbers, 0,

678
00:34:48,780 --> 00:34:51,989
1, 2, 3, for what look to be fairly cryptic things.

679
00:34:51,989 --> 00:34:54,070
And indeed, these are the characters that aren't on a

680
00:34:54,070 --> 00:34:56,870
keyboard, special expressions that you need in a computer to

681
00:34:56,870 --> 00:34:58,800
do interesting things, but humans never

682
00:34:58,800 --> 00:34:59,880
actually type them.

683
00:34:59,880 --> 00:35:02,080
So 53 indeed represents 5.

684
00:35:02,080 --> 00:35:08,320
Now, just as a final sanity check, what number should they

685
00:35:08,320 --> 00:35:10,356
represent in just a moment?

686
00:35:10,356 --> 00:35:10,752
AUDIENCE: 48.

687
00:35:10,752 --> 00:35:11,150
DAVID J. MALAN: OK.

688
00:35:11,150 --> 00:35:11,920
So 48.

689
00:35:11,920 --> 00:35:12,770
And indeed, go ahead.

690
00:35:12,770 --> 00:35:14,020
Round four.

691
00:35:16,510 --> 00:35:20,540
16 plus 32 is, indeed, 48.

692
00:35:20,540 --> 00:35:22,830
And so a big round of applause, if we could, for our

693
00:35:22,830 --> 00:35:25,382
eight volunteers here.

694
00:35:25,382 --> 00:35:25,870
Thanks.

695
00:35:25,870 --> 00:35:27,822
You can keep this one.

696
00:35:27,822 --> 00:35:29,286
If you--

697
00:35:29,286 --> 00:35:30,536
Very well done.

698
00:35:33,140 --> 00:35:34,360
Any direction is fine.

699
00:35:34,360 --> 00:35:37,950
>> So, we now have a way not only of thinking about how to

700
00:35:37,950 --> 00:35:40,810
represent data and actually representing it physically,

701
00:35:40,810 --> 00:35:43,650
but also doing higher-level things on top of it.

702
00:35:43,650 --> 00:35:45,640
Indeed, this is going to be a theme throughout computer

703
00:35:45,640 --> 00:35:48,630
science of building more and more interesting complex

704
00:35:48,630 --> 00:35:52,170
things on top of fairly simple ideas, in this

705
00:35:52,170 --> 00:35:54,040
case just 0s and 1s.

706
00:35:54,040 --> 00:35:57,270
In terms of why this is useful, well even though in a

707
00:35:57,270 --> 00:35:59,760
course like this we'll focus on fundamentals and on

708
00:35:59,760 --> 00:36:03,120
programming and on solving of problems, you can go off in

709
00:36:03,120 --> 00:36:04,970
computer science in any number of directions.

710
00:36:04,970 --> 00:36:07,600
In this case here, this is a chart that you have at the

711
00:36:07,600 --> 00:36:11,120
back page of your unofficial guide to CS at Harvard, one of

712
00:36:11,120 --> 00:36:12,450
today's two printouts.

713
00:36:12,450 --> 00:36:14,270
This suggests the many different directions in which

714
00:36:14,270 --> 00:36:16,270
you can go after a course like this.

715
00:36:16,270 --> 00:36:19,170
Learning about artificial intelligence, about graphics,

716
00:36:19,170 --> 00:36:22,230
about machine learning, about language itself.

717
00:36:22,230 --> 00:36:23,975
Realize, too, that there are yet other paths.

718
00:36:23,975 --> 00:36:26,550
There are more mathematical paths in computer science.

719
00:36:26,550 --> 00:36:29,020
If you're not even able to take something like CS50 this

720
00:36:29,020 --> 00:36:30,930
fall, there's introductory courses in the spring.

721
00:36:30,930 --> 00:36:33,420
Computer Science 1, for instance, is yet another

722
00:36:33,420 --> 00:36:35,920
on-ramp to this new world.

723
00:36:35,920 --> 00:36:38,230
>> Now as an aside in the interest of solving problems

724
00:36:38,230 --> 00:36:41,380
related to courses, realize that CS50 set out some time

725
00:36:41,380 --> 00:36:44,405
ago to try to solve one of these problems, problem known

726
00:36:44,405 --> 00:36:47,500
as my.harvard, which many of you might be using to actually

727
00:36:47,500 --> 00:36:48,480
shop for courses.

728
00:36:48,480 --> 00:36:51,900
But if not, check out a tool like this, as well as other

729
00:36:51,900 --> 00:36:54,130
descendants that some of our past students

730
00:36:54,130 --> 00:36:55,660
and staff have created.

731
00:36:55,660 --> 00:36:57,970
But in Harvard Courses, which is a web-based tool--

732
00:36:57,970 --> 00:37:00,680
something that you will be capable of designing and

733
00:37:00,680 --> 00:37:04,610
deploying yourself, as well as yet other things as well, by

734
00:37:04,610 --> 00:37:05,920
the end of the semester.

735
00:37:05,920 --> 00:37:08,890
Realize that this builds upon an open data set, in this case

736
00:37:08,890 --> 00:37:11,350
a course catalog, and allows students in this case to

737
00:37:11,350 --> 00:37:14,180
explore a fairly complex data set.

738
00:37:14,180 --> 00:37:16,710
>> We dug up last night a few statistics based on the few

739
00:37:16,710 --> 00:37:18,200
thousand folks who have been using this over

740
00:37:18,200 --> 00:37:19,380
the past few days.

741
00:37:19,380 --> 00:37:22,630
If you've been curious to know how many courses your friends

742
00:37:22,630 --> 00:37:25,900
actually tend to shop, well, today it's data suggests that

743
00:37:25,900 --> 00:37:29,260
7.6 is the average number of courses on

744
00:37:29,260 --> 00:37:30,470
someone's shopping list.

745
00:37:30,470 --> 00:37:33,730
And now I'll give you, also, the statistic of the most

746
00:37:33,730 --> 00:37:36,500
number of courses on someone's shopping list.

747
00:37:36,500 --> 00:37:38,220
And we all probably know someone like this.

748
00:37:41,700 --> 00:37:44,610
201 is this year's record.

749
00:37:44,610 --> 00:37:47,220
Now some of our former students and staff actually

750
00:37:47,220 --> 00:37:52,210
put together a clip to paint a picture for you of what this

751
00:37:52,210 --> 00:37:55,220
path of computer science and CS50 itself is.

752
00:37:55,220 --> 00:37:58,350
Let me go ahead and pull up, thanks to Mr. Hahvahd here, a

753
00:37:58,350 --> 00:38:02,390
video produced by some of your predecessors.

754
00:38:02,390 --> 00:38:04,680
If we could keep the lights up for this.

755
00:38:07,860 --> 00:39:08,101
[VIDEO PLAYBACK]

756
00:39:08,101 --> 00:39:11,987
[END VIDEO PLAYBACK]

757
00:39:11,987 --> 00:39:14,160
>> SPEAKER 14: (SINGING) We take our time with some scratch,

758
00:39:14,160 --> 00:39:17,410
for loops, events, we can match, compiling using our

759
00:39:17,410 --> 00:39:20,370
bash, this term won't be a bore.

760
00:39:20,370 --> 00:39:23,970
Hacking fun, some free meals, lectures are simply unreal,

761
00:39:23,970 --> 00:39:28,200
our fair is such a big deal, there's so much to adore.

762
00:39:28,200 --> 00:39:29,784
Go, David Malan.

763
00:39:29,784 --> 00:39:31,810
Walkthroughs, I'm not bailing.

764
00:39:31,810 --> 00:39:33,794
Office hours, no one's failing.

765
00:39:33,794 --> 00:39:36,274
Where you think you're coding, baby?

766
00:39:36,274 --> 00:39:40,242
Hey, I just met you and this is crazy,

767
00:39:40,242 --> 00:39:42,226
but here's our reason.

768
00:39:42,226 --> 00:39:44,210
Take CS50.

769
00:39:44,210 --> 00:39:48,674
It's hard to code right without you, baby.

770
00:39:48,674 --> 00:39:50,658
But here's our reason.

771
00:39:50,658 --> 00:39:52,146
Take CS50.

772
00:39:52,146 --> 00:39:56,610
Hey, I just met you and this is crazy,

773
00:39:56,610 --> 00:39:58,098
but here's our reason.

774
00:39:58,098 --> 00:40:00,082
Take CS50.

775
00:40:00,082 --> 00:40:06,530
And every star firm wants to hire me, another reason.

776
00:40:06,530 --> 00:40:08,018
Take CS50.

777
00:40:08,018 --> 00:40:12,978
Before you came into my life, I coded so bad, I coded so

778
00:40:12,978 --> 00:40:15,954
bad, I coded so, so bad.

779
00:40:15,954 --> 00:40:19,922
Before you came into my life, I coded so bad

780
00:40:19,922 --> 00:40:22,402
and I can't go back.

781
00:40:22,402 --> 00:40:37,410
Take CS50.

782
00:40:37,410 --> 00:40:40,490
>> DAVID J. MALAN: I had no idea that was going to happen.

783
00:40:40,490 --> 00:40:44,030
So, a more serious look at what lies ahead.

784
00:40:44,030 --> 00:40:45,980
So in terms of the expectations of this course,

785
00:40:45,980 --> 00:40:48,490
you're indeed expected to attend or watch the course's

786
00:40:48,490 --> 00:40:51,600
lectures, submit a problem set, take two quizzes, submit

787
00:40:51,600 --> 00:40:52,590
a final project.

788
00:40:52,590 --> 00:40:56,250
In terms of grades, realize that my comment at the opening

789
00:40:56,250 --> 00:40:58,190
about pass/fail, something that we very much take to

790
00:40:58,190 --> 00:40:59,310
heart in CS50.

791
00:40:59,310 --> 00:41:02,970
There is not nearly enough of a culture at Harvard of trying

792
00:41:02,970 --> 00:41:04,940
something and risking failure.

793
00:41:04,940 --> 00:41:07,010
Indeed, we had numbers of students, and myself, in

794
00:41:07,010 --> 00:41:09,910
particular, who were worried about hurting your GPA or

795
00:41:09,910 --> 00:41:13,170
getting a B in something like CS50.

796
00:41:13,170 --> 00:41:16,250
And the opportunity to take a course like this, and other

797
00:41:16,250 --> 00:41:19,080
gateway courses at the introductory level, pass/fail

798
00:41:19,080 --> 00:41:22,190
is a very underutilized opportunity at this college,

799
00:41:22,190 --> 00:41:22,620
in general.

800
00:41:22,620 --> 00:41:25,420
And so please know even I enrolled in this course

801
00:41:25,420 --> 00:41:28,430
initially for pass/fail credit alone.

802
00:41:28,430 --> 00:41:30,710
And even though I did switch at the end of the day, it was

803
00:41:30,710 --> 00:41:33,280
those five initial weeks, up to the fifth Monday of the

804
00:41:33,280 --> 00:41:36,020
semester which is the cutoff, that allowed me to actually

805
00:41:36,020 --> 00:41:39,270
put foot into these new waters and actually try something

806
00:41:39,270 --> 00:41:41,520
very unfamiliar and very uncomfortable

807
00:41:41,520 --> 00:41:43,400
for me at the time.

808
00:41:43,400 --> 00:41:47,130
>> So in terms, now, of what role the various angles via which

809
00:41:47,130 --> 00:41:50,630
you can approach this course serve, so lectures, it's up to

810
00:41:50,630 --> 00:41:53,330
you if you engage with us in person at this venue.

811
00:41:53,330 --> 00:41:57,590
Indeed, we know statistically that roughly 40% of you will

812
00:41:57,590 --> 00:41:59,530
kind of come and go over the course of the semester.

813
00:41:59,530 --> 00:42:03,000
And 10% of you, we will never see again after today.

814
00:42:03,000 --> 00:42:05,400
And that's perfectly fine, to be honest.

815
00:42:05,400 --> 00:42:07,810
One of the defining characteristics of CS50 is

816
00:42:07,810 --> 00:42:10,160
that there are these innumerable resources, some of

817
00:42:10,160 --> 00:42:12,570
which we'll rattle through in just a moment, including

818
00:42:12,570 --> 00:42:15,320
lectures and sections and things called walkthroughs and

819
00:42:15,320 --> 00:42:16,720
office hours and the like.

820
00:42:16,720 --> 00:42:18,890
And it's more resources than the typical student should

821
00:42:18,890 --> 00:42:21,580
have to or could physically take advantage of.

822
00:42:21,580 --> 00:42:23,650
But that's because of the disparate learning styles that

823
00:42:23,650 --> 00:42:25,370
any student body manifests.

824
00:42:25,370 --> 00:42:28,120
And so in lectures, the primary role, as I see it, is

825
00:42:28,120 --> 00:42:31,650
not to verbally push out fairly complex material and to

826
00:42:31,650 --> 00:42:34,670
necessarily deliver all of the intricacies of the

827
00:42:34,670 --> 00:42:36,910
fundamentals that we'll explore this semester, but

828
00:42:36,910 --> 00:42:39,530
rather to do things like we've been doing thus far already,

829
00:42:39,530 --> 00:42:42,350
these examples, involving humans onstage, trying to

830
00:42:42,350 --> 00:42:45,340
paint a mental picture, and also create, dare I say, some

831
00:42:45,340 --> 00:42:46,450
of these memorable moments.

832
00:42:46,450 --> 00:42:49,200
So that even as you struggle with certain topics, you have

833
00:42:49,200 --> 00:42:51,770
these memories like, oh, even though that was fairly

834
00:42:51,770 --> 00:42:54,770
abstract, the math, I got lost with carrying the 1, like it

835
00:42:54,770 --> 00:42:57,510
really, at the end of the day, is not all that dissimilar to

836
00:42:57,510 --> 00:42:58,920
something I already know.

837
00:42:58,920 --> 00:43:01,590
And so the role that lectures will serve, either in person

838
00:43:01,590 --> 00:43:05,120
here in Sanders or online on video, is really to set the

839
00:43:05,120 --> 00:43:08,170
stage mentally for you each week for the various concepts

840
00:43:08,170 --> 00:43:10,240
and problems that we'll be diving into.

841
00:43:10,240 --> 00:43:13,790
>> In terms of the high-level concepts, most of these words

842
00:43:13,790 --> 00:43:16,340
might flow over your head for the moment, and that's fine.

843
00:43:16,340 --> 00:43:18,495
For those of you who come into the course more comfortable

844
00:43:18,495 --> 00:43:20,150
will know of some of these topics.

845
00:43:20,150 --> 00:43:23,150
But typically for that 10% of the class for which they have

846
00:43:23,150 --> 00:43:25,810
much more background, taking AP computer science,

847
00:43:25,810 --> 00:43:28,200
programming since they were 12, realize that there will be

848
00:43:28,200 --> 00:43:31,230
opportunities in sections and in problem sets to go all the

849
00:43:31,230 --> 00:43:34,240
more into depth into various topics, filling in whatever

850
00:43:34,240 --> 00:43:36,710
gaps you might have from your high school or prior

851
00:43:36,710 --> 00:43:37,460
background.

852
00:43:37,460 --> 00:43:39,930
In terms of the languages, realize that what language we

853
00:43:39,930 --> 00:43:42,550
use in CS50 is largely irrelevant at

854
00:43:42,550 --> 00:43:43,220
the end of the day.

855
00:43:43,220 --> 00:43:47,090
We happen to use, primarily, a language called C. Toward the

856
00:43:47,090 --> 00:43:49,010
end of the semester, we introduce web-centric

857
00:43:49,010 --> 00:43:51,110
languages like PHP and JavaScript.

858
00:43:51,110 --> 00:43:54,280
But we and others could teach a course like this in most any

859
00:43:54,280 --> 00:43:55,610
modern high-level language.

860
00:43:55,610 --> 00:43:58,520
Python and Ruby and others are quite popular these days.

861
00:43:58,520 --> 00:44:00,970
Because realize at the end of the day, you're not learning

862
00:44:00,970 --> 00:44:04,180
in this course C. You're not learning PHP or JavaScript.

863
00:44:04,180 --> 00:44:07,140
You're learning how to solve problems, whether web-based,

864
00:44:07,140 --> 00:44:10,480
computer-based, or data-oriented itself, using

865
00:44:10,480 --> 00:44:12,700
these simply as tools.

866
00:44:12,700 --> 00:44:14,730
>> Now, in terms of the logistics, you'll use

867
00:44:14,730 --> 00:44:16,670
something, eventually, called the CS50 Appliance.

868
00:44:16,670 --> 00:44:19,350
Does not matter if you have a Mac, a PC, a Linux computer,

869
00:44:19,350 --> 00:44:20,010
or the like.

870
00:44:20,010 --> 00:44:22,510
You'll have freely available software starting next week

871
00:44:22,510 --> 00:44:25,460
with which to use the CS50 Appliance, a virtual

872
00:44:25,460 --> 00:44:28,020
environment that you'll use on your own computer so that you

873
00:44:28,020 --> 00:44:31,120
and all of your classmates have a uniform Linux desktop

874
00:44:31,120 --> 00:44:31,890
in this case.

875
00:44:31,890 --> 00:44:34,390
It's the problem sets, though, in which you'll really get

876
00:44:34,390 --> 00:44:35,780
your hands dirty in the course.

877
00:44:35,780 --> 00:44:37,630
And at the end of the day, it's the problem sets, I

878
00:44:37,630 --> 00:44:39,680
think, that really define a student's

879
00:44:39,680 --> 00:44:41,380
experience in this course.

880
00:44:41,380 --> 00:44:43,460
Realize that many of the problem sets will be released

881
00:44:43,460 --> 00:44:46,430
in two editions, a standard addition that we expect and

882
00:44:46,430 --> 00:44:49,550
encourage 90% of the class to dive into.

883
00:44:49,550 --> 00:44:51,620
But we also release some problem sets in

884
00:44:51,620 --> 00:44:52,920
so-called hacker additions.

885
00:44:52,920 --> 00:44:54,900
And you know it's the hacker addition because on every page

886
00:44:54,900 --> 00:44:57,840
with a watermark it says hacker addition on it.

887
00:44:57,840 --> 00:45:00,000
And that's for this demographic of you who have AP

888
00:45:00,000 --> 00:45:02,890
computer science with 10 years of programming under your belt

889
00:45:02,890 --> 00:45:05,640
and are looking to fill those gaps and to have more formal,

890
00:45:05,640 --> 00:45:07,370
rather than self-taught, training, perhaps.

891
00:45:07,370 --> 00:45:10,410
Realize that there is a very substantial demographic in the

892
00:45:10,410 --> 00:45:13,090
class that has precisely that same goal.

893
00:45:13,090 --> 00:45:14,320
You'll have five late days.

894
00:45:14,320 --> 00:45:16,490
Problem sets are generally due on Thursdays, but you can

895
00:45:16,490 --> 00:45:18,440
extend five of those deadlines using these

896
00:45:18,440 --> 00:45:19,700
things called late days.

897
00:45:19,700 --> 00:45:22,590
And we'll also drop your lowest score at the end of the

898
00:45:22,590 --> 00:45:25,400
semester per the particulars in the syllabus.

899
00:45:25,400 --> 00:45:27,800
>> But another defining characteristic of CS50 over

900
00:45:27,800 --> 00:45:29,830
the years has become office hours.

901
00:45:29,830 --> 00:45:32,720
It's an opportunity that you saw visually in photos a bit

902
00:45:32,720 --> 00:45:35,850
ago in which we gather-- previously in house dining

903
00:45:35,850 --> 00:45:37,780
halls, prior to that in the basement of the Science

904
00:45:37,780 --> 00:45:40,310
Center, and this year in Annenberg Hall-- four nights a

905
00:45:40,310 --> 00:45:43,330
week from 8:00 PM to 11:00 PM where you'll have this very

906
00:45:43,330 --> 00:45:46,320
much shared experience of working on, struggling

907
00:45:46,320 --> 00:45:49,300
through, certain problems, but with a substantial support

908
00:45:49,300 --> 00:45:50,380
structure in place.

909
00:45:50,380 --> 00:45:53,180
Indeed, the way this will work is you'll arrive at Annenberg

910
00:45:53,180 --> 00:45:55,160
if you have some question during the week, you'll bring

911
00:45:55,160 --> 00:45:58,060
your laptop, you'll sit down, grab some food, and you'll log

912
00:45:58,060 --> 00:46:01,520
into CS50 Discuss, a web-based utility that the teaching

913
00:46:01,520 --> 00:46:04,480
staff has developed that will allow you to post questions

914
00:46:04,480 --> 00:46:07,910
and see follow-ups in a typical discussion forum

915
00:46:07,910 --> 00:46:10,550
sense, using labels and the like and auto complete to

916
00:46:10,550 --> 00:46:11,680
search the data.

917
00:46:11,680 --> 00:46:14,850
But you'll also be able to, during the hours of office

918
00:46:14,850 --> 00:46:16,980
hours, have your questions escalated to

919
00:46:16,980 --> 00:46:18,360
actual human beings.

920
00:46:18,360 --> 00:46:21,430
Indeed, the goal ultimately is so that one, we begin to build

921
00:46:21,430 --> 00:46:24,040
up over the course of the semester a corpus of hopefully

922
00:46:24,040 --> 00:46:26,970
really useful information, common answers to common

923
00:46:26,970 --> 00:46:29,640
questions, so that you yourself can solve problems

924
00:46:29,640 --> 00:46:33,490
and get unstuck as quickly as possible, but while having the

925
00:46:33,490 --> 00:46:37,210
teaching staff, usually 20 to 30 of the teaching fellows and

926
00:46:37,210 --> 00:46:39,660
course assistants, on staff at once.

927
00:46:39,660 --> 00:46:42,420
>> We will have what's called the CS50 Greeter in Annenberg.

928
00:46:42,420 --> 00:46:46,710
And when we determine that, you know what, this question,

929
00:46:46,710 --> 00:46:48,780
we can't really answer effectively online.

930
00:46:48,780 --> 00:46:49,800
We need to see your computer.

931
00:46:49,800 --> 00:46:50,710
We want to talk to you one-on-one.

932
00:46:50,710 --> 00:46:51,340
On one.

933
00:46:51,340 --> 00:46:53,400
You're really struggling and you, therefore, want to talk

934
00:46:53,400 --> 00:46:56,010
one-on-one alongside someone, you'll be dispatched to the

935
00:46:56,010 --> 00:46:58,730
CS50 Greeter, a teaching fellow holding, literally, an

936
00:46:58,730 --> 00:47:01,770
iPad that has students' names on one side, teaching staff's

937
00:47:01,770 --> 00:47:02,790
names on the other.

938
00:47:02,790 --> 00:47:04,760
We will click your name followed by the name of a

939
00:47:04,760 --> 00:47:06,860
teaching staff, and your computer screen will start

940
00:47:06,860 --> 00:47:11,170
blinking saying please go see Alice or please go see Bob at

941
00:47:11,170 --> 00:47:11,980
the staff table.

942
00:47:11,980 --> 00:47:14,450
And so in this way, we will be able to dispatch things as

943
00:47:14,450 --> 00:47:17,610
efficiently as possible, as well as guide you toward

944
00:47:17,610 --> 00:47:19,910
solutions all the more readily.

945
00:47:19,910 --> 00:47:22,970
In sections, these will be opportunities for more

946
00:47:22,970 --> 00:47:25,570
intimate hands-on opportunities with one of the

947
00:47:25,570 --> 00:47:29,040
teaching fellows and 12 to 16 or so of your classmates in

948
00:47:29,040 --> 00:47:31,900
which each week we'll have problems in the problem set

949
00:47:31,900 --> 00:47:33,800
that ask a number of conceptual questions and a

950
00:47:33,800 --> 00:47:35,740
number of bit-sized programming questions that you

951
00:47:35,740 --> 00:47:37,690
could figure out on your own, and you could work on your

952
00:47:37,690 --> 00:47:40,020
own, but in the context of section where we work through

953
00:47:40,020 --> 00:47:42,870
collectively some of those problems and go where the

954
00:47:42,870 --> 00:47:44,570
different conversation takes us.

955
00:47:44,570 --> 00:47:48,010
>> In addition, in section will you have opportunities to

956
00:47:48,010 --> 00:47:50,450
review submissions of homework that you've made, your

957
00:47:50,450 --> 00:47:53,620
classmates, sometimes anonymized, always via opt-in

958
00:47:53,620 --> 00:47:55,810
if you would like to share the work that you've submitted.

959
00:47:55,810 --> 00:47:58,030
So it will really be a two-directional conversation,

960
00:47:58,030 --> 00:48:00,840
an opportunity to review your own work in a much more

961
00:48:00,840 --> 00:48:04,080
dynamic sense, rather than simply looking at a PDF or a

962
00:48:04,080 --> 00:48:06,920
printout and thinking about it for a few seconds and not

963
00:48:06,920 --> 00:48:08,920
necessarily absorbing the feedback that the teaching

964
00:48:08,920 --> 00:48:10,150
staff have provided.

965
00:48:10,150 --> 00:48:12,540
And you'll use a tool here called CS50 Spaces.

966
00:48:12,540 --> 00:48:15,670
For those unfamiliar, this is the language known as C at top

967
00:48:15,670 --> 00:48:17,500
left, and you'll get to know this over time.

968
00:48:17,500 --> 00:48:19,640
But this is a web-based utility that we'll use in

969
00:48:19,640 --> 00:48:22,310
section that will allow you and your 15 or so classmates

970
00:48:22,310 --> 00:48:24,100
to login with your teaching fellow at the

971
00:48:24,100 --> 00:48:24,980
front of the room.

972
00:48:24,980 --> 00:48:26,750
You'll be able to write code in this window.

973
00:48:26,750 --> 00:48:28,720
You'll be able to chat electronically, if you're not

974
00:48:28,720 --> 00:48:30,930
actually at section at that particular moment.

975
00:48:30,930 --> 00:48:33,940
And your teaching fellow, when it comes time to discuss

976
00:48:33,940 --> 00:48:38,645
Alice's or Bob's solution in class, the teaching fellow can

977
00:48:38,645 --> 00:48:41,380
click a button and voila, project onto the screen,

978
00:48:41,380 --> 00:48:44,250
whatever that student has been working on at that particular

979
00:48:44,250 --> 00:48:44,990
point in time.

980
00:48:44,990 --> 00:48:47,130
So for those of you who have friends who have taken CS50 in

981
00:48:47,130 --> 00:48:49,750
the past, realize that sections have been significantly

982
00:48:49,750 --> 00:48:52,600
rebooted this year to be all the more active, all the more

983
00:48:52,600 --> 00:48:55,960
dynamic, and really a two-way conversation between teaching

984
00:48:55,960 --> 00:48:58,190
staff and students.

985
00:48:58,190 --> 00:48:59,020
>> And walkthroughs.

986
00:48:59,020 --> 00:49:01,560
So for these problem sets, we also offer not only the

987
00:49:01,560 --> 00:49:04,210
specification itself, which is generally a fairly detailed

988
00:49:04,210 --> 00:49:07,380
PDF, but also things known as walkthroughs whereby one

989
00:49:07,380 --> 00:49:11,000
member of the teaching staff will lead a weekly session

990
00:49:11,000 --> 00:49:13,210
that literally walks you through the problem set, giving

991
00:49:13,210 --> 00:49:15,720
you hints and advice and starting points and is meant

992
00:49:15,720 --> 00:49:17,250
to ask the very frequently asked

993
00:49:17,250 --> 00:49:19,360
question, where do I begin?

994
00:49:19,360 --> 00:49:21,990
Well, you begin either by diving into the spec on its

995
00:49:21,990 --> 00:49:25,100
own or by attending or watching these walkthroughs.

996
00:49:25,100 --> 00:49:27,420
The first walkthrough, in fact, will be this Friday.

997
00:49:27,420 --> 00:49:29,520
They'll be on Fridays, not so much because we think it'll be

998
00:49:29,520 --> 00:49:32,160
a popular time but because we can then film them very early

999
00:49:32,160 --> 00:49:34,380
in the week to get them online by the weekend so that you

1000
00:49:34,380 --> 00:49:37,670
have as many days as possible to actually engage in that

1001
00:49:37,670 --> 00:49:38,390
content as well.

1002
00:49:38,390 --> 00:49:40,900
But more on that in lecture this Friday.

1003
00:49:40,900 --> 00:49:43,620
>> Now in terms of the support structure, the most

1004
00:49:43,620 --> 00:49:46,430
significant statistic is perhaps the 108 teaching

1005
00:49:46,430 --> 00:49:47,950
fellows and course assistants that this

1006
00:49:47,950 --> 00:49:49,730
course currently has.

1007
00:49:49,730 --> 00:49:52,030
If some of you who don't have conflicting classes would like

1008
00:49:52,030 --> 00:49:55,070
to join me up here on stage, it is these guys who will

1009
00:49:55,070 --> 00:49:57,990
ultimately really define your experience in the course.

1010
00:50:06,750 --> 00:50:09,290
I had a lot of teaching fellows teaching me classes in

1011
00:50:09,290 --> 00:50:11,650
the day, and I remember very few of those frankly.

1012
00:50:11,650 --> 00:50:15,590
But to date, I still remember among those few, my CS50 TF

1013
00:50:15,590 --> 00:50:18,100
who really helped me answer questions, who really helped

1014
00:50:18,100 --> 00:50:20,690
me when I was struggling, and really was a partner in this

1015
00:50:20,690 --> 00:50:23,630
experience of learning a very new world.

1016
00:50:23,630 --> 00:50:26,410
In a little bit, all of these guys will join you outside for

1017
00:50:26,410 --> 00:50:29,010
cake, which is a tradition of CS50, in the transept of

1018
00:50:29,010 --> 00:50:30,030
Memorial Hall.

1019
00:50:30,030 --> 00:50:32,320
>> But allow me first to introduce you to Nate

1020
00:50:32,320 --> 00:50:35,820
Hardison, again, Rob Bowden, and Tommy MacWilliam, this

1021
00:50:35,820 --> 00:50:37,460
year's course heads.

1022
00:50:37,460 --> 00:50:39,250
If you guys would join me here in the middle.

1023
00:50:45,250 --> 00:50:47,910
They have all prepared some inspirational remarks.

1024
00:50:51,540 --> 00:50:52,550
>> TOMMY MACWILLIAM: I didn't prepare anything

1025
00:50:52,550 --> 00:50:53,665
inspirational.

1026
00:50:53,665 --> 00:50:55,020
But my name is Tommy.

1027
00:50:55,020 --> 00:50:56,140
I'm a senior in Mather.

1028
00:50:56,140 --> 00:50:57,325
I'm studying computer science.

1029
00:50:57,325 --> 00:50:59,490
I'm really excited to be on the hedge team and going

1030
00:50:59,490 --> 00:51:01,365
through the CS50 journey with you.

1031
00:51:01,365 --> 00:51:04,010
What I really love about CS50 is how it really teaches you

1032
00:51:04,010 --> 00:51:06,040
to think about problems in a new way.

1033
00:51:06,040 --> 00:51:08,570
This is really a skill that's gonna be invaluable no matter

1034
00:51:08,570 --> 00:51:09,950
what field you go into.

1035
00:51:09,950 --> 00:51:12,660
And not only that, but we offer more free candy than any

1036
00:51:12,660 --> 00:51:13,910
other course on campus.

1037
00:51:20,088 --> 00:51:22,210
Yeah, and so I'm really looking forward to seeing what

1038
00:51:22,210 --> 00:51:23,560
everyone builds this semester.

1039
00:51:23,560 --> 00:51:25,730
And if anyone has any questions now or throughout

1040
00:51:25,730 --> 00:51:27,770
the semester, definitely feel free to reach out to me and

1041
00:51:27,770 --> 00:51:29,020
I'd be happy to help.

1042
00:51:31,450 --> 00:51:31,780
>> ROB BOWDEN: Hi.

1043
00:51:31,780 --> 00:51:32,530
I'm Rob Bowden.

1044
00:51:32,530 --> 00:51:33,780
I'm a senior in Kirkland.

1045
00:51:37,900 --> 00:51:39,760
Yeah, that's right.

1046
00:51:39,760 --> 00:51:41,930
We're all really excited for this next semester.

1047
00:51:41,930 --> 00:51:43,230
We hope you're all excited.

1048
00:51:47,710 --> 00:51:49,410
I wasn't expecting that.

1049
00:51:49,410 --> 00:51:50,390
Yeah.

1050
00:51:50,390 --> 00:51:52,200
So we put so much effort into making this

1051
00:51:52,200 --> 00:51:53,370
semester really great.

1052
00:51:53,370 --> 00:51:55,960
And as long as you're willing to put in the effort, there is

1053
00:51:55,960 --> 00:51:57,840
so much you can get out of this course.

1054
00:51:57,840 --> 00:52:00,130
Ah, we--

1055
00:52:00,130 --> 00:52:01,380
yeah.

1056
00:52:04,640 --> 00:52:08,300
You can get a lot of fun out of this course.

1057
00:52:08,300 --> 00:52:10,500
We wouldn't have a staff of 108 if you couldn't get a lot

1058
00:52:10,500 --> 00:52:11,370
of fun out of it.

1059
00:52:11,370 --> 00:52:15,300
So, just try to be involved and you won't regret it.

1060
00:52:23,190 --> 00:52:23,890
>> NATE HARDISON: Hi, guys.

1061
00:52:23,890 --> 00:52:24,800
I'm Nate.

1062
00:52:24,800 --> 00:52:27,570
I'm the preceptor for the course.

1063
00:52:27,570 --> 00:52:29,060
I'm really excited to be here as well.

1064
00:52:29,060 --> 00:52:30,960
This is my first year here.

1065
00:52:30,960 --> 00:52:34,210
I hope you all take this course and enjoy it as much as

1066
00:52:34,210 --> 00:52:35,820
I've enjoyed it so far.

1067
00:52:35,820 --> 00:52:38,570
And if you ever want to learn how to count to 9 or 10 in

1068
00:52:38,570 --> 00:52:41,730
binary, come talk to me.

1069
00:52:41,730 --> 00:52:43,760
>> DAVID J. MALAN: So at the risk of leaving these guys here on

1070
00:52:43,760 --> 00:52:46,980
stage a bit awkwardly, let's whirl through just a few of

1071
00:52:46,980 --> 00:52:49,370
the things that await before we adjourn for cake.

1072
00:52:49,370 --> 00:52:50,580
What is it that lies ahead?

1073
00:52:50,580 --> 00:52:53,490
Well, if we take a quick look back at last year, in problem

1074
00:52:53,490 --> 00:52:56,010
set 0, your predecessors dove into a programming

1075
00:52:56,010 --> 00:52:58,700
language called Scratch, a graphical programming language

1076
00:52:58,700 --> 00:53:01,220
you'll use in the first days of the course starting this

1077
00:53:01,220 --> 00:53:04,390
Friday to learn some concepts unfamiliar to some of you.

1078
00:53:04,390 --> 00:53:06,750
But realize there will be an advanced aspect of this for

1079
00:53:06,750 --> 00:53:08,950
those of you with prior background.

1080
00:53:08,950 --> 00:53:11,450
>> In last year's problem set 2, students dove into the

1081
00:53:11,450 --> 00:53:14,780
world of cryptography, the art of enciphering or scrambling

1082
00:53:14,780 --> 00:53:17,770
information, implementing programs that encrypted data.

1083
00:53:17,770 --> 00:53:21,490
And in the hacker addition last year did students proceed

1084
00:53:21,490 --> 00:53:24,570
to crack or decode the passwords in a typical

1085
00:53:24,570 --> 00:53:28,090
computer's xe password file by coming up with algorithms and

1086
00:53:28,090 --> 00:53:31,810
heuristics for brute force figuring out what someone's

1087
00:53:31,810 --> 00:53:33,640
password on a computer system was.

1088
00:53:33,640 --> 00:53:36,630
Last year, too, in problem set 3, did students impli-- in

1089
00:53:36,630 --> 00:53:37,680
problem set 4-- did students

1090
00:53:37,680 --> 00:53:39,280
implement the game of Sudoku.

1091
00:53:39,280 --> 00:53:42,250
And in the hacker addition that year did students not

1092
00:53:42,250 --> 00:53:45,650
just implement how to play the game, but actually a solver

1093
00:53:45,650 --> 00:53:47,800
whereby the computer can provide you, the human, with

1094
00:53:47,800 --> 00:53:50,130
hints by more rapidly than you've solving

1095
00:53:50,130 --> 00:53:51,420
that particular problem.

1096
00:53:51,420 --> 00:53:54,130
>> In problem set 5, we did forensics, this art of

1097
00:53:54,130 --> 00:53:57,020
recovering information that was accidentally or very

1098
00:53:57,020 --> 00:53:59,320
deliberately deleted from a computer.

1099
00:53:59,320 --> 00:54:02,100
Last year, the teaching staff and I strolled around campus

1100
00:54:02,100 --> 00:54:04,990
taking photographs of people, places, and things, and then

1101
00:54:04,990 --> 00:54:07,830
accidentally formatted the media card on our camera that

1102
00:54:07,830 --> 00:54:08,900
had all those photos.

1103
00:54:08,900 --> 00:54:09,650
But no problem.

1104
00:54:09,650 --> 00:54:13,780
We made a forensic image of this media card, handed it out

1105
00:54:13,780 --> 00:54:16,320
to all students in the class, and challenged them to write

1106
00:54:16,320 --> 00:54:20,110
programs that recovered all of the JPEGs from that card.

1107
00:54:20,110 --> 00:54:22,010
And this is actually one of our favorite problem sets.

1108
00:54:22,010 --> 00:54:24,680
>> And I dug up in an email from one of your predecessors,

1109
00:54:24,680 --> 00:54:27,270
which was great fun to read sometime ago.

1110
00:54:27,270 --> 00:54:30,240
He wrote-- this is from Matt-- dear David, yesterday my

1111
00:54:30,240 --> 00:54:33,710
sister accidentally formatted her camera's SD card and lost

1112
00:54:33,710 --> 00:54:35,630
a year's worth of memorable photos.

1113
00:54:35,630 --> 00:54:38,710
She unfortunately isn't the best at backing up her data.

1114
00:54:38,710 --> 00:54:41,230
But this situation reminded me of pset 5, so I thought I

1115
00:54:41,230 --> 00:54:44,260
would try to run her SD card through the recover program

1116
00:54:44,260 --> 00:54:46,310
that I wrote all the way back in October.

1117
00:54:46,310 --> 00:54:48,550
So after four hours of figuring out how to create a

1118
00:54:48,550 --> 00:54:51,210
raw image from the formatted SD card--

1119
00:54:51,210 --> 00:54:53,370
Google proved to be pretty unhelpful in this regard until

1120
00:54:53,370 --> 00:54:55,480
ironically I happened to come across your instructions on

1121
00:54:55,480 --> 00:54:58,070
the Internet--

1122
00:54:58,070 --> 00:55:00,490
after tinkering around with some of the command arguments,

1123
00:55:00,490 --> 00:55:02,320
I managed to create the forensic image.

1124
00:55:02,320 --> 00:55:04,490
And after installing and configuring the CS50

1125
00:55:04,490 --> 00:55:07,520
Appliance, I managed to run the forensic image through my

1126
00:55:07,520 --> 00:55:13,110
program and recover all 1,027 of my sister's photographs.

1127
00:55:13,110 --> 00:55:14,340
Right, Matt.

1128
00:55:14,340 --> 00:55:15,165
>> So in last year's--

1129
00:55:15,165 --> 00:55:23,080
[APPLAUSE]

1130
00:55:23,080 --> 00:55:25,680
In last year's problem set 6, we gave the students a

1131
00:55:25,680 --> 00:55:29,910
dictionary of 150,000 English words and challenged them to

1132
00:55:29,910 --> 00:55:32,570
write a spell checker that answered queries of the form

1133
00:55:32,570 --> 00:55:33,930
is this word spelled correctly or

1134
00:55:33,930 --> 00:55:35,705
incorrectly as fast as possible.

1135
00:55:35,705 --> 00:55:38,470
And in an opt-in basis were students allowed to then

1136
00:55:38,470 --> 00:55:41,290
challenge classmates by posting their results, the

1137
00:55:41,290 --> 00:55:44,200
amount of RAM that they used, the number of CPU cycles or

1138
00:55:44,200 --> 00:55:46,910
seconds that they used, so that students were then ranked

1139
00:55:46,910 --> 00:55:47,900
on the course's website.

1140
00:55:47,900 --> 00:55:51,170
Again, purely optional aspect of it, but great fun in that

1141
00:55:51,170 --> 00:55:54,540
very often would a student get to position number 10 or so on

1142
00:55:54,540 --> 00:55:57,210
the big board on the website, go off to dinner, and then

1143
00:55:57,210 --> 00:55:59,920
come back and realize his roommate had just edged in

1144
00:55:59,920 --> 00:56:03,270
front of him or her on the big board, thereby pouring another

1145
00:56:03,270 --> 00:56:07,170
two or three hours just to one up his or her roommate.

1146
00:56:07,170 --> 00:56:08,550
>> So we look forward to something similar

1147
00:56:08,550 --> 00:56:09,580
this year as well.

1148
00:56:09,580 --> 00:56:12,140
In problem set 7 did we steer in the direction of web

1149
00:56:12,140 --> 00:56:14,840
programming, actually solving problems in the ever

1150
00:56:14,840 --> 00:56:18,170
increasingly common environment of a web browser.

1151
00:56:18,170 --> 00:56:20,900
Know decreasingly do we download software on Macs and

1152
00:56:20,900 --> 00:56:23,650
PCs, but increasingly do we do it all within the web.

1153
00:56:23,650 --> 00:56:27,530
>> And indeed last year, some 88% of students' final projects in

1154
00:56:27,530 --> 00:56:29,660
the course were web-based.

1155
00:56:29,660 --> 00:56:32,020
And those, too, are skills that you will derive from this

1156
00:56:32,020 --> 00:56:33,230
class by course's end.

1157
00:56:33,230 --> 00:56:36,090
Because what awaits at course's end is the CS50 Fair,

1158
00:56:36,090 --> 00:56:39,060
this exhibition that's based on the idea of a science fair.

1159
00:56:39,060 --> 00:56:41,750
But in this version of a fair do all students in the class

1160
00:56:41,750 --> 00:56:45,400
bring their laptops and their friends and family and others

1161
00:56:45,400 --> 00:56:48,310
to Northwest Science, a large building on campus, set up

1162
00:56:48,310 --> 00:56:51,030
their laptop, get some food, get some popcorn and drink,

1163
00:56:51,030 --> 00:56:53,990
and then exhibit their final projects for all those in

1164
00:56:53,990 --> 00:56:57,210
attendance who last year numbered some 2,500 attendees

1165
00:56:57,210 --> 00:56:58,320
from across campus.

1166
00:56:58,320 --> 00:57:02,100
And expressions like this and like this were not

1167
00:57:02,100 --> 00:57:03,370
uncommon at the fair.

1168
00:57:03,370 --> 00:57:06,590
>> Leading up to the fair is the CS50 Hackathon, an opportunity

1169
00:57:06,590 --> 00:57:09,350
to hop on a Harvard shuttle, head down the street to

1170
00:57:09,350 --> 00:57:12,700
Microsoft at 8:00 PM, and not go home until 7:00 AM.

1171
00:57:12,700 --> 00:57:16,280
We serve first dinner at 8:00 PM, second dinner at 1:00 AM,

1172
00:57:16,280 --> 00:57:18,900
and for those still standing at 5:00 AM, do we treat to

1173
00:57:18,900 --> 00:57:20,400
pancakes at IHOP.

1174
00:57:20,400 --> 00:57:23,470
And the Hackathon is an opportunity, as pictured here,

1175
00:57:23,470 --> 00:57:25,930
to dive into your final projects, whether working on

1176
00:57:25,930 --> 00:57:29,470
your own or with friends in a collaborative environment,

1177
00:57:29,470 --> 00:57:31,970
where the entire teaching staff is working well into the

1178
00:57:31,970 --> 00:57:35,430
night with an ample supply of Hong Kong Chinese food.

1179
00:57:35,430 --> 00:57:39,550
At 5:00 AM will such images as these be quite common this

1180
00:57:39,550 --> 00:57:40,940
year as well.

1181
00:57:40,940 --> 00:57:44,390
>> So as we adjourn in a moment for cake, keep in mind that

1182
00:57:44,390 --> 00:57:48,500
76% of the people in this room have no prior experience.

1183
00:57:48,500 --> 00:57:51,090
And as per the syllabus, what ultimately matters in this

1184
00:57:51,090 --> 00:57:53,740
course is not so much where you end up relative to your

1185
00:57:53,740 --> 00:57:57,400
classmates, but where you in week 11 end up relative to

1186
00:57:57,400 --> 00:57:59,400
yourself in week 0.

1187
00:57:59,400 --> 00:58:01,850
This is CS50.