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  • PROFESSOR: Good morning.

  • Try it again.

  • Good morning.

  • STUDENTS: Good morning.

  • PROFESSOR: Thank you.

  • This is 6.00, also known as Introduction to Computer

  • Science and Programming.

  • My name is Eric Grimson, I have together Professor John Guttag

  • over here, we're going to be lecturing the course this term.

  • I want to give you a heads up; you're getting some serious

  • firepower this term.

  • John was department head for ten years, felt like a century,

  • and in course six, I'm the current department

  • head in course six.

  • John's been lecturing for thirty years, roughly.

  • All right, I'm the young guy, I've only been lecturing

  • for twenty-five years.

  • You can tell, I have less grey hair than he does.

  • What I'm trying to say to you is, we take this

  • course really seriously.

  • We hope you do as well.

  • But we think it's really important for the department

  • to help everybody learn about computation, and that's

  • what this course is about.

  • What I want to do today is three things: I'm going to

  • start-- actually, I shouldn't say start, I'm going to do a

  • little bit of administrivia, the kinds of things you need

  • to know about how we're going to run the course.

  • I want to talk about the goal of the course, what it is

  • you'll be able to do at the end of this course when you get

  • through it, and then I want to begin talking about the

  • concepts and tools of computational thinking, which

  • is what we're primarily going to focus on here.

  • We're going to try and help you learn how to think like a

  • computer scientist, and we're going to begin talking about

  • that towards the end of this lecture and of course

  • throughout the rest of the lectures that carry on.

  • Right, let's start with the goals.

  • I'm going to give you goals in two levels.

  • The strategic goals are the following: we want to help

  • prepare freshmen and sophomores who are interested in majoring

  • in course six to get an easy entry into the department,

  • especially for those students who don't have a lot of prior

  • programming experience.

  • If you're in that category, don't panic, you're

  • going to get it.

  • We're going to help you ramp in and you'll certainly be able to

  • start the course six curriculum and do just fine and

  • still finish on target.

  • We don't expect everybody to be a course six major, contrary to

  • popular opinion, so for those are you not in that category,

  • the second thing we want to do is we want to help students who

  • don't plan to major in course six to feel justifiably

  • confident in their ability to write and read small

  • pieces of code.

  • For all students, what we want to do is we want to give you

  • an understanding of the role computation can and cannot play

  • in tackling technical problems.

  • So that you will come away with a sense of what you can do,

  • what you can't do, and what kinds of things you should use

  • to tackle complex problems.

  • And finally, we want to position all students so that

  • you can easily, if you like, compete for things like your

  • office and summer jobs.

  • Because you'll have an appropriate level of confidence

  • and competence in your ability to do computational

  • problem solving.

  • Those are the strategic goals.

  • Now, this course is primarily aimed at students who

  • have little or no prior programming experience.

  • As a consequence, we believe that no student here is

  • under-qualified for this course: you're all MIT

  • students, you're all qualified to be here.

  • But we also hope that there aren't any students here who

  • are over-qualified for this course.

  • And what do I mean by that?

  • If you've done a lot prior programming, this is probably

  • not the best course for you, and if you're in that category,

  • I would please encourage you to talk to John or I after class

  • about what your goals are, what kind of experience you have,

  • and how we might find you a course that better

  • meets your goals.

  • Second reason we don't want over-qualified students in the

  • class, it sounds a little nasty, but the second reason

  • is, an over-qualified student, somebody who's, I don't know,

  • programmed for Google for the last five years, is going to

  • have an easy time in this course, but we don't want

  • such a student accidentally intimidating the rest of you.

  • We don't want you to feel inadequate when you're

  • simply inexperienced.

  • And so, it really is a course aimed at students with little

  • or no prior programming experience.

  • And again, if you're not in that category, talk to John

  • or I after class, and we'll help you figure out where

  • you might want to go.

  • OK.

  • Those are the top-level goals of the course.

  • Let's talk sort of at a more tactical level, about what do

  • we want you to know in this course.

  • What we want you to be able to do by the time you

  • leave this course?

  • So here are the skills that we would like you to acquire.

  • Right, the first skill we want you to acquire, is we want you

  • to be able to use the basic tools of computational thinking

  • to write small scale programs.

  • I'm going to keep coming back to that idea, but I'm going to

  • call it computational thinking.

  • And that's so you can write small pieces of code.

  • And small is not derogatory here, by the way, it just says

  • the size of things you're going to be able to do.

  • Second skill we want you to have at the end of this course

  • is the ability to use a vocabulary of computational

  • tools in order to be able to understand programs

  • written by others.

  • So you're going to be able to write, you're going

  • to be able to read.

  • This latter skill, by the way, is incredibly valuable.

  • Because you won't want to do everything from scratch

  • yourself, you want to be able to look at what is being

  • created by somebody else and understand what is inside of

  • there, whether it works correctly and how you

  • can build on it.

  • This is one of the few places where plagiarism

  • is an OK thing.

  • It's not bad to, if you like, learn from the skills of others

  • in order to create something you want to write.

  • Although we'll come back to plagiarism as a

  • bad thing later on.

  • Third thing we want you to do, is to understand the

  • fundamental both capabilities and limitations of

  • computations, and the costs associated with them.

  • And that latter statement sounds funny, you don't think

  • of computations having limits, but they do.

  • There're some things that cannot be computed.

  • We want you to understand where those limits are.

  • So you're going to be able to understand

  • abilities and limits.

  • And then, finally, the last tactical skill that you're

  • going to get out of this course is you're going to have the

  • ability to map scientific problems into a

  • computational frame.

  • So you're going to be able to take a description of

  • a problem and map it into something computational.

  • Now if you think about it, boy, it sounds like grammar school.

  • We're going to teach you to read, we're going to teach you

  • to write, we're going to teach you to understand what you can

  • and cannot do, and most importantly, we're going to try

  • and give you the start of an ability to take a description

  • of a problem from some other domain, and figure out how to

  • map it into that domain of computation so you can do

  • the reading and writing that you want to do.

  • OK, in a few minutes we're going to start talking then

  • about what is computation, how are we going to start building

  • those tools, but that's what you should take away, that's

  • what you're going to gain out of this course by the

  • time you're done.

  • Now, let me take a sidebar for about five minutes to talk

  • about course administration, the administrivia, things that

  • we're going to do in the course, just so you know

  • what the rules are.

  • Right, so, class is two hours of lecture a week.

  • You obviously know where and you know when,

  • because you're here.

  • Tuesdays and Thursdays at 11:00.

  • One hour of recitation a week, on Fridays, and we'll come back

  • in a second to how you're going to get set up for that.

  • And nine hours a week of outside-the-class work.

  • Those nine hours are going to be primarily working on problem

  • sets, and all the problems sets are going to involve

  • programming in Python, which is the language we're going

  • to be using this term.

  • Now, one of the things you're going to see is the first

  • problem sets are pretty easy.

  • Actually, that's probably wrong, John, right?

  • They're very easy.

  • And we're going to ramp up.

  • By the time you get to the end of the term, you're going to be

  • dealing with some fairly complex things, so one of the

  • things you're going to see is, we're going to make heavy use

  • of libraries, or code written by others.

  • It'll allow you to tackle interesting problems I'll have

  • you to write from scratch, but it does mean that this skill

  • here is going to be really valuable.

  • You need to be able to read that code and understand it,

  • as well as write your own.

  • OK.

  • Two quizzes.

  • During the term, the dates have already been scheduled.

  • John, I forgot to look them up, I think it's October 2nd and

  • November 4th, it'll be on the course website.

  • My point is, go check the course website, which by

  • the way is right there.

  • If you have, if you know you have a conflict with one of

  • those quiz dates now, please see John or I right away.

  • We'll arrange something ahead of time.

  • But if you-- The reason I'm saying that is, you know, you

  • know that you're getting married that day for example,

  • we will excuse you from the quiz to get married.

  • We'll expect you come right back to do the quiz by the way,

  • but the-- Boy, tough crowd.

  • All right.

  • If you have a conflict, please let us know.

  • Second thing is, if you have an MIT documented special need for

  • taking quizzes, please see John or I well in advance.

  • At least two weeks before the quiz.

  • Again, we'll arrange for this, but you need to give us

  • enough warning so that we can deal with that.

  • OK, the quizzes are open book.

  • This course is not about memory.

  • It's not how well you can memorize facts: in fact, I

  • think both John and I are a little sensitive to memory

  • tests, given our age, right John?

  • This is not about how you memorize things, it's

  • about how you think.

  • So they're open note, open book.

  • It's really going to test your ability to think.

  • The grades for the course will be assigned roughly, and I use

  • the word roughly because we reserve the right to move these

  • numbers around a little bit, but basically in the following

  • percentages: 55% of your grade comes from the problem sets,

  • the other 45% come from the quizzes.

  • And I should've said there's two quizzes and a final exam.

  • I forgot, that final exam during final period.

  • So the quiz percentages are 10%, 15%, and 20%.

  • Which makes up the other 45%.

  • OK.

  • Other administrivia.

  • Let me just look through my list here.

  • First problem set, problem set zero, has already been posted.

  • This is a really easy one.

  • We intend it to be a really easy problem set.

  • It's basically to get you to load up Python on your machine

  • and make sure you understand how to interact with it.

  • The first problem set will be posted shortly, it's also

  • pretty boring-- somewhat like my lectures but not John's--

  • and that means, you know, we want you just to get

  • going on things.

  • Don't worry, we're going to make them more interesting

  • as you go along.

  • Nonetheless, I want to stress that none of these problems

  • sets are intended to be lethal.

  • We're not using them to weed you out, we're using

  • them to help you learn.

  • So if you run into a problem set that just,

  • you don't get, all right?

  • Seek help.

  • Could be psychiatric help, could be a TA.

  • I recommend the TA.

  • My point being, please come and talk to somebody.

  • The problems are set up so that, if you start down the

  • right path, it should be pretty straight-forward

  • to work it through.

  • If you start down a plausible but incorrect path, you can

  • sometimes find yourself stuck in the weeds somewhere, and we

  • want to bring you back in.

  • So part of the goal here is, this should not be a grueling,

  • exhausting kind of task, it's really something that should be

  • helping you learn the material.

  • If you need help, ask John, myself, or the TAs.

  • That's what we're here for.

  • OK.

  • We're going to run primarily a paperless subject, that's

  • why the website is there.

  • Please check it, that's where everything's going

  • to be posted in terms of things you need to know.

  • In particular, please go to it today, you will find a form

  • there that you need to fill out to register for, or sign up

  • for rather, a recitation.

  • Recitations are on Friday.

  • Right now, we have them scheduled at 9:00, 10:00,

  • 11:00, 12:00, 1:00, and 2:00.

  • We may drop one of the recitations, just depending

  • on course size, all right?

  • So we reserve the right, unfortunately, to have

  • to move you around.

  • My guess is that 9:00 is not going to be a tremendously

  • popular time, but maybe you'll surprise me.

  • Nonetheless, please go in and sign up.

  • We will let you sign up for whichever recitation

  • makes sense for you.

  • Again, we reserve the right to move people around if we have

  • to, just to balance load, but we want you to find something

  • that fits your schedule rather than ours.

  • OK.

  • Other things.

  • There is no required text.

  • If you feel exposed without a text book, you really have to

  • have a textbook, you'll find one recommended-- actually I'm

  • going to reuse that word, John, at least suggest it,

  • on the course website.

  • I don't think either of us are thrilled with the text, it's

  • the best we've probably found for Python, it's OK.

  • If you need it, it's there.

  • But we're going to basically not rely on any specific text.

  • Right.

  • Related to that: attendance here is obviously

  • not mandatory.

  • You ain't in high school anymore.

  • I think both of us would love to see your smiling faces, or

  • at least your faces, even if you're not smiling

  • at us every day.

  • Point I want to make about this, though, is that we are

  • going to cover a lot of material that is not in the

  • assigned readings, and we do have assigned readings

  • associated with each one of these lectures.

  • If you choose not to show up today-- or sorry, you did

  • choose to show up today, if you choose not to show up in future

  • days-- we'll understand, but please also understand that the

  • TAs won't have a lot of patience with you if you're

  • asking a question about something that was either

  • covered in the readings, or covered in the lecture and is

  • pretty straight forward.

  • All right?

  • We expect you to behave responsibly and

  • we will as well.

  • All right.

  • I think the last thing I want to say is, we will not be

  • handing out class notes.

  • Now this sounds like a draconian measure;

  • let me tell you why.

  • Every study I know of, and I suspect every one John knows,

  • about learning, stresses that students learn best

  • when they take notes.

  • Ironically, even if they never look at them.

  • OK.

  • The process of writing is exercising both halves of your

  • brain, and it's actually helping you learn, and so

  • taking notes is really valuable thing.

  • Therefore we're not going to distribute notes.

  • What we will distribute for most lectures is a handout

  • that's mostly code examples that we're going to do.

  • I don't happen to have one today because we're not

  • going to do a lot of code.

  • We will in future.

  • Those notes are going to make no sense, I'm guessing, outside

  • of the lecture, all right?

  • So it's not just, you can swing by 11:04 and grab a copy and go

  • off and catch some more sleep.

  • What we recommend is you use those notes to take your own

  • annotations to help you understand what's going on,

  • but we're not going to provide class notes.

  • We want you to take your own notes to help you, if you like,

  • spur your own learning process.

  • All right.

  • And then finally, I want to stress that John, myself,

  • all of the staff, our job is to help you learn.

  • That's what we're here for.

  • It's what we get excited about.

  • If you're stuck, if you're struggling, if you're

  • not certain about something, please ask.

  • We're not mind readers, we can't tell when you're

  • struggling, other than sort of seeing the expression on your

  • face, we need your help in identifying that.

  • But all of the TAs, many of whom are sitting down in the

  • front row over here, are here to help, so come and ask.

  • At the same time, remember that they're students too.

  • And if you come and ask a question that you could have

  • easily answered by doing the reading, coming to lecture, or

  • using Google, they're going to have less patience.

  • But helping you understand things that really are a

  • conceptual difficulty is what they're here for and what

  • we're here for, so please come and talk to us.

  • OK.

  • That takes care of the administrivia preamble.

  • John, things we add?

  • PROFESSOR GUTTAG: Two more quick things.

  • This semester, your class is being videotaped

  • for OpenCourseware.

  • If any of you don't want your image recorded and posted on

  • the web, you're supposed to sit in the back three rows.

  • PROFESSOR GRIMSON: Ah, thank you.

  • I forgot.

  • PROFESSOR GUTTAG: --Because the camera may pan.

  • I think you're all very good-looking and give MIT

  • a good image, so please, feel free to be filmed.

  • PROFESSOR GRIMSON: I'll turn around, so if you want to,

  • you know, move to the back, I won't see who moves.

  • Right.

  • Great.

  • Thank you, John.

  • PROFESSOR GUTTAG: So that, the other thing I want to

  • mention is, recitations are also very important.

  • We will be covering material in recitations that're not in the

  • lectures, not in the reading, and we do expect you to

  • attend recitations.

  • PROFESSOR GRIMSON: Great.

  • Thanks, John.

  • Any questions about the administrivia?

  • I know it's boring, but we need to do it so you know what

  • the ground rules are.

  • Good.

  • OK.

  • Let's talk about computation.

  • As I said, our strategic goal, our tactical goals, are to

  • help you think like a computer scientist.

  • Another way of saying it is, we want to give you the skill so

  • that you can make the computer do what you want it to do.

  • And we hope that at the end of the class, every time you're

  • confronted with some technical problem, one of your first

  • instincts is going to be, "How do I write the piece of code

  • that's going to help me solve that?"

  • So we want to help you think like a computer scientist.

  • All right.

  • And that, is an interesting statement.

  • What does it mean, to think like a computer scientist?

  • Well, let's see.

  • The primary knowledge you're going to take away from this

  • course is this notion of computational problem solving,

  • this ability to think in computational modes of thought.

  • And unlike in a lot of introductory courses, as a

  • consequence, having the ability to memorize is

  • not going to help you.

  • It's really learning those notions of the tools

  • that you want to use.

  • What in the world does it mean to say computational

  • mode of thought?

  • It sounds like a hifalutin phrase you use when you're

  • trying to persuade a VC to fund you.

  • Right.

  • So to answer this, we really have to ask a different

  • question, a related question; so, what's computation?

  • It's like a strange statement, right?

  • What is computation?

  • And part of the reason for putting it up is that I want

  • to, as much as possible, answer that question by separating

  • out the mechanism, which is the computer, from

  • computational thinking.

  • Right.

  • The artifact should not be what's driving this.

  • It should be the notion of, "What does it mean

  • to do computation?"

  • Now, to answer that, I'm going to back up one more level.

  • And I'm going to pose what sounds like a philosophy

  • question, which is, "What is knowledge?" And you'll see in

  • about two minutes why I'm going to do this.

  • But I'm going to suggest that I can divide knowledge into

  • at least two categories.

  • OK, and what is knowledge?

  • And the two categories I'm going to divide them

  • into are declarative and imperative knowledge.

  • Right.

  • What in the world is declarative knowledge?

  • Think of it as statements of fact.

  • It's assertions of truth.

  • Boy, in this political season, that's a really dangerous

  • phrase to use, right?

  • But it's a statement of fact.

  • I'll stay away from the political comments.

  • Let me give you an example of this.

  • Right.

  • Here's a declarative statement.

  • The square root of x is that y such that y squared

  • equals x, y's positive.

  • You all know that.

  • But what I want you to see here, is that's

  • a statement of fact.

  • It's a definition.

  • It's an axiom.

  • It doesn't help you find square roots.

  • If I say x is 2, I want to know, what's the square root of

  • 2, well if you're enough of a geek, you'll say 1.41529 or

  • whatever the heck it is, but in general, this doesn't help

  • you find the square root.

  • The closest it does is it would let you test.

  • You know, if you're wandering through Harvard Square and you

  • see an out-of-work Harvard grad, they're handing out

  • examples of square roots, they'll give you an example and

  • you can test it to see, is the square root of 2,

  • 1.41529 or whatever.

  • I don't even get laughs at Harvard jokes, John,

  • I'm going to stop in a second here, all right?

  • All right, so what am I trying to say here?

  • It doesn't -- yeah, exactly.

  • We're staying away from that, really quickly, especially

  • with the cameras rolling.

  • All right.

  • What am I trying to say?

  • It tells you how you might test something but it

  • doesn't tell you how to.

  • And that's what imperative knowledge is.

  • Imperative knowledge is a description of how

  • to deduce something.

  • So let me give you an example of a piece of

  • imperative knowledge.

  • All right, this is actually a very old piece of imperative

  • knowledge for computing square roots, it's attributed to Heron

  • of Alexandria, although I believe that the Babylonians

  • are suspected of knowing it beforehand.

  • But here is a piece of imperative knowledge.

  • All right?

  • I'm going to start with a guess, I'm going to call it g.

  • And then I'm going to say, if g squared is close to x, stop.

  • And return g.

  • It's a good enough answer.

  • Otherwise, I'm going to get a new guess by taking g, x over

  • g, adding them, and dividing by two.

  • Then you take the average of g and x over g.

  • Don't worry about how came about, Heron found this out.

  • But that gives me a new guess, and I'm going to repeat.

  • That's a recipe.

  • That's a description of a set of steps.

  • Notice what it has, it has a bunch of nice things that

  • we want to use, right?

  • It's a sequence of specific instructions that

  • I do in order.

  • Along the way I have some tests, and depending on the

  • value of that test, I may change where I am in that

  • sequence of instructions.

  • And it has an end test, something that tells

  • me when I'm done and what the answer is.

  • This tells you how to find square roots.

  • it's how-to knowledge.

  • It's imperative knowledge.

  • All right.

  • That's what computation basically is about.

  • We want to have ways of capturing this process.

  • OK, and that leads now to an interesting question, which

  • would be, "How do I build a mechanical process to capture

  • that set of computations?" So I'm going to suggest that

  • there's an easy way to do it-- I realized I did the boards in

  • the wrong order here-- one of the ways I could do it is,

  • you could imagine building a little circuit to do this.

  • If I had a couple of elements of stored values in it, I had

  • some wires to move things around, I had a little thing to

  • do addition, little thing to do division, and a something to do

  • the testing, I could build a little circuit that would

  • actually do this computation.

  • OK.

  • That, strange as it sounds, is actually an example of the

  • earliest computers, because the earliest computers were what we

  • call fixed-program computers, meaning that they had a piece

  • of circuitry designed to do a specific computation.

  • And that's what they would do: they would do that

  • specific computation.

  • You've seen these a lot, right?

  • A good example of this: calculator.

  • It's basically an example of a fixed-program computer.

  • It does arithmetic.

  • If you want play video games on it, good luck.

  • If you want to do word processing on it, good luck.

  • It's designed to do a specific thing.

  • It's a fixed-program computer.

  • In fact, a lot of the other really interesting early ones

  • similarly have this flavor, to give an example: I never know

  • how to pronounce this, Atanasoff, 1941.

  • One of the earliest computational things was a

  • thing designed by a guy named Atanasoff, and it basically

  • solved linear equations.

  • Handy thing to do if you're doing 1801, all right, or

  • 1806, or whatever you want to do those things in.

  • All it could do, though, was solve those equations.

  • One of my favorite examples of an early computer was done by

  • Alan Turing, one of the great computer scientists of all

  • time, called the bombe, which was designed to break codes.

  • It was actually used during WWII to break German

  • Enigma codes.

  • And what it was designed to do, was to solve

  • that specific problem.

  • The point I'm trying to make is, fixed-program computers

  • is where we started, but it doesn't really get us to

  • where we'd like to be.

  • We want to capture this idea of problem solving.

  • So let's see how we'd get there.

  • So even within this framework of, given a description of a

  • computation as a set of steps, in the idea that I could build

  • a circuit to do it, let me suggest for you what would be a

  • wonderful circuit to build.

  • Suppose you could build a circuit with the following

  • property: the input to this circuit would be any

  • other circuit diagram.

  • Give it a circuit diagram for some computation, you give it

  • to the circuit, and that circuit would wonderfully

  • reconfigure itself to act like the circuits diagram.

  • Which would mean, it could act like a calculator.

  • Or, it could act like Turing's bombe.

  • Or, it could act like a square root machine.

  • So what would that circuit look like?

  • You can imagine these tiny little robots wandering

  • around, right?

  • Pulling wires and pulling out components and

  • stacking them together.

  • How would you build a circuit that could take a circuit

  • diagram in and make a machine act like that circuit?

  • Sounds like a neat challenge.

  • Let me change the game slightly.

  • Suppose instead, I want a machine that can take a recipe,

  • the description of a sequence of steps, take that as its

  • input, and then that machine will now act like what is

  • described in that recipe.

  • Reconfigure itself, emulate it, however you want to use the

  • words, it's going to change how it does the computation.

  • That would be cool.

  • And that exists.

  • It's called an interpreter.

  • It is the basic heart of every computer.

  • What it is doing, is saying, change the game.

  • This is now an example of a stored-program computer.

  • What that means, in a stored-program computer, is

  • that I can provide to the computer a sequence of

  • instructions describing the process I want it to execute.

  • And inside of the machine, and things we'll talk about, there

  • is a process that will allow that sequence to be executed as

  • described in that recipe, so it can behave like any thing that

  • I can describe in one of those recipes.

  • All right.

  • That actually seems like a really nice thing to have, and

  • so let me show you what that would basically look like.

  • Inside of a stored-program computer, we would have the

  • following: we have a memory, it's connected to two things;

  • control unit, in what's called an ALU, an arithmetic logic

  • unit, and this can take in input, and spit out output, and

  • inside this stored-program computer, excuse me, you have

  • the following: you have a sequence of instructions.

  • And these all get stored in there.

  • Notice the difference.

  • The recipe, the sequence of instructions, is actually

  • getting read in, and it's treated just like data.

  • It's inside the memory of the machine, which means we have

  • access to it, we can change it, we can use it to build new

  • pieces of code, as well as we can interpret it.

  • One other piece that goes into this computer-- I

  • never remember where to put the PC, John, control?

  • ALU?

  • Separate?

  • I'll put it separate-- you have a thing called

  • a program counter.

  • And here's the basis of the computation.

  • That program counter points to some location in memory,

  • typically to the first instruction in the sequence.

  • And those instructions, by the way, are very simple: they're

  • things like, take the value out of two places in memory, and

  • run them through the multiplier in here, a little piece of

  • circuitry, and stick them back into someplace in memory.

  • Or take this value out of memory, run it through some

  • other simple operation, stick it back in memory.

  • Having executed this instruction, that counter

  • goes up by one and we move to the next one.

  • We execute that instruction, we move to the next one.

  • Oh yeah, it looks a whole lot like that.

  • Some of those instructions will involve tests: they'll

  • say, is something true?

  • And if the test is true, it will change the value of this

  • program counter to point to some other place in the memory,

  • some other point in that sequence of instructions,

  • and you'll keep processing.

  • Eventually you'll hopefully stop, and a value gets spit

  • out, and you're done.

  • That's the heart of a computer.

  • Now that's a slight misstatement.

  • The process to control it is intriguing and interesting, but

  • the heart of the computer is simply this notion that we

  • build our descriptions, our recipes, on a sequence of

  • primitive instructions.

  • And then we have a flow of control.

  • And that flow of control is what I just described.

  • It's moving through a sequence of instructions, occasionally

  • changing where we are as we move around.

  • OK.

  • The thing I want you to take away from this, then, is to

  • think of this as, this is, if you like, a recipe.

  • And that's really what a program is.

  • It's a sequence of instructions.

  • Now, one of things I left hanging is, I said, OK, you

  • build it out of primitives.

  • So one of the questions is, well, what are the right

  • primitives to use?

  • And one of the things that was useful here is, that we

  • actually know that the set of primitives that you want to

  • use is very straight-forward.

  • OK, but before I do that, let me drive home this idea

  • of why this is a recipe.

  • Assuming I have a set of primitive instructions that I

  • can describe everything on, I want to know what can I build.

  • Well, I'm going to do the same analogy to a real recipe.

  • So, real recipe.

  • I don't know.

  • Separate six eggs.

  • Do something.

  • Beat until the-- sorry, beat the whites until they're stiff.

  • Do something until an end test is true.

  • Take the yolks and mix them in with the sugar and water-- No.

  • Sugar and flour I guess is probably what I want, sugar

  • and water is not going to do anything interesting for me

  • here-- mix them into something else.

  • Do a sequence of things.

  • A traditional recipe actually is based on a small set of

  • primitives, and a good chef with, or good cook, I should

  • say, with that set of primitives, can create an

  • unbounded number of great dishes.

  • Same thing holds true in programming.

  • Right.

  • Given a fixed set of primitives, all right,

  • a good programmer can program anything.

  • And by that, I mean anything that can be described in one of

  • these process, you can capture in that set of primitives.

  • All right, the question is, as I started to say, is, "What are

  • the right primitives?" So there's a little bit of, a

  • little piece of history here, if you like.

  • In 1936, that same guy, Alan Turing, showed that with six

  • simple primitives, anything that could be described in a

  • mechanical process, it's actually algorithmically, could

  • be programmed just using those six primitives.

  • Think about that for a second.

  • That's an incredible statement.

  • It says, with six primitives, I can rule the world.

  • With six primitives, I can program anything.

  • A couple of really interesting consequences of that, by the

  • way, one of them is, it says, anything you can do in one

  • programming language, you can do in another

  • programming language.

  • And there is no programming language that is better-- well

  • actually, that's not quite true, there are some better at

  • doing certain kinds of things-- but there's nothing that you

  • can do in C that you can't do in Fortran.

  • It's called Turing compatibility.

  • Anything you can do with one, you can do with another,

  • it's based on that fundamental result.

  • OK.

  • Now, fortunately we're not going to start with Turing's

  • six primitives, this would be really painful programming,

  • because they're down at the level of, "take this value and

  • write it onto this tape." First of all, we don't have tapes

  • anymore in computers, and even if we did, you don't want to be

  • programming at that level.

  • What we're going to see with programming language is

  • that we're going to use higher-level abstracts.

  • A broader set of primitives, but nonetheless the same

  • fundamental thing holds.

  • With those six primitives, you can do it.

  • OK.

  • So where are we here?

  • What we're saying is, in order to do computation, we want to

  • describe recipes, we want to describe this sequence of steps

  • built on some primitives, and we want to describe the flow of

  • control that goes through those sequence of steps

  • as we carry on.

  • So the last thing we need before we can start talking

  • about real programming is, we need to describe those recipes.

  • All right, And to describe the recipes, we're going

  • to want a language.

  • We need to know not only what are the primitives, but how do

  • we make things meaningful in that language.

  • Language.

  • There we go.

  • All right.

  • Now, it turns out there are-- I don't know, John, hundreds?

  • Thousands?

  • Of programming languages?

  • At least hundreds-- of programming languages around.

  • PROFESSOR JOHN GUTTAG: [UNINTELLIGIBLE]

  • PROFESSOR ERIC GRIMSON: True.

  • Thank you.

  • You know, they all have, you know, their pluses and minuses.

  • I have to admit, in my career here, I think I've taught in

  • at least three languages, I suspect you've taught

  • more, five or six, John?

  • Both of us have probably programmed in more than those

  • number of languages, at least programmed that many, since we

  • taught in those languages.

  • One of the things you want to realize is, there

  • is no best language.

  • At least I would argue that, I think John would agree.

  • We might both agree we have our own nominees for

  • worst language, there are some of those.

  • There is no best language.

  • All right?

  • They all are describing different things.

  • Having said that, some of them are better suited for

  • some things than others.

  • Anybody here heard of MATLAB Maybe programmed in MATLAB?

  • It's great for doing things with vectors and matrices

  • and things that are easily captured in that framework.

  • But there's some things that are a real pain

  • to do in MATLAB.

  • So MATLAB's great for that kind of thing.

  • C is a great language for programming things that control

  • data networks, for example.

  • I happen to be, and John teases me about this regularly, I'm an

  • old-time Lisp programmer, and that's how I was trained.

  • And I happen to like Lisp and Scheme, it's a great language

  • when you're trying to deal with problems where you have

  • arbitrarily structured data sets.

  • It's particularly good at that.

  • So the point I want to make here is that there's no

  • particularly best language.

  • What we're going to do is simply use a language that

  • helps us understand.

  • So in this course, the language we're going to use is Python.

  • Which is a pretty new language, it's growing in popularity, it

  • has a lot of the elements of some other languages because

  • it's more recent, it inherits things from it's

  • pregenitors, if you like.

  • But one of the things I want to stress is, this course

  • is not about Python.

  • Strange statement.

  • You do need to know how to use it, but it's not about the

  • details of, where do the semi-colons go in Python.

  • All right?

  • It's about using it to think.

  • And what you should take away from this course is having

  • learned how to design recipes, how to structure recipes,

  • how to do things in modes in Python.

  • Those same tools easily transfer to any other language.

  • You can pick up another language in a week, couple

  • of weeks at most, once you know how to do Python.

  • OK.

  • In order to talk about Python and languages, I want to do one

  • last thing to set the stage for what we're going to do here,

  • and that's to talk about the different dimensions

  • of a language.

  • And there're three I want to deal with.

  • The first one is, whether this is a high-level

  • or low-level language.

  • That basically says, how close are you the

  • guts of the machine?

  • A low-level language, we used to call this assembly

  • programming, you're down at the level of, your primitives are

  • literally moving pieces of data from one location of memory to

  • another, through a very simple operation.

  • A high-level language, the designer has created a much

  • richer set of primitive things.

  • In a high-level language, square root might simply be a

  • primitive that you can use, rather than you having

  • to go over and code it.

  • And there're trade-offs between both.

  • Second dimension is, whether this is a general versus

  • a targeted language.

  • And by that I mean, do the set of primitives support a broad

  • range of applications, or is it really aimed at a very

  • specific set of applications?

  • I'd argue that MATLAB is basically a targeted language,

  • it's targeted at matrices and vectors and things like that.

  • And the third one I want to point out is, whether this

  • is an interpreted versus a compiled language.

  • What that basically says is the following: in an interpreted

  • language, you take what's called the source code, the

  • thing you write, it may go through a simple checker but it

  • basically goes to the interpreter, that thing inside

  • the machine that's going to control the flow of going

  • through each one of the instructions, and

  • give you an output.

  • So the interpreter is simply operating directly on

  • your code at run time.

  • In a compiled language, you have an intermediate step, in

  • which you take the source code, it runs through what's called a

  • checker or a compiler or both, and it creates what's

  • called object code.

  • And that does two things: one, it helps catch bugs in your

  • code, and secondly it often converts it into a more

  • efficient sequence of instructions before you

  • actually go off and run it.

  • All right?

  • And there's trade-offs between both.

  • I mean, an interpreted language is often easier to debug,

  • because you can still see your raw code there, but it's

  • not always as fast.

  • A compiled language is usually much faster in

  • terms of its execution.

  • And it's one of the things you may want to trade off.

  • Right.

  • In the case of Python, it's a high-level language.

  • I would argue, I think John would agree with

  • me, it's basically a general-purpose language.

  • It happens to be better suited for manipulating strings than

  • numbers, for example, but it's really a general-purpose

  • language.

  • And it's primarily-- I shouldn't say primarily, it

  • is an interpreted language.

  • OK?

  • As a consequence, it's not as good as helping debug, but it

  • does let you-- sorry, that's the wrong way of saying-- it's

  • not as good at catching some things before you run them, it

  • is easier at some times in debugging as you go

  • along on the fly.

  • OK.

  • So what does Python look like?

  • In order to talk about Python-- actually, I'm going to do it

  • this way-- we need to talk about how to write

  • things in Python.

  • Again, you have to let me back up slightly and set the stage.

  • Our goal is to build recipes.

  • You're all going to be great chefs by the

  • time you're done here.

  • All right?

  • Our goal is to take problems and break them down into these

  • computational steps, these sequence of instructions

  • that'll allow us to capture that process.

  • To do that, we need to describe: not only, what are

  • the primitives, but how do we capture things legally in that

  • language, and interact with the computer?

  • And so for that, we need a language.

  • We're about to start talking about the elements of the

  • language, but to do that, we also need to separate out one

  • last piece of distinction.

  • Just like with a natural language, we're going to

  • separate out syntax versus semantics.

  • So what's syntax?

  • Syntax basically says, what are the legal expressions

  • in this language?

  • Boy, my handwriting is atrocious, isn't it?

  • There's a English sequence of words.

  • It's not since syntactically correct, right?

  • It's not a sentence.

  • There's no verb in there anywhere, it's just

  • a sequence of nouns.

  • Same thing in our languages.

  • We have to describe how do you put together legally

  • formed expressions.

  • OK?

  • And as we add constructs to the language, we're

  • going to talk about.

  • Second thing we want to talk about very briefly as we go

  • along is the semantics of the language.

  • And here we're going to break out two pieces; static

  • semantics and full semantics.

  • Static semantics basically says which programs are meaningful.

  • Which expressions make sense.

  • Here's an English sentence.

  • It's syntactically correct.

  • Right?

  • Noun phrase, verb, noun phrase.

  • I'm not certain it's meaningful, unless you are in

  • the habit of giving your furniture personal names.

  • What's the point?

  • Again, you can have things that are syntactically legal but not

  • semantically meaningful, and static semantics is going to be

  • a way of helping us decide what expressions, what pieces of

  • code, actually have real meaning to it.

  • All right?

  • The last piece of it is, in addition to having static

  • semantics, we have sort of full semantics.

  • Which is, what does the program mean?

  • Or, said a different way, what's going to

  • happen when I run it?

  • That's the meaning of the expression.

  • That's what you want.

  • All right?

  • You want to know, what's the meaning of this piece of code?

  • When I run it, what's going to happen?

  • That's what I want to build.

  • The reason for pulling this out is, what you're going to see

  • is, that in most languages, and certainly in Python-- we got

  • lots of help here-- all right, Python comes built-in with

  • something that will check your static, sorry, your

  • syntax for you.

  • And in fact, as a sidebar, if you turn in a problem set that

  • is not syntactically correct, there's a simple button that

  • you push that will check your syntax.

  • If you've turned in a program that's not syntactically

  • correct, the TAs give you a zero.

  • Because it said you didn't even take the time to make sure

  • the syntax is correct.

  • The system will help you find it.

  • In Python, it'll find it, I think one bug at

  • a time, right John?

  • It finds one syntax error at a time, so you have to be a

  • little patient to do it, but you can check that

  • the syntax is right.

  • You're going to see that we get some help here on the static

  • semantics, and I'm going to do an example in a second, meaning

  • that the system, some languages are better than others on it,

  • but it will try and help you catch some things that are not

  • semantically correct statically.

  • In the case of Python, it does that I think all at run time.

  • I'm looking to you again, John, I think there's

  • no pre-time checks.

  • Its-- sorry?

  • PROFESSOR JOHN GUTTAG: [UNINTELLIGIBLE]

  • PROFESSOR ERIC GRIMSON: There is some.

  • OK.

  • Most of them, I think though, are primarily caught at run

  • time, and that's a little bit of a pain because you don't see

  • it until you go and run the code, and there are some,

  • actually we're going to see an example I think in a second

  • where you find it, but you do get some help there.

  • The problem is, things that you catch here are actually

  • the least worrisome bugs.

  • They're easy to spot, you can't run the program with them

  • there, so you're not going to get weird answers.

  • Not everything is going to get caught in static

  • semantics checking.

  • Some things are going to slide through, and that's

  • actually a bother.

  • It's a problem.

  • Because it says, your program will still give you a value,

  • but it may not be what you intended, and you can't always

  • tell, and that may propagate it's way down through a whole

  • bunch of other computations before it causes some

  • catastrophic failure.

  • So actually, the problem with static semantics is you'd

  • like it to catch everything, you don't always get it.

  • Sadly we don't get much help here.

  • Which is where we'd like it.

  • But that's part of your job.

  • OK.

  • What happens if you actually have something that's both

  • syntactically correct, and appears to have correct static

  • semantics, and you run it?

  • It could run and give you the right answer, it could crash,

  • it could loop forever, it could run and apparently

  • give you the right answer.

  • And you're not always going to be able to tell.

  • Well, you'll know when it crashes, that doesn't help you

  • very much, but you can't always tell whether something's stuck

  • in an infinite loop or whether it's simply taking a

  • long time to compute.

  • You'd love to have a system that spots that for you,

  • but it's not possible.

  • And so to deal with this last one, you need to develop style.

  • All right?

  • Meaning, we're going to try to help you with how to develop

  • good programming style, but you need to write in a way in which

  • it is going to be easy for you to spot the places that cause

  • those semantic bugs to occur.

  • All right.

  • If that sounds like a really long preamble, it is.

  • Let's start with Python.

  • But again, my goal here is to let you see what computation's

  • about, why we need to do it, I'm going to remind you one

  • last time, our goal is to be able to have a set of

  • primitives that we combine into complex expressions, which we

  • can then abstract to treat as primitives, and we want to use

  • that sequence of instructions in this flow of control

  • computing, in order to deduce new information.

  • That imperative knowledge that we talked about right there.

  • So I'm going to start today, we have about five or ten minutes

  • left, I think, in order-- sorry, five minutes left-- in

  • order to do this with some beginnings of Python, and we're

  • going to pick this up obviously, next time, so;

  • simple parts of Python.

  • In order to create any kinds of expressions, we're

  • going to need values.

  • Primitive data elements.

  • And in Python, we have two to start with; we have numbers,

  • and we have strings.

  • Numbers is what you'd expect.

  • There's a number.

  • There's another number.

  • All right?

  • Strings are captured in Python with an open quote and some

  • sequence of characters followed by a closed quote.

  • Associated with every data type in Python is a type, which

  • identifies the kind of thing it is.

  • Some of these are obvious.

  • Strings are just a type on their own.

  • But for numbers, for example, we can have a variety of types.

  • So this is something that we would call an

  • integer, or an INT.

  • And this is something we would call a floating

  • point, or a float.

  • Or if you want to think of it as a real number.

  • And there's some others that we can see.

  • We're going to build up this taxonomy if you like, but the

  • reason it's relevant is, associated with each one of

  • those types is a set of operators that expect certain

  • types of input in order to do their job.

  • And given those types of input, will get back output.

  • All right.

  • In order to deal with this, let me show you an example, and

  • I hope that comes up, great.

  • What I have here is a Python shell, and I'm going to

  • just show you some simple examples of how we start

  • building expressions.

  • And this'll lead into what you're going to see next

  • time as well as what you're going to do tomorrow.

  • So.

  • Starting with the shell, I can type in expressions.

  • Actually, let me back up and do this in video.

  • I can type in a number, I get back a number, I can type in a

  • string, I get back the string.

  • Strings, by the way, can have spaces in them, they can have

  • other characters, it's simply a sequence of things, and notice,

  • by the way, that the string five-- sorry, the string's

  • digit five digit two is different than the number 52.

  • The quotes are around them to make that distinction.

  • We're going to see why in a second.

  • What I'm doing, by the way, here is I'm simply typing

  • in expressions to that interpreter.

  • It's using its set of rules to deduce the value and

  • print them back out.

  • Things I might like to do in here is, I might like to

  • do combinations of things with these.

  • So we have associated with simple things,

  • a set of operations.

  • So for numbers, we have the things you'd expect,

  • the arithmetics.

  • And let me show you some examples of that.

  • And actually, I'm going to do one other distinction here.

  • What I typed in, things like-- well, let me start this

  • way-- there's an expression.

  • And in Python the expression is, operand, operator, operand,

  • when we're doing simple expressions like this, and if I

  • give it to the interpreter, it gives me back exactly what

  • you'd expect, which is that value.

  • OK?

  • The distinction I'm going to make is, that's an expression.

  • The interpreter is going to get a value for it.

  • When we start building up code, we're going to use commands.

  • Or statements.

  • Which are actually things that take in a value

  • and ask the computer to do something with it.

  • So I can similarly do this, which is going to look strange

  • because it's going to give me the same value back out, but it

  • actually did a slightly different thing.

  • And notice, by the way, when I typed it how print showed

  • up in a different color?

  • That's the Python saying, that is a command, that is a

  • specific command to get the value of the expression

  • and print it back out.

  • When we start writing code, you're going to see that

  • difference, but for now, don't worry about it, I just

  • want to plant that idea.

  • OK.

  • Once we've got that, we can certainly, though,

  • do things like this.

  • Notice the quotes around it.

  • And it treats it as a string, it's simply getting me back the

  • value of that string, 52 times 7, rather than the value of it.

  • Now, once we've got that, we can start doing things.

  • And I'm going to use print here-- if I could type, in

  • order to just to get into that, I can't type, here we go-- in

  • order to get into the habit.

  • I can print out a string.

  • I can print out-- Ah!-- Here's a first example of something

  • that caught one of my things.

  • This is a static semantic error.

  • So what went on here?

  • I gave it an expression that had an operand in there.

  • It expected arithmetic types.

  • But I gave two strings.

  • And so it's complaining at me, saying, you can't do this.

  • I don't know how to take two strings and

  • multiply them together.

  • Unfortunately-- now John you may disagree with me on this

  • one-- unfortunately in Python you can, however, do

  • things like this.

  • What do you figure that's going to do?

  • Look legal?

  • The string three times the number three?

  • Well it happens to give me three threes in a row.

  • I hate this.

  • I'm sorry, John, I hate this.

  • Because this is overloading that multiplication operator

  • with two different tasks.

  • It's saying, if you give me two numbers, I'll

  • do the right thing.

  • If you give me a number and a string, I'm going to

  • concatenate them together, it's really different operations,

  • but nonetheless, it's what it's going to do.

  • STUDENT: [UNINTELLIGIBLE]

  • PROFESSOR ERIC GRIMSON: There you go.

  • You know, there will be a rebuttal phase a little later

  • on, just like with the political debates, and he likes

  • it as a feature, I don't like it, you can tell he's not a

  • Lisp programmer and I am.

  • All right.

  • I want to do just a couple more quick examples.

  • Here's another one.

  • Ah-ha!

  • Give you an example of a syntax error.

  • Because 52A doesn't make sense.

  • And you might say, wait a minute, isn't that a string,

  • and the answer's no, I didn't say it's a string by

  • putting quotes around it.

  • And notice how the machine responds differently to it.

  • In this case it says, this is a syntax error, and it's actually

  • highlighting where it came from so I can go back and fix it.

  • All right.

  • Let's do a couple of other simple examples.

  • All right?

  • I can do multiplication.

  • I've already seen that.

  • I can do addition.

  • Three plus five.

  • I can take something to a power, double star, just take

  • three to the fifth power.

  • I can do division, right?

  • Whoa.

  • Right?

  • Three divided by five is zero?

  • Maybe in Bush econom-- no, I'm not going to do any political

  • comments today, I will not say that, all right?

  • What happened?

  • Well, this is one of the places where you have to be careful.

  • It's doing integer division.

  • So, three divided by five is zero, with a

  • remainder of three.

  • So this is the correct answer.

  • If I wanted to get full, real division, I should make

  • one of them a float.

  • And yes, you can look at that and say, well is that right?

  • Well, up to some level of accuracy, yeah, that's .6 is

  • what I'd like to get out.

  • All right.

  • I can do other things.

  • In a particular, I have similar operations on strings.

  • OK, I can certainly print out strings, but I can actually add

  • strings together, and just as you saw, I can multiply

  • strings, you can kind of guess what this is going to do.

  • It is going to merge them together into one thing.

  • I want-- I know I'm running you slightly over, I want to do one

  • last example, it's, I also want to be able to do, have

  • variables to store things.

  • And to do that, in this it says, if I have a value, I want

  • to keep it around, to do that, I can do things like this.

  • What does that statement do?

  • It says, create a name for a variable-- which I just did

  • there, in fact, let me type it in-- mystring, with an equal

  • sign, which is saying, assign or bind to that name the value

  • of the following expression.

  • As a consequence, I can now refer to that just by its name.

  • If I get the value of mystring, there it is, or if I say, take

  • mystring and add to it the string, mylastname, and

  • print it back out.

  • So this is the first start of this.

  • What have we done?

  • We've got values, numbers and strings.

  • We have operations to associate with them.

  • I just threw a couple up here.

  • You're going to get a chance to explore them, and you'll see

  • not only are there the standard numerics for strings, there are

  • things like length or plus or other things you

  • can do with them.

  • And once I have values, I want to get a hold of them so

  • I can give them names.

  • And that's what I just did when I bound that.

  • I said, use the name mystring to be bound to or have the

  • value of Eric, so I can refer to it anywhere else

  • that I want to use it.

  • And I apologize for taking you over, we'll come back to this

  • next time, please go to the website to sign up for

  • recitation for tomorrow.

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Lec 1 | 麻省理工學院 6.00 計算機科學與程序設計導論,2008年秋季。 (Lec 1 | MIT 6.00 Introduction to Computer Science and Programming, Fall 2008)

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    Jason 發佈於 2021 年 01 月 14 日
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