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PROFESSOR: Good morning.

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Try it again.

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Good morning.

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STUDENTS: Good morning.

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00:00:25,820 --> 00:00:27,510
PROFESSOR: Thank you.

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This is 6.00, also known as
Introduction to Computer

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Science and Programming.

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My name is Eric Grimson, I have
together Professor John

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Guttag over here, we're
going to be lecturing

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the course this term.

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

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firepower this term.

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John was department head for
ten years, felt like a

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

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head in course six.

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John's been lecturing for
thirty years, roughly.

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

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twenty-five years.

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

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

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course really seriously.

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We hope you do as well.

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But we think it's really
important for the department

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

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this course is about.

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

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

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little bit of administrivia, the
kinds of things you need

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

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

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

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get through it, and then I want
to begin talking about

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the concepts and tools of
computational thinking, which

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

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

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

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that towards the end of this
lecture and of course

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throughout the rest of the
lectures that carry on.

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Right, let's start
with the goals.

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

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The strategic goals are the
following: we want to help

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prepare freshmen and sophomores
who are interested

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in majoring in course six to
get an easy entry into the

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department, especially for those
students who don't have

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a lot of prior programming
experience.

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

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going to get it.

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

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able to start the course six
curriculum and do just fine

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and still finish on target.

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

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contrary to popular opinion,
so for those are you not in

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that category, the second thing
we want to do is we want

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to help students who don't plan
to major in course six to

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feel justifiably confident in
their ability to write and

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read small pieces of code.

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

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an understanding of the role
computation can and cannot

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play in tackling technical
problems. So that you will

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come away with a sense of what
you can do, what you can't do,

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and what kinds of things you
should use to tackle complex

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problems.

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And finally, we want to position
all students so that

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

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office and summer jobs.

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Because you'll have an
appropriate level of

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confidence and competence
in your ability to do

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computational problem solving.

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Those are the strategic goals.

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Now, this course is primarily
aimed at students who have

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little or no prior programming
experience.

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As a consequence, we believe
that no student here is

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

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students, you're all qualified
to be here.

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

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are over-qualified
for this course.

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And what do I mean by that?

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

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

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category, I would please
encourage you to talk to John

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or I after class about what your
goals are, what kind of

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experience you have, and how
we might find you a course

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that better meets your goals.

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

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

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

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

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

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a student accidentally
intimidating the rest of you.

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

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inexperienced.

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

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or no prior programming
experience.

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

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

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want to go.

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OK.

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Those are the top-level
goals of the course.

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

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we want you to know
in this course.

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

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you leave this course?

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

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

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to be able to use the basic
tools of computational

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thinking to write small scale
programs. I'm going to keep

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coming back to that idea,
but I'm going to call it

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computational thinking.

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

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

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

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

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is the ability to use a
vocabulary of computational

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tools in order to be
able to understand

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programs written by others.

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

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to be able to read.

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

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Because you won't want to do
everything from scratch

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

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created by somebody else and
understand what is inside of

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there, whether it works
correctly and how you can

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build on it.

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This is one of the
few places where

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plagiarism is an OK thing.

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

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others in order to create
something you want to write.

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Although we'll come back
to plagiarism as a

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bad thing later on.

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

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fundamental both capabilities
and limitations of

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computations, and the costs
associated with them.

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

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of computations having
limits, but they do.

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There're some things that
cannot be computed.

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We want you to understand
where those limits are.

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So you're going to be
able to understand

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abilities and limits.

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

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

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the ability to map scientific
problems into a

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computational frame.

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

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problem and map it into
something computational.

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Now if you think about
it, boy, it sounds

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like grammar school.

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

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

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

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try and give you the start
of an ability to take a

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description of a problem from
some other domain, and figure

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out how to map it into that
domain of computation so you

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can do the reading and writing
that you want to do.

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

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

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

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

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you're done.

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

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about course administration, the
administrivia, things that

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

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the rules are.

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

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You obviously know where
and you know when,

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because you're here.

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Tuesdays and Thursdays
at 11:00.

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

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

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

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Those nine hours are going to
be primarily working on

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problem sets, and all the
problems sets are going to

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involve programming in Python,
which is the language we're

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going to be using this term.

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

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problem sets are pretty easy.

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

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They're very easy.

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And we're going to ramp up.

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

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

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00:07:59,330 --> 00:08:01,540
things you're going to see is,
we're going to make heavy use

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of libraries, or code
written by others.

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

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

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here is going to be
really valuable.

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

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as well as write your own.

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OK.

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Two quizzes.

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During the term, the dates have
already been scheduled.

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

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

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

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way is right there.

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

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

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We'll arrange something
ahead of time.

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But if you--

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The reason I'm saying that is,
you know, you know that you're

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getting married that day for
example, we will excuse you

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from the quiz to get married.

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

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way, but the--

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Boy, tough crowd.

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All right.

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If you have a conflict,
please let us know.

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

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00:09:03,240 --> 00:09:07,230
for taking quizzes, please see
John or I well in advance.

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00:09:07,230 --> 00:09:08,730
At least two weeks
before the quiz.

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00:09:08,730 --> 00:09:10,950
Again, we'll arrange for this,
but you need to give us enough

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00:09:10,950 --> 00:09:13,830
warning so that we can
deal with that.

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00:09:13,830 --> 00:09:16,920
OK, the quizzes are open book.

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00:09:16,920 --> 00:09:20,330
This course is not
about memory.

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00:09:20,330 --> 00:09:22,190
It's not how well you can
memorize facts: in fact, I

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00:09:22,190 --> 00:09:24,180
think both John and I are a
little sensitive to memory

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tests, given our age,
right John?

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00:09:26,230 --> 00:09:28,370
This is not about how you
memorize things, it's about

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how you think.

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So they're open note,
open book.

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

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

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00:09:38,250 --> 00:09:40,110
the word roughly because we
reserve the right to move

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00:09:40,110 --> 00:09:42,390
these numbers around a little
bit, but basically in the

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00:09:42,390 --> 00:09:44,940
following percentages: 55% of
your grade comes from the

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problem sets, the other 45%
come from the quizzes.

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00:09:48,750 --> 00:09:50,810
And I should've said there's two
quizzes and a final exam.

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I forgot, that final exam
during final period.

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

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Which makes up the other 45%.

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00:09:59,030 --> 00:10:00,220
OK.

238
00:10:00,220 --> 00:10:02,420
Other administrivia.

239
00:10:02,420 --> 00:10:05,180
Let me just look through
my list here.

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00:10:05,180 --> 00:10:07,930
First problem set, problem set
zero, has already been posted.

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00:10:07,930 --> 00:10:09,800
This is a really easy one.

242
00:10:09,800 --> 00:10:11,600
We intend it to be a really
easy problem set.

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00:10:11,600 --> 00:10:14,610
It's basically to get you to
load up Python on your machine

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00:10:14,610 --> 00:10:17,470
and make sure you understand
how to interact with it.

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00:10:17,470 --> 00:10:19,820
The first problem set will be
posted shortly, it's also

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00:10:19,820 --> 00:10:23,630
pretty boring-- somewhat like
my lectures but not John's--

247
00:10:23,630 --> 00:10:25,510
and that means, you know,
we want you just to

248
00:10:25,510 --> 00:10:26,260
get going on things.

249
00:10:26,260 --> 00:10:27,980
Don't worry, we're going to make
them more interesting as

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00:10:27,980 --> 00:10:28,950
you go along.

251
00:10:28,950 --> 00:10:31,540
Nonetheless, I want to stress
that none of these problems

252
00:10:31,540 --> 00:10:33,740
sets are intended
to be lethal.

253
00:10:33,740 --> 00:10:36,140
We're not using them to weed you
out, we're using them to

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00:10:36,140 --> 00:10:36,770
help you learn.

255
00:10:36,770 --> 00:10:39,850
So if you run into a problem
set that just, you

256
00:10:39,850 --> 00:10:41,640
don't get, all right?

257
00:10:41,640 --> 00:10:43,290
Seek help.

258
00:10:43,290 --> 00:10:46,080
Could be psychiatric help,
could be a TA.

259
00:10:46,080 --> 00:10:47,830
I recommend the TA.

260
00:10:47,830 --> 00:10:50,450
My point being, please come
and talk to somebody.

261
00:10:50,450 --> 00:10:53,310
The problems are set up so that,
if you start down the

262
00:10:53,310 --> 00:10:55,520
right path, it should be pretty
straight-forward to

263
00:10:55,520 --> 00:10:56,640
work it through.

264
00:10:56,640 --> 00:11:00,130
If you start down a plausible
but incorrect path, you can

265
00:11:00,130 --> 00:11:02,920
sometimes find yourself stuck in
the weeds somewhere, and we

266
00:11:02,920 --> 00:11:03,980
want to bring you back in.

267
00:11:03,980 --> 00:11:08,040
So part of the goal here is,
this should not be a grueling,

268
00:11:08,040 --> 00:11:10,980
exhausting kind of task, it's
really something that should

269
00:11:10,980 --> 00:11:12,750
be helping you learn
the material.

270
00:11:12,750 --> 00:11:15,000
If you need help, ask John,
myself, or the TAs.

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00:11:15,000 --> 00:11:17,540
That's what we're here for.

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00:11:17,540 --> 00:11:18,530
OK.

273
00:11:18,530 --> 00:11:22,110
We're going to run primarily a
paperless subject, that's why

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00:11:22,110 --> 00:11:23,130
the website is there.

275
00:11:23,130 --> 00:11:24,760
Please check it, that's where
everything's going to be

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00:11:24,760 --> 00:11:27,120
posted in terms of things
you need to know.

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00:11:27,120 --> 00:11:30,680
In particular, please go to it
today, you will find a form

278
00:11:30,680 --> 00:11:33,440
there that you need to fill out
to register for, or sign

279
00:11:33,440 --> 00:11:35,840
up for rather, a recitation.

280
00:11:35,840 --> 00:11:37,110
Recitations are on Friday.

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00:11:37,110 --> 00:11:39,090
Right now, we have them
scheduled at 9:00, 10:00,

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00:11:39,090 --> 00:11:41,790
11:00, 12:00, 1:00, and 2:00.

283
00:11:41,790 --> 00:11:45,480
We may drop one of the
recitations, just depending on

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00:11:45,480 --> 00:11:46,610
course size, all right?

285
00:11:46,610 --> 00:11:48,770
So we reserve the right,
unfortunately, to have to move

286
00:11:48,770 --> 00:11:49,890
you around.

287
00:11:49,890 --> 00:11:52,120
My guess is that 9:00 is not
going to be a tremendously

288
00:11:52,120 --> 00:11:54,510
popular time, but maybe
you'll surprise me.

289
00:11:54,510 --> 00:11:56,040
Nonetheless, please
go in and sign up.

290
00:11:56,040 --> 00:11:58,520
We will let you sign up for
whichever recitation makes

291
00:11:58,520 --> 00:11:59,740
sense for you.

292
00:11:59,740 --> 00:12:02,180
Again, we reserve the right to
move people around if we have

293
00:12:02,180 --> 00:12:04,340
to, just to balance load, but we
want you to find something

294
00:12:04,340 --> 00:12:08,350
that fits your schedule
rather than ours.

295
00:12:08,350 --> 00:12:09,370
OK.

296
00:12:09,370 --> 00:12:10,920
Other things.

297
00:12:10,920 --> 00:12:12,960
There is no required text.

298
00:12:12,960 --> 00:12:17,420
If you feel exposed without a
text book, you really have to

299
00:12:17,420 --> 00:12:20,126
have a textbook, you'll find one
recommended-- actually I'm

300
00:12:20,126 --> 00:12:23,230
going to reuse that word, John,
at least suggest it, on

301
00:12:23,230 --> 00:12:24,020
the course website.

302
00:12:24,020 --> 00:12:26,190
I don't think either of us are
thrilled with the text, it's

303
00:12:26,190 --> 00:12:28,630
the best we've probably found
for Python, it's OK.

304
00:12:28,630 --> 00:12:29,950
If you need it, it's there.

305
00:12:29,950 --> 00:12:33,990
But we're going to basically not
rely on any specific text.

306
00:12:33,990 --> 00:12:34,270
Right.

307
00:12:34,270 --> 00:12:36,620
Related to that: attendance
here is

308
00:12:36,620 --> 00:12:38,600
obviously not mandatory.

309
00:12:38,600 --> 00:12:40,450
You ain't in high
school anymore.

310
00:12:40,450 --> 00:12:42,900
I think both of us would love to
see your smiling faces, or

311
00:12:42,900 --> 00:12:44,320
at least your faces,
even if you're not

312
00:12:44,320 --> 00:12:46,910
smiling at us every day.

313
00:12:46,910 --> 00:12:49,300
Point I want to make about this,
though, is that we are

314
00:12:49,300 --> 00:12:52,040
going to cover a lot of material
that is not in the

315
00:12:52,040 --> 00:12:53,840
assigned readings, and we do
have assigned readings

316
00:12:53,840 --> 00:12:57,760
associated with each one
of these lectures.

317
00:12:57,760 --> 00:13:00,900
If you choose not to show up
today-- or sorry, you did

318
00:13:00,900 --> 00:13:03,000
choose to show up today, if you
choose not to show up in

319
00:13:03,000 --> 00:13:05,750
future days-- we'll understand,
but please also

320
00:13:05,750 --> 00:13:08,420
understand that the TAs won't
have a lot of patience with

321
00:13:08,420 --> 00:13:10,970
you if you're asking a question
about something that

322
00:13:10,970 --> 00:13:12,910
was either covered in the
readings, or covered in the

323
00:13:12,910 --> 00:13:14,820
lecture and is pretty
straight forward.

324
00:13:14,820 --> 00:13:14,930
All right?

325
00:13:14,930 --> 00:13:16,630
We expect you to behave
responsibly

326
00:13:16,630 --> 00:13:18,900
and we will as well.

327
00:13:18,900 --> 00:13:20,950
All right.

328
00:13:20,950 --> 00:13:22,950
I think the last thing I want
to say is, we will not be

329
00:13:22,950 --> 00:13:26,110
handing out class notes.

330
00:13:26,110 --> 00:13:27,920
Now this sounds like a
draconian measure;

331
00:13:27,920 --> 00:13:29,070
let me tell you why.

332
00:13:29,070 --> 00:13:31,660
Every study I know of, and I
suspect every one John knows,

333
00:13:31,660 --> 00:13:35,560
about learning, stresses that
students learn best when they

334
00:13:35,560 --> 00:13:36,940
take notes.

335
00:13:36,940 --> 00:13:40,460
Ironically, even if they
never look at them.

336
00:13:40,460 --> 00:13:40,770
OK.

337
00:13:40,770 --> 00:13:44,850
The process of writing is
exercising both halves of your

338
00:13:44,850 --> 00:13:46,840
brain, and it's actually helping
you learn, and so

339
00:13:46,840 --> 00:13:48,610
taking notes is really
valuable thing.

340
00:13:48,610 --> 00:13:50,820
Therefore we're not going
to distribute notes.

341
00:13:50,820 --> 00:13:53,470
What we will distribute for
most lectures is a handout

342
00:13:53,470 --> 00:13:55,800
that's mostly code examples
that we're going to do.

343
00:13:55,800 --> 00:13:57,310
I don't happen to have one today
because we're not going

344
00:13:57,310 --> 00:13:58,410
to do a lot of code.

345
00:13:58,410 --> 00:13:59,970
We will in future.

346
00:13:59,970 --> 00:14:02,980
Those notes are going to make
no sense, I'm guessing,

347
00:14:02,980 --> 00:14:04,870
outside of the lecture,
all right?

348
00:14:04,870 --> 00:14:08,280
So it's not just, you can swing
by 11:04 and grab a copy

349
00:14:08,280 --> 00:14:10,150
and go off and catch
some more sleep.

350
00:14:10,150 --> 00:14:13,120
What we recommend is you use
those notes to take your own

351
00:14:13,120 --> 00:14:15,910
annotations to help you
understand what's going on,

352
00:14:15,910 --> 00:14:17,840
but we're not going to
provide class notes.

353
00:14:17,840 --> 00:14:20,720
We want you to take your own
notes to help you, if you

354
00:14:20,720 --> 00:14:23,820
like, spur your own
learning process.

355
00:14:23,820 --> 00:14:24,170
All right.

356
00:14:24,170 --> 00:14:28,620
And then finally, I want to
stress that John, myself, all

357
00:14:28,620 --> 00:14:32,120
of the staff, our job is
to help you learn.

358
00:14:32,120 --> 00:14:32,950
That's what we're here for.

359
00:14:32,950 --> 00:14:35,000
It's what we get
excited about.

360
00:14:35,000 --> 00:14:38,080
If you're stuck, if you're
struggling, if you're not

361
00:14:38,080 --> 00:14:40,930
certain about something,
please ask.

362
00:14:40,930 --> 00:14:42,950
We're not mind readers, we
can't tell when you're

363
00:14:42,950 --> 00:14:44,990
struggling, other than sort of
seeing the expression on your

364
00:14:44,990 --> 00:14:48,180
face, we need your help
in identifying that.

365
00:14:48,180 --> 00:14:50,070
But all of the TAs, many of whom
are sitting down in the

366
00:14:50,070 --> 00:14:53,690
front row over here, are here
to help, so come and ask.

367
00:14:53,690 --> 00:14:56,870
At the same time, remember that
they're students too.

368
00:14:56,870 --> 00:14:59,010
And if you come and ask a
question that you could have

369
00:14:59,010 --> 00:15:02,740
easily answered by doing the
reading, coming to lecture, or

370
00:15:02,740 --> 00:15:05,580
using Google, they're going
to have less patience.

371
00:15:05,580 --> 00:15:07,430
But helping you understand
things that really are a

372
00:15:07,430 --> 00:15:10,420
conceptual difficulty is what
they're here for and what

373
00:15:10,420 --> 00:15:14,080
we're here for, so please
come and talk to us.

374
00:15:14,080 --> 00:15:15,390
OK.

375
00:15:15,390 --> 00:15:17,550
That takes care of the
administrivia preamble.

376
00:15:17,550 --> 00:15:18,420
John, things we add?

377
00:15:18,420 --> 00:15:34,890
PROFESSOR GUTTAG: Two
more quick things.

378
00:15:34,890 --> 00:15:35,750
This semester, your class
is being videotaped for

379
00:15:35,750 --> 00:15:35,780
OpenCourseware.

380
00:15:35,780 --> 00:15:36,092
If any of you don't want your
image recorded and posted on

381
00:15:36,092 --> 00:15:38,520
the web, you're supposed to sit
in the back three rows.

382
00:15:38,520 --> 00:15:39,243
PROFESSOR GRIMSON:
Ah, thank you.

383
00:15:39,243 --> 00:15:39,390
I forgot.

384
00:15:39,390 --> 00:15:40,260
PROFESSOR GUTTAG: --Because
the camera may pan.

385
00:15:40,260 --> 00:15:40,460
I think you're all very
good-looking and give MIT a

386
00:15:40,460 --> 00:15:40,650
good image, so please, feel
free to be filmed.

387
00:15:40,650 --> 00:15:45,600
PROFESSOR GRIMSON: I'll turn
around, so if you want to, you

388
00:15:45,600 --> 00:15:48,050
know, move to the back,
I won't see who moves.

389
00:15:48,050 --> 00:15:48,340
Right.

390
00:15:48,340 --> 00:15:48,570
Great.

391
00:15:48,570 --> 00:15:49,090
Thank you, John.

392
00:15:49,090 --> 00:15:57,320
PROFESSOR GUTTAG: So that, the
other thing I want to mention

393
00:15:57,320 --> 00:16:00,130
is, recitations are also
very important.

394
00:16:00,130 --> 00:16:00,449
We will be covering material in
recitations that're not in

395
00:16:00,449 --> 00:16:03,108
the lectures, not in the
reading, and we do expect you

396
00:16:03,108 --> 00:16:03,630
to attend recitations.

397
00:16:03,630 --> 00:16:04,794
PROFESSOR GRIMSON: Great.

398
00:16:04,794 --> 00:16:06,210
Thanks, John.

399
00:16:06,210 --> 00:16:08,050
Any questions about
the administrivia?

400
00:16:08,050 --> 00:16:10,280
I know it's boring, but we need
to do it so you know what

401
00:16:10,280 --> 00:16:12,820
the ground rules are.

402
00:16:12,820 --> 00:16:13,420
Good.

403
00:16:13,420 --> 00:16:14,280
OK.

404
00:16:14,280 --> 00:16:16,970
Let's talk about computation.

405
00:16:16,970 --> 00:16:19,870
As I said, our strategic goal,
our tactical goals, are to

406
00:16:19,870 --> 00:16:23,280
help you think like a computer
scientist. Another way of

407
00:16:23,280 --> 00:16:25,350
saying it is, we want to give
you the skill so that you can

408
00:16:25,350 --> 00:16:28,220
make the computer do what
you want it to do.

409
00:16:28,220 --> 00:16:30,530
And we hope that at the end of
the class, every time you're

410
00:16:30,530 --> 00:16:32,990
confronted with some technical
problem, one of your first

411
00:16:32,990 --> 00:16:35,900
instincts is going to be, "How
do I write the piece of code

412
00:16:35,900 --> 00:16:37,620
that's going to help
me solve that?"

413
00:16:37,620 --> 00:16:39,120
So we want to help you
think like a computer

414
00:16:39,120 --> 00:16:41,990
scientist. All right.

415
00:16:41,990 --> 00:16:45,970
And that, is an interesting
statement.

416
00:16:45,970 --> 00:16:55,690
What does it mean, to think
like a computer scientist?

417
00:16:55,690 --> 00:16:59,100
Well, let's see.

418
00:16:59,100 --> 00:17:00,480
The primary knowledge you're
going to take away from this

419
00:17:00,480 --> 00:17:02,630
course is this notion of
computational problem solving,

420
00:17:02,630 --> 00:17:04,670
this ability to think in

421
00:17:04,670 --> 00:17:07,590
computational modes of thought.

422
00:17:07,590 --> 00:17:10,890
And unlike in a lot of
introductory courses, as a

423
00:17:10,890 --> 00:17:12,870
consequence, having the ability
to memorize is not

424
00:17:12,870 --> 00:17:13,460
going to help you.

425
00:17:13,460 --> 00:17:16,260
It's really learning those
notions of the tools that you

426
00:17:16,260 --> 00:17:18,170
want to use.

427
00:17:18,170 --> 00:17:19,180
What in the world does
it mean to say

428
00:17:19,180 --> 00:17:20,450
computational mode of thought?

429
00:17:20,450 --> 00:17:22,340
It sounds like a hifalutin
phrase you use when you're

430
00:17:22,340 --> 00:17:24,830
trying to persuade
a VC to fund you.

431
00:17:24,830 --> 00:17:25,390
Right.

432
00:17:25,390 --> 00:17:27,530
So to answer this, we really
have to ask a different

433
00:17:27,530 --> 00:17:31,490
question, a related question;
so, what's computation?

434
00:17:31,490 --> 00:17:32,690
It's like a strange
statement, right?

435
00:17:32,690 --> 00:17:35,430
What is computation?

436
00:17:35,430 --> 00:17:38,840
And part of the reason for
putting it up is that I want

437
00:17:38,840 --> 00:17:41,500
to, as much as possible,
answer that question by

438
00:17:41,500 --> 00:17:45,620
separating out the mechanism,
which is the computer, from

439
00:17:45,620 --> 00:17:47,420
computational thinking.

440
00:17:47,420 --> 00:17:47,590
Right.

441
00:17:47,590 --> 00:17:49,570
The artifact should not be
what's driving this.

442
00:17:49,570 --> 00:17:51,090
It should be the notion of,
"What does it mean to do

443
00:17:51,090 --> 00:17:53,240
computation?"

444
00:17:53,240 --> 00:17:56,110
Now, to answer that, I'm going
to back up one more level.

445
00:17:56,110 --> 00:17:57,900
And I'm going to pose what
sounds like a philosophy

446
00:17:57,900 --> 00:18:01,070
question, which is, "What is
knowledge?" And you'll see in

447
00:18:01,070 --> 00:18:02,130
about two minutes why I'm
going to do this.

448
00:18:02,130 --> 00:18:04,815
But I'm going to suggest that I
can divide knowledge into at

449
00:18:04,815 --> 00:18:07,060
least two categories.

450
00:18:07,060 --> 00:18:08,390
OK, and what is knowledge?

451
00:18:08,390 --> 00:18:12,280
And the two categories I'm going
to divide them into are

452
00:18:12,280 --> 00:18:19,870
declarative and imperative
knowledge.

453
00:18:19,870 --> 00:18:20,150
Right.

454
00:18:20,150 --> 00:18:22,840
What in the world is declarative
knowledge?

455
00:18:22,840 --> 00:18:25,120
Think of it as statements
of fact.

456
00:18:25,120 --> 00:18:27,610
It's assertions of truth.

457
00:18:27,610 --> 00:18:29,740
Boy, in this political season,
that's a really dangerous

458
00:18:29,740 --> 00:18:30,630
phrase to use, right?

459
00:18:30,630 --> 00:18:32,920
But it's a statement of fact.

460
00:18:32,920 --> 00:18:34,350
I'll stay away from the
political comments.

461
00:18:34,350 --> 00:18:35,930
Let me give you an
example of this.

462
00:18:35,930 --> 00:18:36,130
Right.

463
00:18:36,130 --> 00:18:37,830
Here's a declarative
statement.

464
00:18:37,830 --> 00:18:46,980
The square root of x is that y
such that y squared equals x,

465
00:18:46,980 --> 00:18:48,940
y's positive.

466
00:18:48,940 --> 00:18:50,800
You all know that.

467
00:18:50,800 --> 00:18:52,280
But what I want you to
see here, is that's a

468
00:18:52,280 --> 00:18:54,550
statement of fact.

469
00:18:54,550 --> 00:18:55,220
It's a definition.

470
00:18:55,220 --> 00:18:55,750
It's an axiom.

471
00:18:55,750 --> 00:19:00,700
It doesn't help you
find square roots.

472
00:19:00,700 --> 00:19:02,865
If I say x is 2, I want to know,
what's the square root

473
00:19:02,865 --> 00:19:06,380
of 2, well if you're enough of
a geek, you'll say 1.41529 or

474
00:19:06,380 --> 00:19:10,340
whatever the heck it is, but in
general, this doesn't help

475
00:19:10,340 --> 00:19:12,660
you find the square root.

476
00:19:12,660 --> 00:19:15,600
The closest it does is it would
let you test. You know,

477
00:19:15,600 --> 00:19:17,290
if you're wandering through
Harvard Square and you see an

478
00:19:17,290 --> 00:19:19,540
out-of-work Harvard grad,
they're handing out examples

479
00:19:19,540 --> 00:19:21,450
of square roots, they'll give
you an example and you can

480
00:19:21,450 --> 00:19:23,666
test it to see, is the
square root of

481
00:19:23,666 --> 00:19:26,930
2, 1.41529 or whatever.

482
00:19:26,930 --> 00:19:29,090
I don't even get laughs at
Harvard jokes, John, I'm going

483
00:19:29,090 --> 00:19:31,550
to stop in a second
here, all right?

484
00:19:31,550 --> 00:19:33,410
All right, so what am I
trying to say here?

485
00:19:33,410 --> 00:19:36,560
It doesn't -- yeah, exactly.

486
00:19:36,560 --> 00:19:38,580
We're staying away from that,
really quickly, especially

487
00:19:38,580 --> 00:19:39,850
with the cameras rolling.

488
00:19:39,850 --> 00:19:40,130
All right.

489
00:19:40,130 --> 00:19:41,260
What am I trying to say?

490
00:19:41,260 --> 00:19:44,140
It tells you how you might test
something but it doesn't

491
00:19:44,140 --> 00:19:46,380
tell you how to.

492
00:19:46,380 --> 00:19:48,330
And that's what imperative
knowledge is.

493
00:19:48,330 --> 00:19:51,190
Imperative knowledge
is a description of

494
00:19:51,190 --> 00:19:52,380
how to deduce something.

495
00:19:52,380 --> 00:19:54,780
So let me give you an
example of a piece

496
00:19:54,780 --> 00:19:56,510
of imperative knowledge.

497
00:19:56,510 --> 00:19:58,940
All right, this is actually a
very old piece of imperative

498
00:19:58,940 --> 00:20:00,970
knowledge for computing square
roots, it's attributed to

499
00:20:00,970 --> 00:20:04,400
Heron of Alexandria, although I
believe that the Babylonians

500
00:20:04,400 --> 00:20:07,110
are suspected of knowing
it beforehand.

501
00:20:07,110 --> 00:20:09,860
But here is a piece of
imperative knowledge.

502
00:20:09,860 --> 00:20:10,830
All right?

503
00:20:10,830 --> 00:20:17,540
I'm going to start with a guess,
I'm going to call it g.

504
00:20:17,540 --> 00:20:26,910
And then I'm going to say, if g
squared is close to x, stop.

505
00:20:26,910 --> 00:20:28,530
And return g.

506
00:20:28,530 --> 00:20:30,560
It's a good enough answer.

507
00:20:30,560 --> 00:20:38,850
Otherwise, I'm going to get a
new guess by taking g, x over

508
00:20:38,850 --> 00:20:42,880
g, adding them, and
dividing by two.

509
00:20:42,880 --> 00:20:44,610
Then you take the average
of g and x over g.

510
00:20:44,610 --> 00:20:47,080
Don't worry about how came
about, Heron found this out.

511
00:20:47,080 --> 00:20:56,210
But that gives me a new guess,
and I'm going to repeat.

512
00:20:56,210 --> 00:20:58,680
That's a recipe.

513
00:20:58,680 --> 00:21:01,840
That's a description
of a set of steps.

514
00:21:01,840 --> 00:21:04,030
Notice what it has, it has a
bunch of nice things that we

515
00:21:04,030 --> 00:21:05,210
want to use, right?

516
00:21:05,210 --> 00:21:08,750
It's a sequence of specific
instructions

517
00:21:08,750 --> 00:21:10,790
that I do in order.

518
00:21:10,790 --> 00:21:13,600
Along the way I have some tests,
and depending on the

519
00:21:13,600 --> 00:21:17,040
value of that test, I may change
where I am in that

520
00:21:17,040 --> 00:21:18,570
sequence of instructions.

521
00:21:18,570 --> 00:21:20,460
And it has an end test,
something that tells me when

522
00:21:20,460 --> 00:21:22,350
I'm done and what
the answer is.

523
00:21:22,350 --> 00:21:24,480
This tells you how to
find square roots.

524
00:21:24,480 --> 00:21:25,300
it's how-to knowledge.

525
00:21:25,300 --> 00:21:27,530
It's imperative knowledge.

526
00:21:27,530 --> 00:21:27,800
All right.

527
00:21:27,800 --> 00:21:31,060
That's what computation
basically is about.

528
00:21:31,060 --> 00:21:35,930
We want to have ways of
capturing this process.

529
00:21:35,930 --> 00:21:37,780
OK, and that leads now to an
interesting question, which

530
00:21:37,780 --> 00:21:43,290
would be, "How do I build a
mechanical process to capture

531
00:21:43,290 --> 00:21:46,910
that set of computations?" So
I'm going to suggest that

532
00:21:46,910 --> 00:21:50,030
there's an easy way to do it--

533
00:21:50,030 --> 00:21:53,474
I realized I did the boards in
the wrong order here-- one of

534
00:21:53,474 --> 00:21:55,460
the ways I could do it is, you
could imagine building a

535
00:21:55,460 --> 00:21:57,630
little circuit to do this.

536
00:21:57,630 --> 00:22:00,080
If I had a couple of elements of
stored values in it, I had

537
00:22:00,080 --> 00:22:02,100
some wires to move things
around, I had a little thing

538
00:22:02,100 --> 00:22:05,000
to do addition, little thing
to do division, and a

539
00:22:05,000 --> 00:22:07,160
something to do the testing, I
could build a little circuit

540
00:22:07,160 --> 00:22:09,700
that would actually do
this computation.

541
00:22:09,700 --> 00:22:11,860
OK.

542
00:22:11,860 --> 00:22:15,410
That, strange as it sounds, is
actually an example of the

543
00:22:15,410 --> 00:22:18,440
earliest computers, because the
earliest computers were

544
00:22:18,440 --> 00:22:31,750
what we call fixed-program
computers, meaning that they

545
00:22:31,750 --> 00:22:34,890
had a piece of circuitry
designed to do a specific

546
00:22:34,890 --> 00:22:35,395
computation.

547
00:22:35,395 --> 00:22:38,200
And that's what they would do:
they would do that specific

548
00:22:38,200 --> 00:22:40,060
computation.

549
00:22:40,060 --> 00:22:41,300
You've seen these
a lot, right?

550
00:22:41,300 --> 00:22:47,540
A good example of this:
calculator.

551
00:22:47,540 --> 00:22:51,610
It's basically an example of
a fixed-program computer.

552
00:22:51,610 --> 00:22:53,460
It does arithmetic.

553
00:22:53,460 --> 00:22:55,860
If you want play video games
on it, good luck.

554
00:22:55,860 --> 00:22:58,130
If you want to do word
processing on it, good luck.

555
00:22:58,130 --> 00:23:00,690
It's designed to do
a specific thing.

556
00:23:00,690 --> 00:23:03,400
It's a fixed-program computer.

557
00:23:03,400 --> 00:23:05,710
In fact, a lot of the other
really interesting early ones

558
00:23:05,710 --> 00:23:09,970
similarly have this flavor, to
give an example: I never know

559
00:23:09,970 --> 00:23:14,310
how to pronounce this,
Atanasoff, 1941.

560
00:23:14,310 --> 00:23:16,340
One of the earliest
computational things was a

561
00:23:16,340 --> 00:23:18,870
thing designed by a guy named
Atanasoff, and it basically

562
00:23:18,870 --> 00:23:22,750
solved linear equations.

563
00:23:22,750 --> 00:23:26,030
Handy thing to do if you're
doing 1801, all right, or

564
00:23:26,030 --> 00:23:29,330
1806, or whatever you want
to do those things in.

565
00:23:29,330 --> 00:23:31,730
All it could do, though, was
solve those equations.

566
00:23:31,730 --> 00:23:36,200
One of my favorite examples of
an early computer was done by

567
00:23:36,200 --> 00:23:39,300
Alan Turing, one of the great
computer scientists of all

568
00:23:39,300 --> 00:23:43,580
time, called the bombe, which
was designed to break codes.

569
00:23:43,580 --> 00:23:45,110
It was actually used during
WWII to break

570
00:23:45,110 --> 00:23:46,060
German Enigma codes.

571
00:23:46,060 --> 00:23:48,520
And what it was designed
to do, was to solve

572
00:23:48,520 --> 00:23:49,540
that specific problem.

573
00:23:49,540 --> 00:23:53,140
The point I'm trying to make is,
fixed-program computers is

574
00:23:53,140 --> 00:23:55,210
where we started, but it doesn't
really get us to where

575
00:23:55,210 --> 00:23:55,680
we'd like to be.

576
00:23:55,680 --> 00:23:58,440
We want to capture this idea
of problem solving.

577
00:23:58,440 --> 00:24:01,260
So let's see how
we'd get there.

578
00:24:01,260 --> 00:24:05,390
So even within this framework
of, given a description of a

579
00:24:05,390 --> 00:24:08,460
computation as a set of steps,
in the idea that I could build

580
00:24:08,460 --> 00:24:10,410
a circuit to do it, let me
suggest for you what would be

581
00:24:10,410 --> 00:24:13,340
a wonderful circuit to build.

582
00:24:13,340 --> 00:24:15,080
Suppose you could build a
circuit with the following

583
00:24:15,080 --> 00:24:18,580
property: the input to this
circuit would be any other

584
00:24:18,580 --> 00:24:20,370
circuit diagram.

585
00:24:20,370 --> 00:24:22,680
Give it a circuit diagram for
some computation, you give it

586
00:24:22,680 --> 00:24:26,270
to the circuit, and that circuit
would wonderfully

587
00:24:26,270 --> 00:24:30,990
reconfigure itself to act like
the circuits diagram.

588
00:24:30,990 --> 00:24:33,000
Which would mean, it could
act like a calculator.

589
00:24:33,000 --> 00:24:35,150
Or, it could act like
Turing's bombe.

590
00:24:35,150 --> 00:24:38,070
Or, it could act like a
square root machine.

591
00:24:38,070 --> 00:24:39,360
So what would that circuit
look like?

592
00:24:39,360 --> 00:24:42,200
You can imagine these tiny
little robots wandering

593
00:24:42,200 --> 00:24:42,700
around, right?

594
00:24:42,700 --> 00:24:44,860
Pulling wires and pulling
out components and

595
00:24:44,860 --> 00:24:45,710
stacking them together.

596
00:24:45,710 --> 00:24:47,990
How would you build a circuit
that could take a circuit

597
00:24:47,990 --> 00:24:53,030
diagram in and make a machine
act like that circuit?

598
00:24:53,030 --> 00:24:55,170
Sounds like a neat challenge.

599
00:24:55,170 --> 00:24:59,010
Let me change the
game slightly.

600
00:24:59,010 --> 00:25:02,160
Suppose instead, I want a
machine that can take a

601
00:25:02,160 --> 00:25:07,350
recipe, the description of a
sequence of steps, take that

602
00:25:07,350 --> 00:25:12,230
as its input, and then that
machine will now act like what

603
00:25:12,230 --> 00:25:15,240
is described in that recipe.

604
00:25:15,240 --> 00:25:17,590
Reconfigure itself, emulate it,
however you want to use

605
00:25:17,590 --> 00:25:19,780
the words, it's going to
change how it does the

606
00:25:19,780 --> 00:25:21,900
computation.

607
00:25:21,900 --> 00:25:23,830
That would be cool.

608
00:25:23,830 --> 00:25:24,890
And that exists.

609
00:25:24,890 --> 00:25:26,240
It's called an interpreter.

610
00:25:26,240 --> 00:25:29,460
It is the basic heart
of every computer.

611
00:25:29,460 --> 00:25:33,330
What it is doing, is saying,
change the game.

612
00:25:33,330 --> 00:25:40,820
This is now an example of a
stored-program computer.

613
00:25:40,820 --> 00:25:48,990
What that means, in a
stored-program computer, is

614
00:25:48,990 --> 00:25:51,920
that I can provide to the
computer a sequence of

615
00:25:51,920 --> 00:25:55,520
instructions describing the
process I want it to execute.

616
00:25:55,520 --> 00:25:58,150
And inside of the machine, and
things we'll talk about, there

617
00:25:58,150 --> 00:26:02,190
is a process that will allow
that sequence to be executed

618
00:26:02,190 --> 00:26:06,130
as described in that recipe,
so it can behave like any

619
00:26:06,130 --> 00:26:09,920
thing that I can describe
in one of those recipes.

620
00:26:09,920 --> 00:26:10,810
All right.

621
00:26:10,810 --> 00:26:14,115
That actually seems like a
really nice thing to have, and

622
00:26:14,115 --> 00:26:19,210
so let me show you what that
would basically look like.

623
00:26:19,210 --> 00:26:22,990
Inside of a stored-program
computer, we would have the

624
00:26:22,990 --> 00:26:31,650
following: we have a memory,
it's connected to two things;

625
00:26:31,650 --> 00:26:37,870
control unit, in what's called
an ALU, an arithmetic logic

626
00:26:37,870 --> 00:26:46,460
unit, and this can take in
input, and spit out output,

627
00:26:46,460 --> 00:26:50,080
and inside this stored-program
computer, excuse me, you have

628
00:26:50,080 --> 00:26:55,140
the following: you have a
sequence of instructions.

629
00:26:55,140 --> 00:27:03,720
And these all get
stored in there.

630
00:27:03,720 --> 00:27:05,280
Notice the difference.

631
00:27:05,280 --> 00:27:07,550
The recipe, the sequence of
instructions, is actually

632
00:27:07,550 --> 00:27:10,020
getting read in, and it's
treated just like data.

633
00:27:10,020 --> 00:27:12,770
It's inside the memory of the
machine, which means we have

634
00:27:12,770 --> 00:27:15,500
access to it, we can change it,
we can use it to build new

635
00:27:15,500 --> 00:27:19,210
pieces of code, as well as
we can interpret it.

636
00:27:19,210 --> 00:27:21,400
One other piece that goes
into this computer--

637
00:27:21,400 --> 00:27:23,720
I never remember where to put
the PC, John, control?

638
00:27:23,720 --> 00:27:25,270
ALU?

639
00:27:25,270 --> 00:27:26,480
Separate?

640
00:27:26,480 --> 00:27:29,400
I'll put it separate--
you have a thing

641
00:27:29,400 --> 00:27:31,280
called a program counter.

642
00:27:31,280 --> 00:27:34,380
And here's the basis
of the computation.

643
00:27:34,380 --> 00:27:38,400
That program counter points to
some location in memory,

644
00:27:38,400 --> 00:27:43,050
typically to the first
instruction in the sequence.

645
00:27:43,050 --> 00:27:45,390
And those instructions, by the
way, are very simple: they're

646
00:27:45,390 --> 00:27:48,880
things like, take the value out
of two places in memory,

647
00:27:48,880 --> 00:27:51,400
and run them through the
multiplier in here, a little

648
00:27:51,400 --> 00:27:53,340
piece of circuitry, and
stick them back into

649
00:27:53,340 --> 00:27:54,840
someplace in memory.

650
00:27:54,840 --> 00:27:57,490
Or take this value out of
memory, run it through some

651
00:27:57,490 --> 00:28:00,570
other simple operation, stick
it back in memory.

652
00:28:00,570 --> 00:28:03,240
Having executed this
instruction, that counter goes

653
00:28:03,240 --> 00:28:05,500
up by one and we move
to the next one.

654
00:28:05,500 --> 00:28:08,670
We execute that instruction,
we move to the next one.

655
00:28:08,670 --> 00:28:13,960
Oh yeah, it looks a whole
lot like that.

656
00:28:13,960 --> 00:28:16,760
Some of those instructions will
involve tests: they'll

657
00:28:16,760 --> 00:28:18,820
say, is something true?

658
00:28:18,820 --> 00:28:22,120
And if the test is true, it will
change the value of this

659
00:28:22,120 --> 00:28:25,660
program counter to point to
some other place in the

660
00:28:25,660 --> 00:28:28,220
memory, some other point in that
sequence of instructions,

661
00:28:28,220 --> 00:28:30,030
and you'll keep processing.

662
00:28:30,030 --> 00:28:32,100
Eventually you'll hopefully
stop, and a value gets spit

663
00:28:32,100 --> 00:28:34,470
out, and you're done.

664
00:28:34,470 --> 00:28:35,520
That's the heart
of a computer.

665
00:28:35,520 --> 00:28:37,140
Now that's a slight
misstatement.

666
00:28:37,140 --> 00:28:39,490
The process to control it is
intriguing and interesting,

667
00:28:39,490 --> 00:28:42,720
but the heart of the computer is
simply this notion that we

668
00:28:42,720 --> 00:28:46,190
build our descriptions, our
recipes, on a sequence of

669
00:28:46,190 --> 00:28:47,910
primitive instructions.

670
00:28:47,910 --> 00:28:50,100
And then we have a
flow of control.

671
00:28:50,100 --> 00:28:51,780
And that flow of control is
what I just described.

672
00:28:51,780 --> 00:28:53,990
It's moving through a sequence
of instructions, occasionally

673
00:28:53,990 --> 00:28:57,840
changing where we are
as we move around.

674
00:28:57,840 --> 00:28:58,200
OK.

675
00:28:58,200 --> 00:29:02,210
The thing I want you to take
away from this, then, is to

676
00:29:02,210 --> 00:29:06,840
think of this as, this is,
if you like, a recipe.

677
00:29:06,840 --> 00:29:19,020
And that's really what
a program is.

678
00:29:19,020 --> 00:29:21,930
It's a sequence of
instructions.

679
00:29:21,930 --> 00:29:23,650
Now, one of things I left
hanging is, I said, OK, you

680
00:29:23,650 --> 00:29:24,520
build it out of primitives.

681
00:29:24,520 --> 00:29:25,960
So one of the questions is,
well, what are the right

682
00:29:25,960 --> 00:29:28,100
primitives to use?

683
00:29:28,100 --> 00:29:31,120
And one of the things that was
useful here is, that we

684
00:29:31,120 --> 00:29:33,130
actually know that the set of
primitives that you want to

685
00:29:33,130 --> 00:29:37,390
use is very straight-forward.

686
00:29:37,390 --> 00:29:39,510
OK, but before I do that, let me
drive home this idea of why

687
00:29:39,510 --> 00:29:42,090
this is a recipe.

688
00:29:42,090 --> 00:29:44,380
Assuming I have a set of
primitive instructions that I

689
00:29:44,380 --> 00:29:47,010
can describe everything on, I
want to know what can I build.

690
00:29:47,010 --> 00:29:49,710
Well, I'm going to do the same
analogy to a real recipe.

691
00:29:49,710 --> 00:29:51,360
So, real recipe.

692
00:29:51,360 --> 00:29:51,850
I don't know.

693
00:29:51,850 --> 00:29:54,230
Separate six eggs.

694
00:29:54,230 --> 00:29:55,620
Do something.

695
00:29:55,620 --> 00:29:57,200
Beat until the-- sorry,
beat the whites

696
00:29:57,200 --> 00:29:59,240
until they're stiff.

697
00:29:59,240 --> 00:30:02,030
Do something until an
end test is true.

698
00:30:02,030 --> 00:30:04,730
Take the yolks and mix them in
with the sugar and water--

699
00:30:04,730 --> 00:30:05,060
No.

700
00:30:05,060 --> 00:30:06,825
Sugar and flour I guess is
probably what I want, sugar

701
00:30:06,825 --> 00:30:08,430
and water is not going to do
anything interesting for me

702
00:30:08,430 --> 00:30:11,010
here-- mix them into
something else.

703
00:30:11,010 --> 00:30:13,310
Do a sequence of things.

704
00:30:13,310 --> 00:30:17,400
A traditional recipe actually
is based on a small set of

705
00:30:17,400 --> 00:30:21,230
primitives, and a good chef
with, or good cook, I should

706
00:30:21,230 --> 00:30:23,060
say, with that set of
primitives, can create an

707
00:30:23,060 --> 00:30:26,040
unbounded number of
great dishes.

708
00:30:26,040 --> 00:30:28,270
Same thing holds true
in programming.

709
00:30:28,270 --> 00:30:29,020
Right.

710
00:30:29,020 --> 00:30:37,560
Given a fixed set of primitives,
all right, a good

711
00:30:37,560 --> 00:30:43,630
programmer can program
anything.

712
00:30:43,630 --> 00:30:45,720
And by that, I mean anything
that can be described in one

713
00:30:45,720 --> 00:30:47,400
of these process, you can
capture in that set of

714
00:30:47,400 --> 00:30:49,580
primitives.

715
00:30:49,580 --> 00:30:51,480
All right, the question is, as
I started to say, is, "What

716
00:30:51,480 --> 00:30:54,220
are the right primitives?" So
there's a little bit of, a

717
00:30:54,220 --> 00:30:55,830
little piece of history
here, if you like.

718
00:30:55,830 --> 00:31:01,860
In 1936, that same guy, Alan
Turing, showed that with six

719
00:31:01,860 --> 00:31:05,850
simple primitives, anything that
could be described in a

720
00:31:05,850 --> 00:31:08,730
mechanical process, it's
actually algorithmically,

721
00:31:08,730 --> 00:31:12,990
could be programmed just using
those six primitives.

722
00:31:12,990 --> 00:31:14,730
Think about that for a second.

723
00:31:14,730 --> 00:31:16,320
That's an incredible
statement.

724
00:31:16,320 --> 00:31:20,100
It says, with six primitives,
I can rule the world.

725
00:31:20,100 --> 00:31:23,470
With six primitives, I
can program anything.

726
00:31:23,470 --> 00:31:25,620
A couple of really interesting
consequences of that, by the

727
00:31:25,620 --> 00:31:29,770
way, one of them is, it says,
anything you can do in one

728
00:31:29,770 --> 00:31:31,920
programming language,
you can do in

729
00:31:31,920 --> 00:31:33,810
another programming language.

730
00:31:33,810 --> 00:31:36,440
And there is no programming
language that is better-- well

731
00:31:36,440 --> 00:31:37,790
actually, that's not quite true,
there are some better at

732
00:31:37,790 --> 00:31:39,580
doing certain kinds of things--
but there's nothing

733
00:31:39,580 --> 00:31:43,340
that you can do in C that
you can't do in Fortran.

734
00:31:43,340 --> 00:31:45,250
It's called Turing
compatibility.

735
00:31:45,250 --> 00:31:46,890
Anything you can do with one,
you can do with another, it's

736
00:31:46,890 --> 00:31:49,250
based on that fundamental
result.

737
00:31:49,250 --> 00:31:50,390
OK.

738
00:31:50,390 --> 00:31:53,310
Now, fortunately we're not going
to start with Turing's

739
00:31:53,310 --> 00:31:56,600
six primitives, this would be
really painful programming,

740
00:31:56,600 --> 00:31:59,770
because they're down at the
level of, "take this value and

741
00:31:59,770 --> 00:32:01,550
write it onto this tape." First
of all, we don't have

742
00:32:01,550 --> 00:32:04,330
tapes anymore in computers, and
even if we did, you don't

743
00:32:04,330 --> 00:32:05,780
want to be programming
at that level.

744
00:32:05,780 --> 00:32:07,410
What we're going to see with
programming language is that

745
00:32:07,410 --> 00:32:09,370
we're going to use higher-level
abstracts.

746
00:32:09,370 --> 00:32:12,170
A broader set of primitives,
but nonetheless the same

747
00:32:12,170 --> 00:32:13,150
fundamental thing holds.

748
00:32:13,150 --> 00:32:16,570
With those six primitives,
you can do it.

749
00:32:16,570 --> 00:32:18,080
OK.

750
00:32:18,080 --> 00:32:19,920
So where are we here?

751
00:32:19,920 --> 00:32:22,410
What we're saying is, in order
to do computation, we want to

752
00:32:22,410 --> 00:32:24,820
describe recipes, we want to
describe this sequence of

753
00:32:24,820 --> 00:32:28,110
steps built on some primitives,
and we want to

754
00:32:28,110 --> 00:32:30,520
describe the flow of control
that goes through those

755
00:32:30,520 --> 00:32:33,210
sequence of steps
as we carry on.

756
00:32:33,210 --> 00:32:35,050
So the last thing we need before
we can start talking

757
00:32:35,050 --> 00:32:36,500
about real programming
is, we need to

758
00:32:36,500 --> 00:32:39,030
describe those recipes.

759
00:32:39,030 --> 00:32:41,330
All right, And to describe
the recipes, we're

760
00:32:41,330 --> 00:32:54,180
going to want a language.

761
00:32:54,180 --> 00:32:57,860
We need to know not only what
are the primitives, but how do

762
00:32:57,860 --> 00:33:01,960
we make things meaningful
in that language.

763
00:33:01,960 --> 00:33:03,080
Language.

764
00:33:03,080 --> 00:33:05,680
There we go.

765
00:33:05,680 --> 00:33:07,510
All right.

766
00:33:07,510 --> 00:33:08,680
Now, it turns out there are--

767
00:33:08,680 --> 00:33:09,820
I don't know, John, hundreds?

768
00:33:09,820 --> 00:33:10,220
Thousands?

769
00:33:10,220 --> 00:33:11,470
Of programming languages?

770
00:33:11,470 --> 00:33:13,410
At least hundreds-- of
programming languages around.

771
00:33:13,410 --> 00:33:16,065
PROFESSOR JOHN GUTTAG:
[UNINTELLIGIBLE]

772
00:33:16,065 --> 00:33:16,492
PROFESSOR ERIC GRIMSON: True.

773
00:33:16,492 --> 00:33:18,620
Thank you.

774
00:33:18,620 --> 00:33:20,120
You know, they all
have, you know,

775
00:33:20,120 --> 00:33:21,290
their pluses and minuses.

776
00:33:21,290 --> 00:33:23,910
I have to admit, in my career
here, I think I've taught in

777
00:33:23,910 --> 00:33:26,080
at least three languages, I
suspect you've taught more,

778
00:33:26,080 --> 00:33:27,570
five or six, John?

779
00:33:27,570 --> 00:33:29,600
Both of us have probably
programmed in more than those

780
00:33:29,600 --> 00:33:31,700
number of languages, at least
programmed that many, since we

781
00:33:31,700 --> 00:33:33,840
taught in those languages.

782
00:33:33,840 --> 00:33:35,040
One of the things you
want to realize is,

783
00:33:35,040 --> 00:33:36,930
there is no best language.

784
00:33:36,930 --> 00:33:38,630
At least I would argue that,
I think John would agree.

785
00:33:38,630 --> 00:33:40,860
We might both agree we have
our own nominees for worst

786
00:33:40,860 --> 00:33:43,120
language, there are
some of those.

787
00:33:43,120 --> 00:33:44,750
There is no best language.

788
00:33:44,750 --> 00:33:44,920
All right?

789
00:33:44,920 --> 00:33:46,380
They all are describing
different things.

790
00:33:46,380 --> 00:33:48,873
Having said that, some of them
are better suited for some

791
00:33:48,873 --> 00:33:51,400
things than others.

792
00:33:51,400 --> 00:33:55,970
Anybody here heard of MATLAB
Maybe programmed in MATLAB?

793
00:33:55,970 --> 00:33:58,220
It's great for doing things with
vectors and matrices and

794
00:33:58,220 --> 00:34:01,630
things that are easily captured
in that framework.

795
00:34:01,630 --> 00:34:02,710
But there's some things
that are a real

796
00:34:02,710 --> 00:34:03,490
pain to do in MATLAB.

797
00:34:03,490 --> 00:34:05,670
So MATLAB's great for
that kind of thing.

798
00:34:05,670 --> 00:34:07,850
C is a great language for
programming things that

799
00:34:07,850 --> 00:34:10,620
control data networks,
for example.

800
00:34:10,620 --> 00:34:12,860
I happen to be, and John
teases me about this

801
00:34:12,860 --> 00:34:14,920
regularly, I'm an old-time Lisp
programmer, and that's

802
00:34:14,920 --> 00:34:16,710
how I was trained.

803
00:34:16,710 --> 00:34:19,190
And I happen to like Lisp and
Scheme, it's a great language

804
00:34:19,190 --> 00:34:20,970
when you're trying to deal with
problems where you have

805
00:34:20,970 --> 00:34:23,400
arbitrarily structured
data sets.

806
00:34:23,400 --> 00:34:25,550
It's particularly
good at that.

807
00:34:25,550 --> 00:34:27,800
So the point I want to make
here is that there's no

808
00:34:27,800 --> 00:34:30,330
particularly best language.

809
00:34:30,330 --> 00:34:32,350
What we're going to do is simply
use a language that

810
00:34:32,350 --> 00:34:33,060
helps us understand.

811
00:34:33,060 --> 00:34:34,620
So in this course, the
language we're

812
00:34:34,620 --> 00:34:38,060
going to use is Python.

813
00:34:38,060 --> 00:34:39,490
Which is a pretty new language,
it's growing in

814
00:34:39,490 --> 00:34:42,580
popularity, it has a lot of
the elements of some other

815
00:34:42,580 --> 00:34:44,750
languages because it's more
recent, it inherits things

816
00:34:44,750 --> 00:34:48,390
from it's pregenitors,
if you like.

817
00:34:48,390 --> 00:34:50,270
But one of the things I want to
stress is, this course is

818
00:34:50,270 --> 00:34:54,310
not about Python.

819
00:34:54,310 --> 00:34:55,500
Strange statement.

820
00:34:55,500 --> 00:34:58,000
You do need to know how to use
it, but it's not about the

821
00:34:58,000 --> 00:35:00,330
details of, where do the
semi-colons go in Python.

822
00:35:00,330 --> 00:35:02,400
All right?

823
00:35:02,400 --> 00:35:04,050
It's about using it to think.

824
00:35:04,050 --> 00:35:06,520
And what you should take away
from this course is having

825
00:35:06,520 --> 00:35:10,470
learned how to design recipes,
how to structure recipes, how

826
00:35:10,470 --> 00:35:13,120
to do things in modes
in Python.

827
00:35:13,120 --> 00:35:15,045
Those same tools
easily transfer

828
00:35:15,045 --> 00:35:16,790
to any other language.

829
00:35:16,790 --> 00:35:18,640
You can pick up another language
in a week, couple of

830
00:35:18,640 --> 00:35:22,640
weeks at most, once you
know how to do Python.

831
00:35:22,640 --> 00:35:23,280
OK.

832
00:35:23,280 --> 00:35:25,420
In order to talk about Python
and languages, I want to do

833
00:35:25,420 --> 00:35:28,250
one last thing to set the stage
for what we're going to

834
00:35:28,250 --> 00:35:30,510
do here, and that's to talk
about the different dimensions

835
00:35:30,510 --> 00:35:31,650
of a language.

836
00:35:31,650 --> 00:35:33,560
And there're three I
want to deal with.

837
00:35:33,560 --> 00:35:35,660
The first one is, whether
this is a high-level

838
00:35:35,660 --> 00:35:41,120
or low-level language.

839
00:35:41,120 --> 00:35:42,830
That basically says,
how close are you

840
00:35:42,830 --> 00:35:43,770
the guts of the machine?

841
00:35:43,770 --> 00:35:45,950
A low-level language, we used
to call this assembly

842
00:35:45,950 --> 00:35:48,200
programming, you're down at the
level of, your primitives

843
00:35:48,200 --> 00:35:51,340
are literally moving pieces of
data from one location of

844
00:35:51,340 --> 00:35:54,080
memory to another, through
a very simple operation.

845
00:35:54,080 --> 00:35:57,100
A high-level language, the
designer has created a much

846
00:35:57,100 --> 00:35:59,530
richer set of primitive
things.

847
00:35:59,530 --> 00:36:02,230
In a high-level language, square
root might simply be a

848
00:36:02,230 --> 00:36:04,200
primitive that you can use,
rather than you having to go

849
00:36:04,200 --> 00:36:06,010
over and code it.

850
00:36:06,010 --> 00:36:08,540
And there're trade-offs
between both.

851
00:36:08,540 --> 00:36:12,710
Second dimension is, whether
this is a general versus a

852
00:36:12,710 --> 00:36:15,980
targeted language.

853
00:36:15,980 --> 00:36:18,860
And by that I mean, do the set
of primitives support a broad

854
00:36:18,860 --> 00:36:22,130
range of applications, or is
it really aimed at a very

855
00:36:22,130 --> 00:36:23,410
specific set of applications?

856
00:36:23,410 --> 00:36:25,710
I'd argue that MATLAB is
basically a targeted language,

857
00:36:25,710 --> 00:36:29,470
it's targeted at matrices and
vectors and things like that.

858
00:36:29,470 --> 00:36:31,540
And the third one I want to
point out is, whether this is

859
00:36:31,540 --> 00:36:41,810
an interpreted versus
a compiled language.

860
00:36:41,810 --> 00:36:44,440
What that basically says
is the following: in an

861
00:36:44,440 --> 00:36:46,760
interpreted language, you take
what's called the source code,

862
00:36:46,760 --> 00:36:49,740
the thing you write, it may go
through a simple checker but

863
00:36:49,740 --> 00:36:52,270
it basically goes to the
interpreter, that thing inside

864
00:36:52,270 --> 00:36:54,210
the machine that's going to
control the flow of going

865
00:36:54,210 --> 00:36:55,970
through each one of
the instructions,

866
00:36:55,970 --> 00:36:57,400
and give you an output.

867
00:36:57,400 --> 00:37:00,420
So the interpreter is simply
operating directly on your

868
00:37:00,420 --> 00:37:02,430
code at run time.

869
00:37:02,430 --> 00:37:05,050
In a compiled language, you have
an intermediate step, in

870
00:37:05,050 --> 00:37:07,370
which you take the source code,
it runs through what's

871
00:37:07,370 --> 00:37:10,240
called a checker or a compiler
or both, and it creates what's

872
00:37:10,240 --> 00:37:11,890
called object code.

873
00:37:11,890 --> 00:37:16,250
And that does two things: one,
it helps catch bugs in your

874
00:37:16,250 --> 00:37:19,160
code, and secondly it often
converts it into a more

875
00:37:19,160 --> 00:37:21,970
efficient sequence of
instructions before you

876
00:37:21,970 --> 00:37:24,000
actually go off and run it.

877
00:37:24,000 --> 00:37:24,190
All right?

878
00:37:24,190 --> 00:37:25,400
And there's trade-offs
between both.

879
00:37:25,400 --> 00:37:27,520
I mean, an interpreted language
is often easier to

880
00:37:27,520 --> 00:37:30,280
debug, because you can still see
your raw code there, but

881
00:37:30,280 --> 00:37:32,680
it's not always as fast. A
compiled language is usually

882
00:37:32,680 --> 00:37:34,770
much faster in terms
of its execution.

883
00:37:34,770 --> 00:37:37,900
And it's one of the things you
may want to trade off.

884
00:37:37,900 --> 00:37:38,120
Right.

885
00:37:38,120 --> 00:37:43,300
In the case of Python, it's
a high-level language.

886
00:37:43,300 --> 00:37:45,660
I would argue, I think John
would agree with me, it's

887
00:37:45,660 --> 00:37:47,095
basically a general-purpose
language.

888
00:37:47,095 --> 00:37:50,010
It happens to be better suited
for manipulating strings than

889
00:37:50,010 --> 00:37:51,770
numbers, for example,
but it's really a

890
00:37:51,770 --> 00:37:53,490
general-purpose language.

891
00:37:53,490 --> 00:37:55,003
And it's primarily--

892
00:37:55,003 --> 00:37:58,100
I shouldn't say primarily, it
is an interpreted language.

893
00:37:58,100 --> 00:37:59,360
OK?

894
00:37:59,360 --> 00:38:03,000
As a consequence, it's not as
good as helping debug, but it

895
00:38:03,000 --> 00:38:04,370
does let you-- sorry, that's the
wrong way of saying-- it's

896
00:38:04,370 --> 00:38:07,280
not as good at catching some
things before you run them, it

897
00:38:07,280 --> 00:38:09,030
is easier at some times
in debugging as you go

898
00:38:09,030 --> 00:38:11,470
along on the fly.

899
00:38:11,470 --> 00:38:11,800
OK.

900
00:38:11,800 --> 00:38:13,410
So what does Python look like?

901
00:38:13,410 --> 00:38:15,766
In order to talk about Python--
actually, I'm going

902
00:38:15,766 --> 00:38:22,170
to do it this way-- we need
to talk about how to

903
00:38:22,170 --> 00:38:22,860
write things in Python.

904
00:38:22,860 --> 00:38:26,070
Again, you have to let me back
up slightly and set the stage.

905
00:38:26,070 --> 00:38:28,820
Our goal is to build recipes.

906
00:38:28,820 --> 00:38:29,960
You're all going to be
great chefs by the

907
00:38:29,960 --> 00:38:30,660
time you're done here.

908
00:38:30,660 --> 00:38:32,150
All right?

909
00:38:32,150 --> 00:38:35,550
Our goal is to take problems and
break them down into these

910
00:38:35,550 --> 00:38:37,890
computational steps, these
sequence of instructions

911
00:38:37,890 --> 00:38:40,460
that'll allow us to capture
that process.

912
00:38:40,460 --> 00:38:42,690
To do that, we need to describe:
not only, what are

913
00:38:42,690 --> 00:38:45,630
the primitives, but how do we
capture things legally in that

914
00:38:45,630 --> 00:38:47,420
language, and interact
with the computer?

915
00:38:47,420 --> 00:38:49,450
And so for that, we
need a language.

916
00:38:49,450 --> 00:38:51,190
We're about to start talking
about the elements of the

917
00:38:51,190 --> 00:38:54,000
language, but to do that, we
also need to separate out one

918
00:38:54,000 --> 00:38:58,290
last piece of distinction.

919
00:38:58,290 --> 00:38:59,780
Just like with a natural
language, we're going to

920
00:38:59,780 --> 00:39:02,910
separate out syntax
versus semantics.

921
00:39:02,910 --> 00:39:03,600
So what's syntax?

922
00:39:03,600 --> 00:39:09,750
Syntax basically says, what are
the legal expressions in

923
00:39:09,750 --> 00:39:16,960
this language?

924
00:39:16,960 --> 00:39:22,910
Boy, my handwriting is
atrocious, isn't it?

925
00:39:22,910 --> 00:39:25,890
There's a English sequence
of words.

926
00:39:25,890 --> 00:39:27,750
It's not since syntactically
correct, right?

927
00:39:27,750 --> 00:39:28,910
It's not a sentence.

928
00:39:28,910 --> 00:39:30,350
There's no verb in there
anywhere, it's just

929
00:39:30,350 --> 00:39:31,400
a sequence of nouns.

930
00:39:31,400 --> 00:39:32,940
Same thing in our languages.

931
00:39:32,940 --> 00:39:36,670
We have to describe how do you
put together legally formed

932
00:39:36,670 --> 00:39:38,190
expressions.

933
00:39:38,190 --> 00:39:39,070
OK?

934
00:39:39,070 --> 00:39:41,030
And as we add constructs to the
language, we're going to

935
00:39:41,030 --> 00:39:42,780
talk about.

936
00:39:42,780 --> 00:39:45,550
Second thing we want to talk
about very briefly as we go

937
00:39:45,550 --> 00:39:48,680
along is the semantics
of the language.

938
00:39:48,680 --> 00:39:50,610
And here we're going to break
out two pieces; static

939
00:39:50,610 --> 00:39:53,230
semantics and full semantics.

940
00:39:53,230 --> 00:40:01,950
Static semantics basically
says which programs are

941
00:40:01,950 --> 00:40:05,730
meaningful.

942
00:40:05,730 --> 00:40:09,090
Which expressions make sense.

943
00:40:09,090 --> 00:40:17,740
Here's an English sentence.

944
00:40:17,740 --> 00:40:20,090
It's syntactically correct.

945
00:40:20,090 --> 00:40:20,300
Right?

946
00:40:20,300 --> 00:40:23,090
Noun phrase, verb,
noun phrase.

947
00:40:23,090 --> 00:40:25,460
I'm not certain it's meaningful,
unless you are in

948
00:40:25,460 --> 00:40:29,620
the habit of giving your
furniture personal names.

949
00:40:29,620 --> 00:40:30,240
What's the point?

950
00:40:30,240 --> 00:40:32,770
Again, you can have things that
are syntactically legal

951
00:40:32,770 --> 00:40:35,870
but not semantically meaningful,
and static

952
00:40:35,870 --> 00:40:38,980
semantics is going to be a way
of helping us decide what

953
00:40:38,980 --> 00:40:41,360
expressions, what pieces of
code, actually have real

954
00:40:41,360 --> 00:40:41,950
meaning to it.

955
00:40:41,950 --> 00:40:43,450
All right?

956
00:40:43,450 --> 00:40:47,160
The last piece of it is, in
addition to having static

957
00:40:47,160 --> 00:40:53,910
semantics, we have sort
of full semantics.

958
00:40:53,910 --> 00:40:58,170
Which is, what does
the program mean?

959
00:40:58,170 --> 00:40:59,830
Or, said a different way,
what's going to

960
00:40:59,830 --> 00:41:08,600
happen when I run it?

961
00:41:08,600 --> 00:41:09,770
That's the meaning of
the expression.

962
00:41:09,770 --> 00:41:10,660
That's what you want.

963
00:41:10,660 --> 00:41:10,790
All right?

964
00:41:10,790 --> 00:41:13,060
You want to know, what's the
meaning of this piece of code?

965
00:41:13,060 --> 00:41:14,470
When I run it, what's
going to happen?

966
00:41:14,470 --> 00:41:16,360
That's what I want to build.

967
00:41:16,360 --> 00:41:18,200
The reason for pulling this out
is, what you're going to

968
00:41:18,200 --> 00:41:21,700
see is, that in most languages,
and certainly in

969
00:41:21,700 --> 00:41:29,090
Python-- we got lots of help
here-- all right, Python comes

970
00:41:29,090 --> 00:41:31,840
built-in with something that
will check your static, sorry,

971
00:41:31,840 --> 00:41:33,440
your syntax for you.

972
00:41:33,440 --> 00:41:37,880
And in fact, as a sidebar, if
you turn in a problem set that

973
00:41:37,880 --> 00:41:40,510
is not syntactically correct,
there's a simple button that

974
00:41:40,510 --> 00:41:42,620
you push that will check
your syntax.

975
00:41:42,620 --> 00:41:44,320
If you've turned in a program
that's not syntactically

976
00:41:44,320 --> 00:41:46,400
correct, the TAs give
you a zero.

977
00:41:46,400 --> 00:41:48,040
Because it said you didn't even
take the time to make

978
00:41:48,040 --> 00:41:49,050
sure the syntax is correct.

979
00:41:49,050 --> 00:41:50,840
The system will help
you find it.

980
00:41:50,840 --> 00:41:53,110
In Python, it'll find it,
I think one bug at

981
00:41:53,110 --> 00:41:53,820
a time, right John?

982
00:41:53,820 --> 00:41:55,530
It finds one syntax error at
a time, so you have to be a

983
00:41:55,530 --> 00:41:57,130
little patient to do it,
but you can check that

984
00:41:57,130 --> 00:41:59,630
the syntax is right.

985
00:41:59,630 --> 00:42:06,050
You're going to see that we
get some help here on the

986
00:42:06,050 --> 00:42:08,080
static semantics, and I'm going
to do an example in a

987
00:42:08,080 --> 00:42:11,860
second, meaning that the system,
some languages are

988
00:42:11,860 --> 00:42:15,930
better than others on it, but it
will try and help you catch

989
00:42:15,930 --> 00:42:20,040
some things that are not
semantically correct

990
00:42:20,040 --> 00:42:21,380
statically.

991
00:42:21,380 --> 00:42:23,940
In the case of Python, it does
that I think all at run time.

992
00:42:23,940 --> 00:42:26,670
I'm looking to you again,
John, I think there's no

993
00:42:26,670 --> 00:42:27,490
pre-time checks.

994
00:42:27,490 --> 00:42:27,690
Its-- sorry?

995
00:42:27,690 --> 00:42:28,930
PROFESSOR JOHN GUTTAG:
[UNINTELLIGIBLE]

996
00:42:28,930 --> 00:42:31,260
PROFESSOR ERIC GRIMSON:
There is some.

997
00:42:31,260 --> 00:42:32,890
OK.

998
00:42:32,890 --> 00:42:35,150
Most of them, I think though,
are primarily caught at run

999
00:42:35,150 --> 00:42:36,990
time, and that's a little bit
of a pain because you don't

1000
00:42:36,990 --> 00:42:38,860
see it until you go and run the
code, and there are some,

1001
00:42:38,860 --> 00:42:40,590
actually we're going to see an
example I think in a second

1002
00:42:40,590 --> 00:42:43,800
where you find it, but you
do get some help there.

1003
00:42:43,800 --> 00:42:47,250
The problem is, things that you
catch here are actually

1004
00:42:47,250 --> 00:42:49,800
the least worrisome bugs.

1005
00:42:49,800 --> 00:42:52,800
They're easy to spot, you can't
run the program with

1006
00:42:52,800 --> 00:42:55,770
them there, so you're not going
to get weird answers.

1007
00:42:55,770 --> 00:42:58,190
Not everything is going
to get caught in

1008
00:42:58,190 --> 00:42:59,360
static semantics checking.

1009
00:42:59,360 --> 00:43:00,790
Some things are going to
slide through, and

1010
00:43:00,790 --> 00:43:03,040
that's actually a bother.

1011
00:43:03,040 --> 00:43:04,060
It's a problem.

1012
00:43:04,060 --> 00:43:07,280
Because it says, your program
will still give you a value,

1013
00:43:07,280 --> 00:43:10,010
but it may not be what you
intended, and you can't always

1014
00:43:10,010 --> 00:43:12,260
tell, and that may propagate
it's way down through a whole

1015
00:43:12,260 --> 00:43:14,600
bunch of other computations
before it causes some

1016
00:43:14,600 --> 00:43:16,400
catastrophic failure.

1017
00:43:16,400 --> 00:43:19,290
So actually, the problem with
static semantics is you'd like

1018
00:43:19,290 --> 00:43:21,890
it to catch everything, you
don't always get it.

1019
00:43:21,890 --> 00:43:23,550
Sadly we don't get
much help here.

1020
00:43:23,550 --> 00:43:25,390
Which is where we'd like it.

1021
00:43:25,390 --> 00:43:27,440
But that's part of your job.

1022
00:43:27,440 --> 00:43:27,780
OK.

1023
00:43:27,780 --> 00:43:30,130
What happens if you actually
have something that's both

1024
00:43:30,130 --> 00:43:32,880
syntactically correct, and
appears to have correct static

1025
00:43:32,880 --> 00:43:33,780
semantics, and you run it?

1026
00:43:33,780 --> 00:43:37,990
It could run and give you the
right answer, it could crash,

1027
00:43:37,990 --> 00:43:43,070
it could loop forever, it could
run and apparently give

1028
00:43:43,070 --> 00:43:45,270
you the right answer.

1029
00:43:45,270 --> 00:43:47,410
And you're not always going
to be able to tell.

1030
00:43:47,410 --> 00:43:49,080
Well, you'll know when it
crashes, that doesn't help you

1031
00:43:49,080 --> 00:43:51,020
very much, but you can't
always tell whether

1032
00:43:51,020 --> 00:43:52,760
something's stuck in an infinite
loop or whether it's

1033
00:43:52,760 --> 00:43:54,880
simply taking a long
time to compute.

1034
00:43:54,880 --> 00:43:57,240
You'd love to have a system that
spots that for you, but

1035
00:43:57,240 --> 00:43:58,780
it's not possible.

1036
00:43:58,780 --> 00:44:00,620
And so to deal with
this last one, you

1037
00:44:00,620 --> 00:44:02,630
need to develop style.

1038
00:44:02,630 --> 00:44:06,210
All right?

1039
00:44:06,210 --> 00:44:09,420
Meaning, we're going to try to
help you with how to develop

1040
00:44:09,420 --> 00:44:12,500
good programming style, but you
need to write in a way in

1041
00:44:12,500 --> 00:44:15,630
which it is going to be easy
for you to spot the places

1042
00:44:15,630 --> 00:44:19,940
that cause those semantic
bugs to occur.

1043
00:44:19,940 --> 00:44:20,380
All right.

1044
00:44:20,380 --> 00:44:23,610
If that sounds like a really
long preamble, it is.

1045
00:44:23,610 --> 00:44:24,710
Let's start with Python.

1046
00:44:24,710 --> 00:44:28,270
But again, my goal here is to
let you see what computation's

1047
00:44:28,270 --> 00:44:30,290
about, why we need to do it,
I'm going to remind you one

1048
00:44:30,290 --> 00:44:32,820
last time, our goal is to
be able to have a set of

1049
00:44:32,820 --> 00:44:35,590
primitives that we combine
into complex expressions,

1050
00:44:35,590 --> 00:44:38,820
which we can then abstract to
treat as primitives, and we

1051
00:44:38,820 --> 00:44:43,100
want to use that sequence of
instructions in this flow of

1052
00:44:43,100 --> 00:44:47,220
control computing, in order
to deduce new information.

1053
00:44:47,220 --> 00:44:50,530
That imperative knowledge that
we talked about right there.

1054
00:44:50,530 --> 00:44:52,850
So I'm going to start today,
we have about five or ten

1055
00:44:52,850 --> 00:44:54,580
minutes left, I think, in
order-- sorry, five minutes

1056
00:44:54,580 --> 00:44:56,500
left-- in order to do this
with some beginnings of

1057
00:44:56,500 --> 00:44:58,970
Python, and we're going to pick
this up obviously, next

1058
00:44:58,970 --> 00:45:02,730
time, so; simple parts
of Python.

1059
00:45:02,730 --> 00:45:04,710
In order to create any kinds of
expressions, we're going to

1060
00:45:04,710 --> 00:45:06,640
need values.

1061
00:45:06,640 --> 00:45:07,780
Primitive data elements.

1062
00:45:07,780 --> 00:45:13,520
And in Python, we have two to
start with; we have numbers,

1063
00:45:13,520 --> 00:45:16,580
and we have strings.

1064
00:45:16,580 --> 00:45:18,720
Numbers is what you'd expect.

1065
00:45:18,720 --> 00:45:21,200
There's a number.

1066
00:45:21,200 --> 00:45:21,920
There's another number.

1067
00:45:21,920 --> 00:45:24,030
All right?

1068
00:45:24,030 --> 00:45:28,400
Strings are captured in Python
with an open quote and some

1069
00:45:28,400 --> 00:45:33,030
sequence of characters followed
by a closed quote.

1070
00:45:33,030 --> 00:45:37,340
Associated with every data
type in Python is a type,

1071
00:45:37,340 --> 00:45:40,040
which identifies the kind
of thing it is.

1072
00:45:40,040 --> 00:45:41,390
Some of these are obvious.

1073
00:45:41,390 --> 00:45:44,960
Strings are just a type
on their own.

1074
00:45:44,960 --> 00:45:46,630
But for numbers, for example,
we can have

1075
00:45:46,630 --> 00:45:47,980
a variety of types.

1076
00:45:47,980 --> 00:45:49,530
So this is something that
we would call an

1077
00:45:49,530 --> 00:45:50,780
integer, or an INT.

1078
00:45:50,780 --> 00:45:54,300
And this is something
we would call a

1079
00:45:54,300 --> 00:45:57,130
floating point, or a float.

1080
00:45:57,130 --> 00:45:59,480
Or if you want to think of
it as a real number.

1081
00:45:59,480 --> 00:46:01,690
And there's some others
that we can see.

1082
00:46:01,690 --> 00:46:04,380
We're going to build up this
taxonomy if you like, but the

1083
00:46:04,380 --> 00:46:07,180
reason it's relevant is,
associated with each one of

1084
00:46:07,180 --> 00:46:11,130
those types is a set of
operators that expect certain

1085
00:46:11,130 --> 00:46:13,370
types of input in order
to do their job.

1086
00:46:13,370 --> 00:46:16,480
And given those types of input,
will get back output.

1087
00:46:16,480 --> 00:46:17,050
All right.

1088
00:46:17,050 --> 00:46:19,590
In order to deal with this, let
me show you an example,

1089
00:46:19,590 --> 00:46:21,880
and I hope that comes
up, great.

1090
00:46:21,880 --> 00:46:25,040
What I have here is a Python
shell, and I'm going to just

1091
00:46:25,040 --> 00:46:27,570
show you some simple examples
of how we start building

1092
00:46:27,570 --> 00:46:28,370
expressions.

1093
00:46:28,370 --> 00:46:30,550
And this'll lead into what
you're going to see next time

1094
00:46:30,550 --> 00:46:32,610
as well as what you're
going to do tomorrow.

1095
00:46:32,610 --> 00:46:33,710
So.

1096
00:46:33,710 --> 00:46:37,480
Starting with the shell, I
can type in expressions.

1097
00:46:37,480 --> 00:46:38,870
Actually, let me back up
and do this in video.

1098
00:46:38,870 --> 00:46:42,630
I can type in a number, I get
back a number, I can type in a

1099
00:46:42,630 --> 00:46:46,990
string, I get back the string.

1100
00:46:46,990 --> 00:46:49,690
Strings, by the way, can have
spaces in them, they can have

1101
00:46:49,690 --> 00:46:52,510
other characters, it's simply
a sequence of things, and

1102
00:46:52,510 --> 00:46:59,870
notice, by the way, that the
string five-- sorry, the

1103
00:46:59,870 --> 00:47:02,110
string's digit five digit
two is different

1104
00:47:02,110 --> 00:47:03,930
than the number 52.

1105
00:47:03,930 --> 00:47:05,680
The quotes are around them
to make that distinction.

1106
00:47:05,680 --> 00:47:07,770
We're going to see
why in a second.

1107
00:47:07,770 --> 00:47:09,820
What I'm doing, by the way, here
is I'm simply typing in

1108
00:47:09,820 --> 00:47:11,240
expressions to that
interpreter.

1109
00:47:11,240 --> 00:47:13,820
It's using its set of rules to
deduce the value and print

1110
00:47:13,820 --> 00:47:15,540
them back out.

1111
00:47:15,540 --> 00:47:17,260
Things I might like to do in
here is, I might like to do

1112
00:47:17,260 --> 00:47:19,200
combinations of things
with these.

1113
00:47:19,200 --> 00:47:23,900
So we have associated with
simple things, a set of

1114
00:47:23,900 --> 00:47:27,300
operations.

1115
00:47:27,300 --> 00:47:32,440
So for numbers, we have the
things you'd expect, the

1116
00:47:32,440 --> 00:47:33,020
arithmetics.

1117
00:47:33,020 --> 00:47:35,880
And let me show you some
examples of that.

1118
00:47:35,880 --> 00:47:38,440
And actually, I'm going to do
one other distinction here.

1119
00:47:38,440 --> 00:47:42,760
What I typed in, things like--
well, let me start this way--

1120
00:47:42,760 --> 00:47:44,850
there's an expression.

1121
00:47:44,850 --> 00:47:48,630
And in Python the expression
is, operand, operator,

1122
00:47:48,630 --> 00:47:51,150
operand, when we're doing simple
expressions like this,

1123
00:47:51,150 --> 00:47:53,790
and if I give it to the
interpreter, it gives me back

1124
00:47:53,790 --> 00:47:56,080
exactly what you'd expect,
which is that value.

1125
00:47:56,080 --> 00:47:57,290
OK?

1126
00:47:57,290 --> 00:47:59,670
The distinction I'm going to
make is, that's an expression.

1127
00:47:59,670 --> 00:48:01,450
The interpreter is going
to get a value for it.

1128
00:48:01,450 --> 00:48:03,000
When we start building
up code, we're

1129
00:48:03,000 --> 00:48:04,160
going to use commands.

1130
00:48:04,160 --> 00:48:05,120
Or statements.

1131
00:48:05,120 --> 00:48:08,220
Which are actually things that
take in a value and ask the

1132
00:48:08,220 --> 00:48:09,750
computer to do something
with it.

1133
00:48:09,750 --> 00:48:13,980
So I can similarly do this,
which is going to look strange

1134
00:48:13,980 --> 00:48:16,135
because it's going to give me
the same value back out, but

1135
00:48:16,135 --> 00:48:17,640
it actually did a slightly
different thing.

1136
00:48:17,640 --> 00:48:19,910
And notice, by the way, when I
typed it how print showed up

1137
00:48:19,910 --> 00:48:21,390
in a different color?

1138
00:48:21,390 --> 00:48:24,020
That's the Python saying, that
is a command, that is a

1139
00:48:24,020 --> 00:48:26,260
specific command to get the
value of the expression and

1140
00:48:26,260 --> 00:48:27,060
print it back out.

1141
00:48:27,060 --> 00:48:29,490
When we start writing code,
you're going to see that

1142
00:48:29,490 --> 00:48:31,180
difference, but for now, don't
worry about it, I just want to

1143
00:48:31,180 --> 00:48:33,020
plant that idea.

1144
00:48:33,020 --> 00:48:33,190
OK.

1145
00:48:33,190 --> 00:48:35,130
Once we've got that, we
can certainly, though,

1146
00:48:35,130 --> 00:48:39,000
do things like this.

1147
00:48:39,000 --> 00:48:42,240
Notice the quotes around it.

1148
00:48:42,240 --> 00:48:44,630
And it treats it as a string,
it's simply getting me back

1149
00:48:44,630 --> 00:48:47,900
the value of that string,
52 times 7, rather than

1150
00:48:47,900 --> 00:48:49,800
the value of it.

1151
00:48:49,800 --> 00:48:52,030
Now, once we've got that, we
can start doing things.

1152
00:48:52,030 --> 00:48:53,720
And I'm going to use print
here-- if I could type, in

1153
00:48:53,720 --> 00:48:55,720
order to just to get into that,
I can't type, here we

1154
00:48:55,720 --> 00:48:57,660
go-- in order to get
into the habit.

1155
00:48:57,660 --> 00:49:00,480
I can print out a string.

1156
00:49:00,480 --> 00:49:07,010
I can print out--

1157
00:49:07,010 --> 00:49:09,130
Ah!--

1158
00:49:09,130 --> 00:49:10,860
Here's a first example
of something that

1159
00:49:10,860 --> 00:49:11,910
caught one of my things.

1160
00:49:11,910 --> 00:49:15,000
This is a static
semantic error.

1161
00:49:15,000 --> 00:49:16,110
So what went on here?

1162
00:49:16,110 --> 00:49:18,470
I gave it an expression that
had an operand in there.

1163
00:49:18,470 --> 00:49:21,290
It expected arithmetic types.

1164
00:49:21,290 --> 00:49:23,800
But I gave two strings.

1165
00:49:23,800 --> 00:49:26,010
And so it's complaining at me,
saying, you can't do this.

1166
00:49:26,010 --> 00:49:27,810
I don't know how to take
two strings and

1167
00:49:27,810 --> 00:49:30,020
multiply them together.

1168
00:49:30,020 --> 00:49:32,280
Unfortunately-- now John you may
disagree with me on this

1169
00:49:32,280 --> 00:49:34,780
one-- unfortunately in Python
you can, however,

1170
00:49:34,780 --> 00:49:37,570
do things like this.

1171
00:49:37,570 --> 00:49:39,950
What do you figure that's
going to do?

1172
00:49:39,950 --> 00:49:41,400
Look legal?

1173
00:49:41,400 --> 00:49:45,080
The string three times
the number three?

1174
00:49:45,080 --> 00:49:49,850
Well it happens to give me
three threes in a row.

1175
00:49:49,850 --> 00:49:50,400
I hate this.

1176
00:49:50,400 --> 00:49:51,510
I'm sorry, John, I hate this.

1177
00:49:51,510 --> 00:49:55,060
Because this is overloading that
multiplication operator

1178
00:49:55,060 --> 00:49:56,360
with two different tasks.

1179
00:49:56,360 --> 00:49:57,580
It's saying, if you give
me two numbers, I'll

1180
00:49:57,580 --> 00:49:58,320
do the right thing.

1181
00:49:58,320 --> 00:50:00,510
If you give me a number and
a string, I'm going to

1182
00:50:00,510 --> 00:50:02,130
concatenate them together,
it's really different

1183
00:50:02,130 --> 00:50:05,380
operations, but nonetheless,
it's what it's going to do.

1184
00:50:05,380 --> 00:50:11,130
STUDENT: [UNINTELLIGIBLE]

1185
00:50:11,130 --> 00:50:12,700
PROFESSOR ERIC GRIMSON:
There you go.

1186
00:50:12,700 --> 00:50:14,690
You know, there will be a
rebuttal phase a little later

1187
00:50:14,690 --> 00:50:16,380
on, just like with the political
debates, and he

1188
00:50:16,380 --> 00:50:18,520
likes it as a feature, I don't
like it, you can tell he's not

1189
00:50:18,520 --> 00:50:20,810
a Lisp programmer and I am.

1190
00:50:20,810 --> 00:50:21,020
All right.

1191
00:50:21,020 --> 00:50:22,710
I want to do just a couple
more quick examples.

1192
00:50:22,710 --> 00:50:23,410
Here's another one.

1193
00:50:23,410 --> 00:50:24,980
Ah-ha!

1194
00:50:24,980 --> 00:50:28,220
Give you an example
of a syntax error.

1195
00:50:28,220 --> 00:50:31,110
Because 52A doesn't
make sense.

1196
00:50:31,110 --> 00:50:32,400
And you might say, wait a
minute, isn't that a string,

1197
00:50:32,400 --> 00:50:34,870
and the answer's no, I didn't
say it's a string by putting

1198
00:50:34,870 --> 00:50:35,692
quotes around it.

1199
00:50:35,692 --> 00:50:38,430
And notice how the machine
responds differently to it.

1200
00:50:38,430 --> 00:50:41,290
In this case it says, this is
a syntax error, and it's

1201
00:50:41,290 --> 00:50:43,120
actually highlighting where
it came from so I can go

1202
00:50:43,120 --> 00:50:44,940
back and fix it.

1203
00:50:44,940 --> 00:50:45,710
All right.

1204
00:50:45,710 --> 00:50:48,800
Let's do a couple of other
simple examples.

1205
00:50:48,800 --> 00:50:49,160
All right?

1206
00:50:49,160 --> 00:50:50,640
I can do multiplication.

1207
00:50:50,640 --> 00:50:51,320
I've already seen that.

1208
00:50:51,320 --> 00:50:52,960
I can do addition.

1209
00:50:52,960 --> 00:50:54,460
Three plus five.

1210
00:50:54,460 --> 00:50:58,020
I can take something to a power,
double star, just take

1211
00:50:58,020 --> 00:50:59,900
three to the fifth power.

1212
00:50:59,900 --> 00:51:03,390
I can do division, right?

1213
00:51:03,390 --> 00:51:04,360
Whoa.

1214
00:51:04,360 --> 00:51:05,420
Right?

1215
00:51:05,420 --> 00:51:08,090
Three divided by five is zero?

1216
00:51:08,090 --> 00:51:10,010
Maybe in Bush econom-- no, I'm
not going to do any political

1217
00:51:10,010 --> 00:51:12,350
comments today, I will not
say that, all right?

1218
00:51:12,350 --> 00:51:14,090
What happened?

1219
00:51:14,090 --> 00:51:15,280
Well, this is one of
the places where

1220
00:51:15,280 --> 00:51:16,500
you have to be careful.

1221
00:51:16,500 --> 00:51:19,070
It's doing integer division.

1222
00:51:19,070 --> 00:51:22,110
So, three divided by five
is zero, with a

1223
00:51:22,110 --> 00:51:23,090
remainder of three.

1224
00:51:23,090 --> 00:51:24,120
So this is the correct answer.

1225
00:51:24,120 --> 00:51:28,240
If I wanted to get full, real
division, I should make one of

1226
00:51:28,240 --> 00:51:30,810
them a float.

1227
00:51:30,810 --> 00:51:32,500
And yes, you can look at that
and say, well is that right?

1228
00:51:32,500 --> 00:51:35,410
Well, up to some level of
accuracy, yeah, that's .6 is

1229
00:51:35,410 --> 00:51:36,380
what I'd like to get out.

1230
00:51:36,380 --> 00:51:38,430
All right.

1231
00:51:38,430 --> 00:51:40,720
I can do other things.

1232
00:51:40,720 --> 00:51:46,600
In a particular, I have similar
operations on strings.

1233
00:51:46,600 --> 00:51:48,660
OK, I can certainly print out
strings, but I can actually

1234
00:51:48,660 --> 00:51:51,250
add strings together, and just
as you saw, I can multiply

1235
00:51:51,250 --> 00:51:53,670
strings, you can kind of guess
what this is going to do.

1236
00:51:53,670 --> 00:51:58,820
It is going to merge them
together into one thing.

1237
00:51:58,820 --> 00:51:59,190
I want--

1238
00:51:59,190 --> 00:52:00,730
I know I'm running you slightly
over, I want to do

1239
00:52:00,730 --> 00:52:03,940
one last example, it's, I also
want to be able to do, have

1240
00:52:03,940 --> 00:52:05,290
variables to store things.

1241
00:52:05,290 --> 00:52:11,140
And to do that, in this it says,
if I have a value, I

1242
00:52:11,140 --> 00:52:12,870
want to keep it around,
to do that, I can

1243
00:52:12,870 --> 00:52:20,490
do things like this.

1244
00:52:20,490 --> 00:52:21,370
What does that statement do?

1245
00:52:21,370 --> 00:52:24,140
It says, create a name for a
variable-- which I just did

1246
00:52:24,140 --> 00:52:28,180
there, in fact, let me type it
in-- mystring, with an equal

1247
00:52:28,180 --> 00:52:32,580
sign, which is saying, assign or
bind to that name the value

1248
00:52:32,580 --> 00:52:35,430
of the following expression.

1249
00:52:35,430 --> 00:52:37,040
As a consequence, I
can now refer to

1250
00:52:37,040 --> 00:52:39,920
that just by its name.

1251
00:52:39,920 --> 00:52:43,190
If I get the value of mystring,
there it is, or if I

1252
00:52:43,190 --> 00:52:49,760
say, take mystring and add to it
the string, mylastname, and

1253
00:52:49,760 --> 00:52:51,970
print it back out.

1254
00:52:51,970 --> 00:52:53,610
So this is the first
start of this.

1255
00:52:53,610 --> 00:52:54,950
What have we done?

1256
00:52:54,950 --> 00:52:57,880
We've got values, numbers
and strings.

1257
00:52:57,880 --> 00:52:59,610
We have operations to
associate with them.

1258
00:52:59,610 --> 00:53:00,730
I just threw a couple up here.

1259
00:53:00,730 --> 00:53:02,220
You're going to get a chance
to explore them, and you'll

1260
00:53:02,220 --> 00:53:04,910
see not only are there the
standard numerics for strings,

1261
00:53:04,910 --> 00:53:06,960
there are things like length
or plus or other things you

1262
00:53:06,960 --> 00:53:08,140
can do with them.

1263
00:53:08,140 --> 00:53:10,500
And once I have values, I want
to get a hold of them so I can

1264
00:53:10,500 --> 00:53:11,270
give them names.

1265
00:53:11,270 --> 00:53:12,820
And that's what I just did
when I bound that.

1266
00:53:12,820 --> 00:53:16,650
I said, use the name mystring
to be bound to or have the

1267
00:53:16,650 --> 00:53:19,470
value of Eric, so I can refer
to it anywhere else that I

1268
00:53:19,470 --> 00:53:20,830
want to use it.

1269
00:53:20,830 --> 00:53:23,020
And I apologize for taking you
over, we'll come back to this

1270
00:53:23,020 --> 00:53:25,460
next time, please go to the
website to sign up for

1271
00:53:25,460 --> 00:53:27,430
recitation for tomorrow.