Lec 4 | MIT 6.00 Introduction to Computer Science and Programming, Fall 2008

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MIT OpenCourseWare at ocw.mit.edu . PROFESSOR ERIC GRIMSON: As
I’ve done in the previous lectures, let me set the stage
for what we’ve been doing, so we can use that to talk about
what we’re going to do today. So far, we have the following
in our language. Right, we have assignment. We have conditionals. We have input/output. And we have looping
constructs. These are things like
FOR and WHILE loops. And of course we’ve got some
data to go with that. One of the things we said last
time was, with that set of things, the ability to give
values– sorry, to give names to values– the ability to make
decisions, the ability to loop as a function of that, the
ability get things in and out, we said that that actually
gave us a language we said was Turing-complete. And that meant, in English, that
this was enough to write any program. Now that’s a slight lie– or
actually in these days of political debates, a slight
misspeaking, a wonderful word– it is technically
correct. It is enough to allow us to
write any program, but it’s not enough to allow us to easily
write any program. And so I joked, badly, I’ll
agree, at the end of last lecture, that we can just stop
now, go straight to the final exam, because this is all
you need to know. The point is, yes it’s enough to
start with, but we want to add things to this that let
us problem solve well. And one of the things to think
about is, even though I’ve got all of that, let’s think about
what I could do, if I wanted to write a piece of code. Right now, you’ve got to
write it in one file. It’s a long sequence of
instructions, it starts at the beginning, walks through, may
jump around a little bit, but eventually comes down
at the end. It’s okay for the things you’re
doing for the early problem sets. Ten lines of code. Twenty lines of code. Imagine instead, you’re writing
code that’s a hundred thousand lines, a million
lines, of code. You don’t want to write
it in this form. All right, and the reason you
don’t want to do that is, well, several. First of all, it’s really hard
to figure out where everything is, where everything goes,
making sure I’m in the right place. To use an ancient expression
from the 1970’s, which only John and I will appreciate, it’s
really hard to grok what that code is doing, to
understand what it’s trying to make happen. And the reason that that’s the
case is, what we don’t have, are two important things. We don’t have decomposition, and
we don’t have abstraction. And that’s what we’re
going to add today. So what does that mean? Those are fancy terms.
Decomposition is a way of putting structure
onto the code. It’s a way of breaking the
code up into modules. Modules that makes sense
on their own. Modules that we can reuse
in multiple places. Modules that, if you
like, isolate components of the process. And abstraction is related to
that, abstraction is going to let us suppress details. It’s going to let us bury away
the specifics of something, and treat that computation
like a black box. And by black box, I mean,
literally behaves like a mysterious little black box. You put some inputs in, it has
a contract that says if you put the right kind of inputs
in you’ll get a specific output coming out, but you
don’t have to know what’s inside of that box. And that abstraction is
really important. Again, imagine if I’m a writing
a piece of code. I want to just use it, I
shouldn’t have to worry about what variables I use inside of
it, I have shouldn’t have to worry about where that is in the
code, I should be able to just abstract it away. And that’s what we want to add
today, are those two things. Now, our mechanism for doing
that– or at least one mechanism, I shouldn’t say the
only one– one mechanism for doing that is going to be to add
functions to our language. Now, the point of a function is
that it’s going to provide both of these things, so the
first thing it’s going to do is, it’s going to let us
break up into modules. Second thing they’re going to do
is let us suppress detail. And in essence what that does
is, the functions, and we’re going to look at a bunch of
examples in a second, these functions are going to give us
a way to, in some or in one way of thinking about it is
to create new primitives. And I’m going to put those in
quotes, it’s a generalization. What do I mean by that? The idea of a function is, that
I’m going to capture a common pattern of computation. Computing square root. I’m going to capture it in a
piece of code, I’m going to be able to refer to it by a name,
and I’m going to suppress the details, meaning inside of that
computation, you don’t need to know what it does. You just need to know, if I
give it the right kind of input, it’ll give me back an
input that satisfies the contract that I set up. And that in essence says, I’ve
just created the equivalent of a new primitive. Same way that I have
multiplication or division as a primitive, functions are going
to give me, or somebody else who wrote them for me as
part of a library, a new primitive that I’m going
to be able to use. And that gives me a lot of power
in terms of what I want to have inside of the language. OK. So, let’s look at an example. To try to see what we’re
going to do with this. Before I do that though, let me
try and give you an analogy to keep this in mind of why we
want to basically build these abstractions and what
we need in order to have them work together. So here’s the supposed
to say silly analogy. You can tell my jokes
are always bad. John’s are much better, by the
way, which is why Thursday will be a much better lay–
a better lecture. But here’s the example. You’ve been hired by PBS to
produce a nice thirteen-hour documentary, or drama,
that’s going to run. And, you know, you start by
saying, OK, thirteen hours, I’m going to break it up into
thirteen different chunks. I’m going to assign each chunk
to a different writer. And they’re going to go off and
write that element, that hour’s worth of stuff. You can imagine what you get:
each hours worth of drama, if you like, may be great, but it
may have absolutely nothing to do with the other
twelve hours. And unless, you know, you’ve
been hired to do Pirandello’s Six Characters In Search Of An
Author, this is not a great thing, because you
get something that is really confusing. Now, what’s the point
of the analogy? What do I need for
those writers to all interact together? I need a specification. I need a contract that says,
here’s what I want in terms of things that you’re going to take
as input, to begin your part of the drama, here’s what
you’re going to produce at the output, and the details
of what they do inside are up to them. An idea of abstraction, that
idea of specification, is exactly what we want to use
inside of our functions. We won’t make you write dramas
like Pirandello, but we’re going to try make you at
least write good code. And that’s we’re going
to try and do. All right. Let’s set the stage for it. Up on the screen, I’ve got– I commented it out, but I’ve
got a piece of code that you’ve seen before,
right up here. OK? What is that? It’s the piece of code we wrote
for computing square roots, square roots of actually
perfect squares. [UNINTELLIGIBLE] Just to remind you what it
does, we bound x to some value, we set up an initial
variable called ANS or answer, and then we run through
a little loop. All right, we’re– well
actually, I should say that better, we first check to see,
is x greater than or equal to zero, if it’s not, then we come
down here and we print something out, otherwise we run
through a little loop to get the answer, and then
we check it and we spit something out. It does the computation,
that’s fine. Suppose I want to compute square
roots a lot of places in a big chunk of code. Right now, I have to take that
piece of code and replicate it everywhere I want in
my larger file. And I’ve got to worry about,
is somebody else using ANS, answer, as a variable, in which
case I’ve got to be really careful. Is somebody else using
x as a variable? I’ve got to deal with a
lot of those details. I want to abstract that. And the abstraction you
see, right here. I’m going to highlight it for
a second so you can see it. I want you to look at it
on the handout as well. This is the creation
of a function. And I want to describe both the
syntax, what we’re doing, and then the semantics of
how do we use it and what does that mean. So. Here’s the syntax
of the function. First of all, we have
a keyword. def. Definition or define, depending
on which sort of piece of history
you come from. This is a keyword to Python that
says, when it reads this in the file, it says, I’m
creating a definition. I’m creating a function. And that’s follow– so this
is, let me say this is a keyboard– that is followed
immediately by a name. And this equates to that. In this case, sqrt,
square root. I’m saying, this is the
name I’m going to give to this function. This is the name to which I’m
going to refer when I want to use this function. All right? And notice, immediately after
that name, we have an open and close paren with another
variable name inside of that. And this defines formal
parameters of this function. Yup. PROFESSOR JOHN GUTTAG:
does indeed, thank you. This is me being a Scheme
hacker, not a Python hacker. Yes, def has to be lowercase
or won’t recognize it. Thank you, John. OK. def’s the keyword. I’m creating a function.
sqrt– again, I’m being careful about case-sensitive,
I’m using all lowercase here, followed by an open paren, and
I said, formal parameters. We’ll see there could be
more than one there. We’re going to come back to what
they mean in a second, but for now, think of them as,
or think of x, in this case, as the place holder. This place holder is saying, if
you give me a value for x, inside the body of this function
I’m going to use that value everywhere I see x. Question. STUDENT: [INAUDIBLE] PROFESSOR ERIC GRIMSON: Ah,
we’re going to come back to this in a second. But the question was, do
I always need an input? I can have functions with no
parameters, that’s fine, I will still need the open and
close paren there to identify that I have no parameters. We’re going to see an
example in a second. Good question. Actually, I’ve got to get rid of
this candy, so since it was a good question, here you go. Nice catch. Almost. Sorry. OK. No, I’m not. I’m sorry. I thought you had it, and then
I’ve got the wrong glasses on and I realized you didn’t,
so I will, ah come back to that later. What are we doing here? We got definition, we got name,
we got a set of formal parameters. Right. If you look at the rest of that
code, gee, it looks a lot like what I had elsewhere. Of what I had outside
of it, right? It’s running through a
similar set of loops. So in some sets, as long as x
has the value I want, it ought to do the right thing. However, there’s a couple of
other changes there that we want to highlight. In particular, notice– let me
highlight it for you, if I can find it with the wrong glasses
on– we’ve got these return commands. So return is another keyword. And it basically says, when you
get to this point in the computation, stop
the computation. Literally, return the control
from this function, and take the value of the next
expression, and return that as the value of the whole
computation. Now, the one that we’re most
interested in is the one where, in fact, it gets out ANS,
so you see down here in the code, there’s a spot where
it’s going to return the value of ANS, which is what
we want, right? That’s the thing that holds the
value that we intended to have. But there’s another couple of
places in that code where it’s got this funky-looking thing,
return none, and notice none’s in a different color. None is a special value, and
it has the following slightly-odd behavior: it is
a value, we can return it– God bless– but what none says
is, there is no value coming back from this computation. So when it is returned, and
we’ll see this in a second, to the interpreter, it
doesn’t print. OK. It simply doesn’t
print anything. Nonetheless, it is actually a
value and we can use it, for example, to do comparisons. If we want to know, did this
function return a value or not, rather than reserving, say,
-1 or some other special thing which you might want to
use some other ways, it literally returns this very
special value that says, there is no actual value return
from this computation. OK. Note, by the way, if I chase
through each possible path, like there’s some IFs in here,
there’s some places to go, at least in this piece of code,
every possible path through this code ends in a return. And that’s a good programming
discipline, to make sure that happens. There’s an exception, which
we’ll see in a second, but I’ll highlight, which is, if
we get to the end of the procedure, there’s sort of
an implicit return there. In fact, a return of
none, in that case. It comes out of it. But if I look at this, right? If I come into this code, I’m
going to check this branch first, if it’s not true, ah,
there’s a return at the end of that branch. If it is true, I do that, and
then I’ve got a second test. If it’s true, I return,
otherwise a return. So there’s a return branch
on every possible path through the code. And that’s valuable, it’s
something you want to think about as your right your own. OK. What do I do to use this,
in particular? How do I invoke this? OK, so I’m going to invoke a
function by passing in values for the parameters. And in this case, that literally
means typing sqrt, with some value inside
the parens. OK. Now, let’s just try this out
to see what happens. I’m going to make sure I’ve got
it there, so if I type, for example, sqrt
of 16, ah-ha! What did it do? Well, let’s talk about
what it did. What this invocation does, is
the following: it binds, and I’m going to say this
specifically for this example, rather than general,
it binds x to 16. Just as you would have done with
an assignment statement up in the top level thing. But this binding is local. Meaning it only holds within
the confines of the code of this procedure. Relative to that, think of that
as creating what we, I’m going to call a new
environment. Relative to that, it does all
the other execution we would do, including, notice the first
instruction there is to the set up a binding for ANS. So answer, or ANS, is also
bound only locally. Meaning, inside the
confines of this environment of this procedure. ANS starts off with a value of
0 and now we just run through that loop just like
we did before. Writing Increments it slowly,
checking to see if ANS squared is bigger than x, and when it
gets to that point, it checks to see, is it actually a perfect
square or not, and it returns it. And once it returns it, it
returns a value from that return, that in this case
is just printed out. All right. Now I want to say a
couple of things about these local bindings. I’m going to repeat this
a second time, because it’s important. These local bindings do not
affect any global bindings. What does that mean? Let me show you a little
example, and then we’ll come back to this. I’ve got a little
function here. See, I’ve defined f of x to be
a function that takes a value of x in, changes x to x+1, and
then just returns the value. OK. So it’s just adding 1 to x. But I want you to see now what
happens if I use this. Let’s bind x to the
value of 3. It’s creating a binding for x
in this global environment. This is what the interpreter
sees. All right? In fact, if I look at
x, its value is 3. Let’s bind z eh let’s bind z
to the– if I could type it would help– say, f of 3. OK? So the value is z is 4, it’s
what I expect, right? Locally x got bound to 3, I
added 1 to it, whoop-dee-doo, I get back a 4. But what’s the value of x? It’s still 3. The way to think of this is,
again, I’ve got multiple scopes, or multiple frames, or
if we’re going to come back to those terms, I’m going to use
the word environment, because I’m an old-time Lisp hacker,
multiple environments in which there are bindings. So let me spell this out in just
a little bit more detail. What this is saying
is the following. When I’m talking to the
interpreter, when I’m typing things in as I just did, to that
Python environment, I’m getting what I’m going to
call global bindings. I’m going to draw a
little chart here. Think of this as the, as the
world of the interpreter, in that I’ve got things like x
bound to the value of 3. When I call or invoke a
function, think of it as creating a local table. Inside that local table, I bind
the formal parameter, which is what I do I did
16 right to some value. This x only gets seen by sqrt. Inside of there, I can bind
other things, like ANS gets locally bound to 0, and then
it increments around and eventually we return
that value out. When I get to a return from
sqrt, some value is returned back to the interpreter, and
that table goes away. But that table does not affect
any bindings for other instances of the variable
like x for ANS. OK. Let’s look at a couple of
examples, just to sort of stress that. And one of the things I
wanted to show is, OK. Again, I can now use a function
just as if it was a primitive, so this is just an
assignment and I going to take test to be the value of that, of
course nothing gets printed because that was an assignment
statement. All right? So if I called sqrt alone, that
return value is done, but in this case I bound it to
test, so I can go look at test, and there it is. What happens if I do that? OK. If you look at the code, it
printed out, it’s not a perfect square, which is what I
wanted, but now, what’s the value of test? OK, I bound test to something,
if I look at it, it doesn’t print anything, but–
if I could type– I can ask, is test bound to
that special name none? The answer is yes. Boy, this seems like
a nuance, right? But it’s a valuable thing. It says, in each case, I return
some useful value from this procedure. I can check it, so if this
was part of some other computation, I want to
know, did it find a perfect square or not? I don’t have to go read what it
printed out in the screen. This has returned a value
that I can use. Because I could do a test to
say, is this a return value? If it’s not, I’ll do something
else with it. So the binding is still
there, it simply doesn’t print it out. OK. What do we have out of this? Simple, seems like,
at least addition. We’ve added this notion
of a function. I’ve highlighted some of the key
things we got here, right? We have that def keyword, we’ve
got a name, we’ve got a list– or I shouldn’t say
a word list, we have a collection of formal parameters
that we’re going to use– we have a body, and the
body looks just like the normal instructions we’d use,
although by the way, we ought to be able to use functions
inside the body, which we’re going to do in a second, and
then we’re going to simply return some values
out of this. Now I started by saying,
build these functions. I’m trying to get both
decomposition and abstraction. Well, you hopefully can see
the decomposition, right? I now have a module. OK, let me set the stage. Imagine I wanted to do sqrt, or
square root– no, I’m going to use sqrt, that’s the name I’m
using here– square root a hundred different places
in some piece of code. Without function, I’d have to
copy that piece of code everywhere. Now I got one just simple
thing, and I simply have isolated that module inside
of that function. What about abstraction? Well, I’ve got part of what
I want for abstraction. Abstraction, again, says I’m
going to suppress details. Now that I’ve written sqrt, I
can just use it anywhere I want in the code. You’ve got to rely on the fact
that I wrote it correctly, but you can basically suppress the
details of how it’s used. There’s one more piece that we’d
like to get out of that, and that is– you may have been
wondering, what’s with the funky stuttering here of
three double-quotes in a row. All right? And that is a specification. Which is a really valuable
thing to have. So what is the specification
going to do? It is my place, as a programmer,
to write information to the user. This is me writing one hour of
that episode of Pirandello and telling the other authors,
here’s what I’m assuming as you use it. So it’s up to me to do it right,
but if I do it, I’m going to specify, what does
this function do? What does it expect as input,
and any other information I want to pass on. And notice, by the way, if I
do that, I’m going to come down here, and I type sqrt and
open the paren, ah-ha! It shows me what the creator, in
this case actually I stole this from John so what Professor
Guttag put up as his specification for this
piece of code. Now, it’s not guaranteed
it’s right, right? You’re trusting the programmer
did it right, but this now tells you something. What is this? This is a wonderful piece
of abstraction. It is saying, you don’t need
to know squat about what’s inside the body of
this function. You don’t have to worry about
the parameter names, because they’re going to be preserved,
you don’t need to worry about how I’m doing it, this tells you
how you can use this, in order to use it correctly. Of course, I can then close
it off, and off we go. All right, so that notion of
abstraction and I was going to come back– we’re going to come
back to multiple times during the term– and it’s not
just abstraction, it’s the idea of a specification. And just to look ahead a little
bit, you could easily imagine that I might want to
not just put a statement in there, what the specs are,
I might want to put some constraints. Some specific things to check
for, to make sure that you’re calling the code right. And it becomes a powerful way of
reasoning about the code, a powerful way of using the code,
so those notions of specs are really important. Look, part of the reason I’m
flaming at you is, something like square root, it seems dumb
to write specs on it. Everybody knows what this
is going to do. But you want to get into that
discipline of good hygiene, good style. You want to write the specs so
that everybody does in fact know what this piece of code is
doing, and you’re writing it each time around. OK. Now that we’ve got functions,
let’s see what we can do as a problem-solving tool
using them. In a particular, I’ve already
said I want to get this notion of modularity, it’s a module I
can isolate, and I want to get the notion of abstracting away
the details, let’s see how we can actually use that to
actually write some reasonably interesting pieces of code, but
in particular, to see how we can use it to capture the
ideas of decomposition and abstraction. So I’m going to shift gears. Start with a simple problem. Boy, we’re suddenly be
transported to Nebraska. Or where I grew up,
Saskatchewan. All right, we’ve got
a farm air problem. I got a farmer, walks out into
his yard, one morning. This farmer has a bunch of pigs
in a punch– it’s been a long day– a bunch of pigs
and a bunch of chickens. And he walks out into the
farmyard and he observes 20 heads and 56 legs. And for sake of argument, there
are no amputees among the chickens and the pigs. And the question is, so how many
pigs does he have, and how many chickens does he have? Wow. What a deep problem, right? But you’re going to see
why we’re going to use this in a second. So you know how to solve
this, this is a fifth-grade problem, right? And what’s the way
to solve this? System of linear equations. What are the equations here? Well, I could say, you know,
the number of pigs plus the number of chickens
equals 20, right? Because we’ve got 20 heads. And then what else do I have? Four times the number of pigs
plus two times the number of chickens, assuming they’re
not next to a nuclear reactor, is 56. And then how would
you solve this? Well, it’s, you sort of know
how you’d do it if this was grammar school right? You’d pull out your pencil and
paper, you can do it as a matrix inversion if you know
how to do that, or you can just simply do substitution
of one equation into another to solve it. That’s certainly one way to do
it, but for computers that’s not necessarily the
easiest way. So another way of solving it is
to do something we already saw last time, which is
basically, why not simply enumerate all possible examples
and check them? You could say, I could have
zero chickens and 20 pigs, does that work? I’ve got one chicken and
nineteen pigs, does that work? I’ve got two chickens
and eighteen pigs, you get the idea. So I’m going to solve this by
enumerate and check, which is an example of what’s called
a brute-force algorithm. Meaning, I’m just going
to write a little loop that does that. All right, so let’s go
back to our code. That’s right, let me pull
this over a little bit, so I can see it. And what I’d like you to look
at, I’m going to highlight it just for a second here, is
those two pieces of code. OK? Let’s start with solve. OK. Here’s the idea of solve. I’m going to have it take in as
input how many legs I got, how many heads do I have,
and I just want to write a little loop. OK. I know how to do that, right? Write a little loop, all
I’m going to do, is run a FOR loop here. I’m going to let the number of
chickens be in this range. Remember what range does, it
gives me a set or a collection or a tuple of integers from 0
up to 1 – is the last value, so it’s going to give me
everything from 0 up to the total number of heads. Knowing that, I’m going to say,
OK, how many pigs are there, well that’s just how
we’re, however many I had total, minus that amount, and
then I can see, how many legs does that give, and then I can
check, that the number of legs that I would get for that
solution, is it even equal to the number of legs I
started with, ah! Interesting. A return. In particular, I’m going
to return a tuple. So, a pair or collection
of those two values. If it isn’t, then I’m going to
go back around the loop, and notice what happens. If I get all the way around the
loop, that is, all the way through that FOR loop and I
never find a path that takes me through here, then the last
thing I’m going to do is return a pair or a tuple
with a special simple number none twice. Yep. Are you telling me I want
parens there and not, and not braces? All right. I hate this language, because I
always want to have parens. Every time you see a square
bracket, put a paren in. All right? Thank you, Christy. I’ll get it eventually . Having done that, right,
notice what I’ve got. First of all, two parameters. It’s OK. All it says is, when I call
this, I need to pass in two parameters for this to work. All right? Now, if I want to use that,
I’m going to use a second piece of code here,
called Barnyard. I’m going to read in a couple
of values, convert them into integers, and then I’m
going to use solve to get a solution out. And what do I know
about solve? It is going to give me back a
tuple a collection of two things, and so check
out the syntax. I can give two names, which
will get bound to the two parts of that return tuple. OK, pigs will be the first part,
chickens will be the second part. OK, and then once I’ve got that,
well, notice: I can then check to see, did I return
that special symbol none? Is the first part. That says, I took the branch
through here that eventually got to the end and said, there
wasn’t a solution, in which case I’m going to print out, there ain’t
no solution, otherwise I’ll print out the pieces. All right, let’s check it out. Ah, what did I say? Twenty and 56, Right? OK, notice the form. I’ve got two parameters, they’re
separated by a comma. Ah, right. Sorry? Yeah, but I see– it’s legs and
heads, but it should not still have– Oh, sorry. Thank you. I’ve been doing the
wrong thing. I want Barnyard this way, and if
I had looked when I opened the paren, it would have shown
me a closed paren with no parameters. Aren’t you glad I make mistakes,
so you can see how well I can fix from these? All right. Now I call that, and it says,
tell me how many heads you want, give it a 20, and tell
it how many legs you want, give it 56, and it prints
out the answers. I know, whoop-dee-doo. But notice what’s
inside if here. First of all, notice
the modularity. I’ve used solve. All right? Right there. I’ve captured it as
a computation. It’s buried away, all the
details are suppressed. I can use that to return values,
which I can then use elsewhere, which I did– and
if I just come back and highlight this– inside
of that computation. But I don’t have to know, inside
of Barnyard, what the values are used inside
of solve. I don’t know what the names of
the variables are, I don’t care, I can basically suppress
away that detail. Second thing we saw is, that
using this as a computation, I can return multiple values. Which is actually of
real value to me here as I use that. OK. Yeah. Question. STUDENT: [INAUDIBLE] PROFESSOR ERIC GRIMSON: Ah. The question was, when it
returns, how does it distinguish between local
and other things? So let me try and answer that. Inside of solve, solve creates
an environment where inside of that, it has bindings for the
parameters it’s going to use. All right? Like, number of– wait, what
did we call a were solve– number of legs and
number of heads. OK, those are bound locally. When solve is done, it wraps up,
if you like, a value that it returns. Which is that. That expression, or that
value, or that value, literally gets passed back out
of that local environment to the value that comes
back out of it. So in particular, what’s solved
returns is a pair. It could be the pair of none,
none, it could be the pair of, you know, whatever the answer
was that we put up there. That value comes back out and
is now available inside the scope of Barnyard. OK. And Barnyard then uses that. Question? STUDENT: [INAUDIBLE] PROFESSOR ERIC GRIMSON: Here? So the question is, why is this
return on the same level as the FOR? Why do you think? STUDENT: [INAUDIBLE] PROFESSOR ERIC GRIMSON: No. Good question. All right? So what’s going to
happen here? If I’m inside this FOR, OK, and
I’m running around, if I ever hit a place where this
test is true, I’m going to execute that return,
that return returns from the entire procedure. OK? So the return comes back
from the procedure. So the question was, why is
this return down at this level, it says, well if I ever
execute out of this FOR loop, I get to the end of the FOR
loop without hitting that branch that took me through the
return, then and only then do I want to actually say,
gee, I got to this place, there isn’t any value to return,
I’m going to return none and none. I’m still trying to get rid
of this candy, Halloween’s coming, where were we? There’s one, thank you. I don’t think I’m going
to make it, I did. Thank you. Make sense? The answer is no, I want parens
to create tuple and I get really confused about
the difference between lists and tuples. For now, the code is working. Yes is the answer, all right? And we’re having a difference
of opinion as to whether we should use a tuple or
a list here, right? But the answer is
yes, you can. And my real answer is, go try it
out, because obviously you can tell I frequently do this
the wrong way and the TAs give me a hard time every time. John. PROFESSOR JOHN GUTTAG:
see next week, tuples and lists are very close
to the same thing. In almost any place where you
can get away with using tuples you can use lists. PROFESSOR ERIC GRIMSON: Yes. PROFESSOR JOHN GUTTAG: But
want to emphasize word is almost, because we’ll see a
couple of places where if it expects a tuple and you
use a list you’ll get an error message. But we’ll see all
that next week. PROFESSOR ERIC GRIMSON: Right,
when the real pro comes in to pick up the pieces I’m leaving
behind for him. OK. Let me pull this back up. What we’re doing now is we’re
building this encapsulation. Now one of the things you notice
here by the way is, you know, this in essence just
solves the simple problems. Suppose I now add one
other piece to this. The farmer is not keeping a
great set of things so in addition to pigs, and chickens
he raises spiders. I have no idea why. He’s making silk I guess. Right? Why am I giving you
this example? I want to show you how easy
it is to change the code. But, notice, once I’ve
added this I actually have a problem. This is now an under-constrained
problem. I have more unknowns than
I have equations. So you know from algebra I can’t
actually solve this. There may be multiple
solutions to this. What would I have to do
to change my code? And the answer is fortunately
not a lot. So I’m going to ask you to
look now at this set of things, which is solve
1 and Barnyard 1. OK. The change is, well, on Barnyard
1 it looks much the same as it did for Barnyard. Right, I’m going to read in the
values of the number of heads and the number of legs. I’m going to use solve 1 as
before, but now I’m going to bind out three variables. And then I’m going to do
a similar thing to print things out. But would the solver do? Well here what I’d like
to do is to run through a couple of loops. Right, how would I solve
this problem? You can use the same enumerate
and check idea, but now say gee let me pick how many
pigs there are. Is that the one I used first? Sorry, let me pick the number
of spiders there are. Having chosen the number of
spiders, let me pick how many chickens I have. With those two
in place, I now know how many pigs I must have
and I can run through the same solution. The reason I’m showing you this
is this is another very standard structure. I now have two nested loops. One running through a choice for
one parameter, another one running through a choice
for a second parameter. And then the rest
of the solution looks much like before. I’m going to get the total
number of legs out. I’m going to check to see
if it’s right or not. And again I’m going to return
either a three tuple there or a three tuple there. It’s part of what I want,
because I’m going to bind those values out. And if I run that example,
Barnyard 1, I don’t know we’ll give it 20 heads, 56 legs;
and it find a solution. I ought to be able to
run something else. I don’t know, give
me some numbers. How many heads? Pick an integer, somebody. STUDENT: 5. PROFESSOR ERIC GRIMSON: 5. All right. How many legs? 10? All right. We got an easy one. Let’s just for the
heck of it — I should have found some
better examples before I tried this. No mutant spiders here. OK, so what have I done? I just added a little
bit more now. I’m now running through
a pair of loops. Again notice the encapsulation,
that nice abstraction going on, which
is what I want. Once I get to this stage though
by the way, there might be more than one solution. Because in an under-constrained
problem there could be multiple
solutions. So suppose I want to capture
all of them or print all of them out. Well I ought to be able
to do that by simply generalizing the loop. And that’s what the next
piece of code on your a hand out shows you. I’m just going to let
you look at this. If you look at solve 2, it’s
going to run through the same kind of loop, printing out
all of the answers. But it’s going to keep going. In other words it doesn’t just
return when it finds one, it’s going to run through
all of them. All right? Sounds like a reasonable
thing to do. Notice one last piece. If I’m going to do that, run
through all possible answers, I still want to know,
gee, what if there aren’t any answers? How do I return that case? And that shows you one other
nice little thing we want to do, which is if I look in this
code notice I set up a variable up here called Solution
Found, initially bound to false. The rest of that code’s
a pair of loops. Pick the number of spiders. Pick the number of chickens. That sets up the
number of pigs. Figure out the legs. See if it’s right. If it is right, I’m going to
print out the information but I’m also going to change
that variable to true. And that allows me then, at the
end of that pair of loops when I get down to this point
right here, I can check to see did I find any solution and if
not in that case print out there is no solution. So this gives you another nice
piece which is I can now look for first solution, I can look
for all solutions, and I can maintain some internal variables
that let me know what I found. A trick that you’re going to
use a lot as you write your own functions. All right, I want to end up with
the last 10 minutes with a different variation on how
to use functions to think about problems. And that is to introduce the idea of recursion. How many of you have heard
the term used before? How may have you heard the term
used before in terms of programming languages? Great. For the rest you,
don’t sweat it. This is a highfalutin term that
computer scientists use to try and make them look like
they’re smarter than they really are. But it is a very handy way of
thinking about, not just how to program, but how to
break problems down into nice sized chunks. And the idea behind recursion
I’m going to describe with a simple example. And then I’m going to show you
how we can actually use it. The idea of recursion is that
I’m going to take a problem and break it down into a simpler
version of the same problem plus some steps
that I can execute. I’m go to show you an example
of a procedure, sorry a function, in a second. But let me give you actually
an analogy. If you look at US law, and you
look at the definition of the US legal code that defines
the notion of a natural born US citizen. It’s actually a wonderful
recursive definition. So what’s the definition? If you’re born in the United
States you are by definition a natural born US citizen. We call that a base case. It’s basically the
simplest possible solution to the problem. Now if you were not born in
the United States, you may still be, under definition, a
natural born US citizen if you’re born outside this United
States, both of your parents are citizens of the
United States and at least one parent has lived in
the United States. There’s a wonderful
legal expression. But notice what that is. It’s a recursive definition. How do you know that your
parents, at least one of your parents satisfies
the definition? Well I’ve reduced the problem
from am I a natural born US citizen to is one
of my parents a natural born US citizen? And that may generalize again
and it keeps going until you either get back to Adam
and Eve, I guess. I don’t think they were born in
the US as far as I know, or you find somebody who satisfies
that definition or you find that none
of your parents actually are in that category. But that second one is called
the inductive step, or the recursive step. And in my words it says break
the problem into a simpler version of the same problem
and some other steps. And I think this is best
illustrated by giving you a simple little piece of code. I use simple advisedly here. This is actually a piece of code
that is really easy to think about recursively and is
much more difficult to think about in other ways. And the piece of code is suppose
I have a spring and I want to know if it’s
a palindrome. Does it read the same
thing left to right as right to left. OK? How would I solve that? If the string has no elements
in it it is obviously a palindrome. If the string has one element
in it, it’s a palindrome. There’s the base case. If it’s longer than one,
what do I want to do? Well I’d like to check the two
end points to see are they the same character? And if they are, then oh,
I just need to know is everything else in the
middle a palindrome? I know it sounds simple, but
notice what I just did. I just used a recursive
definition. I just reduced it to a smaller
version of the same problem. That is if I can write code that
would solve all instances of smaller size strings, then
what I just described will solve the larger size one. And in fact that’s exactly what
I have. I would like you to look at this piece of
code right here called isPalindrome. Notice what it says. I’m going to pass in a string,
call it s, binds it locally, and it says the following. It says if this is a string of
length 0 or 1, I’m done. I’m going to return
the answer true. Otherwise I’m going to check to
see is the first and last, there’s that – 1 indexing, is
the first and last element of the string the same? And if that’s true is everything
in the string, starting at the first element
and removing the last element, a palindrome? Let me remind you. By saying first element remember
we start at 0 as the initial indexing point. Wonderful recursive
definition. OK, let’s try it out. Go back over here and we’re
going to say isPalindrome. How did I actually spell this? Palindrome with a capital P.
Only in New York, in Canada we pronounce it Palindrome. When you’re teaching it you get
to call it your way, I’m going to call it my way. Sorry John, you’re
absolutely right. OK. Notice by the way, there’s
that nice speck going on saying put a string here. It’s going to return true if
it’s a PAIL-indrome and false if it’s a PAL-indrome. And it says true. Now maybe you’re
bugged by this. I know you’re bugged by my
bad humor, but too bad. Maybe you’re bugged by this,
saying wait a minute, how does this thing stop? This is the kind of definition
that your high school geometry teacher would have rapped
your knuckles over. You can’t define things in
terms of themselves. This is an inductive
definition. Actually we could prove
inductively that it holds, but how do we know it stops? Well notice what the computation
is doing. it’s looking first to see am I in the
base case, which I’m done. If I’m not I’m just going to
reduce this to a smaller computation. And as long as that smaller
computation reduces to another smaller computation, eventually
I ought to get to the place where I’m down
in that base case. And to see that I’ve written
another version of this, which I’m going to use here, where I’m
going to give it a little indentation. I’m going to call this
palindrome 1. Sorry about that. Palindrome 1. I’m going to give it a
little indentation so that we can see this. OK. Code is right here. And all it’s doing is when I’m
getting into the different places I’m simply printing out
information about where I am. What I want you to see is notice
what happened here. OK. I’m calling palindrome
with that. It first calls it
on that problem. And the code over here says,
OK gee, if I’m in the base case do something. I’m not, so come down here check
that the two end points a and a are the same and
call this again also. Notice what happens. There’s this nice unwrapping
of the problem. I just doubled the indentation
each time so you can see it. So each successive call, notice
what’s happening. The argument is getting
reduced. And we’re going another
level in. When we get down to this point,
we’re calling it with just a string of length one. At that point we’re in the base
case and we can unwrap this computation. We say, ah, that’s now true. So I can return true here. Given that that’s true and I
already checked the two end points, that’s true,
that’s true. And I unwrap the computation
to get back. You are going to have to
go play with this. Rock it if you like to try
and see where it goes. But I want to stress again, as
long as I do the base case right and my inductive or
recursive step reduces it to a smaller version of the same
problem, the code will in fact converge and give me
out an answer. All right, I want to show you
one last example of using recursion because we’re going
to come back to this. This is a classic example
of using recursion. And that is dating from
the 1200s and it is due to Fibonacci. Does anyone know the history
of what Fibonacci was trying to do? Sorry, let me re-ask that. Fibonacci. Which actually is son of Bonacci
which is the name of his father who was apparently
a very friendly guy. First of all, does anyone know
what a Fibonacci number is? Wow. STUDENT: [INAUDIBLE] PROFESSOR ERIC GRIMSON: Right,
we’re going to do that in a second, but the answer is
Fibonacci numbers, we define the first two. Which are both defined to be,
or I can define them in multiple ways, 0 and 1. And then the next Fibonacci
number is the sum of the previous two. And the next number is the
sum of the previous two. Do you know the history
Exactly. Thank you. Bad throw, I’m playing
for the Yankees. Sorry John. The answer is Fibonacci actually
was actually trying to count rabbits back
in the 1200s. The idea was that rabbits could
mate after a month, at age one month. And so he said, if you start off
with a male and a female, at the end of one month they
have an offspring. Let’s assume they have
two offspring. At the end of the next month
let’s assume those offspring have offspring. Again a male and female. The question was how many
rabbits do you have at the end of a year? At the end of two years? At the end of more than that
number of years, and so. We can do this with the
following level definition. We’re going to let pairs of 0,
the number of pairs at month 0, actually it would not
be 0 it would be 1. We let the number of pairs
at month 1 be 1. And then the number of pairs
at month n is the number of pairs at month n – 1 plus
the number of pairs at month n – 2. The sum of the previous two. If I write Fibonacci, you
see it right there. And the reason I want to show
you this is to notice that the recursion can be doubled. So this says, given a value x,
if it’s either 0 or 1, either of those two cases,
just return 1. Otherwise break this down
into two versions of a simpler problem. Fib of x – 1 and fib of x – 2,
and then take the sum of those and return that as the value. Notice if I’m going to have two
different sub problems I need to have two base cases
here to catch this. And if I only had one
it would error out. And as a consequence, I can go
off and ask about rabbits. Let’s see. At the end of 12 months,
not so bad. At the end of two years, we’re
not looking so good. At the end of three years,
we are now in Australia. Overrun with rabbits. In fact I don’t think the thing
ever comes back, so I’m going to stop it because it
really gets hung up here. And I’m going to restart it. What’s the point of this? Again, now that I can think
about things recursively, I can similarly break things down
into simpler versions of the same problem. It could be one version. It could be multiple versions. And we’re going to come back
throughout the term to think about how to code programs
that reflect this. The last point I want to make
to you is, you’ve started writing programs that you
would think of as being inherently iterative. They’re running through
a loop. It’s a common way of thinking
about problems. Some problems are naturally tackled
that way. There are other problems that
are much more naturally thought of in a recursive
fashion. And I would suggest palindrome
as a great example of that. That’s easy to think
about recursively. It’s much harder to think
about iteratively. And you want to get into the
habit of deciding which is the right one for you to use. And with that, we’ll
see you next time.

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