# Lec 6 | MIT 6.00 Introduction to Computer Science and Programming, Fall 2008

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MIT OpenCourseWare at ocw.mit.edu . PROFESSOR JOHN GUTTAG:

All right. That said, let’s continue, and

if you remember last time, we ended up looking at this thing

I called square roots bi. This was using something called

a bisection method, which is related to something

called binary search, which we’ll see lots more of later,

to find square roots. And the basic idea was that we

had some sort of a line, and we knew the answer was somewhere

between this point and this point. The line is totally ordered. And what that means, is that

anything here is smaller than anything to its right. So the integers are totally

ordered, the reals are totally ordered, lots of

things are, the rationals are totally ordered. And that idea was, we make a

guess in the middle, we test it so this is kind of a guess

and check, and if the answer was too big, then we knew

that we should be looking over here. If it was too small, we knew

we should be looking over here, and then we

would repeat. So this is very similar, this

is a kind of recursive thinking we talked about

earlier, where we take our problem and we make it smaller,

we solve a smaller problem, et cetera. All right. So now, we’ve got it, I’ve

got the code up for you. I want you to notice the

specifications to start. We’re assuming that x is greater

than or equal to 0, and epsilon is strictly greater

than 0, and we’re going to return some value y

such that y squared is within epsilon of x. I’d last time talked about the

two assert statements. In some sense, strictly speaking

they shouldn’t be necessary, because the fact that

my specification starts with an assumption, says, hey

you, who might call square root, make sure that the things

you call me with obey the assumption. On the other hand, as I said,

never trust a programmer to do the right thing, so we’re

going to check it. And just in case the assumptions

are not true, we’re just going to stop

dead in our tracks. All right. Then we’re going to set low to–

low and high, and we’re going to perform exactly the

process I talked about. And along the way, I’m keeping

track of how many iterations, at the end I’ll print how many

iterations I took, before I return the final guess. All right, let’s test it. So one of the things I want you

to observe here, is that instead of sitting there and

typing away a bunch of test cases, I took the trouble to

write a function, called test bi in this case. All right, so what that’s doing,

is it’s taking the things I would normally type,

and putting them in a function, which I

can then call. Why is that better

than typing them? Why was it worth creating

a function to do this? Pardon? STUDENT:: [INAUDIBLE] PROFESSOR JOHN GUTTAG: Then I

can I can use it again and again and again. Exactly. By putting it in a function, if

I find a bug and I change my program, I can just run

the function again. The beauty of this is, it keeps

me from getting lazy, and not only testing my program

and the thing that found the bug, but in all the

things that used to work. We’ll talk more about this

later, but it often happens that when you change your

program to solve one problem, you break it, and things that

used to work don’t work. And so what you want to do, and

again we’ll come back to this later in the term,

is something called regression testing. This has nothing to do with

linear regression. And that’s basically trying to

make sure our program has not regressed, as to say,

gone backwards in how well it works. And so we always test

it on everything. All right? So I’ve created this function,

let’s give it a shot and see what happens. We’ll run test bi. Whoops! All right, well let’s

look at our answers. I first tested it on the square

root of 4, and in one iteration it found 2. I like that answer. I then tested it on the square

root of 9, and as I mentioned last time, I didn’t find 3. I was not crushed. You know, I was not really

disappointed, it found something close enough

to 3 that I’m happy. All right. I tried it on 2, I surely didn’t

expect a precise and exact answer to that, but I

got something, and if you square this, you’ll find

the answer kept pretty darn close to 2. I then tried it on

0.25 One quarter. And what happened was

not what I wanted. As you’ll see, it crashed. It didn’t really crash, it found

an assert statement. So if you look at the bottom

of the function, you’ll see that, in fact, I checked

for that. I assert the counter is less

than or equal to 0. I’m checking that I didn’t leave

my program because I didn’t find an answer. Well, this is a good thing, it’s

better than my program running forever, but it’s a bad

thing because I don’t have it the square root of 0.25. What went wrong here? Well, let’s think about

it for a second. You look like– someone

looks like they’re dying to give an answer. No, you just scratching

your head? All right. Remember, I said when we do

a bisection method, we’re assuming the answer lies

somewhere between the lower bound and the upper bound. Well, what is the square

root of a quarter? It is a half. Well, what– where did

I tell my program to look for an answer? Between 0 and x. So the problem was, the answer

was over here somewhere, and so I’m never going to find it

cleverly searching in this region, right? So the basic idea was fine, but

I failed to satisfy the initial condition that the

answer had to be between the lower bound and the

upper bound. Right? And why did I do that? Because I forgot what happens

when you look at fractions. So what should I do? Actually I lied, by the way,

when I said the answer was over there. Where was the answer? Somebody? It was over here. Because the square root of a

quarter is not smaller than a quarter, it’s bigger

than a quarter. Right? A half is strictly greater

than a quarter. So it wasn’t on the region. So how– what’s the fix? Should be a pretty simple fix,

in fact we should be able to do it on the fly, here. What should I change? Do I need to change

the lower bound? Is the square root ever going

to be less than 0? Doesn’t need to be, so, what

should I do about the upper bound here? Oh, I could cheat and make, OK,

the upper bound a half, but that wouldn’t

be very honest. What would be a good

thing to do here? Pardon? I could square x, but maybe

I should just do something pretty simple here. Suppose– whoops. Suppose I make it the

max of x and 1. Then if I’m looking for the

square root of something less than 1, I know it will be

in my region, right? All right, let’s save

this, and run it and see what happens. Sure enough, it worked and, did

we get– we got the right answer, 0.5 All right? And by the way, I checked

all of my previous ones, and they work too. All right. Any questions about

bisection search? One of the things I want you to

notice here is the number iterations is certainly

not constant. Yeah, when I will looked at 4,

it was a nice number like 1, 9 looked like it took me 18, 2

took me 14, if we try some big numbers it might take

even longer. These numbers are small, but

sometimes when we look at really harder problems, we got

ourselves in a position where we do care about the number of

iterations, and we care about something called the speed

of convergence. Bisection methods were known to

the ancient Greeks, and it is believed by many, even

to the Babylonians. And as I mentioned last time,

this was the state of the art until the 17th century. At which point, things

got better. So, let’s think about it, and

let’s think about what we’re actually doing when

we solve this. When we look for something like

the square root of x, what we’re really doing,

is solving an equation. We’re looking at the equation

f of guess equals the guess squared minus x. Right, that’s what that is equal

to, and we’re trying to solve the equation that

f of guess equals 0. Looking for the root

of this equation. So if we looked at it

pictorially, what we’ve got here is, we’re looking at f of

x, I’ve plotted it here, and we’re asking where it

crosses the x axis. Sorry for the overloading

of the word x. And I’m looking here at 16. Square root of 16, and my plot

basically shows it crosses at 4 and– well, I think

that’s minus 4. The perspective is tricky–

and so we’re trying to find the roots. Now Isaac Newton and/or Joseph

Raphson figured out how to do this kind of thing for all

differentiable functions. Don’t worry about

what that means. The basic idea is, you take a

guess, and you — whoops — and you find the tangent

of that guess. So let’s say I guessed 3. I look for the tangent

of the curve at 3. All right, so I’ve got the

tangent, and then my next guess is going to be where the

tangent crosses the x axis. So instead of dividing it in

half, I’m using a different method to find the next guess. The utility of this relies upon

the observation that, most of the time– and I want to

emphasize this, most of the time, that implies not all of

the time– the tangent line is a good approximation

to the curve for values near the solution. And therefore, the x intercept

of the tangent will be closer to the right answer than

the current guess. Is that always true,

by the way? Show me a place where that’s

not true, where the tangent line will be really bad. Yeah. Suppose I choose it right

down there, I guess 0. Well, the tangent there will not

even have an x intercept. So I’m really going to

be dead in the water. This is the sort of thing that

people who do numerical programming worry about

all the time. And there are a lot of a little

tricks they use to deal with that, they’ll perturb

it a little bit, things like that. You should not, at

this point, be worrying about those things. This method, interestingly

enough, is actually the method used in most hand calculators. So if you’ve got a calculator

that has a square root button, it’s actually in the calculator running Newton’s method. Now I know you thought it was

going to do that thing you learned in high school for

finding square roots, which I never could quite understand,

but no. It uses Newton’s method

to do it. So how do we find the intercept

of the tangent, the x intercept? Well this is where derivatives

come in. What we know is that the slope

of the tangent is given by the first derivative of the

function f at the point of the guess. So the slope of the guess

is the first derivative. Right. Which dy over dx. Change in y divided

by change in x. So we can use some algebra,

which I won’t go through here, and what we would find is that

for square root, the derivative, written f prime of

the i’th guess is equal to two times the i’th guess. Well, should have left myself

a little more room, sorry about that. All right? You could work this out. Right? The derivative of the

square root is not a complicated thing. Therefore, and here’s the key

thing we need to keep in mind, we’ll know that we can choose

guess i plus 1 to be equal to the old guess, guess i, minus

whatever the value is of the new guess– of the old rather,

the old guess– divided by twice the old guess. All right, again this is

straightforward kind of algebraic manipulations

to get here. So let’s look at an example. Suppose we start looking for

the square root of 16 with the guess 3. What’s the value of the

function f of 3? Well, it’s going to be, we

looked at our function there, guess squared, 3 times 3 is 9 I

think, minus 16, that’s what x is in this case, which

equals minus 7. That being the case, what’s

my next guess? Well I start with my old guess,

3, minus f of my old guess, which is minus 7, divided

by twice my old guess, which is 6, minus the minus,

and I get as my new guess 4.1666 or thereabouts. So you can see I’ve missed,

but I am closer. And then I would reiterate this

process using that as guess i, and do it again. One way to think about this

intuitively, if the derivative is very large, the function

is changing quickly, and therefore we want to

take small steps. All right. If the derivative is small, it’s

not changing, maybe want to take a larger step,

but let’s not worry about that, all right? Does this method work

all the time? Well, we already saw no, if my

initial guess is zero, I don’t get anywhere. In fact, my program crashes

because I end up trying to divide by zero, a really

bad thing. Hint: if you implement Newton’s

method, do not make your first guess zero. All right, so let’s look

at the code for that. All right so– yeah, how do I

get to the code for that? That’s interesting. All right. So we have that square

root NR. NR for Newton Raphson. First thing I want you to

observe is its specification is identical to the

specification of square root bi. What that’s telling me is that

if you’re a user of this, you don’t care how it’s

implemented, you care what it does. And therefore, it’s fine that

the specifications are identical, in fact it’s a good

thing, so that means if someday Professor Grimson

invents something that’s better than Newton Raphson, we

can all re-implement our square root functions and none

of the programs that use it will have to change,

as long as the specification is the same. All right, so, not much

to see about this. As I said, the specifications is

the same, same assertions, and the– it’s basically the

same program as the one we were just looking at, but I’m

starting with a different guess, in this case x over 2,

well I’m going to, couple of different guesses we can start

with, we can experiment with different guesses and see

whether we get the same answer, and in fact, if we did,

we would see we didn’t get this, we got different

answers, but correct answers. Actually now, we’ll just

comment that out. I’m going to compute the

difference, just as I did on the board, and off we’ll go. All right. Now, let’s try and compare

these things. And what we’re going to look at

is another procedure, you have the code for these things

on your handout so we won’t worry, don’t need to show you

the code, but let’s look at how we’re going to test it. I’m doing a little trick by the

way, I’m using raw input in my function here, as a just

a way to stop the display. This way I can torture

you between tests by asking you questions. Making it stop. All right, so, we’ll

try some things. We’ll see what it does. Starting with that, well, let’s

look at some of the things it will do. Yeah, I’ll save it.. It’s a little bit annoying, but

it makes the font bigger. All right, so we’ve tested it,

and we haven’t tested it yet, we have tested it but, we

haven’t seen it, well, you know what I’m going to do? I’m going to tort– I’m going to make the font

smaller so we can see more. Sorry about this. Those of you in the back, feel

free to move forward. All right. So we’ve got it, now

let’s test it. So we’re going to do

here, we’re going to run compare methods. Well we’re seeing this famous

computers are no damn good. All right. So we’re going to try it on 2,

and at least we’ll notice for 2, that the bisection method

took eight iterations, the Newton Raphson only took three, so it was more efficient. They came up with slightly

different answers, but both answers are within .01 which

is what I gave it here for epsilon, so we’re OK. So even though they have

different answers, they both satisfy the same specification, so we have no problem. All right? Try it again, just for fun. I gave it here a different

epsilon, and you’ll note, we get different answers. Again, that’s OK. Notice here, when I asked for

a more precise answer, bisection took a lot more

iterations, but Newton Raphson took only one extra iteration to

get that extra precision in the answer. So we’re sort of getting the

notion that Newton Raphson maybe is considerably better on

harder problems. Which, by the way, it is. We’ll make it an even harder

problem, by making it looking an even smaller epsilon, and

again, what you’ll see is, Newton Raphson just crept up by

one, didn’t take it long, and got the better answer,

where bisection gets worse and worse. So as you can see, as we

escalate the problem difficulty, the difference

between the good method and the not quite as good

method gets bigger and bigger and bigger. That’s an important observation,

and as we get to the part of the course, we

talk about computational complexity, you’ll see that what

we really care about is not how efficient the program

is on easy problems, but how efficient it is on hard problems. All right. Look at another example. All right, here I gave it

a big number, 123456789. And again, I don’t want to

bore you, but you can see what’s going on here

with this trend. So here’s an interesting

question. You may notice that it’s always

printing out the same number of digits. Why should this be? If you look at it here,

what’s going on? Something very peculiar

is happening here. We’re looking at it, and we’re

getting some funny answers. This gets back to what I talked

about before, about some of the precision of

floating point numbers. And the thing I’m trying to

drive home to you here is perhaps the most important

lesson we’ll talk about all semester. Which is, answers

can be wrong. People tend to think, because

the computer says it’s so, it must be so. That the computer is–

speaks for God. And therefore it’s infallible. Maybe it speaks for the Pope. It speaks for something

that’s infallible. But in fact, it is not. And so, something I find myself

repeating over and over again to myself, to my graduate

students, is, when you get an answer from the

computer, always ask yourself, why do I believe it? Do I think it’s the

right answer? Because it isn’t necessarily. So if we look at what we’ve got

here, we’ve got something rather peculiar, right? What’s peculiar about what

this computer is now printing for us? Why should I be really

suspicious about what I see in the screen here? STUDENT: [INAUDIBLE] PROFESSOR JOHN GUTTAG: Well, not

only is it different, it’s really different, right? If it were just a little bit

different, I could say, all right, I have a different

approximation. But when it’s this different,

something is wrong. Right? We’ll, later in the term when

we get to more detailed numerical things, look

at what’s wrong. You can run into issues of

things like overflow, underflow, with floating point

numbers, and when you see a whole bunches of ones, it’s

particularly a good time to be suspicious. Anyway the only point I’m making

here is, paranoia is a healthy human trait. All right. We can look at some other things

which will work better. And we’ll now move on. OK. So we’ve looked at how to

solve square root we’ve, looked at two problems, I’ve

tried to instill in you this sense of paranoia which is so

valuable, and now we’re going to pull back and return to

something much simpler than numbers, and that’s Python. All right? Numbers are hard. That’s why we teach whole

semesters worth of courses in number theory. Python it’s easy, which is why

we do it in about four weeks. All right. I want to return to some

non-scalar types. So we’ve been looking, the last

couple of lectures, at floating point numbers

and integers. We’ve looked so far really

at two non-scalar types. And those were tuples written

with parentheses, and strings. The key thing about

both of them is that they were immutable. And I responded to at least one

email about this issue, someone quite correctly said

tuple are immutable, how can I change one? My answer is, you can’t change

one, but you can create a new one that is almost like the

old one but different in a little bit. Well now we’re going to talk

about some mutable types. Things you can change. And we’re going to start with

one that you, many of you, have already bumped

into, perhaps by accident, which are lists. Lists differ from strings in two

ways; one way is that it’s mutable, the other way is that

the values need not be characters. They can be numbers, they can

be characters, they can be strings, they can even

be other lists. So let’s look at some

examples here. What we’ll do, is we’ll work

on two boards at once. So I could write a statement

like, techs, a variable, is equal to the list, written with

the square brace, not a parenthesis, MIT, Cal

Tech, closed brace. What that basically does, is it

takes the variable techs, and it now makes it point to a

list with two items in it. One is the string MIT and one

is the string Cal Tech. So let’s look at it. And we’ll now run another little

test program, show lists, and I printed it,

and it prints the list MIT, Cal Tech. Now suppose I introduce a new

variable, we’ll call it ivys, and we say that is equal to the

list Harvard, Yale, Brown. Three of the ivy league

colleges. What that does is, I have a new

variable, ivys, and it’s now pointing to another, what we

call object, in Python and Java, and many other languages,

think of these things that are sitting

there in memory somewhere as objects. And I won’t write it all out,

I’ll just write it’s got Harvard as one in it, and

then it’s got Yale, and then it’s got Brown. And I can now print ivys. And it sure enough prints what

we expected it to print. Now, let’s say I have univs, for

universities, equals the empty list. That would create

something over here called univs, another variable, and it

will point to the list, an object that contains

nothing in it. This is not the same as none. It’s it does have a value, it

just happens to be the list that has nothing in it. And the next thing I’m

going to write is univs dot append tex. What is this going to do? It’s going to take this list and

add to it something else. Let’s look at the code. I’m going to print it, and

let’s see what it prints. It’s kind of interesting. Whoops. Why did it do that? That’s not what I expected. It’s going to print that. The reason it printed that is

I accidentally had my finger on the control key, which said

print the last thing you had. Why does it start with square

braced square brace? I take it– yes, go ahead. STUDENT: So you’re adding

a list to a list? PROFESSOR JOHN GUTTAG: So I’m

adding a list to a list. What have I– what I’ve appended to

the empty list is not the elements MIT and Cal Tech

but the list that contains those elements. So I’ve appended this

whole object. Since that object is itself

a list, what I get is a list of lists. Now I should mention this

notation here append is what is in Python called a method. Now we’ll hear lots more about

methods when we get to classes and inheritance, but really, a

method is just a fancy word for a function with

different syntax. Think of this as a function that

takes two arguments, the first of which is univs and the

second of which is techs. And this is just a different

syntax for writing that function call. Later in the term, we’ll see

why we have this syntax and why it wasn’t just a totally

arbitrary brain-dead decision by the designers of Python,

and many languages before Python, but in fact is a

pretty sensible thing. But for now, think of this as

just another way to write a function call. All right, people

with me so far? Now let’s say we wanted as the

next thing we’ll do, is we’re going to append the ivys

to univ. Stick another list on it. All right. So we’ll do that, and now we get

MIT, Cal Tech, followed by that list followed by the list

Harvard, Yale, Brown. So now we have a list containing

two lists. What are we going to try next? Well just to see what we know

what we’re doing, let’s look at this code here. I’ve written a little for

loop, which is going to iterate over all of the elements

in the list. So remember, before we wrote things

like for i in range 10, which iterated over a list or

tuple of numbers, here you can iterate over any list, and so

we’re going to just going to take the list called univs

and iterate over it. So the first thing we’ll do is,

we’ll print the element, in this case it will

be a list, right? Because it’s a list with

two lists in it. Then the next thing in the loop,

we’re going to enter a nested loop, and say for every

college in the list e, we’re going to print the name

of the college. So now if we look what we get–

do you not want to try and execute that?– it’ll

first print the list containing MIT and Cal Tech,

and then separately the strings MIT and Cal Tech, and

then the list containing Harvard, Yale, and Brown, and

then the strings Harvard, Yale, and Brown. So we’re beginning to see this

is a pretty powerful notion, these lists, and that

we can do a lot of interesting things with them. Suppose I don’t want all of this

structure, and I want to do what’s called flattening the

list. Well I can do that by, instead of using the

method append, use the concatenation operator. So I can concatenate techs

plus ivys and assign that result to univs, and then when

I print it you’ll notice I just get a list of

five strings. So plus and append do very

different things. Append sticks the list on the

end of the list, append flattens it, one level

of course. If I had lists of lists of

lists, then it would only take out the first level of it. OK, very quiet here. Any questions about

any of this? All right. Because we’re about to get

to the hard part Sigh. All right. Let’s look at the– well,

suppose I want to, quite understandably, eliminate

Harvard. All right, I then get

down here, where I’m going to remove it. So this is again another method,

this is remove, takes two arguments, the first

is ivys, the second is the string Harvard. It’s going to search through the

list until the first time it finds Harvard and then it’s

going to yank it away. So what happened here? Did I jump to the wrong place? STUDENT: You hit two returns. PROFESSOR JOHN GUTTAG:

I hit two returns. Pardon? STUDENT: You hit two returns. One was at STUDENT: Pardo PROFESSOR JOHN GUTTAG:

This one. STUDENT: No, up one. PROFESSOR JOHN GUTTAG: Up one. STUDENT: Right. PROFESSOR JOHN GUTTAG: But

why is Harvard there? STUDENT: I’m sorry, I didn’t

write it down. PROFESSOR JOHN GUTTAG: Let’s

look at it again. All right, it’s time to

interrupt the world, and we’ll just type into the shell. Let’s see what we get here. All right, so let’s just see

what we got, we’ll print univs. Nope, not defined. All right, well let’s do a list

equals, and we’ll put some interesting things in it,

we’ll put a number in it, because we can put a number,

we’ll put MIT in it, because we can put strings, we’ll put

another number in it, 3.3, because we can put floating

points, we can put all sorts of things in this list. We can

put a list in the list again, as we’ve seen before. So let’s put the list containing

the string a, and I’ll print out, so now we see

something pretty interesting about a list, that we can mix up

all sorts of things in it, and that’s OK. You’ll notice I have the string

with the number 1, a string with MIT, and then it

just a plain old number, not a string, again it didn’t quite

give me 3.3 for reasons we’ve talked before, and now

it in the list a. So, suppose I want to

remove something. What should we try and remove

from this list? Anybody want to vote? Pardon? All right, someone wants

to remove MIT. Sad but true. Now what do we get

if we print l? MIT is gone. How do I talk about the

different pieces of l? Well I can do this. l sub 0–

whoops– will give me the first element of the list,

just as we could do with strings, and I can look at l

sub minus 1 to get the last element of the list, so I can

do all the strings, all the things that I could

do with strings. It’s extremely powerful,

but what we haven’t seen yet is mutation. Well, we have seen

mutation, right? Because notice that what remove

did, it was it actually changed the list. Didn’t create

a new list. The old l is still there, but it’s

different than it used to be. So this is very different from

what we did with slicing, where we got a new copy

of something. Here we took the old one

and we just changed it. On Thursday, we’ll look at why

that allows you to do lots of things more conveniently than

you can do without mutation.