Practice English Speaking&Listening with: Lec 12 | MIT 6.00 Introduction to Computer Science and Programming, Fall 2008

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PROFESSOR: I want to take a few minutes at the start of

today's lecture to wrap up a few more

things about debugging.

And then I'll move on to the main event.

Just a few small points.

First of all, keep in mind that the bug is probably not

where you think it is.

Because if a bug were there, you'd have already found it.

There are some simple things that you should always look at

when you're looking for bugs.

Reversed order of arguments.

You might have a function that takes to floats and you just

passed them in the wrong order.

You may have noticed that when Professor Grimson and I used

examples in class, we were pretty careful how we name our


And I often end up using the same names for the actuals and

the formals.

And this just helps me not get confused about what order to

do things in.


These are just dumb little things to look for.

Did you spell all the identifiers the way

you think you did.

The problem is, when you read code you see what

you expect to see.

And if you've typed in l when you should've typed a 1, or a

1 when you should've typed an l, you're likely to miss it.

If you've made a mistake with upper and lower case.

Things like that.

So just be very careful looking at that.


A very common bug is to initialize a variable.

Go through a loop, and then forget to reinitialize it when

it needs to be reinitialized again.

So it's initialized outside the loop when it should be

initialized inside the loop.

Or conversely, inside the loop when it should

be outside the loop.

So look carefully at when variables are being


Object versus value equality.

Particularly when you use double equals, are you asking

whether you've got the same object or the same value?

They mean very different things, as we have seen.

Keep track of that.

Related to that is aliasing.

And you'll see that, by the way, on the problem set that

we're posting tomorrow.

Where we're going to ask you to just look at some code,

that there's some issues there with the fact that you alias

things, maybe on purpose, maybe on accident.

And by that I mean two different ways to get to the

same value, the same object.

Frequently that introduces bugs into the program.

A particular instance of that is deep versus shallow copy.

When you decide to make a copy of something like a list, you

have to think hard about are you only copying the top level

of the list, but if it's, say, a list that contains a list,

are you also getting a new copy of

everything it contains?

It doesn't matter if it's a list of integers or a list of

strings or a list of tuples, but it matters a lot if it's a

list of lists.

Or a list of anything that could be mutable.

So when you copy, what are you getting?

When you copy a dictionary, for example.

Think through all of that.

And again, a related problem that people run into is


You call a function and it returns a value, but maybe it,

on purpose or on accident, modifies one of the actual


So you could each make a list of things.

Every experienced programmer over time develops a personal

model of the mistakes they make.

I know the kinds of things I get wrong.

And so when I'm looking for a bug, I always say, oh, have

you done this dumb thing you always do.

So be a little bit introspective.

Keep track of the mistakes you make.

And if your program doesn't work, that should be your

first guess.

Couple of other hints.

Keep a record of what you've tried.

People will look at things and they'll come in and that'll

talk to a TA, and the TA will say, did you try this?

And the person will say, I don't know.

That leads you to end up doing the same thing

over and over again.

This gets back to my main theme of Tuesday, which is be


Know what you've already tried, don't waste your time

trying it again.

Think about reconsidering your assumptions.

A huge one is, are you actually running the code

you're looking at in your editor.

This is a mistake I make all the time in Python.

I sit there in idle, and I edit some code.

I then click on the shell, rather than hitting F5, try

and run it and say, it's not doing what I

thought I should do.

And I'll sit there staring at the output, staring at the

code, not realizing that that output was not

produced by that code.

It's a mistake I've learned that I make.

It's an easy one to make in Python.

Lots of assumptions like that.

You thought you knew what the built-in function sort did,

method sort did.

Well, does it do what you think it does?

Does append do what you think it does?

Go back and say, alright, I've obviously, some

assumption is not right.

When you're debugging somebody else's code, debug the code,

not the comments.

I've often made the mistake of believing the comments

somebody wrote in your code about what some function does.

Sometimes it's good to believe it.

But sometimes you have to read it.

Important thing.

When it gets really tough, get help.

Swallow your pride.

Any one of my graduate students will tell you that

from time to time I walk into their office and say, do you

have a minute?

And if they foolishly say yes, I drag them back to my office

and say I'm stuck on this bug, what am I doing wrong?

And it's amazing what -- well, a, they're smarter than I am

which helps.

But even if they weren't smarter than I am, just a

fresh set of eyes.

I've read through the same thing twenty times and I've

missed something obvious.

Someone who's never seen it before looks at and says, did

you really mean to do this?

Are you sure you didn't want to do for i in range of list,

rather than for i in list?

That sort of thing.

Makes a big difference to just get that set of eyes.

And in particular, try and explain to somebody else what

you think the program is doing.

So I'll sit there with the student and I'll say, I'm

going to try and explain to you why what I think this

program is doing.

And probably 80% of the time, halfway through my

explanation, I say, oh, never mind.

I'm embarrassed.

And I send them away.

Because just the act of explaining it to him or her

has helped me understand it.

So when you're really stuck, just get somebody.

And try and explain to them why you think your program is

doing what it's doing.

Another good thing to do when you're really

stuck is walk away.

Take a break.

Come back and look at it with fresh eyes of your own.

Effectively, what you're doing here is perhaps trading

latency for efficiency.

It may, if you go back, come back and two hours later,

maybe you'll, it'll take you at least two hours to have

found the bugs, because you've been away for two hours.

And maybe if you'd stayed there and worked really hard

you'd have found it an hour and fifty eight minutes.

But is it really worth the two minutes?

This is another reason, by the way, to start it

long before it's due.

That you actually have the leisure to walk away.

All right.

What do you do when you've found the bug and

you need to fix it?

Remember the old saw, haste makes waste.

I don't know this but I'll bet Benjamin Franklin said this.

Don't rush into anything.

Think about the fix, don't make the first change that

comes to mind and see if it works.

Ask yourself, will it fix all of the symptoms you've seen?

Or if not, are the symptoms independent, will at least fix

some of them?

What are the ramifications of the proposed change.

Will it break other things?

That's a big issue.

You can fix one thing, you break something else.

So think through what this change might break.

Does it allow you to tidy up other things?

This is important, I think, that code

should not always grow.

We all have a tendency to fix code by adding code.

And the program just gets bigger and bigger and bigger.

The more code you have, the harder it is to get it right.

So, sometimes, what you need to, so you just pull back.

And say, well, let me just tidy things up.

That, by the way, is also a good debugging tool.

Sometimes when I'm really stuck, I say, alright let me

stop looking for the bug.

Let me just clean up the code a little bit.

Make my program prettier.

And in the process of tidying it up and making it prettier,

I'll often by accident find the bug.

So it's a good trick to remember.

Finally, and this is very important, make sure that you

can revert.

There's nothing more frustrating then spending four

hours debugging, and realizing at the end of four hours your

program is worse than it was when you started.

And you can't get back to where it was when you started.

So if you look at one of my directories, you'll find that

I've been pretty anal about saving old versions.

I can always pretty much get back to some place I've been.

So if I've made a set of changes and I realize I've

broken something that used to work, I can find a version of

the code in which it used to work, and figure out what was

going on there.

Disk space is cheap.

Don't hesitate to save your old versions.

It's a good thing.

Alright, that's the end of my, maybe sermon is the right word

on debugging.

Polemic, I don't know.

I hope it will be helpful.

And I hope you'll remember some of these things as you

try and get your programs to work.

I now want to get back to algorithms. And where we've

sort of been for a while, and where we

will stay for a while.

More fundamentally, I want to get back to what I think of as

the main theme of 6.00, which is taking a problem and

finding some way to formulate the problem so that we can use

computing to help us get an answer.

And in particular, we're going to spend the next few lectures

looking at a class of problems known as optimization

problems. In general, every optimization

problem is the same.

It has two parts.

Some function that you're either attempting to maximize

or minimize the value of.

And these are duals.

And some set of constraints that must be honored.

Possibly an empty set of constraints.

So what are some of the classic optimization problems?

Well one of the most well-known is the so-called

shortest path problem.

Probably almost every one of you has used a

shortest path algorithm.

For example, if you go to Mapquest, or Google Maps and

ask how do I get from here to there.

You give it the function, probably in this case to

minimize, and it gives you a choice of functions.

Minimize time, minimize distance.

And maybe you give it a set of constraints.

I don't want to drive on highways.

And it tries to find the shortest way, subject to those

constraints, to get from Point A to Point B. And there are

many, many other instances of this kind of thing.

And tomorrow it's recitation, we'll spend quite a bit of

time on shortest path problems.

Another classic optimization problem is

the traveling salesman.

Actually, I should probably, be modern, call it the

traveling salesperson, the traveling salesperson problem.

So the problem here, roughly, is given, a number of cities,

and say the cost of traveling from city to city by airplane,

what's the least cost round trip that you can find?

So you start at one place, you have to go to a number of

other places.

End up where you started.

It's not quite the same as the shortest path, and figure out

the way to do that that involves

spending the least money.

Or the least time, or something else.

What are some of the other classic optimization problems?

There's bin packing.

Filling up some container with objects of varying size and,

perhaps, shape.

So you've got the trunk of your car and you've got a

bunch of luggage.

More luggage than can actually fit in.

And you're trying to figure out what order

to put things in.

And which ones you can put in.

How to fill up that bin.

Very important in shipping.

People use bin packing algorithms to figure out, for

example, how to load up container ships.

Things of that nature.

Moving vans, all sorts of things of that nature.

In biology and in natural language processing and many

other things, we see a lot of sequence alignment problems.

For example, aligning DNA sequences, or RNA sequences.

And, one we'll spend a fair amount of time on today and

Tuesday is the knapsack problem.

In the old days people used to call backpacks knapsacks.

So we old folks sometimes even still make that mistake.

And the problem there is, you've got a bunch of things.

More than will fit into the knapsack, and you're trying to

figure out which things to take and which things to

leave. As you plan your hiking trip.

How much water should you take.

How many blankets?

How much food?

And you're trying to optimize the value of the objects you

can take subject to the constraint that the backpack

is of finite size.

Now, why am I telling you all of this at

this lightning speed?

It's because I want you to think about it, going forward,

about the issue of problem reduction.

We'll come back to this.

What this basically means is, you're given some problem to

solve, that you've never seen before.

And the first thing you do is ask is it an instance of some

problem that other people have already solved?

So when the folks at Mapquest sat down to do their program,

I guarantee you somebody opened an algorithms book and

said, what have other people done to solve

shortest path problems?

I'll rely on fifty years of clever people rather than

trying to invent my own.

And so frequently what we try and do is, we take a new

problem and map it onto an old problem so that we can use an

old solution.

In order to be able to do that, it's nice to have in our

heads an inventory of previously solved problems. To

which we can reduce the current problem.

So as we go through this semester, we'll look at,

briefly or not so briefly, different previously solved

problems in the hope that at some time in your future, when

you have a problem to deal with, you'll say, I know

that's really like shortest path, or

really like graph coloring.

Let me just take my problem and turn it into that problem,

and use an existing solution.

So we'll start looking in detail at one problem, and

that's the knapsack problem.

Let's see.

Where do I want to start?

Oh yes, OK.

So far, we've been looking at problems that

have pretty fast solutions.

Most optimization problems do not have fast solutions.

That is to say, when you're dealing with a large set of

things, it takes a while to get the right answer.

Consequently, you have to be clever about it.

Typically up till now, we've looked at things that can be

done in sublinear time.

Or, at worst, polynomial time.

We'll now look at a problem that does not fall into that.

And we'll start with what's called the

continuous knapsack problem.

So here's the classic formulation.

Assume that you are a burglar.

And you have a backpack that holds, say, eight pounds'

worth of stuff.

And you've now broken into a house and you're trying to

decide what to take.

Well, let's assume in the continuous world, what you is

you walk into the house and you see something like four

pounds of gold dust. And you see three pounds of silver

dust, and maybe ten pounds of raisins.

And I don't actually know the periodic

table entry for raisins.

So I'll have to write it out.

Well, how would you solve this problem?

First, let's say, what is the problem?

How can we formulate it?

Well, let's assume that what we want to do is, we have

something we want to optimize.

So we're looking for a function to

maximize, in this case.

What might that function be?

Well, let's say it's some number of, some amount of, the

cost of the value of gold.

Times however many pounds of gold.

Plus the cost of silver times however many - no, gold, is a

g, isn't it.

Pounds of silver, plus the cost of raisins times the

number of pounds of raisins.

So that's the function I want to optimize.

I want to maximize that function.

And the constraint is what?

It's that the pounds of gold plus the pounds of silver plus

the pounds of raisins is no greater than eight.

So I've got a function to maximize and a constraint that

must be obeyed.

Now, the strategy here is pretty clear.

As often is for the continuous problem.

What's the strategy?

I pour in the gold till I run out of gold.

I pour in the silver until I run out of silver.

And then I take as many raisins as will fit in and I

leave. Right?

I hope almost every one of you could figure out that was the

right strategy.

If not, you're not well suited to a life of crime.

What I just described is an

instance of a greedy algorithm.

In a greedy algorithm, at every step you do what

maximizes your value at that step.

So there's no planning ahead.

You just do what's ever best. It's like when someone gets

their food and they start by eating dessert.

Just to make sure they get to the best part

before they're full.

In this case, a greedy algorithm actually gives us

the best possible solution.

That's not always so.

Now, you've actually all

implemented a greedy algorithm.

Or are in the process thereof.

Where have we implemented a greedy algorithm, or have been

asked to do a greedy algorithm?

Well, there are not that many things you guys have been

working on this semester.



PROFESSOR: Exactly right.

So what you were doing there, it was a really good throw.

But it was a really good answer you gave. So I'll

forgive you the bad hands.

You were asked to choose the word that gave

you the maximum value.

And then do it again with whatever letters you had left.

Was that guaranteed to win?

To give you the best possible scores?


Suppose, for example, you had the letters this, doglets.

Well, the highest scoring word might have been something like

Doges, these guys used to rule Venice, but if you did that

you'd been left with the letters l and t, which are

kind of hard to use.

So you've optimized the first step.

But now you're stuck with something

that's not very useful.

Whereas in fact, maybe you would have been better to go

with dog, dogs, and let.

So what we see here is an example of something very

important and quite general.

Which was that locally optimal decisions do not always lead

to a global optimums. So you can't just repeatedly do the

apparently local thing and expect to

necessarily get to it.

Now, as it happens with the continuous knapsack problem as

we've formulated it, greedy is good.

But let's look for a slight variant of it, where greedy is

not so good.

And that's what's called the zero-one knapsack problem.

This is basically a discrete version of

the knapsack problem.

The formulation is that we have n items and at every step

we have to either take the whole item

or none of the item.

In the continuous problem, the gold dust was assumed to be

infinitely small.

And so you could take as much of it as you wanted.

Here it's as if you had gold bricks.

You get to take the whole brick or no brick at all.

Each item has a weight and a value, and we're trying to

optimize it as before.

So let's look at an example of a zero-one knapsack problem.

Again we'll go back to our burglar.

So the burglar breaks into the house and finds the following

items available.

And you'll see in your handout a list of items and their

value and how much, what they weight.

Finds a watch, a nice Bose radio, a beautiful Tiffany

vase, and a large velvet Elvis.

And now this burglar finds, in fact, two of each of those.

Person is a real velvet Elvis fan and needed two

copies of this one.

Alright, and now he's trying to decide what to take.

Well if the knapsack were large enough the thief would

take it all and run, but let's assume that it can only hold

eight pounds, as before.

And therefore the thief has choices to make.

Well, there are three types of thieves I want to consider:

the greedy thief, the slow thief, and you.

We'll start with the greedy thief.

Well, the greedy thief follows the greedy algorithm.

What do you get if you follow the greedy algorithm?

What's the first thing the thief does?

Takes the most valuable item, which is a watch.

And then what does he do after that?

Takes another watch.

And then?



PROFESSOR: And then?



Not unless he wants to break the vase into little pieces

and stuff it in the corners.

The backpack is now full, right?

There's no more room.

So the greedy thief take that and leaves.

But it's not an optimal solution.

What should the thief have done?

What's the best thing you can do?

Instead of taking that one vase, the thief

could take two radios.

And get more value.

So the greedy thief, in some sense, gets the wrong answer.

But maybe isn't so dumb.

While greedy algorithms are not guaranteed to get you the

right answer all the time, they're often

very good to use.

And what they're good about is, they're easy to implement.

And they're fast to run.

You can imagine coding the solution up

and it's pretty easy.

And when it runs, it's pretty fast. Just takes the most

valuable, the next most valuable, the next most

valuable, I'm done.

And the thief leaves, and is gone.

So that's a good thing.

On the other hand, it's often the case in the world that

that's not good enough.

And we're not looking for an OK solution, but we're looking

for the best solution.

Optimal means best. And that's what the slow thief does.

So the slow thief thinks the following.

Well, what I'll do is I'll put stuff in the

backpack until it's full.

I'll compute its value.

Then I'll empty the backpack out, put another combination

of stuff compute its value, try all possible ways of

filling up the backpack, and then when I'm done, I'll know

which was the best. And that's the one I'll do.

So he's packing and unpacking, packing and unpacking, trying

all possible combinations of objects that will obey the


And then choosing the winner.

Well, this is like an algorithm we've seen before.

It's not greedy.

What is this?

What category of algorithm is that?



STUDENT: Brute force.

PROFESSOR: Brute force, exhaustive

enumeration, exactly.

We're exhausting all possibilities.

And then choosing the winner.

Well, that's what the slow thief tried.

Unfortunately it took so long that before he finished the

owner returned home, called the police and the thief ended

up in jail.

It happens.

Fortunately, while sitting in jail awaiting trial, the slow

thief decided to figure what was wrong.

And, amazingly enough, he had studied mathematics.

And had a blackboard in the cell.

So he was able to work it out.

So he first said, well, let me try and figure out what I was

really doing and why it took so long.

So first, let's think about what was the function the slow

thief was attempting to maximize.

The summation, from i equals 1 to n, where n is the number of

items, so I might label watch 1-0, watch 2-2, I don't care

that they're both watches.

They're two separate items. And then what I want to

maximize is the sum of the price of item i times whether

or not I took x i.

So think of x as a vector of 0's and 1's.

Hence the name of the problem.

If I'm going to keep that item, item i, if I'm going to

take it, I give it a 1.

If I'm not going to take it I give it a 0.

And so I just take the value of that item times whether or

not it's 0 or 1.

So my goal here is to choose x such that this is maximized.

Choose x such that that function is maximized, subject

to a constraint.

And the constraint, is it the sum from 1 to n of the weight

of the item, times x i, is less than or equal to c, the

maximum weight I'm allowed to put in my backpack.

In this case it was eight.

So now I have a nice, tidy mathematical formulation of

the problem.

And that's often the first step in problem reduction.

Is to go from a problem that has a bunch of words, and try

and write it down as a nice, tight mathematical


So now I know the formulation is to find x, the vector x,

such that this constraint is obeyed and

this function is maximized.

That make sense to everybody?

Any questions about that?


So as the thief had thought, we can clearly solve this

problem by generating all possible values of x and

seeing which one solves this problem.

But now the thief started scratching his head and said,

well, how many possible values of x are there?

Well, how can we think about that?

Well, a nice way to think about that is to

say, I've got a vector.

So there's the vector with eight 0's in it.

Three, four, five, six, seven, eight.

Indicating I didn't take any of the items.

There's the vector, and again you'll see this in your

handout, says I only took the first item.

There's the one that says I only took the second item.

There's the one that says I took the first

and the second item.

And at the last, I have all 1's.

This series look like anything familiar to you?

These are binary numbers, right?

Eight digit binary numbers.

Zero, one, two, three.

What's the biggest number I can represent

with eight of these?


Well, suppose we had decimal numbers.

And I said I'm giving you three decimal digits.

What's the biggest number you can represent with three

decimal digits?




Which is roughly what?

10 to the?






Exactly right.

So, in this case it's 2 to the 8th.

Because I have eight digits.

But exactly right.

More generally, it's 2 to the n.

Where n is the number of possible items I have to

choose from.

Well, now that we've figured that out, what we're seeing is

that the brute force algorithm is exponential.

In the number of items we have to choose from.

Not in the number that we take, but the number we have

to think about taking.

Exponential growth is a scary thing.

So now we can look at these two graphs,

look at the top one.

So there, we've compared n squared, quadratic growth,

which by the way Professor Grimson told you was bad, to,

really bad, which is exponential growth.

And in fact, if you look at the top figure it looks as

exponential or, quadratic isn't even growing at all.

You see how really fast exponential growth is?

You get to fifteen items and we're up at seventy thousand

already and counting.

The bottom graph has exactly the same data.

But what I've done is, I've use the logarithmic y-axis.

Later in the term, we'll spend quite a lot of time talking

about how do we visualize data.

How do we make sense of data.

I've done that because you can see here that the quadratic

one is actually growing.

It's just growing a lot more slowly.

So the moral here is simple one.

Exponential algorithms are typically not useful. n does

not have to get very big for exponential to fail.

Now, imagine that you're trying to pack a ship and

you've got ten thousand items to choose from.

2 to the 10,000 is a really big number.

So what we see immediately, and the slow thief decided

just before being incarcerated for years and years, was that

it wasn't possible to do it that way.

He threw up his hands and said, it's an unsolvable

problem, I should have been greedy, there's no

good way to do this.

That gets us to the smart thief.

Why is this thief smart?

Because she took 600.

And she learned that in fact there is a good way to solve

this problem.

And that's what we're going to talk about next.

And that's something called dynamic programming.

A lot of people think this is a really hard and fancy

concept, and they teach in advanced algorithms classes.

And they do, but in fact as you'll see it's

really pretty simple.

A word of warning.

Don't try and figure out why it's called dynamic


It makes no sense at all.

It was invented by a mathematician called Bellman.

And he was at the time being paid by the Defense Department

to work on something else.

And he didn't want them to know what he was doing.

So he made up a name that he was sure they would have no

clue what it meant.

Unfortunately, we now have lived with it forever, so

don't think of it as actually being anything dynamic


It's just a name.

It's very useful, and why we spend time on it for solving a

broad range of problems that on their surface are

exponentially difficult.

And, in fact, getting very fast solutions to them.

The key thing in dynamic programming, and we'll return

to both of these, is you're looking for a situation where

there are overlapping sub-problems and what's called

optimal substructure.

Don't expect to know what these mean yet.

Hopefully by the end of the day, Tuesday, they will both

make great sense to you.

Let's first look at the overlapping

sub-problems example.

You have on your handout, a recursive

implementation of Fibonacci.

Which we've seen before.

What I've done is I've augmented it with this global

called num calls just so I can keep track of how many times

it gets called.

And let's see what happens if we call fib of 5.

Well, quite a few steps.

What's the thing that you notice about this output?

There's something here that should tip us off that maybe

we're not doing this the most efficient way possible.

I called fib with 5 and then it calls it with 4.

And then that call calls fib with 3.

So I'm doing 4, 3, 2, 2, 1, 0.

And I'm doing 1, 2.

Well, what we see is I'm calling fib a lot of times

with the same argument.

And it makes sense.

Because I start with fib of 5, and then I have to do fib of 4

and fib of 3.

Well, fib of 4 is going to also have to do a fib of 3 and

a fib of 2 and a fib of 1, and a fib of 0.

And then the fib of 3 is going to do a fib of 2 and a fib of

1 and a fib of 0.

And so I'm doing the same thing over and over again.

That's because I have what are called overlapping

sub-problems. I have used divide and conquer, as we seen

before, to recursively break it into smaller problems. But

the smaller problem of fib of 4 and the smaller problem of

fib of 3 overlap with each other.

And that leads to a lot of redundant computation.

And I've done fib of 5, which is a small number.

If we look at some other things, for example, let's get

rid of this.

Let's try see fib of 10.

Well, there's a reason I chose 10 rather than, say, 20.

Here fib got called 177 times.

Roughly speaking, the analysis of fib is actually quite

complex, of a recursive fib.

And I won't go through it, but what you can see is it's more

or less, it is in fact, exponential.

But it's not 2 to the something.

It's a more complicated thing.

But it grows quite quickly.

And the reason it does is because of this overlapping.

On Tuesday we'll talk about a different way to implement

Fibonacci, where the growth will be much less dramatic.

Thank you.

The Description of Lec 12 | MIT 6.00 Introduction to Computer Science and Programming, Fall 2008