Explore
For Enterprise
Join for Free
Log In

Гуманитарные науки и искусства
Бизнес
Компьютерные науки
Наука о данных
Информационные технологии
здоровье
Математика и логика
Личное развитие
Естественные и технические науки
Социальные науки
Изучение языков
Степени
Сертификаты

Полный обзор Coursera

Обзор
Поиск
Для организаций
Войти
Присоединиться бесплатно

Worked Exercise: Lists

Loading...

Структуры данных в Python

Мичиганский университет

4.9 (оценок: 34,517) | Зарегистрировано учащихся: 290K

Курс 2 из 5 — Python для всех Специализация

This course will introduce the core data structures of the Python programming language. We will move past the basics of procedural programming and explore how we can use the Python built-in data structures such as lists, dictionaries, and tuples to perform increasingly complex data analysis. This course will cover Chapters 6-10 of the textbook “Python for Everybody”. This course covers Python 3.

Просмотреть программу курса

Получаемые навыки

Python Syntax And Semantics, Data Structure, Tuple, Python Programming

Рецензии

4.9 (оценок: 34,517)

5 stars
30,453 ratings
4 stars
3,668 ratings
3 stars
313 ratings
2 stars
41 ratings
1 star
42 ratings

ME

Jul 19, 2019

This was a super-fun course! The materials were very easy to follow, and delivered in a very friendly and engaging manner. I highly recommend this course to anyone just getting started in programming.

JL

Feb 01, 2017

Challenging for someone with zero experience with coding. Took a fairly long time to grind through the first two courses in the specialisation, but the lectures and course material were very helpful.

Из урока

Chapter Eight: Lists

As we want to solve more complex problems in Python, we need more powerful variables. Up to now we have been using simple variables to store numbers or strings where we have a single value in a variable. Starting with lists we will store many values in a single variable using an indexing scheme to store, organize, and retrieve different values from within a single variable. We call these multi-valued variables "collections" or "data structures".

Worked Exercise: Lists11:33

Преподаватели

Charles Russell Severance
Professor

Текст видео

Hello and welcome to Python for Everybody.

I'm Charles Severance and now we're going to take a look at how

we would write some code to do some parsing, read some data.

As a matter of fact, we're going to read you our famous mailbox data, look for

lines that begin with From space, and extract the third word.

As a matter of fact, we already have some of this code already written,

we're going to debug it.

We're going to look at code and we're going to debug it.

So here we go, here we have it.

And so, it's a pretty basic program. It opens a file, loops through the file,

throws away the whitespace, splits it into words and checks to see if this

zeroth word, the first word, is From and if it's not we skip and read the next line.

And otherwise, if we find the line that starts with From space, then

we print the third word which is wds sub 2, okay?

So this is what we've got and

we carefully saved this file into the same folder that we've got, ex_08.

And so let's go ahead, cd desktop python for everybody,

ex_08.

And so, this is some files.

We've got our day of the week python and our mbox-short, so

that's sitting there. Okay?

And so let's run this program.

This is the program that we've got right here,

python3 dow.py and it doesn't work.

And, by now you've seen a few tracebacks and there you go.

So, when you look at a traceback you think to yourself, well I made a mistake.

And you've gotten pretty good at looking at that line.

So there you are.

You're like, this is the line.

There must be something wrong on this line.

And you want to change it but that line's not actually the problem in this

particular thing and so you've got to be careful sometimes.

And one of the things that you didn't notice in this one right away is that it

actually worked.

It printed the first line out.

So if we take a look at our dataset, it found the line started with From space, it

split it, and it printed out the third word.

And it blew up later.

And so part of the problem is we don't know what it was doing when it blew up.

And so the first thing I like to do in this kind of a situation is

find the line and make sure there's a print statement right before it.

And some I'm going to print words:, and then ,wds.

I want to print right before the line that blows up.

So that I know really when this finally does blow up what was going on

in that line.

So I'm going to run it again.

And oop, did I forget to save it?

No, I forgot to save it.

Look at that. See the little blue dot?

Forgot to save it.

And so now we see a whole bunch of output.

And we see that it's actually doing a whole of work before it's blowing up.

And so you see that it prints the words out from that first line and

prints out Saturday, which is exactly what we expect.

It's the third word in the line.

And then it reads a whole bunch of stuff.

And what it's doing now is ignoring.

Let me just put something here.

I'm going to say print('Ignore'),

So I can keep track of when these lines are being ignored.

So let's run it again and have the word Ignore pop up, right?

And so it's doing a lot of ignoring.

It prints and finds these words, prints out Saturday, reads this line and ignores it,

reads this line and ignores it, reads this line and ignores it.

So a lot of stuff's going on here that you might not realize.

And so we have to take a look at what the problem is.

So it is now blowing up wds sub 0.

And now we can scroll down and

we can look at exactly what happened right before the traceback.

So we really now know exactly what happened before the traceback.

And the interesting thing is that there is an empty string, I mean, empty array.

There's an array with zero items.

So I'm going to print the line out too, print line:.

Now I haven't changed my program at all.

I'm just trying to figure out what's going on here.

So I'll save that, and then I'm going to run it, and we've got a lot of stuff and

it's still working.

It reads a line, it reads the line, splits it into words, and

then prints out Saturday, which is the third word on the line.

Now, here it reads a line and this line is a blank line.

And because it's a blank line, the split returns no words, and

that's what blows up.

And the problem now is oh, wait a sec, list index out of range.

So wds sub 0 is not valid, which is the first word, when there are no words.

So this is a statement that works most of time.

Now, you might think oh, I want to just put a try and except in there.

Well, the right thing to do is to say to yourself oh, wait a second.

If I don't have enough words, if the length of the words is less than 1, continue.

So basically it's going to come through here, it's going to split it and

if we don't have any words, meaning it's a blank line, then we're going to skip it.

So let's run that.

So now this ran all the way to the end, it did a lot of stuff,

and it did not blow up. Specifically, it didn't have a traceback.

Another way to protect this would be to, let me take this part out.

This is called a guardian pattern.

Right? Guardian pattern, because this, this is dangerous.

This could blow up but this, it won't blow up if it makes it past here and

then it won't come through there under the conditions that are causing it to blow up.

Another way to do this might be to protect it as follows, to say, oh, wait a sec,

if the line is a blank line, no, continue.

So now what we're going to do is we're going to skip blank lines.

I even say this,

print('Skip Blank').

So if it's a blank, we're going to skip blank and keep going.

This will skip blank lines.

It will come through here and this will skip lines that don't have From but

because we're not processing blank lines, wds sub 0 always works.

So I can run this code and it works again.

So, here we have a blank line, we skipped it.

Here we have a blank line, we skipped it.

Now here, we had a non-blank line.

So we parsed it but then we ignored it.

And then up here, we'll find a From somewhere.

Let's find a From, here it comes.

Nope. Ignore, ignore.

I've got too much debug print,

I can't find it.

Here, I'll just hunt for From with find.

Okay, so there we go.

It's From and we print the thing out.

So we're getting a lot of extra stuff.

So I'm going to comment out some of these debugs.

And I'm actually just going to get rid of this whole skipping of the blank line.

I'm going to do it with the wds.

I'm going to go back to the guardian we had before.

If the number of words that we got, len(wds), is less than 1,

Continue. Okay? So now this going to be a working program.

Oops, I've got to take another print statement out.

Got to take another print statement out.

We sort of know what we're doing here.

Okay, so this looks like a pretty safe thing.

This guardian is protecting this dangerous,

I'll get rid of that one too.

This is the word that could, was our traceback, and nothing else in this thing

changed from when we started except we've added this little guardian.

Now, the interesting thing is if it comes through here, and prints wds sub 2,

what happens if somehow we find the line that has From as its first word and

there's only one word on, this is going to blow up.

So we can make our guardian a little stronger.

And we can say, you know what,

we're going to skip this line if it doesn't have three words in it.

So it has to have at least three words.

And if we see less than three words,

we're going to skip it and just makes the guardian a bit stronger.

And so the program works safely.

And you see these things where you sometimes you want to check to see

reasonable that your assumptions about data are reasonable and

skip things where the data is not reasonable.

Now there, so that's one guardian pattern.

Let me show you a slightly different way to do this and

this is with an or statement.

So I'm going to take this code, copy that, and put it here with or.

Get rid of all this stuff.

Well this is the guardian in a compound statement.

So, what we're saying is if there

are less than three words on the line, or if the first word is not From, continue.

Now, we're doing this in order, because the way it works is or

is true if either that's true or this is true.

But if it knows that this is true, then it doesn't bother checking this, and

the checking of this is what blows up, what causes the traceback.

So if we flip this order, it would fail. If we do did in this order, it will work.

So, let's do this one right, it works.

But if I get this backwards,

it's going to check this before it checks this.

And were going to go back to failing again.

So you've got to get the order of these things right.

The guardian comes before in the or.

The guardian comes before.

And if this is true, than it doesn't check this.

This is called short circuit evaluation where it knows that as long as this

part's true, it doesn't evaluate this second part.

And so, now we have a guardian in the compound statement.

You'll see this a lot.

Sometimes if it's more complex, you do it in multiple statements or

you say your fall-through.

Check for sanity, check for sanity, and only run the code.

So, I hope that that was useful to you.

Looking a little bit about how to debug where you just don't start

chopping on the line that had the problem.

It's not always that line because we never did change that line.

Although,we did change it a little bit at the end, we added this guardian here, but

we also fixed it without it.

Sometimes you add some print statements to figure out

what's going on before you just start chopping on that line.

So again, I hope this helps, thanks.

Ознакомьтесь с нашим каталогом

Присоединяйтесь бесплатно и получайте персонализированные рекомендации, обновления и предложения.

Coursera делает лучшее в мире образование доступным каждому, предлагая онлайн-курсы от ведущих университетов и организаций.

© Coursera Inc., 2019 Все права защищены.

Загрузить из App Store

Загрузить в Google Play

Coursera
О проекте
Руководство
Карьера
Каталог
Сертификаты
Сертификаты MasterTrack™
Степени
Для организаций
Для правительственных организаций

Сообщество
Learners
Partners
Developers
Beta Testers
Translators

На связи
Блог
Facebook
LinkedIn
Twitter
Instagram
YouTube
Технический блог

Еще
Условия
Конфиденциальность
Помощь
Доступность
Пресса
Контакты
Справочник
Филиалы