import re
hand = """This file contains the actual data for your assignment - good luck!

8253
Why should you learn to write programs?
5546  7115
Writing programs (or programming) is a very creative
and 5007 rewarding 7198 activity. 8368  You can write programs for
 many reasons, ranging from making your living to solving
a difficult data analysis problem to having fun to helping
someone else solve a problem.  This book assumes that
everyone needs to know how to program, and that once
you know how to program you will figure out what you want
to do with your newfound skills.

We are surrounded in our daily lives with computers ranging
from laptops to cell phones.  We can think of these computers
as our personal assistants who can take care of many things
4925 on our behalf.  The hardware in our current-day computers
is 2403 essentially 6208 built 7296 to continuously ask us the question,
 What would you like me to do next?

Programmers add an operating system and a set of applications
9769 to the hardware and we end up with a Personal Digital
Assistant that is quite helpful and capable of helping
us do many different things.

Our computers are fast and have vast amounts of memory and
could 1211 be 6710 very 1136 helpful to us if we only knew the language to
 speak to explain to the computer what we would like it to
do next.  If we knew this language, we could tell the
computer to do tasks on our behalf that were repetitive.
Interestingly, the kinds of things computers can do best
are often the kinds of things that we humans find boring
and mind-numbing.

For example, look at the first three paragraphs of this
chapter and tell me the most commonly used word and how
many times the word is used.  While you were able to read
and understand the words in a few seconds, counting them
is 3633 almost 3685 painful 6776 because it is not the kind of problem
 that human minds are designed to solve.  For a computer
the opposite is true, reading and understanding text
from a piece of paper is hard for a computer to do
but counting the words and telling you how many times
the most used word was used is very easy for the
8881 computer: 3526

Our personal information analysis assistant quickly
2937 told us that the word to was used sixteen times in the 7745
first three paragraphs of this chapter.

This very fact that computers are good at things
that humans are not is why you need to become
skilled at talking computer language.  Once you learn
this new language, you can delegate mundane tasks
to your partner (the computer), leaving more time
for you to do the
things that you are uniquely suited for.  You bring
creativity, intuition, and inventiveness to this
9051 partnership.

Creativity and motivation

While this book is not intended for professional programmers, professional
programming can be a very rewarding job both financially and personally.
Building useful, elegant, and clever programs for others to use is a very
creative activity.  Your computer or Personal Digital Assistant (PDA)
usually contains many different programs from many different groups of
programmers, each competing for your attention and interest.  They try
their best to meet your needs and give you a great user experience in the
process.   In some situations, when you choose a piece of software, the
programmers are directly compensated because of your choice.

6661 If we think of programs as the creative output of groups of programmers,
perhaps the following figure is a more sensible version of our PDA:

For now, our primary motivation is not to make money or please end users, but
instead for us to be more productive in handling the data and
information that we will encounter in our lives.
When you first start, you will be both the programmer and the end user of
your programs.  As you gain skill as a programmer and
programming feels more creative to you, your thoughts may turn
toward developing programs for others.

Computer hardware architecture
5776
Before we start learning the language we
speak to give instructions to computers to
develop software, we need to learn a small amount about
how computers are built.

Central Processing Unit (or CPU) is
the 5832 part 880 of 4183 the computer that is built to be obsessed
 with what is next?  If your computer is rated
at three Gigahertz, it means that the CPU will ask What next?
three billion times per second.  You are going to have to
learn how to talk fast to keep up with the CPU.

Main Memory is used to store information
that the CPU needs in a hurry.  The main memory is nearly as
fast as the CPU.  But the information stored in the main
memory vanishes when the computer is turned off.

Secondary Memory is also used to store
1953 information, but it is much slower than the main memory.
The advantage of the secondary memory is that it can
store information even when there is no power to the
9316 computer.  Examples of secondary memory are disk drives
or flash memory (typically found in USB sticks and portable
music players).

Input and Output Devices are simply our
screen, keyboard, mouse, microphone, speaker, touchpad, etc.
They are all of the ways we interact with the computer.

These days, most computers also have a
Network Connection to retrieve information over a network.
We can think of the network as a very slow place to store and
retrieve data that might not always be up.  So in a sense,
the 5275 network 9442 is 9022 a slower and at times unreliable form of
 Secondary Memory.

While most of the detail of how these components work is best left
to computer builders, it helps to have some terminology
so 6735 we 1640 can 3560 talk about these different parts as we write our programs.

As a programmer, your job is to use and orchestrate
each of these resources to solve the problem that you need to solve
and analyze the data you get from the solution.  As a programmer you will
mostly be talking to the CPU and telling it what to
do next.  Sometimes you will tell the CPU to use the main memory,
secondary memory, network, or the input/output devices.

7253 You need to be the person who answers the CPU's What next?
question.  But it would be very uncomfortable to shrink you
down to five mm  tall and insert you into the computer just so you
4658 could issue a command three billion times per second.  So instead, 1269
9171 you must write down your instructions in advance. 352
We call these stored instructions a program and the act
of writing these instructions down and getting the instructions to
be correct programming.

Understanding programming

In the rest of this book, we will try to turn you into a person
who is skilled in the art of programming.  In the end you will be a
programmer --- perhaps not a professional programmer, but
at least you will have the skills to look at a data/information
analysis problem and develop a program to solve the problem.

problem solving

1262 In a sense, you need two skills to be a programmer:

First, you need to know the programming language (Python) -
you need to know the vocabulary and the grammar.  You need to be able
to spell the words in this new language properly and know how to construct
well-formed sentences in this new language.

Second, you need to tell a story.  In writing a story,
you combine words and sentences to convey an idea to the reader.
There is a skill and art in constructing the story, and skill in
story 6014 writing 5030 is 5085 improved by doing some writing and getting some
 feedback.  In programming, our program is the story and the
problem you are trying to solve is the idea.

itemize

Once you learn one programming language such as Python, you will
find it much easier to learn a second programming language such
as JavaScript or C++.  The new programming language has very different
vocabulary and grammar but the problem-solving skills
will be the same across all programming languages.

You will learn the vocabulary and sentences of Python pretty quickly.
It will take longer for you to be able to write a coherent program
to solve a brand-new problem.  We teach programming much like we teach
writing.  We start reading and explaining programs, then we write
simple programs, and then we write increasingly complex programs over time.
At some point you get your muse and see the patterns on your own
and can see more naturally how to take a problem and
write a program that solves that problem.  And once you get
to that point, programming becomes a very pleasant and creative process.

We start with the vocabulary and structure of Python programs.  Be patient
as the simple examples remind you of when you started reading for the first
time.

Words and sentences

Unlike human languages, the Python vocabulary is actually pretty small.
We call this vocabulary the reserved words.  These are words that
have very special meaning to Python.  When Python sees these words in
a Python program, they have one and only one meaning to Python.  Later
as you write programs you will make up your own words that have meaning to
you called variables.   You will have great latitude in choosing
your names for your variables, but you cannot use any of Python's
reserved words as a name for a variable.
1767 1912 9989
When we train a dog, we use special words like
sit, stay, and fetch.  When you talk to a dog and
don't use any of the reserved words, they just look at you with a
quizzical look on their face until you say a reserved word.
For example, if you say,
I wish more people would walk to improve their overall health,
what most dogs likely hear is,
blah blah blah walk blah blah blah blah.
2252 That is because walk is a reserved word in dog language. 6925

The reserved words in the language where humans talk to
Python include the following:

and       del       from      not       while
as        elif      global    or        with
assert    else      if        pass      yield
break     except    import    print
class     exec      in        raise
continue  finally   is        return
def       for       lambda    try

That is it, and unlike a dog, Python is already completely trained.
When you say try, Python will try every time you say it without
fail.

1662 We will learn these reserved words and how they are used in good time,
but for now we will focus on the Python equivalent of speak (in
human-to-dog language).  The nice thing about telling Python to speak
is that we can even tell it what to say by giving it a message in quotes:

And we have even written our first syntactically correct Python sentence.
Our sentence starts with the reserved word print followed
by 6527 a 9911 string 9929 of text of our choosing enclosed in single quotes.
 6835 8529 9748
Conversing with Python
7914  5041
Now that we have a word and a simple sentence that we know in Python,
we need to know how to start a conversation with Python to test
our new language skills.

Before you can converse with Python, you must first install the Python
software on your computer and learn how to start Python on your
computer.  That is too much detail for this chapter so I suggest
that you consult www.py4e.com where I have detailed
instructions and screencasts of setting up and starting Python
on Macintosh and Windows systems.  At some point, you will be in
a terminal or command window and you will type python and
the Python interpreter will start executing in interactive mode
and appear somewhat as follows:
8879 interactive mode

The >>> prompt is the Python interpreter's way of asking you, What
do you want me to do next?  Python is ready to have a conversation with
you.  All you have to know is how to speak the Python language.

Let's say for example that you did not know even the simplest Python language
words or sentences. You might want to use the standard line that astronauts
use when they land on a faraway planet and try to speak with the inhabitants
of the planet:

This is not going so well.  Unless you think of something quickly,
the inhabitants of the planet are likely to stab you with their spears,
put you on a spit, roast you over a fire, and eat you for dinner.

At this point, you should also realize that while Python
is amazingly complex and powerful and very picky about
the syntax you use to communicate with it, Python is
not intelligent.  You are really just having a conversation
with yourself, but using proper syntax.

In a sense, when you use a program written by someone else
the conversation is between you and those other
programmers with Python acting as an intermediary.  Python
3113 is a way for the creators of programs to express how the 4074
conversation is supposed to proceed.  And
in just a few more chapters, you will be one of those
2955 programmers using Python to talk to the users of your program.

Before we leave our first conversation with the Python
interpreter, you should probably know the proper way
to say good-bye when interacting with the inhabitants
of Planet Python:

You will notice that the error is different for the first two
3425 incorrect attempts.   The second error is different because
if is a reserved word and Python saw the reserved word
and thought we were trying to say something but got the syntax
2076 of the sentence wrong. 5384

Terminology: interpreter and compiler

Python is a high-level language intended to be relatively
straightforward for humans to read and write and for computers
to read and process.  Other high-level languages include Java, C++,
PHP, Ruby, Basic, Perl, JavaScript, and many more.  The actual hardware
inside the Central Processing Unit (CPU) does not understand any
of these high-level languages.

The CPU understands a language we call machine language.  Machine
language is very simple and frankly very tiresome to write because it
is represented all in zeros and ones.

Machine language seems quite simple on the surface, given that there
are only zeros and ones, but its syntax is even more complex
and far more intricate than Python.  So very few programmers ever write
machine language.  Instead we build various translators to allow
programmers to write in high-level languages like Python or JavaScript
and these translators convert the programs to machine language for actual
execution 8379 by 2320 the 8056 CPU.

Since machine language is tied to the computer hardware, machine language
is not portable across different types of hardware.  Programs written in
high-level languages can be moved between different computers by using a
different interpreter on the new machine or recompiling the code to create
a machine language version of the program for the new machine.

These programming language translators fall into two general categories:
(one) interpreters and (two) compilers.

An interpreter reads the source code of the program as written by the
programmer, parses the source code, and interprets the instructions on the fly.
Python is an interpreter and when we are running Python interactively,
we can type a line of Python (a sentence) and Python processes it immediately
and is ready for us to type another line of Python.

Some of the lines of Python tell Python that you want it to remember some
value for later.   We need to pick a name for that value to be remembered and
we can use that symbolic name to retrieve the value later.  We use the
term variable to refer to the labels we use to refer to this stored data.

In 4238 this 9076 example, 3214 we ask Python to remember the value six and use the label x
 so we can retrieve the value later.   We verify that Python has actually remembered
the value using x and multiply
it by seven and put the newly computed value in y.  Then we ask Python to print out
the value currently in y.

Even though we are typing these commands into Python one line at a time, Python
is treating them as an ordered sequence of statements with later statements able
to retrieve data created in earlier statements.   We are writing our first
simple paragraph with four sentences in a logical and meaningful order.

It is the nature of an interpreter to be able to have an interactive conversation
as shown above.  A compiler needs to be handed the entire program in a file, and then
2369 it runs a process to translate the high-level source code into machine language 1600
and then the compiler puts the resulting machine language into a file for later
execution.

If you have a Windows system, often these executable machine language programs have a
suffix of .exe or .dll which stand for executable and dynamic link
library respectively.  In Linux and Macintosh, there is no suffix that uniquely marks
a file as executable.

If you were to open an executable file in a text editor, it would look
completely crazy and be unreadable:

It is not easy to read or write machine language, so it is nice that we have
compilers that allow us to write in high-level
languages like Python or C.

Now at this point in our discussion of compilers and interpreters, you should
be wondering a bit about the Python interpreter itself.  What language is
it written in?  Is it written in a compiled language?  When we type
python, what exactly is happening?

The Python interpreter is written in a high-level language called C.
You can look at the actual source code for the Python interpreter by
going to www.python.org and working your way to their source code.
So Python is a program itself and it is compiled into machine code.
When you installed Python on your computer (or the vendor installed it),
you copied a machine-code copy of the translated Python program onto your
system.   In Windows, the executable machine code for Python itself is likely
in a file.

That is more than you really need to know to be a Python programmer, but
sometimes it pays to answer those little nagging questions right at
the beginning.

Writing a program

Typing commands into the Python interpreter is a great way to experiment
with Python's features, but it is not recommended for solving more complex problems.

When we want to write a program,
we use a text editor to write the Python instructions into a file,
which is called a script.  By
convention, Python scripts have names that end with .py.

script

To execute the script, you have to tell the Python interpreter
the name of the file.  In a Unix or Windows command window,
you would type python hello.py as follows:

We call the Python interpreter and tell it to read its source code from
the file hello.py instead of prompting us for lines of Python code
interactively.

You will notice that there was no need to have quit() at the end of
the Python program in the file.   When Python is reading your source code
from a file, it knows to stop when it reaches the end of the file.

9006 What is a program?

The definition of a program at its most basic is a sequence
of Python statements that have been crafted to do something.
Even our simple hello.py script is a program.  It is a one-line
program and is not particularly useful, but in the strictest definition,
it is a Python program.

It might be easiest to understand what a program is by thinking about a problem
that a program might be built to solve, and then looking at a program
that would solve that problem.

Lets say you are doing Social Computing research on Facebook posts and
you are interested in the most frequently used word in a series of posts.
You could print out the stream of Facebook posts and pore over the text
looking for the most common word, but that would take a long time and be very
mistake prone.  You would be smart to write a Python program to handle the
task quickly and accurately so you can spend the weekend doing something
fun.

For example, look at the following text about a clown and a car.  Look at the
text and figure out the most common word and how many times it occurs.

Then imagine that you are doing this task looking at millions of lines of
text.  Frankly it would be quicker for you to learn Python and write a
Python program to count the words than it would be to manually
scan the words.

The even better news is that I already came up with a simple program to
find the most common word in a text file.  I wrote it,
tested it, and now I am giving it to you to use so you can save some time.

You don't even need to know Python to use this program.  You will need to get through
Chapter ten of this book to fully understand the awesome Python techniques that were
used to make the program.  You are the end user, you simply use the program and marvel
at its cleverness and how it saved you so much manual effort.
You simply type the code
into a file called words.py and run it or you download the source
code from http://www.py4e.com/code3/ and run it.

This is a good example of how Python and the Python language are acting as an intermediary
between you (the end user) and me (the programmer).  Python is a way for us to exchange useful
instruction sequences (i.e., programs) in a common language that can be used by anyone who
installs Python on their computer.  So neither of us are talking to Python,
instead we are communicating with each other through Python.

The building blocks of programs

In the next few chapters, we will learn more about the vocabulary, sentence structure,
paragraph structure, and story structure of Python.  We will learn about the powerful
capabilities of Python and how to compose those capabilities together to create useful
programs.

There are some low-level conceptual patterns that we use to construct programs.  These
constructs are not just for Python programs, they are part of every programming language
from machine language up to the high-level languages.

description

Get data from the outside world.  This might be
reading data from a file, or even some kind of sensor like
a microphone or GPS.  In our initial programs, our input will come from the user
typing data on the keyboard.

Display the results of the program on a screen
or store them in a file or perhaps write them to a device like a
speaker to play music or speak text.

Perform statements one after
another in the order they are encountered in the script.

Check for certain conditions and
then execute or skip a sequence of statements.

Perform some set of statements
repeatedly, usually with
some variation.

Write a set of instructions once and give them a name
and then reuse those instructions as needed throughout your program.

description

It sounds almost too simple to be true, and of course it is never
so simple.  It is like saying that walking is simply
putting one foot in front of the other.  The art
of writing a program is composing and weaving these
basic elements together many times over to produce something
that is useful to its users.

The word counting program above directly uses all of
these patterns except for one.

What could possibly go wrong?

As we saw in our earliest conversations with Python, we must
communicate very precisely when we write Python code.  The smallest
deviation or mistake will cause Python to give up looking at your
program.

Beginning programmers often take the fact that Python leaves no
room for errors as evidence that Python is mean, hateful, and cruel.
While Python seems to like everyone else, Python knows them
personally and holds a grudge against them.  Because of this grudge,
Python takes our perfectly written programs and rejects them as
unfit just to torment us.

There is little to be gained by arguing with Python.  It is just a tool.
It has no emotions and it is happy and ready to serve you whenever you
need it.  Its error messages sound harsh, but they are just Python's
call for help.  It has looked at what you typed, and it simply cannot
understand what you have entered.

Python is much more like a dog, loving you unconditionally, having a few
key words that it understands, looking you with a sweet look on its
face (>>>), and waiting for you to say something it understands.
When Python says SyntaxError: invalid syntax, it is simply wagging
its tail and saying, You seemed to say something but I just don't
understand what you meant, but please keep talking to me (>>>).

As your programs become increasingly sophisticated, you will encounter three
general types of errors:

description

These are the first errors you will make and the easiest
to fix.  A syntax error means that you have violated the grammar rules of Python.
Python does its best to point right at the line and character where
it noticed it was confused.  The only tricky bit of syntax errors is that sometimes
the mistake that needs fixing is actually earlier in the program than where Python
noticed it was confused.  So the line and character that Python indicates in
a syntax error may just be a starting point for your investigation.

A logic error is when your program has good syntax but there is a mistake
in the order of the statements or perhaps a mistake in how the statements relate to one another.
A good example of a logic error might be, take a drink from your water bottle, put it
in your backpack, walk to the library, and then put the top back on the bottle.

A semantic error is when your description of the steps to take
is syntactically perfect and in the right order, but there is simply a mistake in
the program.  The program is perfectly correct but it does not do what
you intended for it to do. A simple example would
be if you were giving a person directions to a restaurant and said, ...when you reach
the intersection with the gas station, turn left and go one mile and the restaurant
is a red building on your left.  Your friend is very late and calls you to tell you that
they are on a farm and walking around behind a barn, with no sign of a restaurant.
Then you say did you turn left or right at the gas station? and
they say, I followed your directions perfectly, I have
them written down, it says turn left and go one mile at the gas station.  Then you say,
I am very sorry, because while my instructions were syntactically correct, they
sadly contained a small but undetected semantic error..

description

Again in all three types of errors, Python is merely trying its hardest to
do exactly what you have asked.

The learning journey

As you progress through the rest of the book, don't be afraid if the concepts
don't seem to fit together well the first time.  When you were learning to speak,
it was not a problem  for your first few years that you just made cute gurgling noises.
And it was OK if it took six months for you to move from simple vocabulary to
simple sentences and took five or six more years to move from sentences to paragraphs, and a
few more years to be able to write an interesting complete short story on your own.

We want you to learn Python much more rapidly, so we teach it all at the same time
over the next few chapters.
But it is like learning a new language that takes time to absorb and understand
before it feels natural.
That leads to some confusion as we visit and revisit
topics to try to get you to see the big picture while we are defining the tiny
fragments that make up that big picture.  While the book is written linearly, and
if you are taking a course it will progress in a linear fashion, don't hesitate
to be very nonlinear in how you approach the material.  Look forwards and backwards
and read with a light touch.  By skimming more advanced material without
fully understanding the details, you can get a better understanding of the why?
of programming.  By reviewing previous material and even redoing earlier
exercises, you will realize that you actually learned a lot of material even
if the material you are currently staring at seems a bit impenetrable.

Usually when you are learning your first programming language, there are a few
wonderful Ah Hah! moments where you can look up from pounding away at some rock
with a hammer and chisel and step away and see that you are indeed building
a beautiful sculpture.

If something seems particularly hard, there is usually no value in staying up all
night and staring at it.   Take a break, take a nap, have a snack, explain what you
are having a problem with to someone (or perhaps your dog), and then come back to it with
fresh eyes.  I assure you that once you learn the programming concepts in the book
you will look back and see that it was all really easy and elegant and it simply
took you a bit of time to absorb it.
42
The end""" 
y = re.findall('[0-9]+',hand)

print(y)
sum=0
for x in y:
  sum = sum+int(x)
print(sum)

import re
hand = """This file contains the actual data for your assignment - good luck!

8253
Why should you learn to write programs?
5546  7115
Writing programs (or programming) is a very creative
and 5007 rewarding 7198 activity. 8368  You can write programs for
 many reasons, ranging from making your living to solving
a difficult data analysis problem to having fun to helping
someone else solve a problem.  This book assumes that
everyone needs to know how to program, and that once
you know how to program you will figure out what you want
to do with your newfound skills.

We are surrounded in our daily lives with computers ranging
from laptops to cell phones.  We can think of these computers
as our personal assistants who can take care of many things
4925 on our behalf.  The hardware in our current-day computers
is 2403 essentially 6208 built 7296 to continuously ask us the question,
 What would you like me to do next?

Programmers add an operating system and a set of applications
9769 to the hardware and we end up with a Personal Digital
Assistant that is quite helpful and capable of helping
us do many different things.

Our computers are fast and have vast amounts of memory and
could 1211 be 6710 very 1136 helpful to us if we only knew the language to
 speak to explain to the computer what we would like it to
do next.  If we knew this language, we could tell the
computer to do tasks on our behalf that were repetitive.
Interestingly, the kinds of things computers can do best
are often the kinds of things that we humans find boring
and mind-numbing.

For example, look at the first three paragraphs of this
chapter and tell me the most commonly used word and how
many times the word is used.  While you were able to read
and understand the words in a few seconds, counting them
is 3633 almost 3685 painful 6776 because it is not the kind of problem
 that human minds are designed to solve.  For a computer
the opposite is true, reading and understanding text
from a piece of paper is hard for a computer to do
but counting the words and telling you how many times
the most used word was used is very easy for the
8881 computer: 3526

Our personal information analysis assistant quickly
2937 told us that the word to was used sixteen times in the 7745
first three paragraphs of this chapter.

This very fact that computers are good at things
that humans are not is why you need to become
skilled at talking computer language.  Once you learn
this new language, you can delegate mundane tasks
to your partner (the computer), leaving more time
for you to do the
things that you are uniquely suited for.  You bring
creativity, intuition, and inventiveness to this
9051 partnership.

Creativity and motivation

While this book is not intended for professional programmers, professional
programming can be a very rewarding job both financially and personally.
Building useful, elegant, and clever programs for others to use is a very
creative activity.  Your computer or Personal Digital Assistant (PDA)
usually contains many different programs from many different groups of
programmers, each competing for your attention and interest.  They try
their best to meet your needs and give you a great user experience in the
process.   In some situations, when you choose a piece of software, the
programmers are directly compensated because of your choice.

6661 If we think of programs as the creative output of groups of programmers,
perhaps the following figure is a more sensible version of our PDA:

For now, our primary motivation is not to make money or please end users, but
instead for us to be more productive in handling the data and
information that we will encounter in our lives.
When you first start, you will be both the programmer and the end user of
your programs.  As you gain skill as a programmer and
programming feels more creative to you, your thoughts may turn
toward developing programs for others.

Computer hardware architecture
5776
Before we start learning the language we
speak to give instructions to computers to
develop software, we need to learn a small amount about
how computers are built.

Central Processing Unit (or CPU) is
the 5832 part 880 of 4183 the computer that is built to be obsessed
 with what is next?  If your computer is rated
at three Gigahertz, it means that the CPU will ask What next?
three billion times per second.  You are going to have to
learn how to talk fast to keep up with the CPU.

Main Memory is used to store information
that the CPU needs in a hurry.  The main memory is nearly as
fast as the CPU.  But the information stored in the main
memory vanishes when the computer is turned off.

Secondary Memory is also used to store
1953 information, but it is much slower than the main memory.
The advantage of the secondary memory is that it can
store information even when there is no power to the
9316 computer.  Examples of secondary memory are disk drives
or flash memory (typically found in USB sticks and portable
music players).

Input and Output Devices are simply our
screen, keyboard, mouse, microphone, speaker, touchpad, etc.
They are all of the ways we interact with the computer.

These days, most computers also have a
Network Connection to retrieve information over a network.
We can think of the network as a very slow place to store and
retrieve data that might not always be up.  So in a sense,
the 5275 network 9442 is 9022 a slower and at times unreliable form of
 Secondary Memory.

While most of the detail of how these components work is best left
to computer builders, it helps to have some terminology
so 6735 we 1640 can 3560 talk about these different parts as we write our programs.

As a programmer, your job is to use and orchestrate
each of these resources to solve the problem that you need to solve
and analyze the data you get from the solution.  As a programmer you will
mostly be talking to the CPU and telling it what to
do next.  Sometimes you will tell the CPU to use the main memory,
secondary memory, network, or the input/output devices.

7253 You need to be the person who answers the CPU's What next?
question.  But it would be very uncomfortable to shrink you
down to five mm  tall and insert you into the computer just so you
4658 could issue a command three billion times per second.  So instead, 1269
9171 you must write down your instructions in advance. 352
We call these stored instructions a program and the act
of writing these instructions down and getting the instructions to
be correct programming.

Understanding programming

In the rest of this book, we will try to turn you into a person
who is skilled in the art of programming.  In the end you will be a
programmer --- perhaps not a professional programmer, but
at least you will have the skills to look at a data/information
analysis problem and develop a program to solve the problem.

problem solving

1262 In a sense, you need two skills to be a programmer:

First, you need to know the programming language (Python) -
you need to know the vocabulary and the grammar.  You need to be able
to spell the words in this new language properly and know how to construct
well-formed sentences in this new language.

Second, you need to tell a story.  In writing a story,
you combine words and sentences to convey an idea to the reader.
There is a skill and art in constructing the story, and skill in
story 6014 writing 5030 is 5085 improved by doing some writing and getting some
 feedback.  In programming, our program is the story and the
problem you are trying to solve is the idea.

itemize

Once you learn one programming language such as Python, you will
find it much easier to learn a second programming language such
as JavaScript or C++.  The new programming language has very different
vocabulary and grammar but the problem-solving skills
will be the same across all programming languages.

You will learn the vocabulary and sentences of Python pretty quickly.
It will take longer for you to be able to write a coherent program
to solve a brand-new problem.  We teach programming much like we teach
writing.  We start reading and explaining programs, then we write
simple programs, and then we write increasingly complex programs over time.
At some point you get your muse and see the patterns on your own
and can see more naturally how to take a problem and
write a program that solves that problem.  And once you get
to that point, programming becomes a very pleasant and creative process.

We start with the vocabulary and structure of Python programs.  Be patient
as the simple examples remind you of when you started reading for the first
time.

Words and sentences

Unlike human languages, the Python vocabulary is actually pretty small.
We call this vocabulary the reserved words.  These are words that
have very special meaning to Python.  When Python sees these words in
a Python program, they have one and only one meaning to Python.  Later
as you write programs you will make up your own words that have meaning to
you called variables.   You will have great latitude in choosing
your names for your variables, but you cannot use any of Python's
reserved words as a name for a variable.
1767 1912 9989
When we train a dog, we use special words like
sit, stay, and fetch.  When you talk to a dog and
don't use any of the reserved words, they just look at you with a
quizzical look on their face until you say a reserved word.
For example, if you say,
I wish more people would walk to improve their overall health,
what most dogs likely hear is,
blah blah blah walk blah blah blah blah.
2252 That is because walk is a reserved word in dog language. 6925

The reserved words in the language where humans talk to
Python include the following:

and       del       from      not       while
as        elif      global    or        with
assert    else      if        pass      yield
break     except    import    print
class     exec      in        raise
continue  finally   is        return
def       for       lambda    try

That is it, and unlike a dog, Python is already completely trained.
When you say try, Python will try every time you say it without
fail.

1662 We will learn these reserved words and how they are used in good time,
but for now we will focus on the Python equivalent of speak (in
human-to-dog language).  The nice thing about telling Python to speak
is that we can even tell it what to say by giving it a message in quotes:

And we have even written our first syntactically correct Python sentence.
Our sentence starts with the reserved word print followed
by 6527 a 9911 string 9929 of text of our choosing enclosed in single quotes.
 6835 8529 9748
Conversing with Python
7914  5041
Now that we have a word and a simple sentence that we know in Python,
we need to know how to start a conversation with Python to test
our new language skills.

Before you can converse with Python, you must first install the Python
software on your computer and learn how to start Python on your
computer.  That is too much detail for this chapter so I suggest
that you consult www.py4e.com where I have detailed
instructions and screencasts of setting up and starting Python
on Macintosh and Windows systems.  At some point, you will be in
a terminal or command window and you will type python and
the Python interpreter will start executing in interactive mode
and appear somewhat as follows:
8879 interactive mode

The >>> prompt is the Python interpreter's way of asking you, What
do you want me to do next?  Python is ready to have a conversation with
you.  All you have to know is how to speak the Python language.

Let's say for example that you did not know even the simplest Python language
words or sentences. You might want to use the standard line that astronauts
use when they land on a faraway planet and try to speak with the inhabitants
of the planet:

This is not going so well.  Unless you think of something quickly,
the inhabitants of the planet are likely to stab you with their spears,
put you on a spit, roast you over a fire, and eat you for dinner.

At this point, you should also realize that while Python
is amazingly complex and powerful and very picky about
the syntax you use to communicate with it, Python is
not intelligent.  You are really just having a conversation
with yourself, but using proper syntax.

In a sense, when you use a program written by someone else
the conversation is between you and those other
programmers with Python acting as an intermediary.  Python
3113 is a way for the creators of programs to express how the 4074
conversation is supposed to proceed.  And
in just a few more chapters, you will be one of those
2955 programmers using Python to talk to the users of your program.

Before we leave our first conversation with the Python
interpreter, you should probably know the proper way
to say good-bye when interacting with the inhabitants
of Planet Python:

You will notice that the error is different for the first two
3425 incorrect attempts.   The second error is different because
if is a reserved word and Python saw the reserved word
and thought we were trying to say something but got the syntax
2076 of the sentence wrong. 5384

Terminology: interpreter and compiler

Python is a high-level language intended to be relatively
straightforward for humans to read and write and for computers
to read and process.  Other high-level languages include Java, C++,
PHP, Ruby, Basic, Perl, JavaScript, and many more.  The actual hardware
inside the Central Processing Unit (CPU) does not understand any
of these high-level languages.

The CPU understands a language we call machine language.  Machine
language is very simple and frankly very tiresome to write because it
is represented all in zeros and ones.

Machine language seems quite simple on the surface, given that there
are only zeros and ones, but its syntax is even more complex
and far more intricate than Python.  So very few programmers ever write
machine language.  Instead we build various translators to allow
programmers to write in high-level languages like Python or JavaScript
and these translators convert the programs to machine language for actual
execution 8379 by 2320 the 8056 CPU.

Since machine language is tied to the computer hardware, machine language
is not portable across different types of hardware.  Programs written in
high-level languages can be moved between different computers by using a
different interpreter on the new machine or recompiling the code to create
a machine language version of the program for the new machine.

These programming language translators fall into two general categories:
(one) interpreters and (two) compilers.

An interpreter reads the source code of the program as written by the
programmer, parses the source code, and interprets the instructions on the fly.
Python is an interpreter and when we are running Python interactively,
we can type a line of Python (a sentence) and Python processes it immediately
and is ready for us to type another line of Python.

Some of the lines of Python tell Python that you want it to remember some
value for later.   We need to pick a name for that value to be remembered and
we can use that symbolic name to retrieve the value later.  We use the
term variable to refer to the labels we use to refer to this stored data.

In 4238 this 9076 example, 3214 we ask Python to remember the value six and use the label x
 so we can retrieve the value later.   We verify that Python has actually remembered
the value using x and multiply
it by seven and put the newly computed value in y.  Then we ask Python to print out
the value currently in y.

Even though we are typing these commands into Python one line at a time, Python
is treating them as an ordered sequence of statements with later statements able
to retrieve data created in earlier statements.   We are writing our first
simple paragraph with four sentences in a logical and meaningful order.

It is the nature of an interpreter to be able to have an interactive conversation
as shown above.  A compiler needs to be handed the entire program in a file, and then
2369 it runs a process to translate the high-level source code into machine language 1600
and then the compiler puts the resulting machine language into a file for later
execution.

If you have a Windows system, often these executable machine language programs have a
suffix of .exe or .dll which stand for executable and dynamic link
library respectively.  In Linux and Macintosh, there is no suffix that uniquely marks
a file as executable.

If you were to open an executable file in a text editor, it would look
completely crazy and be unreadable:

It is not easy to read or write machine language, so it is nice that we have
compilers that allow us to write in high-level
languages like Python or C.

Now at this point in our discussion of compilers and interpreters, you should
be wondering a bit about the Python interpreter itself.  What language is
it written in?  Is it written in a compiled language?  When we type
python, what exactly is happening?

The Python interpreter is written in a high-level language called C.
You can look at the actual source code for the Python interpreter by
going to www.python.org and working your way to their source code.
So Python is a program itself and it is compiled into machine code.
When you installed Python on your computer (or the vendor installed it),
you copied a machine-code copy of the translated Python program onto your
system.   In Windows, the executable machine code for Python itself is likely
in a file.

That is more than you really need to know to be a Python programmer, but
sometimes it pays to answer those little nagging questions right at
the beginning.

Writing a program

Typing commands into the Python interpreter is a great way to experiment
with Python's features, but it is not recommended for solving more complex problems.

When we want to write a program,
we use a text editor to write the Python instructions into a file,
which is called a script.  By
convention, Python scripts have names that end with .py.

script

To execute the script, you have to tell the Python interpreter
the name of the file.  In a Unix or Windows command window,
you would type python hello.py as follows:

We call the Python interpreter and tell it to read its source code from
the file hello.py instead of prompting us for lines of Python code
interactively.

You will notice that there was no need to have quit() at the end of
the Python program in the file.   When Python is reading your source code
from a file, it knows to stop when it reaches the end of the file.

9006 What is a program?

The definition of a program at its most basic is a sequence
of Python statements that have been crafted to do something.
Even our simple hello.py script is a program.  It is a one-line
program and is not particularly useful, but in the strictest definition,
it is a Python program.

It might be easiest to understand what a program is by thinking about a problem
that a program might be built to solve, and then looking at a program
that would solve that problem.

Lets say you are doing Social Computing research on Facebook posts and
you are interested in the most frequently used word in a series of posts.
You could print out the stream of Facebook posts and pore over the text
looking for the most common word, but that would take a long time and be very
mistake prone.  You would be smart to write a Python program to handle the
task quickly and accurately so you can spend the weekend doing something
fun.

For example, look at the following text about a clown and a car.  Look at the
text and figure out the most common word and how many times it occurs.

Then imagine that you are doing this task looking at millions of lines of
text.  Frankly it would be quicker for you to learn Python and write a
Python program to count the words than it would be to manually
scan the words.

The even better news is that I already came up with a simple program to
find the most common word in a text file.  I wrote it,
tested it, and now I am giving it to you to use so you can save some time.

You don't even need to know Python to use this program.  You will need to get through
Chapter ten of this book to fully understand the awesome Python techniques that were
used to make the program.  You are the end user, you simply use the program and marvel
at its cleverness and how it saved you so much manual effort.
You simply type the code
into a file called words.py and run it or you download the source
code from http://www.py4e.com/code3/ and run it.

This is a good example of how Python and the Python language are acting as an intermediary
between you (the end user) and me (the programmer).  Python is a way for us to exchange useful
instruction sequences (i.e., programs) in a common language that can be used by anyone who
installs Python on their computer.  So neither of us are talking to Python,
instead we are communicating with each other through Python.

The building blocks of programs

In the next few chapters, we will learn more about the vocabulary, sentence structure,
paragraph structure, and story structure of Python.  We will learn about the powerful
capabilities of Python and how to compose those capabilities together to create useful
programs.

There are some low-level conceptual patterns that we use to construct programs.  These
constructs are not just for Python programs, they are part of every programming language
from machine language up to the high-level languages.

description

Get data from the outside world.  This might be
reading data from a file, or even some kind of sensor like
a microphone or GPS.  In our initial programs, our input will come from the user
typing data on the keyboard.

Display the results of the program on a screen
or store them in a file or perhaps write them to a device like a
speaker to play music or speak text.

Perform statements one after
another in the order they are encountered in the script.

Check for certain conditions and
then execute or skip a sequence of statements.

Perform some set of statements
repeatedly, usually with
some variation.

Write a set of instructions once and give them a name
and then reuse those instructions as needed throughout your program.

description

It sounds almost too simple to be true, and of course it is never
so simple.  It is like saying that walking is simply
putting one foot in front of the other.  The art
of writing a program is composing and weaving these
basic elements together many times over to produce something
that is useful to its users.

The word counting program above directly uses all of
these patterns except for one.

What could possibly go wrong?

As we saw in our earliest conversations with Python, we must
communicate very precisely when we write Python code.  The smallest
deviation or mistake will cause Python to give up looking at your
program.

Beginning programmers often take the fact that Python leaves no
room for errors as evidence that Python is mean, hateful, and cruel.
While Python seems to like everyone else, Python knows them
personally and holds a grudge against them.  Because of this grudge,
Python takes our perfectly written programs and rejects them as
unfit just to torment us.

There is little to be gained by arguing with Python.  It is just a tool.
It has no emotions and it is happy and ready to serve you whenever you
need it.  Its error messages sound harsh, but they are just Python's
call for help.  It has looked at what you typed, and it simply cannot
understand what you have entered.

Python is much more like a dog, loving you unconditionally, having a few
key words that it understands, looking you with a sweet look on its
face (>>>), and waiting for you to say something it understands.
When Python says SyntaxError: invalid syntax, it is simply wagging
its tail and saying, You seemed to say something but I just don't
understand what you meant, but please keep talking to me (>>>).

As your programs become increasingly sophisticated, you will encounter three
general types of errors:

description

These are the first errors you will make and the easiest
to fix.  A syntax error means that you have violated the grammar rules of Python.
Python does its best to point right at the line and character where
it noticed it was confused.  The only tricky bit of syntax errors is that sometimes
the mistake that needs fixing is actually earlier in the program than where Python
noticed it was confused.  So the line and character that Python indicates in
a syntax error may just be a starting point for your investigation.

A logic error is when your program has good syntax but there is a mistake
in the order of the statements or perhaps a mistake in how the statements relate to one another.
A good example of a logic error might be, take a drink from your water bottle, put it
in your backpack, walk to the library, and then put the top back on the bottle.

A semantic error is when your description of the steps to take
is syntactically perfect and in the right order, but there is simply a mistake in
the program.  The program is perfectly correct but it does not do what
you intended for it to do. A simple example would
be if you were giving a person directions to a restaurant and said, ...when you reach
the intersection with the gas station, turn left and go one mile and the restaurant
is a red building on your left.  Your friend is very late and calls you to tell you that
they are on a farm and walking around behind a barn, with no sign of a restaurant.
Then you say did you turn left or right at the gas station? and
they say, I followed your directions perfectly, I have
them written down, it says turn left and go one mile at the gas station.  Then you say,
I am very sorry, because while my instructions were syntactically correct, they
sadly contained a small but undetected semantic error..

description

Again in all three types of errors, Python is merely trying its hardest to
do exactly what you have asked.

The learning journey

As you progress through the rest of the book, don't be afraid if the concepts
don't seem to fit together well the first time.  When you were learning to speak,
it was not a problem  for your first few years that you just made cute gurgling noises.
And it was OK if it took six months for you to move from simple vocabulary to
simple sentences and took five or six more years to move from sentences to paragraphs, and a
few more years to be able to write an interesting complete short story on your own.

We want you to learn Python much more rapidly, so we teach it all at the same time
over the next few chapters.
But it is like learning a new language that takes time to absorb and understand
before it feels natural.
That leads to some confusion as we visit and revisit
topics to try to get you to see the big picture while we are defining the tiny
fragments that make up that big picture.  While the book is written linearly, and
if you are taking a course it will progress in a linear fashion, don't hesitate
to be very nonlinear in how you approach the material.  Look forwards and backwards
and read with a light touch.  By skimming more advanced material without
fully understanding the details, you can get a better understanding of the why?
of programming.  By reviewing previous material and even redoing earlier
exercises, you will realize that you actually learned a lot of material even
if the material you are currently staring at seems a bit impenetrable.

Usually when you are learning your first programming language, there are a few
wonderful Ah Hah! moments where you can look up from pounding away at some rock
with a hammer and chisel and step away and see that you are indeed building
a beautiful sculpture.

If something seems particularly hard, there is usually no value in staying up all
night and staring at it.   Take a break, take a nap, have a snack, explain what you
are having a problem with to someone (or perhaps your dog), and then come back to it with
fresh eyes.  I assure you that once you learn the programming concepts in the book
you will look back and see that it was all really easy and elegant and it simply
took you a bit of time to absorb it.
42
The end""" 
y = re.findall('[0-9]+',hand)

print(y)
sum=0
for x in y:
  sum = sum+int(x)
print(sum)

This file contains the actual data for your assignment - good luck!  8253 Why should you learn to write programs? 5546  7115 Writing programs (or programming) is a very creative and 5007 rewarding 7198 activity. 8368  You can write programs for  many reasons, ranging from making your living to solving a difficult data analysis problem to having fun to helping someone else solve a problem.  This book assumes that everyone needs to know how to program, and that once you know how to program you will figure out what you want to do with your newfound skills.  We are surrounded in our daily lives with computers ranging from laptops to cell phones.  We can think of these computers as our personal assistants who can take care of many things 4925 on our behalf.  The hardware in our current-day computers is 2403 essentially 6208 built 7296 to continuously ask us the question,  What would you like me to do next?  Programmers add an operating system and a set of applications 9769 to the hardware and we end up with a Personal Digital Assistant that is quite helpful and capable of helping us do many different things.  Our computers are fast and have vast amounts of memory and could 1211 be 6710 very 1136 helpful to us if we only knew the language to  speak to explain to the computer what we would like it to do next.  If we knew this language, we could tell the computer to do tasks on our behalf that were repetitive. Interestingly, the kinds of things computers can do best are often the kinds of things that we humans find boring and mind-numbing.  For example, look at the first three paragraphs of this chapter and tell me the most commonly used word and how many times the word is used.  While you were able to read and understand the words in a few seconds, counting them is 3633 almost 3685 painful 6776 because it is not the kind of problem  that human minds are designed to solve.  For a computer the opposite is true, reading and understanding text from a piece of paper is hard for a computer to do but counting the words and telling you how many times the most used word was used is very easy for the 8881 computer: 3526  Our personal information analysis assistant quickly 2937 told us that the word to was used sixteen times in the 7745 first three paragraphs of this chapter.  This very fact that computers are good at things that humans are not is why you need to become skilled at talking computer language.  Once you learn this new language, you can delegate mundane tasks to your partner (the computer), leaving more time for you to do the things that you are uniquely suited for.  You bring creativity, intuition, and inventiveness to this 9051 partnership.  Creativity and motivation  While this book is not intended for professional programmers, professional programming can be a very rewarding job both financially and personally. Building useful, elegant, and clever programs for others to use is a very creative activity.  Your computer or Personal Digital Assistant (PDA) usually contains many different programs from many different groups of programmers, each competing for your attention and interest.  They try their best to meet your needs and give you a great user experience in the process.   In some situations, when you choose a piece of software, the programmers are directly compensated because of your choice.  6661 If we think of programs as the creative output of groups of programmers, perhaps the following figure is a more sensible version of our PDA:  For now, our primary motivation is not to make money or please end users, but instead for us to be more productive in handling the data and information that we will encounter in our lives. When you first start, you will be both the programmer and the end user of your programs.  As you gain skill as a programmer and programming feels more creative to you, your thoughts may turn toward developing programs for others.  Computer hardware architecture 5776 Before we start learning the language we speak to give instructions to computers to develop software, we need to learn a small amount about how computers are built.  Central Processing Unit (or CPU) is the 5832 part 880 of 4183 the computer that is built to be obsessed  with what is next?  If your computer is rated at three Gigahertz, it means that the CPU will ask What next? three billion times per second.  You are going to have to learn how to talk fast to keep up with the CPU.  Main Memory is used to store information that the CPU needs in a hurry.  The main memory is nearly as fast as the CPU.  But the information stored in the main memory vanishes when the computer is turned off.  Secondary Memory is also used to store 1953 information, but it is much slower than the main memory. The advantage of the secondary memory is that it can store information even when there is no power to the 9316 computer.  Examples of secondary memory are disk drives or flash memory (typically found in USB sticks and portable music players).  Input and Output Devices are simply our screen, keyboard, mouse, microphone, speaker, touchpad, etc. They are all of the ways we interact with the computer.  These days, most computers also have a Network Connection to retrieve information over a network. We can think of the network as a very slow place to store and retrieve data that might not always be up.  So in a sense, the 5275 network 9442 is 9022 a slower and at times unreliable form of  Secondary Memory.  While most of the detail of how these components work is best left to computer builders, it helps to have some terminology so 6735 we 1640 can 3560 talk about these different parts as we write our programs.  As a programmer, your job is to use and orchestrate each of these resources to solve the problem that you need to solve and analyze the data you get from the solution.  As a programmer you will mostly be talking to the CPU and telling it what to do next.  Sometimes you will tell the CPU to use the main memory, secondary memory, network, or the input/output devices.  7253 You need to be the person who answers the CPU's What next? question.  But it would be very uncomfortable to shrink you down to five mm  tall and insert you into the computer just so you 4658 could issue a command three billion times per second.  So instead, 1269 9171 you must write down your instructions in advance. 352 We call these stored instructions a program and the act of writing these instructions down and getting the instructions to be correct programming.  Understanding programming  In the rest of this book, we will try to turn you into a person who is skilled in the art of programming.  In the end you will be a programmer --- perhaps not a professional programmer, but at least you will have the skills to look at a data/information analysis problem and develop a program to solve the problem.  problem solving  1262 In a sense, you need two skills to be a programmer:  First, you need to know the programming language (Python) - you need to know the vocabulary and the grammar.  You need to be able to spell the words in this new language properly and know how to construct well-formed sentences in this new language.  Second, you need to tell a story.  In writing a story, you combine words and sentences to convey an idea to the reader. There is a skill and art in constructing the story, and skill in story 6014 writing 5030 is 5085 improved by doing some writing and getting some  feedback.  In programming, our program is the story and the problem you are trying to solve is the idea.  itemize  Once you learn one programming language such as Python, you will find it much easier to learn a second programming language such as JavaScript or C++.  The new programming language has very different vocabulary and grammar but the problem-solving skills will be the same across all programming languages.  You will learn the vocabulary and sentences of Python pretty quickly. It will take longer for you to be able to write a coherent program to solve a brand-new problem.  We teach programming much like we teach writing.  We start reading and explaining programs, then we write simple programs, and then we write increasingly complex programs over time. At some point you get your muse and see the patterns on your own and can see more naturally how to take a problem and write a program that solves that problem.  And once you get to that point, programming becomes a very pleasant and creative process.  We start with the vocabulary and structure of Python programs.  Be patient as the simple examples remind you of when you started reading for the first time.  Words and sentences  Unlike human languages, the Python vocabulary is actually pretty small. We call this vocabulary the reserved words.  These are words that have very special meaning to Python.  When Python sees these words in a Python program, they have one and only one meaning to Python.  Later as you write programs you will make up your own words that have meaning to you called variables.   You will have great latitude in choosing your names for your variables, but you cannot use any of Python's reserved words as a name for a variable. 1767 1912 9989 When we train a dog, we use special words like sit, stay, and fetch.  When you talk to a dog and don't use any of the reserved words, they just look at you with a quizzical look on their face until you say a reserved word. For example, if you say, I wish more people would walk to improve their overall health, what most dogs likely hear is, blah blah blah walk blah blah blah blah. 2252 That is because walk is a reserved word in dog language. 6925  The reserved words in the language where humans talk to Python include the following:  and       del       from      not       while as        elif      global    or        with assert    else      if        pass      yield break     except    import    print class     exec      in        raise continue  finally   is        return def       for       lambda    try  That is it, and unlike a dog, Python is already completely trained. When you say try, Python will try every time you say it without fail.  1662 We will learn these reserved words and how they are used in good time, but for now we will focus on the Python equivalent of speak (in human-to-dog language).  The nice thing about telling Python to speak is that we can even tell it what to say by giving it a message in quotes:  And we have even written our first syntactically correct Python sentence. Our sentence starts with the reserved word print followed by 6527 a 9911 string 9929 of text of our choosing enclosed in single quotes.  6835 8529 9748 Conversing with Python 7914  5041 Now that we have a word and a simple sentence that we know in Python, we need to know how to start a conversation with Python to test our new language skills.  Before you can converse with Python, you must first install the Python software on your computer and learn how to start Python on your computer.  That is too much detail for this chapter so I suggest that you consult www.py4e.com where I have detailed instructions and screencasts of setting up and starting Python on Macintosh and Windows systems.  At some point, you will be in a terminal or command window and you will type python and the Python interpreter will start executing in interactive mode and appear somewhat as follows: 8879 interactive mode  The >>> prompt is the Python interpreter's way of asking you, What do you want me to do next?  Python is ready to have a conversation with you.  All you have to know is how to speak the Python language.  Let's say for example that you did not know even the simplest Python language words or sentences. You might want to use the standard line that astronauts use when they land on a faraway planet and try to speak with the inhabitants of the planet:  This is not going so well.  Unless you think of something quickly, the inhabitants of the planet are likely to stab you with their spears, put you on a spit, roast you over a fire, and eat you for dinner.  At this point, you should also realize that while Python is amazingly complex and powerful and very picky about the syntax you use to communicate with it, Python is not intelligent.  You are really just having a conversation with yourself, but using proper syntax.  In a sense, when you use a program written by someone else the conversation is between you and those other programmers with Python acting as an intermediary.  Python 3113 is a way for the creators of programs to express how the 4074 conversation is supposed to proceed.  And in just a few more chapters, you will be one of those 2955 programmers using Python to talk to the users of your program.  Before we leave our first conversation with the Python interpreter, you should probably know the proper way to say good-bye when interacting with the inhabitants of Planet Python:  You will notice that the error is different for the first two 3425 incorrect attempts.   The second error is different because if is a reserved word and Python saw the reserved word and thought we were trying to say something but got the syntax 2076 of the sentence wrong. 5384  Terminology: interpreter and compiler  Python is a high-level language intended to be relatively straightforward for humans to read and write and for computers to read and process.  Other high-level languages include Java, C++, PHP, Ruby, Basic, Perl, JavaScript, and many more.  The actual hardware inside the Central Processing Unit (CPU) does not understand any of these high-level languages.  The CPU understands a language we call machine language.  Machine language is very simple and frankly very tiresome to write because it is represented all in zeros and ones.  Machine language seems quite simple on the surface, given that there are only zeros and ones, but its syntax is even more complex and far more intricate than Python.  So very few programmers ever write machine language.  Instead we build various translators to allow programmers to write in high-level languages like Python or JavaScript and these translators convert the programs to machine language for actual execution 8379 by 2320 the 8056 CPU.  Since machine language is tied to the computer hardware, machine language is not portable across different types of hardware.  Programs written in high-level languages can be moved between different computers by using a different interpreter on the new machine or recompiling the code to create a machine language version of the program for the new machine.  These programming language translators fall into two general categories: (one) interpreters and (two) compilers.  An interpreter reads the source code of the program as written by the programmer, parses the source code, and interprets the instructions on the fly. Python is an interpreter and when we are running Python interactively, we can type a line of Python (a sentence) and Python processes it immediately and is ready for us to type another line of Python.  Some of the lines of Python tell Python that you want it to remember some value for later.   We need to pick a name for that value to be remembered and we can use that symbolic name to retrieve the value later.  We use the term variable to refer to the labels we use to refer to this stored data.  In 4238 this 9076 example, 3214 we ask Python to remember the value six and use the label x  so we can retrieve the value later.   We verify that Python has actually remembered the value using x and multiply it by seven and put the newly computed value in y.  Then we ask Python to print out the value currently in y.  Even though we are typing these commands into Python one line at a time, Python is treating them as an ordered sequence of statements with later statements able to retrieve data created in earlier statements.   We are writing our first simple paragraph with four sentences in a logical and meaningful order.  It is the nature of an interpreter to be able to have an interactive conversation as shown above.  A compiler needs to be handed the entire program in a file, and then 2369 it runs a process to translate the high-level source code into machine language 1600 and then the compiler puts the resulting machine language into a file for later execution.  If you have a Windows system, often these executable machine language programs have a suffix of .exe or .dll which stand for executable and dynamic link library respectively.  In Linux and Macintosh, there is no suffix that uniquely marks a file as executable.  If you were to open an executable file in a text editor, it would look completely crazy and be unreadable:  It is not easy to read or write machine language, so it is nice that we have compilers that allow us to write in high-level languages like Python or C.  Now at this point in our discussion of compilers and interpreters, you should be wondering a bit about the Python interpreter itself.  What language is it written in?  Is it written in a compiled language?  When we type python, what exactly is happening?  The Python interpreter is written in a high-level language called C. You can look at the actual source code for the Python interpreter by going to www.python.org and working your way to their source code. So Python is a program itself and it is compiled into machine code. When you installed Python on your computer (or the vendor installed it), you copied a machine-code copy of the translated Python program onto your system.   In Windows, the executable machine code for Python itself is likely in a file.  That is more than you really need to know to be a Python programmer, but sometimes it pays to answer those little nagging questions right at the beginning.  Writing a program  Typing commands into the Python interpreter is a great way to experiment with Python's features, but it is not recommended for solving more complex problems.  When we want to write a program, we use a text editor to write the Python instructions into a file, which is called a script.  By convention, Python scripts have names that end with .py.  script  To execute the script, you have to tell the Python interpreter the name of the file.  In a Unix or Windows command window, you would type python hello.py as follows:  We call the Python interpreter and tell it to read its source code from the file hello.py instead of prompting us for lines of Python code interactively.  You will notice that there was no need to have quit() at the end of the Python program in the file.   When Python is reading your source code from a file, it knows to stop when it reaches the end of the file.  9006 What is a program?  The definition of a program at its most basic is a sequence of Python statements that have been crafted to do something. Even our simple hello.py script is a program.  It is a one-line program and is not particularly useful, but in the strictest definition, it is a Python program.  It might be easiest to understand what a program is by thinking about a problem that a program might be built to solve, and then looking at a program that would solve that problem.  Lets say you are doing Social Computing research on Facebook posts and you are interested in the most frequently used word in a series of posts. You could print out the stream of Facebook posts and pore over the text looking for the most common word, but that would take a long time and be very mistake prone.  You would be smart to write a Python program to handle the task quickly and accurately so you can spend the weekend doing something fun.  For example, look at the following text about a clown and a car.  Look at the text and figure out the most common word and how many times it occurs.  Then imagine that you are doing this task looking at millions of lines of text.  Frankly it would be quicker for you to learn Python and write a Python program to count the words than it would be to manually scan the words.  The even better news is that I already came up with a simple program to find the most common word in a text file.  I wrote it, tested it, and now I am giving it to you to use so you can save some time.  You don't even need to know Python to use this program.  You will need to get through Chapter ten of this book to fully understand the awesome Python techniques that were used to make the program.  You are the end user, you simply use the program and marvel at its cleverness and how it saved you so much manual effort. You simply type the code into a file called words.py and run it or you download the source code from http://www.py4e.com/code3/ and run it.  This is a good example of how Python and the Python language are acting as an intermediary between you (the end user) and me (the programmer).  Python is a way for us to exchange useful instruction sequences (i.e., programs) in a common language that can be used by anyone who installs Python on their computer.  So neither of us are talking to Python, instead we are communicating with each other through Python.  The building blocks of programs  In the next few chapters, we will learn more about the vocabulary, sentence structure, paragraph structure, and story structure of Python.  We will learn about the powerful capabilities of Python and how to compose those capabilities together to create useful programs.  There are some low-level conceptual patterns that we use to construct programs.  These constructs are not just for Python programs, they are part of every programming language from machine language up to the high-level languages.  description  Get data from the outside world.  This might be reading data from a file, or even some kind of sensor like a microphone or GPS.  In our initial programs, our input will come from the user typing data on the keyboard.  Display the results of the program on a screen or store them in a file or perhaps write them to a device like a speaker to play music or speak text.  Perform statements one after another in the order they are encountered in the script.  Check for certain conditions and then execute or skip a sequence of statements.  Perform some set of statements repeatedly, usually with some variation.  Write a set of instructions once and give them a name and then reuse those instructions as needed throughout your program.  description  It sounds almost too simple to be true, and of course it is never so simple.  It is like saying that walking is simply putting one foot in front of the other.  The art of writing a program is composing and weaving these basic elements together many times over to produce something that is useful to its users.  The word counting program above directly uses all of these patterns except for one.  What could possibly go wrong?  As we saw in our earliest conversations with Python, we must communicate very precisely when we write Python code.  The smallest deviation or mistake will cause Python to give up looking at your program.  Beginning programmers often take the fact that Python leaves no room for errors as evidence that Python is mean, hateful, and cruel. While Python seems to like everyone else, Python knows them personally and holds a grudge against them.  Because of this grudge, Python takes our perfectly written programs and rejects them as unfit just to torment us.  There is little to be gained by arguing with Python.  It is just a tool. It has no emotions and it is happy and ready to serve you whenever you need it.  Its error messages sound harsh, but they are just Python's call for help.  It has looked at what you typed, and it simply cannot understand what you have entered.  Python is much more like a dog, loving you unconditionally, having a few key words that it understands, looking you with a sweet look on its face (>>>), and waiting for you to say something it understands. When Python says SyntaxError: invalid syntax, it is simply wagging its tail and saying, You seemed to say something but I just don't understand what you meant, but please keep talking to me (>>>).  As your programs become increasingly sophisticated, you will encounter three general types of errors:  description  These are the first errors you will make and the easiest to fix.  A syntax error means that you have violated the grammar rules of Python. Python does its best to point right at the line and character where it noticed it was confused.  The only tricky bit of syntax errors is that sometimes the mistake that needs fixing is actually earlier in the program than where Python noticed it was confused.  So the line and character that Python indicates in a syntax error may just be a starting point for your investigation.  A logic error is when your program has good syntax but there is a mistake in the order of the statements or perhaps a mistake in how the statements relate to one another. A good example of a logic error might be, take a drink from your water bottle, put it in your backpack, walk to the library, and then put the top back on the bottle.  A semantic error is when your description of the steps to take is syntactically perfect and in the right order, but there is simply a mistake in the program.  The program is perfectly correct but it does not do what you intended for it to do. A simple example would be if you were giving a person directions to a restaurant and said, ...when you reach the intersection with the gas station, turn left and go one mile and the restaurant is a red building on your left.  Your friend is very late and calls you to tell you that they are on a farm and walking around behind a barn, with no sign of a restaurant. Then you say did you turn left or right at the gas station? and they say, I followed your directions perfectly, I have them written down, it says turn left and go one mile at the gas station.  Then you say, I am very sorry, because while my instructions were syntactically correct, they sadly contained a small but undetected semantic error..  description  Again in all three types of errors, Python is merely trying its hardest to do exactly what you have asked.  The learning journey  As you progress through the rest of the book, don't be afraid if the concepts don't seem to fit together well the first time.  When you were learning to speak, it was not a problem  for your first few years that you just made cute gurgling noises. And it was OK if it took six months for you to move from simple vocabulary to simple sentences and took five or six more years to move from sentences to paragraphs, and a few more years to be able to write an interesting complete short story on your own.  We want you to learn Python much more rapidly, so we teach it all at the same time over the next few chapters. But it is like learning a new language that takes time to absorb and understand before it feels natural. That leads to some confusion as we visit and revisit topics to try to get you to see the big picture while we are defining the tiny fragments that make up that big picture.  While the book is written linearly, and if you are taking a course it will progress in a linear fashion, don't hesitate to be very nonlinear in how you approach the material.  Look forwards and backwards and read with a light touch.  By skimming more advanced material without fully understanding the details, you can get a better understanding of the why? of programming.  By reviewing previous material and even redoing earlier exercises, you will realize that you actually learned a lot of material even if the material you are currently staring at seems a bit impenetrable.  Usually when you are learning your first programming language, there are a few wonderful Ah Hah! moments where you can look up from pounding away at some rock with a hammer and chisel and step away and see that you are indeed building a beautiful sculpture.  If something seems particularly hard, there is usually no value in staying up all night and staring at it.   Take a break, take a nap, have a snack, explain what you are having a problem with to someone (or perhaps your dog), and then come back to it with fresh eyes.  I assure you that once you learn the programming concepts in the book you will look back and see that it was all really easy and elegant and it simply took you a bit of time to absorb it. 42 The end