
Why should you learn to write programs?

Writing programs (or programming) is a very creative 
and rewarding activity.  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
on our behalf.  The hardware in our current-day computers 
is essentially built to continuously ask us the question, 
What would you like me to do next?

Programmers add an operating system and a set of applications
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 be very 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 almost painful 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
computer:

Our personal information analysis assistant quickly 
told us that the word to was used sixteen times in the
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
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.

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

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 part of 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
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
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 network is 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 we can 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.

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 
could issue a command three billion times per second.  So instead,
you must write down your instructions in advance.
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

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 writing is 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.

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.
That is because walk is a reserved word in dog language.  

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.

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 a string of text of our choosing enclosed in single quotes.

Conversing with Python

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:
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
is a way for the creators of programs to express how the 
conversation is supposed to proceed.  And
in just a few more chapters, you will be one of those
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
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
of the sentence wrong.

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 by the 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 this example, 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 
it runs a process to translate the high-level source code into machine language
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.

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/code/ 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.
