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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

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<head>

<title>The way of the program — Pense Python 2e documentation</title>

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URL_ROOT: './',

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<h1>The way of the program<a class="headerlink" href="#the-way-of-the-program" title="Permalink to this headline">¶</a></h1>

<p>The goal of this book is to teach you to think like a computer

scientist. This way of thinking combines some of the best features of

mathematics, engineering, and natural science. Like mathematicians,

computer scientists use formal languages to denote ideas (specifically

computations). Like engineers, they design things, assembling components

into systems and evaluating tradeoffs among alternatives. Like

scientists, they observe the behavior of complex systems, form

hypotheses, and test predictions.</p>

<p>The single most important skill for a computer scientist is <strong>problem

solving</strong>. Problem solving means the ability to formulate problems,

think creatively about solutions, and express a solution clearly and

accurately. As it turns out, the process of learning to program is an

excellent opportunity to practice problem-solving skills. That’s why

this chapter is called, “The way of the program”.</p>

<p>On one level, you will be learning to program, a useful skill by itself.

On another level, you will use programming as a means to an end. As we

go along, that end will become clearer.</p>

<h2>What is a program?<a class="headerlink" href="#what-is-a-program" title="Permalink to this headline">¶</a></h2>

<p>A <strong>program</strong> is a sequence of instructions that specifies how to

perform a computation. The computation might be something mathematical,

such as solving a system of equations or finding the roots of a

polynomial, but it can also be a symbolic computation, such as searching

and replacing text in a document or something graphical, like processing

an image or playing a video.</p>

<p>The details look different in different languages, but a few basic

instructions appear in just about every language:</p>

<dt>input:</dt>

<dd>Get data from the keyboard, a file, the network, or some other

device.</dd>

<dt>output:</dt>

<dd>Display data on the screen, save it in a file, send it over the

network, etc.</dd>

<dd>Perform basic mathematical operations like addition and

multiplication.</dd>

<dt>conditional execution:</dt>

<dd>Check for certain conditions and run the appropriate code.</dd>

<dt>repetition:</dt>

<dd>Perform some action repeatedly, usually with some variation.</dd>

</dl>

<p>Believe it or not, that’s pretty much all there is to it. Every program

you’ve ever used, no matter how complicated, is made up of instructions

that look pretty much like these. So you can think of programming as the

process of breaking a large, complex task into smaller and smaller

subtasks until the subtasks are simple enough to be performed with one

of these basic instructions.</p>

</div>

<h2>Running Python<a class="headerlink" href="#running-python" title="Permalink to this headline">¶</a></h2>

<p>One of the challenges of getting started with Python is that you might

have to install Python and related software on your computer. If you are

familiar with your operating system, and especially if you are

comfortable with the command-line interface, you will have no trouble

installing Python. But for beginners, it can be painful to learn about

system administration and programming at the same time.</p>

<p>To avoid that problem, I recommend that you start out running Python in

a browser. Later, when you are comfortable with Python, I’ll make

suggestions for installing Python on your computer.</p>

<p>There are a number of web pages you can use to run Python. If you

already have a favorite, go ahead and use it. Otherwise I recommend

PythonAnywhere. I provide detailed instructions for getting started at

<a class="reference external" href="http://tinyurl.com/thinkpython2e">http://tinyurl.com/thinkpython2e</a>.</p>

<p>There are two versions of Python, called Python 2 and Python 3. They are

very similar, so if you learn one, it is easy to switch to the other. In

fact, there are only a few differences you will encounter as a beginner.

This book is written for Python 3, but I include some notes about Python

2.</p>

<p>The Python <strong>interpreter</strong> is a program that reads and executes Python

code. Depending on your environment, you might start the interpreter by

clicking on an icon, or by typing python on a command line. When it

starts, you should see output like this:</p>

<div class="highlight-python"><div class="highlight"><pre>Python 3.4.0 (default, Jun 19 2015, 14:20:21)

[GCC 4.8.2] on linux

Type "help", "copyright", "credits" or "license" for more information.

>>>

</pre></div>

</div>

<p>The first three lines contain information about the interpreter and the

operating system it’s running on, so it might be different for you. But

you should check that the version number, which is 3.4.0 in this

example, begins with 3, which indicates that you are running Python 3.

If it begins with 2, you are running (you guessed it) Python 2.</p>

<p>The last line is a <strong>prompt</strong> that indicates that the interpreter is

ready for you to enter code. If you type a line of code and hit Enter,

the interpreter displays the result:</p>

</pre></div>

</div>

<p>Now you’re ready to get started. From here on, I assume that you know

how to start the Python interpreter and run code.</p>

</div>

<h2>The first program<a class="headerlink" href="#the-first-program" title="Permalink to this headline">¶</a></h2>

<p>Traditionally, the first program you write in a new language is called

“Hello, World!” because all it does is display the words “Hello,

World!”. In Python, it looks like this:</p>

<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="s">'Hello, World!'</span><span class="p">)</span>

</pre></div>

</div>

<p>This is an example of a <strong>print statement</strong>, although it doesn’t

actually print anything on paper. It displays a result on the screen. In

this case, the result is the words</p>

<div class="highlight-python"><div class="highlight"><pre>Hello, World!

</pre></div>

</div>

<p>The quotation marks in the program mark the beginning and end of the

text to be displayed; they don’t appear in the result.</p>

<p>The parentheses indicate that print is a function. We’ll get to

functions in Chapter [funcchap].</p>

<p>In Python 2, the print statement is slightly different; it is not a

function, so it doesn’t use parentheses.</p>

<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">print</span> <span class="s">'Hello, World!'</span>

</pre></div>

</div>

<p>This distinction will make more sense soon, but that’s enough to get

started.</p>

</div>

<h2>Arithmetic operators<a class="headerlink" href="#arithmetic-operators" title="Permalink to this headline">¶</a></h2>

<p>After “Hello, World”, the next step is arithmetic. Python provides

<strong>operators</strong>, which are special symbols that represent computations

like addition and multiplication.</p>

<p>The operators +, -, and perform addition, subtraction, and

multiplication, as in the following examples:</p>

</pre></div>

</div>

<p>The operator / performs division:</p>

</pre></div>

</div>

<p>You might wonder why the result is 42.0 instead of 42. I’ll explain in

the next section.</p>

<p>Finally, the operator * performs exponentiation; that is, it raises a

number to a power:</p>

</pre></div>

</div>

<p>In some other languages, <code class="docutils literal"><span class="pre">^</span></code> is used for exponentiation, but in Python

it is a bitwise operator called XOR. If you are not familiar with

bitwise operators, the result will surprise you:</p>

</pre></div>

</div>

<p>I won’t cover bitwise operators in this book, but you can read about

them at <a class="reference external" href="http://wiki.python.org/moin/BitwiseOperators">http://wiki.python.org/moin/BitwiseOperators</a>.</p>

</div>

<h2>Values and types<a class="headerlink" href="#values-and-types" title="Permalink to this headline">¶</a></h2>

<p>A <strong>value</strong> is one of the basic things a program works with, like a

letter or a number. Some values we have seen so far are 2, 42.0, and

<code class="docutils literal"><span class="pre">'Hello,</span> <span class="pre">World!'</span></code>.</p>

<p>These values belong to different <strong>types</strong>: 2 is an <strong>integer</strong>, 42.0 is

a <strong>floating-point number</strong>, and <code class="docutils literal"><span class="pre">'Hello,</span> <span class="pre">World!'</span></code> is a <strong>string</strong>,

so-called because the letters it contains are strung together.</p>

<p>If you are not sure what type a value has, the interpreter can tell you:</p>

<span class="gp">>>> </span><span class="nb">type</span><span class="p">(</span><span class="s">'Hello, World!'</span><span class="p">)</span>

</pre></div>

</div>

<p>In these results, the word “class” is used in the sense of a category; a

type is a category of values.</p>

<p>Not surprisingly, integers belong to the type int, strings belong to str

and floating-point numbers belong to float.</p>

<p>What about values like <code class="docutils literal"><span class="pre">'2'</span></code> and <code class="docutils literal"><span class="pre">'42.0'</span></code>? They look like numbers,

but they are in quotation marks like strings.</p>

</pre></div>

</div>

<p>They’re strings.</p>

<p>When you type a large integer, you might be tempted to use commas

between groups of digits, as in 1,000,000. This is not a legal <em>integer</em>

in Python, but it is legal:</p>

</pre></div>

</div>

<p>That’s not what we expected at all! Python interprets 1,000,000 as a

comma-separated sequence of integers. We’ll learn more about this kind

of sequence later.</p>

</div>

<h2>Formal and natural languages<a class="headerlink" href="#formal-and-natural-languages" title="Permalink to this headline">¶</a></h2>

<p><strong>Natural languages</strong> are the languages people speak, such as English,

Spanish, and French. They were not designed by people (although people

try to impose some order on them); they evolved naturally.</p>

<p><strong>Formal languages</strong> are languages that are designed by people for

specific applications. For example, the notation that mathematicians use

is a formal language that is particularly good at denoting relationships

among numbers and symbols. Chemists use a formal language to represent

the chemical structure of molecules. And most importantly:</p>

<div><strong>Programming languages are formal languages that have been designed

to express computations.</strong></div></blockquote>

<p>Formal languages tend to have strict <strong>syntax</strong> rules that govern the

structure of statements. For example, in mathematics the statement

<span class="math">3 + 3 = 6</span> has correct syntax, but <span class="math">3 + = 3 \$ 6</span> does not.

In chemistry <span class="math">H_2O</span> is a syntactically correct formula, but

<p>Syntax rules come in two flavors, pertaining to <strong>tokens</strong> and

structure. Tokens are the basic elements of the language, such as words,

numbers, and chemical elements. One of the problems with

<span class="math">3 += 3 \$ 6</span> is that :math:` $ ` is not a legal token in

mathematics (at least as far as I know). Similarly, <span class="math">_2Zz</span> is not

legal because there is no element with the abbreviation <span class="math">Zz</span>.</p>

<p>The second type of syntax rule pertains to the way tokens are combined.

The equation <span class="math">3 += 3</span> is illegal because even though <span class="math">+</span> and

<span class="math">=</span> are legal tokens, you can’t have one right after the other.

Similarly, in a chemical formula the subscript comes after the element

name, not before.</p>

<p>This is @ well-structured Engli$h sentence with invalid t*kens in it.

This sentence all valid tokens has, but invalid structure with.</p>

<p>When you read a sentence in English or a statement in a formal language,

you have to figure out the structure (although in a natural language you

do this subconsciously). This process is called <strong>parsing</strong>.</p>

<p>Although formal and natural languages have many features in

common—tokens, structure, and syntax—there are some differences:</p>

<dt>ambiguity:</dt>

<dd>Natural languages are full of ambiguity, which people deal with by

using contextual clues and other information. Formal languages are

designed to be nearly or completely unambiguous, which means that

any statement has exactly one meaning, regardless of context.</dd>

<dt>redundancy:</dt>

<dd>In order to make up for ambiguity and reduce misunderstandings,

natural languages employ lots of redundancy. As a result, they are

often verbose. Formal languages are less redundant and more concise.</dd>

<dt>literalness:</dt>

<dd>Natural languages are full of idiom and metaphor. If I say, “The

penny dropped”, there is probably no penny and nothing dropping

(this idiom means that someone understood something after a period

of confusion). Formal languages mean exactly what they say.</dd>

</dl>

<p>Because we all grow up speaking natural languages, it is sometimes hard

to adjust to formal languages. The difference between formal and natural

language is like the difference between poetry and prose, but more so:</p>

<dt>Poetry:</dt>

<dd>Words are used for their sounds as well as for their meaning, and

the whole poem together creates an effect or emotional response.

Ambiguity is not only common but often deliberate.</dd>

<dt>Prose:</dt>

<dd>The literal meaning of words is more important, and the structure

contributes more meaning. Prose is more amenable to analysis than

poetry but still often ambiguous.</dd>

<dt>Programs:</dt>

<dd>The meaning of a computer program is unambiguous and literal, and

can be understood entirely by analysis of the tokens and structure.</dd>

</dl>

<p>Formal languages are more dense than natural languages, so it takes

longer to read them. Also, the structure is important, so it is not

always best to read from top to bottom, left to right. Instead, learn to

parse the program in your head, identifying the tokens and interpreting

the structure. Finally, the details matter. Small errors in spelling and

punctuation, which you can get away with in natural languages, can make

a big difference in a formal language.</p>

</div>

<h2>Debugging<a class="headerlink" href="#debugging" title="Permalink to this headline">¶</a></h2>

<p>Programmers make mistakes. For whimsical reasons, programming errors are

called <strong>bugs</strong> and the process of tracking them down is called

<strong>debugging</strong>.</p>

<p>Programming, and especially debugging, sometimes brings out strong

emotions. If you are struggling with a difficult bug, you might feel

angry, despondent, or embarrassed.</p>

<p>There is evidence that people naturally respond to computers as if they

were people. When they work well, we think of them as teammates, and

when they are obstinate or rude, we respond to them the same way we

respond to rude, obstinate people (Reeves and Nass, <em>The Media Equation:

How People Treat Computers, Television, and New Media Like Real People

and Places</em>).</p>

<p>Preparing for these reactions might help you deal with them. One

approach is to think of the computer as an employee with certain

strengths, like speed and precision, and particular weaknesses, like

lack of empathy and inability to grasp the big picture.</p>

<p>Your job is to be a good manager: find ways to take advantage of the

strengths and mitigate the weaknesses. And find ways to use your

emotions to engage with the problem, without letting your reactions

interfere with your ability to work effectively.</p>

<p>Learning to debug can be frustrating, but it is a valuable skill that is

useful for many activities beyond programming. At the end of each

chapter there is a section, like this one, with my suggestions for

debugging. I hope they help!</p>

</div>

<span id="glossary01"></span><h2>Glossary<a class="headerlink" href="#glossary" title="Permalink to this headline">¶</a></h2>

<dt>solução de problemas (<em>problem solving</em>)</dt>

<dd>The process of formulating a problem, finding a solution, and expressing it.</dd>

<dt>linguagem de alto nível (<em>high-level language</em>)</dt>

<dd>A programming language like Python that is designed to be easy for humans to read and write.</dd>

<dt>linguagem de baixo nível (<em>low-level language</em>)</dt>

<dd>A programming language that is designed to be easy for a computer to run; also called “machine language” or “assembly language”.</dd>

<dt>portabilidade (<em>portability</em>)</dt>

<dd>A property of a program that can run on more than one kind of computer.</dd>

<dt>interpretador (<em>interpreter</em>)</dt>

<dd>A program that reads another program and executes it</dd>

<dt><em>prompt</em> (“sinal de pronto”)</dt>

<dd>Characters displayed by the interpreter to indicate that it is ready to take input from the user.</dd>

<dt>programação funcional (<em>program</em>)</dt>

<dd>A set of instructions that specifies a computation.</dd>

<dt><code class="docutils literal"><span class="pre">print</span></code>, instrução (<code class="docutils literal"><span class="pre">print</span></code> <em>statement</em>)</dt>

<dd>An instruction that causes the Python interpreter to display a value on the screen.</dd>

<dt>operador (<em>operator</em>)</dt>

<dd>A special symbol that represents a simple computation like addition, multiplication, or string concatenation.</dd>

<dt>valor (<em>value</em>)</dt>

<dd>One of the basic units of data, like a number or string, that a program manipulates.</dd>

<dd>A category of values. The types we have seen so far are integers (type int), floating-point numbers (type float), and strings (type str).</dd>

<dt>inteiro (<em>integer</em>)</dt>

<dd>A type that represents whole numbers.</dd>

<dt>ponto-flutuante (<em>floating-point</em>)</dt>

<dd>A type that represents numbers with fractional parts.</dd>

<dt><em>string</em> (“cadeia de caracteres”)</dt>

<dd>A type that represents sequences of characters.</dd>

<dt>linguagem natural (<em>natural language</em>)</dt>

<dd>Any one of the languages that people speak that evolved naturally.</dd>

<dt>linguagem formal (<em>formal language</em>)</dt>

<dd>Any one of the languages that people have designed for specific purposes, such as representing mathematical ideas or computer programs; all programming languages are formal languages.</dd>

<dt><em>token</em> (“símbolo”)</dt>

<dd>One of the basic elements of the syntactic structure of a program, analogous to a word in a natural language.</dd>

<dt>sintaxe (<em>syntax</em>)</dt>

<dd>The rules that govern the structure of a program.</dd>

<dt>analisar (<em>parse</em>)</dt>

<dd>To examine a program and analyze the syntactic structure.</dd>

<dd>An error in a program.</dd>

<dt>depuração (<em>debugging</em>)</dt>

<dd>The process of finding and correcting bugs.</dd>

</dl>

</div>

<h2>Exercises<a class="headerlink" href="#exercises" title="Permalink to this headline">¶</a></h2>

<p>It is a good idea to read this book in front of a computer so you can

try out the examples as you go.</p>

<p>Whenever you are experimenting with a new feature, you should try to

make mistakes. For example, in the “Hello, world!” program, what happens

if you leave out one of the quotation marks? What if you leave out both?

What if you spell print wrong?</p>

<p>This kind of experiment helps you remember what you read; it also helps

when you are programming, because you get to know what the error

messages mean. It is better to make mistakes now and on purpose than

later and accidentally.</p>

<li>In a print statement, what happens if you leave out one of the

parentheses, or both?</li>

<li>If you are trying to print a string, what happens if you leave out

one of the quotation marks, or both?</li>

<li>You can use a minus sign to make a negative number like -2. What

happens if you put a plus sign before a number? What about 2++2?</li>

<li>In math notation, leading zeros are ok, as in 02. What happens if you

try this in Python?</li>

<li>What happens if you have two values with no operator between them?</li>

</ol>

<p>Start the Python interpreter and use it as a calculator.</p>

<li>How many seconds are there in 42 minutes 42 seconds?</li>

<li>How many miles are there in 10 kilometers? Hint: there are 1.61

kilometers in a mile.</li>

<li>If you run a 10 kilometer race in 42 minutes 42 seconds, what is your

average pace (time per mile in minutes and seconds)? What is your

average speed in miles per hour?</li>

</ol>

</div>

<h3><a href="index.html">Table Of Contents</a></h3>

<ul>

<li><a class="reference internal" href="#">The way of the program</a><ul>

<li><a class="reference internal" href="#what-is-a-program">What is a program?</a></li>

<li><a class="reference internal" href="#running-python">Running Python</a></li>

<li><a class="reference internal" href="#the-first-program">The first program</a></li>

<li><a class="reference internal" href="#arithmetic-operators">Arithmetic operators</a></li>

<li><a class="reference internal" href="#values-and-types">Values and types</a></li>

<li><a class="reference internal" href="#formal-and-natural-languages">Formal and natural languages</a></li>

<li><a class="reference internal" href="#debugging">Debugging</a></li>

<li><a class="reference internal" href="#glossary">Glossary</a></li>

<li><a class="reference internal" href="#exercises">Exercises</a></li>

</ul>

</li>

</ul>

<h3>Related Topics</h3>

<ul>

<li><a href="index.html">Documentation overview</a><ul>

<li>Previous: <a href="00-front-matter.html" title="previous chapter">Preface</a></li>

<li>Next: <a href="02-statement.html" title="next chapter">Variables, expressions and statements</a></li>

</ul></li>

</ul>

</div>

<li><a href="_sources/01-the-way.txt"

rel="nofollow">Show Source</a></li>

</ul>

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