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<h1 class="chapter" id="sec15">Chapter&#XA0;2&#XA0;&#XA0;Variables, expressions and statements</h1>

<p>One of the most powerful features of a programming language is the

ability to manipulate <span class="c010">variables</span>. A variable is a name that

refers to a value.

<a id="hevea_default88"></a></p>

<h2 class="section" id="sec16">2.1&#XA0;&#XA0;Assignment statements</h2>

<a id="variables"></a>

<a id="hevea_default89"></a>

<a id="hevea_default90"></a></p><p>An <span class="c010">assignment statement</span> creates a new variable and gives

it a value:</p><pre class="verbatim">&gt;&gt;&gt; message = 'And now for something completely different'

&gt;&gt;&gt; n = 17

&gt;&gt;&gt; pi = 3.141592653589793

</pre><p>This example makes three assignments. The first assigns a string

to a new variable named <span class="c004">message</span>;

the second gives the integer <span class="c004">17</span> to <span class="c004">n</span>; the third

assigns the (approximate) value of &#X3C0; to <span class="c004">pi</span>.

<a id="hevea_default91"></a>

<a id="hevea_default92"></a></p><p>A common way to represent variables on paper is to write the name with

an arrow pointing to its value. This kind of figure is

called a <span class="c010">state diagram</span> because it shows what state each of the

variables is in (think of it as the variable&#X2019;s state of mind).

Figure&#XA0;<a href="thinkpython2003.html#fig.state2">2.1</a> shows the result of the previous example.</p><blockquote class="figure"><div class="center"><hr class="c019"></div>

<div class="center"><img src="images/thinkpython2001.png"></div>

<div class="caption"><table class="c001 cellpading0"><tr><td class="c018">Figure 2.1: State diagram.</td></tr>

<a id="fig.state2"></a>

<div class="center"><hr class="c019"></div></blockquote>

<h2 class="section" id="sec17">2.2&#XA0;&#XA0;Variable names</h2>

<a id="hevea_default93"></a></p><p>Programmers generally choose names for their variables that

are meaningful&#X2014;they document what the variable is used for.</p><p>Variable names can be as long as you like. They can contain

both letters and numbers, but they can&#X2019;t begin with a number.

It is legal to use uppercase letters, but it is conventional

to use only lower case for variables names.</p><p>The underscore character, <code>_</code>, can appear in a name.

It is often used in names with multiple words, such as

<code>your_name</code> or <code>airspeed_of_unladen_swallow</code>.

<a id="hevea_default94"></a></p><p>If you give a variable an illegal name, you get a syntax error:</p><pre class="verbatim">&gt;&gt;&gt; 76trombones = 'big parade'

SyntaxError: invalid syntax

&gt;&gt;&gt; more@ = 1000000

SyntaxError: invalid syntax

&gt;&gt;&gt; class = 'Advanced Theoretical Zymurgy'

SyntaxError: invalid syntax

</pre><p><span class="c004">76trombones</span> is illegal because it begins with a number.

<span class="c004">more@</span> is illegal because it contains an illegal character, <span class="c004">@</span>. But what&#X2019;s wrong with <span class="c004">class</span>?</p><p>It turns out that <span class="c004">class</span> is one of Python&#X2019;s <span class="c010">keywords</span>. The

interpreter uses keywords to recognize the structure of the program,

and they cannot be used as variable names.

<a id="hevea_default95"></a></p><p>Python 3 has these keywords:</p><pre class="verbatim">False class finally is return

None continue for lambda try

True def from nonlocal while

and del global not with

as elif if or yield

assert else import pass

break except in raise

</pre><p>You don&#X2019;t have to memorize this list. In most development environments,

keywords are displayed in a different color; if you try to use one

as a variable name, you&#X2019;ll know.</p>

<h2 class="section" id="sec18">2.3&#XA0;&#XA0;Expressions and statements</h2>

<p>An <span class="c010">expression</span> is a combination of values, variables, and operators.

A value all by itself is considered an expression, and so is

a variable, so the following are all legal expressions:

<a id="hevea_default96"></a></p><pre class="verbatim">&gt;&gt;&gt; 42

42

&gt;&gt;&gt; n

17

&gt;&gt;&gt; n + 25

42

</pre><p>When you type an expression at the prompt, the interpreter

<span class="c010">evaluates</span> it, which means that it finds the value of

the expression.

In this example, <span class="c004">n</span> has the value 17 and

<span class="c004">n + 25</span> has the value 42.

<a id="hevea_default97"></a></p><p>A <span class="c010">statement</span> is a unit of code that has an effect, like

creating a variable or displaying a value.

<a id="hevea_default98"></a></p><pre class="verbatim">&gt;&gt;&gt; n = 17

&gt;&gt;&gt; print(n)

</pre><p>The first line is an assignment statement that gives a value to

<span class="c004">n</span>. The second line is a print statement that displays the

value of <span class="c004">n</span>.</p><p>When you type a statement, the interpreter <span class="c010">executes</span> it,

which means that it does whatever the statement says. In general,

statements don&#X2019;t have values.

<a id="hevea_default99"></a></p>

<h2 class="section" id="sec19">2.4&#XA0;&#XA0;Script mode</h2>

<p>So far we have run Python in <span class="c010">interactive mode</span>, which

means that you interact directly with the interpreter.

Interactive mode is a good way to get started,

but if you are working with more than a few lines of code, it can be

clumsy.

<a id="hevea_default100"></a></p><p>The alternative is to save code in a file called a <span class="c010">script</span> and

then run the interpreter in <span class="c010">script mode</span> to execute the script. By

convention, Python scripts have names that end with <span class="c004">.py</span>.

<a id="hevea_default101"></a>

<a id="hevea_default102"></a></p><p>If you know how to create and run a script on your computer, you

are ready to go. Otherwise I recommend using PythonAnywhere again.

I have posted instructions for running in script mode at

<a href="http://tinyurl.com/thinkpython2e"><span class="c004">http://tinyurl.com/thinkpython2e</span></a>.</p><p>Because Python provides both modes,

you can test bits of code in interactive mode before you put them

in a script. But there are differences between interactive mode

and script mode that can be confusing.

<a id="hevea_default103"></a>

<a id="hevea_default104"></a></p><p>For example, if you are using Python as a calculator, you might type</p><pre class="verbatim">&gt;&gt;&gt; miles = 26.2

&gt;&gt;&gt; miles * 1.61

42.182

</pre><p>The first line assigns a value to <span class="c004">miles</span>, but it has no visible

effect. The second line is an expression, so the

interpreter evaluates it and displays the result. It turns out that a

marathon is about 42 kilometers.</p><p>But if you type the same code into a script and run it, you get no

output at all. In script mode an expression, all by itself, has no

visible effect. Python actually evaluates the expression, but it doesn&#X2019;t

display the value unless you tell it to:</p><pre class="verbatim">miles = 26.2

print(miles * 1.61)

</pre><p>This behavior can be confusing at first.</p><p>A script usually contains a sequence of statements. If there

is more than one statement, the results appear one at a time

as the statements execute.</p><p>For example, the script</p><pre class="verbatim">print(1)

x = 2

print(x)

</pre><p>produces the output</p><pre class="verbatim">1

2

</pre><p>The assignment statement produces no output.</p><p>To check your understanding, type the following statements in the

Python interpreter and see what they do:</p><pre class="verbatim">5

x = 5

x + 1

</pre><p>Now put the same statements in a script and run it. What

is the output? Modify the script by transforming each

expression into a print statement and then run it again.</p>

<h2 class="section" id="sec20">2.5&#XA0;&#XA0;Order of operations</h2>

<a id="hevea_default105"></a>

<a id="hevea_default106"></a></p><p>When an expression contains more than one operator, the order of

evaluation depends on the <span class="c010">order of operations</span>. For

mathematical operators, Python follows mathematical convention.

The acronym <span class="c010">PEMDAS</span> is a useful way to

remember the rules:</p><ul class="itemize"><li class="li-itemize"><span class="c010">P</span>arentheses have the highest precedence and can be used

to force an expression to evaluate in the order you want. Since

expressions in parentheses are evaluated first, <span class="c004">2 * (3-1)</span> is 4,

and <span class="c004">(1+1)**(5-2)</span> is 8. You can also use parentheses to make an

expression easier to read, as in <span class="c004">(minute * 100) / 60</span>, even

if it doesn&#X2019;t change the result.</li><li class="li-itemize"><span class="c010">E</span>xponentiation has the next highest precedence, so

<span class="c004">1 + 2**3</span> is 9, not 27, and <span class="c004">2 * 3**2</span> is 18, not 36.</li><li class="li-itemize"><span class="c010">M</span>ultiplication and <span class="c010">D</span>ivision have higher precedence

than <span class="c010">A</span>ddition and <span class="c010">S</span>ubtraction. So <span class="c004">2*3-1</span> is 5, not

4, and <span class="c004">6+4/2</span> is 8, not 5.</li><li class="li-itemize">Operators with the same precedence are evaluated from left to

right (except exponentiation). So in the expression <span class="c004">degrees /

2 * pi</span>, the division happens first and the result is multiplied

by <span class="c004">pi</span>. To divide by 2 &#X3C0;, you can use parentheses or write

<span class="c004">degrees / 2 / pi</span>.</li></ul><p>I don&#X2019;t work very hard to remember the precedence of

operators. If I can&#X2019;t tell by looking at the expression, I use

parentheses to make it obvious.</p>

<h2 class="section" id="sec21">2.6&#XA0;&#XA0;String operations</h2>

<a id="hevea_default107"></a>

<a id="hevea_default108"></a></p><p>In general, you can&#X2019;t perform mathematical operations on strings, even

if the strings look like numbers, so the following are illegal:</p><pre class="verbatim">'2'-'1' 'eggs'/'easy' 'third'*'a charm'

</pre><p>But there are two exceptions, <span class="c004">+</span> and <span class="c004">*</span>.</p><p>The <span class="c004">+</span> operator performs <span class="c010">string concatenation</span>, which means

it joins the strings by linking them end-to-end. For example:

<a id="hevea_default109"></a></p><pre class="verbatim">&gt;&gt;&gt; first = 'throat'

&gt;&gt;&gt; second = 'warbler'

&gt;&gt;&gt; first + second

throatwarbler

</pre><p>The <span class="c004">*</span> operator also works on strings; it performs repetition.

For example, <code>'Spam'*3</code> is <code>'SpamSpamSpam'</code>. If one of the

values is a string, the other has to be an integer.</p><p>This use of <span class="c004">+</span> and <span class="c004">*</span> makes sense by

analogy with addition and multiplication. Just as <span class="c004">4*3</span> is

equivalent to <span class="c004">4+4+4</span>, we expect <code>'Spam'*3</code> to be the same as

<code>'Spam'+'Spam'+'Spam'</code>, and it is. On the other hand, there is a

significant way in which string concatenation and repetition are

different from integer addition and multiplication.

Can you think of a property that addition has

that string concatenation does not?

<a id="hevea_default110"></a></p>

<h2 class="section" id="sec22">2.7&#XA0;&#XA0;Comments</h2>

<a id="hevea_default111"></a></p><p>As programs get bigger and more complicated, they get more difficult

to read. Formal languages are dense, and it is often difficult to

look at a piece of code and figure out what it is doing, or why.</p><p>For this reason, it is a good idea to add notes to your programs to explain

in natural language what the program is doing. These notes are called

<span class="c010">comments</span>, and they start with the <code>#</code> symbol:</p><pre class="verbatim"># compute the percentage of the hour that has elapsed

percentage = (minute * 100) / 60

</pre><p>In this case, the comment appears on a line by itself. You can also put

comments at the end of a line:</p><pre class="verbatim">percentage = (minute * 100) / 60 # percentage of an hour

</pre><p>Everything from the <span class="c004">#</span> to the end of the line is ignored&#X2014;it

has no effect on the execution of the program.</p><p>Comments are most useful when they document non-obvious features of

the code. It is reasonable to assume that the reader can figure out

<em>what</em> the code does; it is more useful to explain <em>why</em>.</p><p>This comment is redundant with the code and useless:</p><pre class="verbatim">v = 5 # assign 5 to v

</pre><p>This comment contains useful information that is not in the code:</p><pre class="verbatim">v = 5 # velocity in meters/second.

</pre><p>Good variable names can reduce the need for comments, but

long names can make complex expressions hard to read, so there is

a tradeoff.</p>

<h2 class="section" id="sec23">2.8&#XA0;&#XA0;Debugging</h2>

<a id="hevea_default112"></a>

<a id="hevea_default113"></a></p><p>Three kinds of errors can occur in a program: syntax errors, runtime

errors, and semantic errors. It is useful

to distinguish between them in order to track them down more quickly.</p><dl class="description"><dt class="dt-description"><span class="c010">Syntax error:</span></dt><dd class="dd-description"> &#X201C;Syntax&#X201D; refers to the structure of a program

and the rules about that structure. For example, parentheses have

to come in matching pairs, so <span class="c004">(1 + 2)</span> is legal, but <span class="c004">8)</span>

is a <span class="c010">syntax error</span>. <a id="hevea_default114"></a> <a id="hevea_default115"></a>

<a id="hevea_default116"></a>

<a id="hevea_default117"></a> <p>If there is a syntax error

anywhere in your program, Python displays an error message and quits,

and you will not be able to run the program. During the first few

weeks of your programming career, you might spend a lot of

time tracking down syntax errors. As you gain experience, you will

make fewer errors and find them faster.</p></dd><dt class="dt-description"><span class="c010">Runtime error:</span></dt><dd class="dd-description"> The second type of error is a runtime error, so

called because the error does not appear until after the program has

started running. These errors are also called <span class="c010">exceptions</span>

because they usually indicate that something exceptional (and bad)

has happened. <a id="hevea_default118"></a> <a id="hevea_default119"></a>

<a id="hevea_default120"></a> <a id="hevea_default121"></a> <a id="hevea_default122"></a><p>Runtime errors are rare in the simple programs you will see in the

first few chapters, so it might be a while before you encounter one.</p></dd><dt class="dt-description"><span class="c010">Semantic error:</span></dt><dd class="dd-description"> The third type of error is &#X201C;semantic&#X201D;, which

means related to meaning. If there is a semantic error in your

program, it will run without generating error messages, but it will

not do the right thing. It will do something else. Specifically,

it will do what you told it to do. <a id="hevea_default123"></a>

<a id="hevea_default124"></a> <a id="hevea_default125"></a><p>Identifying semantic errors can be tricky because it requires you to work

backward by looking at the output of the program and trying to figure

out what it is doing.</p></dd></dl>

<h2 class="section" id="sec24">2.9&#XA0;&#XA0;Glossary</h2>

<dl class="description"><dt class="dt-description"><span class="c010">variable:</span></dt><dd class="dd-description"> A name that refers to a value.

<a id="hevea_default126"></a></dd><dt class="dt-description"><span class="c010">assignment:</span></dt><dd class="dd-description"> A statement that assigns a value to a variable.

<a id="hevea_default127"></a></dd><dt class="dt-description"><span class="c010">state diagram:</span></dt><dd class="dd-description"> A graphical representation of a set of variables and the

values they refer to.

<a id="hevea_default128"></a></dd><dt class="dt-description"><span class="c010">keyword:</span></dt><dd class="dd-description"> A reserved word that is used to parse a

program; you cannot use keywords like <span class="c004">if</span>, <span class="c004">def</span>, and <span class="c004">while</span> as

variable names.

<a id="hevea_default129"></a></dd><dt class="dt-description"><span class="c010">operand:</span></dt><dd class="dd-description"> One of the values on which an operator operates.

<a id="hevea_default130"></a></dd><dt class="dt-description"><span class="c010">expression:</span></dt><dd class="dd-description"> A combination of variables, operators, and values that

represents a single result.

<a id="hevea_default131"></a></dd><dt class="dt-description"><span class="c010">evaluate:</span></dt><dd class="dd-description"> To simplify an expression by performing the operations

in order to yield a single value.</dd><dt class="dt-description"><span class="c010">statement:</span></dt><dd class="dd-description"> A section of code that represents a command or action. So

far, the statements we have seen are assignments and print statements.

<a id="hevea_default132"></a></dd><dt class="dt-description"><span class="c010">execute:</span></dt><dd class="dd-description"> To run a statement and do what it says.

<a id="hevea_default133"></a></dd><dt class="dt-description"><span class="c010">interactive mode:</span></dt><dd class="dd-description"> A way of using the Python interpreter by

typing code at the prompt.

<a id="hevea_default134"></a></dd><dt class="dt-description"><span class="c010">script mode:</span></dt><dd class="dd-description"> A way of using the Python interpreter to read

code from a script and run it.

<a id="hevea_default135"></a></dd><dt class="dt-description"><span class="c010">script:</span></dt><dd class="dd-description"> A program stored in a file.

<a id="hevea_default136"></a></dd><dt class="dt-description"><span class="c010">order of operations:</span></dt><dd class="dd-description"> Rules governing the order in which

expressions involving multiple operators and operands are evaluated.

<a id="hevea_default137"></a></dd><dt class="dt-description"><span class="c010">concatenate:</span></dt><dd class="dd-description"> To join two operands end-to-end.

<a id="hevea_default138"></a></dd><dt class="dt-description"><span class="c010">comment:</span></dt><dd class="dd-description"> Information in a program that is meant for other

programmers (or anyone reading the source code) and has no effect on the

execution of the program.

<a id="hevea_default139"></a></dd><dt class="dt-description"><span class="c010">syntax error:</span></dt><dd class="dd-description"> An error in a program that makes it impossible

to parse (and therefore impossible to interpret).

<a id="hevea_default140"></a></dd><dt class="dt-description"><span class="c010">exception:</span></dt><dd class="dd-description"> An error that is detected while the program is running.

<a id="hevea_default141"></a></dd><dt class="dt-description"><span class="c010">semantics:</span></dt><dd class="dd-description"> The meaning of a program.

<a id="hevea_default142"></a></dd><dt class="dt-description"><span class="c010">semantic error:</span></dt><dd class="dd-description"> An error in a program that makes it do something

other than what the programmer intended.

<a id="hevea_default143"></a></dd></dl>

<h2 class="section" id="sec25">2.10&#XA0;&#XA0;Exercises</h2>

<div class="theorem"><span class="c010">Exercise&#XA0;1</span>&#XA0;&#XA0;<p><em>Repeating my advice from the previous chapter, whenever you learn

a new feature, you should try it out in interactive mode and make

errors on purpose to see what goes wrong.</em></p><ul class="itemize"><li class="li-itemize"><em>We&#X2019;ve seen that <span class="c004">n = 42</span> is legal. What about <span class="c004">42 = n</span>?</em></li><li class="li-itemize"><em>How about <span class="c004">x = y = 1</span>?</em></li><li class="li-itemize"><em>In some languages every statement ends with a semi-colon, <span class="c004">;</span>.

What happens if you put a semi-colon at the end of a Python statement?</em></li><li class="li-itemize"><em>What if you put a period at the end of a statement?</em></li><li class="li-itemize"><em>In math notation you can multiply </em><span class="c009">x</span><em> and </em><span class="c009">y</span><em> like this: </em><span class="c009">x y</span><em>.

What happens if you try that in Python?</em></li></ul></div><div class="theorem"><span class="c010">Exercise&#XA0;2</span>&#XA0;&#XA0;<p><em>Practice using the Python interpreter as a calculator:

</em><a id="hevea_default144"></a></p><ol class="enumerate" type=1><li class="li-enumerate"><em>The volume of a sphere with radius </em><span class="c009">r</span><em> is </em>4/3 &#X3C0; <span class="c009">r</span>³<em>.

What is the volume of a sphere with radius 5?</em></li><li class="li-enumerate"><em>Suppose the cover price of a book is $24.95, but bookstores get a

40% discount. Shipping costs $3 for the first copy and 75 cents

for each additional copy. What is the total wholesale cost for

60 copies?</em></li><li class="li-enumerate"><em>If I leave my house at 6:52 am and run 1 mile at an easy pace

(8:15 per mile), then 3 miles at tempo (7:12 per mile) and 1 mile at

easy pace again, what time do I get home for breakfast?

</em><a id="hevea_default145"></a></li></ol></div>

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