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<!DOCTYPE html>

<html>

<head>

<title>Variables, expressions, and statements</title>

</head>

<body>

<hr />

<h1 class="chapter" id="sec17">Chapter&#XA0;2&#XA0;&#XA0;Variables, expressions, and statements</h1>

<h2 class="section" id="sec18">2.1&#XA0;&#XA0;Values and types</h2>

<a id="hevea_default34"></a>A value is one of the basic things a program works with,

like a letter or a

number. The values we have seen so far

are 1, 2, and

<code>'Hello, World!'</code>These values belong to different types:

2 is an integer, and <code>'Hello, World!'</code> is a string,

so called because it contains a &#X201C;string&#X201D; of letters.

You (and the interpreter) can identify

strings because they are enclosed in quotation marks.<a id="hevea_default35"></a>The print statement also works for integers. We use the

python command to start the interpreter.<pre class="verbatim">python

>>> print 4

</pre>If you are not sure what type a value has, the interpreter can tell you.<pre class="verbatim">>>> type('Hello, World!')

>>> type(17)

</pre>Not surprisingly, strings belong to the type str and

integers belong to the type int. Less obviously, numbers

with a decimal point belong to a type called float,

because these numbers are represented in a

format called floating point.<a id="hevea_default36"></a>

<a id="hevea_default42"></a><pre class="verbatim">>>> type(3.2)

</pre>What about values like <code>'17'</code> and <code>'3.2'</code>?

They look like numbers, but they are in quotation marks like

strings.<a id="hevea_default43"></a><pre class="verbatim">>>> type('17')

>>> type('3.2')

</pre>They&#X2019;re strings.When you type a large integer, you might be tempted to use commas

between groups of three digits, as in 1,000,000. This is not a

legal integer in Python, but it is legal:<pre class="verbatim">>>> print 1,000,000

1 0 0

</pre>Well, that&#X2019;s not what we expected at all! Python interprets 1,000,000 as a comma-separated sequence of integers, which it

prints with spaces between.<a id="hevea_default44"></a>

<a id="hevea_default46"></a>This is the first example we have seen of a semantic error: the code

runs without producing an error message, but it doesn&#X2019;t do the

&#X201C;right&#X201D; thing.

<h2 class="section" id="sec19">2.2&#XA0;&#XA0;Variables</h2>

<a id="hevea_default49"></a>One of the most powerful features of a programming language is the

ability to manipulate variables. A variable is a name that

refers to a value.An assignment statement creates new variables and gives

them values:<pre class="verbatim">>>> message = 'And now for something completely different'

>>> n = 17

>>> pi = 3.1415926535897931

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

to a new variable named message;

the second assigns the integer 17 to n; the third

assigns the (approximate) value of &#X3C0; to pi.To display the value of a variable, you can use a print statement:<pre class="verbatim">>>> print n

>>> print pi

3.14159265359

</pre>The type of a variable is the type of the value it refers to.<pre class="verbatim">>>> type(message)

>>> type(n)

>>> type(pi)

</pre>

<h2 class="section" id="sec20">2.3&#XA0;&#XA0;Variable names and keywords</h2>

<a id="hevea_default50"></a>Programmers generally choose names for their variables that

are meaningful and document what the variable is used for.Variable names can be arbitrarily long. They can contain

both letters and numbers, but they cannot start with a number.

It is legal to use uppercase letters, but it is a good idea

to begin variable names with a lowercase letter (you&#X2019;ll

see why later).The underscore character (<code>_</code>) can appear in a name.

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

<code>my_name</code> or <code>airspeed_of_unladen_swallow</code>.

Variable names can start with an underscore character, but

we generally avoid doing this unless we are writing library

code for others to use.<a id="hevea_default51"></a>If you give a variable an illegal name, you get a syntax error:<pre class="verbatim">>>> 76trombones = 'big parade'

SyntaxError: invalid syntax

>>> more@ = 1000000

SyntaxError: invalid syntax

>>> class = 'Advanced Theoretical Zymurgy'

SyntaxError: invalid syntax

</pre>76trombones is illegal because it begins with a number.

more@ is illegal because it contains an illegal character, @. But what&#X2019;s wrong with class?It turns out that class is one of Python&#X2019;s keywords. The

interpreter uses keywords to recognize the structure of the program,

and they cannot be used as variable names.<a id="hevea_default52"></a>Python reserves 31 keywords<a id="text3" href="#note3">1</a> for its use:<pre class="verbatim">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

</pre>You might want to keep this list handy. If the interpreter complains

about one of your variable names and you don&#X2019;t know why, see if it

is on this list.

<h2 class="section" id="sec21">2.4&#XA0;&#XA0;Statements</h2>

A statement is a unit of code that the Python interpreter can

execute. We have seen two kinds of statements: print

and assignment.<a id="hevea_default53"></a>

<a id="hevea_default55"></a>When you type a statement in interactive mode, the interpreter

executes it and displays the result, if there is one.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.For example, the script<pre class="verbatim">print 1

x = 2

print x

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

</pre>The assignment statement produces no output.

<h2 class="section" id="sec22">2.5&#XA0;&#XA0;Operators and operands</h2>

<a id="hevea_default59"></a>Operators are special symbols that represent computations like

addition and multiplication. The values the operator is applied to

are called operands.The operators +, -, *, /, and **

perform addition, subtraction, multiplication, division, and

exponentiation, as in the following examples:<pre class="verbatim">20+32 hour-1 hour*60+minute minute/60 5**2 (5+9)*(15-7)

</pre>The division operator might not do what you expect:<pre class="verbatim">>>> minute = 59

>>> minute/60

</pre>The value of minute is 59, and in conventional arithmetic 59

divided by 60 is 0.98333, not 0. The reason for the discrepancy is

that Python is performing floor division<a id="text4" href="#note4">2</a>.<a id="hevea_default60"></a>

<a id="hevea_default64"></a>When both of the operands are integers, the result is also an

integer; floor division chops off the fractional

part, so in this example it truncates the answer to zero.If either of the operands is a floating-point number, Python performs

floating-point division, and the result is a float:<pre class="verbatim">>>> minute/60.0

0.98333333333333328

</pre>

<h2 class="section" id="sec23">2.6&#XA0;&#XA0;Expressions</h2>

An expression 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

(assuming that the variable x has been assigned a value):<a id="hevea_default65"></a>

<a id="hevea_default66"></a><pre class="verbatim">17

x + 17

</pre>If you type an expression in interactive mode, the interpreter

evaluates it and displays the result:<pre class="verbatim">>>> 1 + 1

</pre>But in a script, an expression all by itself doesn&#X2019;t

do anything! This is a common

source of confusion for beginners.<div class="theorem">Exercise&#XA0;1&#XA0;&#XA0;

Type the following statements in the Python interpreter to see

what they do:<pre class="verbatim">5

x = 5

x + 1

</pre></div>

<h2 class="section" id="sec24">2.7&#XA0;&#XA0;Order of operations</h2>

<a id="hevea_default69"></a>When more than one operator appears in an expression, the order of

evaluation depends on the rules of precedence. For

mathematical operators, Python follows mathematical convention.

The acronym PEMDAS is a useful way to

remember the rules:<a id="hevea_default70"></a><ul class="itemize"><li class="li-itemize">Parentheses 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, 2 * (3-1) is 4,

and (1+1)**(5-2) is 8. You can also use parentheses to make an

expression easier to read, as in (minute * 100) / 60, even

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

2**1+1 is 3, not 4, and 3*1**3 is 3, not 27.</li><li class="li-itemize">Multiplication and Division have the same precedence,

which is higher than Addition and Subtraction, which also

have the same precedence. So 2*3-1 is 5, not 4, and

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

right. So the expression 5-3-1 is 1, not 3, because the

5-3 happens first and then 1 is subtracted from 2.</li></ul>When in doubt, always put parentheses in your expressions to make sure

the computations are performed in the order you intend.

<h2 class="section" id="sec25">2.8&#XA0;&#XA0;Modulus operator</h2>

<a id="hevea_default72"></a>The modulus operator works on integers and yields the remainder

when the first operand is divided by the second. In Python, the

modulus operator is a percent sign (<code>%</code>). The syntax is the same

as for other operators:<pre class="verbatim">>>> quotient = 7 / 3

>>> print quotient

>>> remainder = 7 % 3

>>> print remainder

</pre>So 7 divided by 3 is 2 with 1 left over.The modulus operator turns out to be surprisingly useful. For

example, you can check whether one number is divisible by another&#X2014;if

x % y is zero, then x is divisible by y.<a id="hevea_default73"></a>You can also extract the right-most digit

or digits from a number. For example, x % 10 yields the

right-most digit of x (in base 10). Similarly, x % 100

yields the last two digits.

<h2 class="section" id="sec26">2.9&#XA0;&#XA0;String operations</h2>

<a id="hevea_default75"></a>The + operator works with strings, but it

is not addition in the mathematical sense. Instead it performs

concatenation, which means joining the strings by

linking them end to end. For example:<a id="hevea_default76"></a><pre class="verbatim">>>> first = 10

>>> second = 15

>>> print first+second

>>> first = '100'

>>> second = '150'

>>> print first + second

100150

</pre>The output of this program is 100150.

<h2 class="section" id="sec27">2.10&#XA0;&#XA0;Asking the user for input</h2>

<a id="hevea_default77"></a>Sometimes we would like to take the value for a variable from the user

via their keyboard.

Python provides a built-in function called <code>raw_input</code> that gets

input from the keyboard<a id="text5" href="#note5">3</a>. When this function is called, the program stops and

waits for the user to type something. When the user presses Return or Enter, the program resumes and <code>raw_input</code>

returns what the user typed as a string.<a id="hevea_default78"></a>

<a id="hevea_default80"></a><pre class="verbatim">>>> input = raw_input()

Some silly stuff

>>> print input

Some silly stuff

</pre>Before getting input from the user, it is a good idea to print a

prompt telling the user what to input. You can pass a string

to <code>raw_input</code> to be displayed to the user before pausing

for input:<a id="hevea_default81"></a><pre class="verbatim">>>> name = raw_input('What is your name?\n')

What is your name?

Chuck

>>> print name

Chuck

</pre>The sequence <code>\n</code> at the end of the prompt represents a newline,

which is a special character that causes a line break.

That&#X2019;s why the user&#X2019;s input appears below the prompt.<a id="hevea_default82"></a>If you expect the user to type an integer, you can try to convert

the return value to int using the int() function:<pre class="verbatim">>>> prompt = 'What...is the airspeed velocity of an unladen swallow?\n'

>>> speed = raw_input(prompt)

What...is the airspeed velocity of an unladen swallow?

>>> int(speed)

>>> int(speed) + 5

</pre>But if the user types something other than a string of digits,

you get an error:<pre class="verbatim">>>> speed = raw_input(prompt)

What...is the airspeed velocity of an unladen swallow?

What do you mean, an African or a European swallow?

>>> int(speed)

ValueError: invalid literal for int()

</pre>We will see how to handle this kind of error later.<a id="hevea_default83"></a>

<h2 class="section" id="sec28">2.11&#XA0;&#XA0;Comments</h2>

<a id="hevea_default85"></a>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.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

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

percentage = (minute * 100) / 60

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

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

</pre>Everything from the # to the end of the line is ignored&#X2014;it

has no effect on the program.Comments are most useful when they document non-obvious features of

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

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

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

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

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

a trade-off.

<h2 class="section" id="sec29">2.12&#XA0;&#XA0;Choosing mnemonic variable names</h2>

<a id="hevea_default86"></a>As long as you follow the simple rules of variable naming, and avoid

reserved words, you have a lot of choice when you name your variables.

In the beginning, this choice can be confusing both when you read a

program and when you write your own programs. For example, the

following three programs are identical in terms of what they accomplish,

but very different when you read them and try to understand them.<pre class="verbatim">a = 35.0

b = 12.50

c = a * b

print c

hours = 35.0

rate = 12.50

pay = hours * rate

print pay

x1q3z9ahd = 35.0

x1q3z9afd = 12.50

x1q3p9afd = x1q3z9ahd * x1q3z9afd

print x1q3p9afd

</pre>The Python interpreter sees all three of these programs as exactly the

same but humans see and understand these programs quite differently.

Humans will most quickly understand the intent

of the second program because the

programmer has chosen variable names that reflect their intent

regarding what data will be stored in each variable.We call these wisely chosen variable names &#X201C;mnemonic variable names&#X201D;. The

word mnemonic<a id="text6" href="#note6">4</a>

means &#X201C;memory aid&#X201D;.

We choose mnemonic variable names to help us remember why we created the variable

in the first place.While this all sounds great, and it is a very good idea to use mnemonic variable

names, mnemonic variable names can get in the way of a beginning programmer&#X2019;s

ability to parse and understand code. This is because beginning programmers

have not yet memorized the reserved words (there are only 31 of them) and sometimes

variables with names that are too descriptive start to look like

part of the language and not just well-chosen variable names.Take a quick look at the following Python sample code which loops through some data.

We will cover loops soon, but for now try to just puzzle through what this means:<pre class="verbatim">for word in words:

print word

</pre>What is happening here? Which of the tokens (for, word, in, etc.) are reserved words

and which are just variable names? Does Python understand at a fundamental level

the notion of words? Beginning programmers have

trouble separating what parts of the

code must be the same as this example and what parts of the code are simply

choices made by the programmer.The following code is equivalent to the above code:<pre class="verbatim">for slice in pizza:

print slice

</pre>It is easier for the beginning programmer to look at this code and know which

parts are reserved words defined by Python and which parts are simply variable

names chosen by the programmer. It is pretty clear that Python has no fundamental

understanding of pizza and slices and the fact that a pizza consists of a set

of one or more slices.But if our program is truly about reading data and looking for words in the data,

pizza and slice are very un-mnemonic variable names. Choosing them

as variable names distracts from the meaning of the program.After a pretty short period of time, you will know the most common reserved words

and you will start to see the reserved words jumping out at you:for word in words:

<code> </code>print word The parts of the code that are defined by

Python (for, in, print, and :) are in bold

and the programmer-chosen variables (word and words) are not in bold.

Many text editors are aware of Python

syntax and will color reserved words differently to give you clues to keep

your variables and reserved words separate.

After a while you will begin to read Python and quickly determine what

is a variable and what is a reserved word.

<h2 class="section" id="sec30">2.13&#XA0;&#XA0;Debugging</h2>

<a id="hevea_default87"></a>At this point, the syntax error you are most likely to make is

an illegal variable name, like class and yield, which

are keywords, or <code>odd~job</code> and <code>US$</code>, which contain

illegal characters.<a id="hevea_default88"></a>

<a id="hevea_default89"></a>If you put a space in a variable name, Python thinks it is two

operands without an operator:<pre class="verbatim">>>> bad name = 5

SyntaxError: invalid syntax

</pre>For syntax errors, the error messages don&#X2019;t help much.

The most common messages are SyntaxError: invalid syntax and

SyntaxError: invalid token, neither of which is very informative.<a id="hevea_default90"></a>

<a id="hevea_default94"></a>The runtime error you are most likely to make is a &#X201C;use before

def;&#X201D; that is, trying to use a variable before you have assigned

a value. This can happen if you spell a variable name wrong:<pre class="verbatim">>>> principal = 327.68

>>> interest = principle * rate

NameError: name 'principle' is not defined

</pre>Variables names are case sensitive, so LaTeX is not the

same as latex.<a id="hevea_default95"></a>

<a id="hevea_default97"></a>At this point, the most likely cause of a semantic error is

the order of operations. For example, to evaluate 1/2 &#X3C0;,

you might be tempted to write<pre class="verbatim">>>> 1.0 / 2.0 * pi

</pre>But the division happens first, so you would get &#X3C0; / 2, which

is not the same thing! There is no way for Python

to know what you meant to write, so in this case you don&#X2019;t

get an error message; you just get the wrong answer.<a id="hevea_default98"></a>

<h2 class="section" id="sec31">2.14&#XA0;&#XA0;Glossary</h2>

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

<a id="hevea_default99"></a></dd><dt class="dt-description">concatenate:</dt><dd class="dd-description"> To join two operands end to end.

<a id="hevea_default100"></a></dd><dt class="dt-description">comment:</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_default101"></a></dd><dt class="dt-description">evaluate:</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">expression:</dt><dd class="dd-description"> A combination of variables, operators, and values that

represents a single result value.

<a id="hevea_default102"></a></dd><dt class="dt-description">floating point:</dt><dd class="dd-description"> A type that represents numbers with fractional

parts.

<a id="hevea_default103"></a></dd><dt class="dt-description">floor division:</dt><dd class="dd-description"> The operation that divides two numbers and chops off

the fractional part.

<a id="hevea_default104"></a></dd><dt class="dt-description">integer:</dt><dd class="dd-description"> A type that represents whole numbers.

<a id="hevea_default105"></a></dd><dt class="dt-description">keyword:</dt><dd class="dd-description"> A reserved word that is used by the compiler to parse a

program; you cannot use keywords like if, def, and while as

variable names.

<a id="hevea_default106"></a></dd><dt class="dt-description">mnemonic:</dt><dd class="dd-description"> A memory aid. We often give variables mnemonic names

to help us remember what is stored in the variable.

<a id="hevea_default107"></a></dd><dt class="dt-description">modulus operator:</dt><dd class="dd-description"> An operator, denoted with a percent sign

(%), that works on integers and yields the remainder when one

number is divided by another.

<a id="hevea_default109"></a></dd><dt class="dt-description">operand:</dt><dd class="dd-description"> One of the values on which an operator operates.

<a id="hevea_default110"></a></dd><dt class="dt-description">operator:</dt><dd class="dd-description"> A special symbol that represents a simple computation like

addition, multiplication, or string concatenation.

<a id="hevea_default111"></a></dd><dt class="dt-description">rules of precedence:</dt><dd class="dd-description"> The set of rules governing the order in which

expressions involving multiple operators and operands are evaluated.

<a id="hevea_default113"></a></dd><dt class="dt-description">statement:</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_default114"></a></dd><dt class="dt-description">string:</dt><dd class="dd-description"> A type that represents sequences of characters.

<a id="hevea_default115"></a></dd><dt class="dt-description">type:</dt><dd class="dd-description"> A category of values. The types we have seen so far

are integers (type int), floating-point numbers (type float), and strings (type str).

<a id="hevea_default116"></a></dd><dt class="dt-description">value:</dt><dd class="dd-description"> One of the basic units of data, like a number or string,

that a program manipulates.

<a id="hevea_default117"></a></dd><dt class="dt-description">variable:</dt><dd class="dd-description"> A name that refers to a value.

<h2 class="section" id="sec32">2.15&#XA0;&#XA0;Exercises</h2>

<div class="theorem">Exercise&#XA0;2&#XA0;&#XA0;

Write a program that uses <code>raw_input</code> to prompt a user for their name

and then welcomes them.<pre class="verbatim">Enter your name: Chuck

Hello Chuck

</pre></div><div class="theorem">Exercise&#XA0;3&#XA0;&#XA0;

Write a program to prompt the user for hours and rate per hour to compute

gross pay.

<pre class="verbatim">Enter Hours: 35

Enter Rate: 2.75

Pay: 96.25

</pre></div>

We won&#X2019;t worry about making sure our pay has exactly two digits after

the decimal place for now. If you want, you can play with the

built-in Python round function to properly round the resulting pay

to two decimal places.<div class="theorem">Exercise&#XA0;4&#XA0;&#XA0;

Assume that we execute the following assignment statements:<pre class="verbatim">width = 17

height = 12.0

</pre>For each of the following expressions, write the value of the

expression and the type (of the value of the expression).<ol class="enumerate" type="1"><li class="li-enumerate">width/2</li><li class="li-enumerate">width/2.0</li><li class="li-enumerate">height/3</li><li class="li-enumerate">1 + 2 * 5</li></ol>Use the Python interpreter to check your answers.

</div><div class="theorem">Exercise&#XA0;5&#XA0;&#XA0;

Write a program which prompts the user for a Celsius temperature,

convert the temperature to Fahrenheit, and print out the converted

temperature.

</div>

<a id="note3" href="#text3">1</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">In Python 3.0, exec is no

longer a keyword, but nonlocal is.</div>

</dd><dt class="dt-thefootnotes"><a id="note4" href="#text4">2</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">In Python 3.0,

the result of this division is a float.

In Python 3.0, the new operator

// performs integer division.</div>

</dd><dt class="dt-thefootnotes"><a id="note5" href="#text5">3</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">In Python 3.0, this function is named

input.</div>

</dd><dt class="dt-thefootnotes"><a id="note6" href="#text6">4</a></dt><dd class="dd-thefootnotes"><div class="footnotetext">See

http://en.wikipedia.org/wiki/Mnemonic

for an extended description of the word &#X201C;mnemonic&#X201D;.</div>

</dd></dl>

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