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Variables, expressions and statements

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<h1><font color="black"><a name="htoc16">Chapter&nbsp;2</a>&nbsp;&nbsp;Variables, expressions and statements</font></h1>

<a name="toc16"></a>

<h2><font color="black"><a name="htoc17">2.1</a>&nbsp;&nbsp;Values and types</font></h2>

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<font color="black">A <b>value</b> is one of the basic things a program works with,

like a letter or a

number. The values we have seen so far

are <tt>1</tt>, <tt>2</tt>, and

<code>'Hello, World!'</code>.<br />

<br />

These values belong to different <b>types</b>:

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

so-called because it contains a "string" of letters.

You (and the interpreter) can identify

strings because they are enclosed in quotation marks.<br />

<br />

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<br />

The <tt>print</tt> statement also works for integers. We use the

<tt>python</tt> command to start the interpreter.

</font><pre><font size="4" color="blue">

python

&gt;&gt;&gt; print 4

4

</font></pre><font color="black">If you are not sure what type a value has, the interpreter can tell you.

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; type('Hello, World!')

&lt;type 'str'&gt;

&gt;&gt;&gt; type(17)

&lt;type 'int'&gt;

</font></pre><font color="black">Not surprisingly, strings belong to the type <tt>str</tt> and

integers belong to the type <tt>int</tt>. Less obviously, numbers

with a decimal point belong to a type called <tt>float</tt>,

because these numbers are represented in a

format called <b>floating-point</b>.</font><br />

<br />

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<pre><font size="4" color="blue">

&gt;&gt;&gt; type(3.2)

&lt;type 'float'&gt;

</font></pre><font color="black">What about values like <code>'17'</code> and <code>'3.2'</code>?

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

strings.</font><br />

<br />

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<pre><font size="4" color="blue">

&gt;&gt;&gt; type('17')

&lt;type 'str'&gt;

&gt;&gt;&gt; type('3.2')

&lt;type 'str'&gt;

</font></pre><font color="black">They're strings.<br />

<br />

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

between groups of three digits, as in <tt>1,000,000</tt>. This is not a

legal integer in Python, but it is legal:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; print 1,000,000

1 0 0

</font></pre><font color="black">Well, that's not what we expected at all! Python interprets <tt>1,000,000</tt> as a comma-separated sequence of integers, which it

prints with spaces between.<br />

<br />

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<br />

This is the first example we have seen of a semantic error: the code

runs without producing an error message, but it doesn't do the

"right" thing.</font><br />

<br />

<a name="toc17"></a>

<h2><font color="black"><a name="htoc18">2.2</a>&nbsp;&nbsp;Variables</font></h2>

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<font color="black">One of the most powerful features of a programming language is the

ability to manipulate <b>variables</b>. A variable is a name that

refers to a value.<br />

<br />

An <b>assignment statement</b> creates new variables and gives

them values:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; message = 'And now for something completely different'

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

&gt;&gt;&gt; pi = 3.1415926535897931

</font></pre><font color="black">This example makes three assignments. The first assigns a string

to a new variable named <tt>message</tt>;

the second assigns the integer <tt>17</tt> to <tt>n</tt>; the third

assigns the (approximate) value of <font face="symbol">p</font> to <tt>pi</tt>.<br />

<br />

To display the value of a variable, you can use a print statement:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; print n

17

&gt;&gt;&gt; print pi

3.14159265359

</font></pre><font color="black">The type of a variable is the type of the value it refers to.

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; type(message)

&lt;type 'str'&gt;

&gt;&gt;&gt; type(n)

&lt;type 'int'&gt;

&gt;&gt;&gt; type(pi)

&lt;type 'float'&gt;

<a name="toc18"></a>

<h2><font color="black"><a name="htoc19">2.3</a>&nbsp;&nbsp;Variable names and keywords</font></h2>

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<font color="black">Programmers generally choose names for their variables that

are meaningful---they document what the variable is used for.<br />

<br />

Variable names can be arbitrarily long. They can contain

both letters and numbers, but they have to begin with a letter.

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

to begin variable names with a lowercase letter (you'll

see why later).<br />

<br />

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>.<br />

<br />

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<br />

If you give a variable an illegal name, you get a syntax error:

</font><pre><font size="4" color="blue">

&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

</font></pre><font color="black"><tt>76trombones</tt> is illegal because it does not begin with a letter.

<tt>more@</tt> is illegal because it contains an illegal character, <tt>@</tt>. But what's wrong with <tt>class</tt>?<br />

<br />

It turns out that <tt>class</tt> is one of Python's <b>keywords</b>. The

interpreter uses keywords to recognize the structure of the program,

and they cannot be used as variable names.<br />

<br />

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<br />

Python reserves 31 keywords^{<a name="text3" href="#note3">1</a>} for its use:

</font><pre><font size="4" color="blue">

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

</font></pre><font color="black">You might want to keep this list handy. If the interpreter complains

about one of your variable names and you don't know why, see if it

is on this list.</font><br />

<br />

<a name="toc19"></a>

<h2><font color="black"><a name="htoc20">2.4</a>&nbsp;&nbsp;Statements</font></h2>

<font color="black">A <b>statement</b> is a unit of code that the Python interpreter can

execute. We have seen two kinds of statements: print

and assignment.<br />

<br />

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<br />

When you type a statement in interactive mode, the interpreter

executes it and displays the result, if there is one.<br />

<br />

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.<br />

<br />

For example, the script

</font><pre><font size="4" color="blue">

print 1

x = 2

print x

</font></pre><font color="black">produces the output

</font><pre><font size="4" color="blue">

1

2

</font></pre><font color="black">The assignment statement produces no output.</font><br />

<br />

<a name="toc20"></a>

<h2><font color="black"><a name="htoc21">2.5</a>&nbsp;&nbsp;Operators and operands</font></h2>

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<font color="black"><b>Operators</b> are special symbols that represent computations like

addition and multiplication. The values the operator is applied to

are called <b>operands</b>.<br />

<br />

The operators <tt>+</tt>, <tt>-</tt>, <tt>*</tt>, <tt>/</tt> and <tt>**</tt>

perform addition, subtraction, multiplication, division and

exponentiation, as in the following examples:

</font><pre><font size="4" color="blue">

20+32 hour-1 hour*60+minute minute/60 5**2 (5+9)*(15-7)

</font></pre><font color="black">The division operator might not do what you expect:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; minute = 59

&gt;&gt;&gt; minute/60

0

</font></pre><font color="black">The value of <tt>minute</tt> 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 <b>floor division</b>^{<a name="text4" href="#note4">2</a>}.<br />

<br />

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<br />

When both of the operands are integers, the result is also an

integer; floor division chops off the fraction

part, so in this example it rounds down to zero.<br />

<br />

If either of the operands is a floating-point number, Python performs

floating-point division, and the result is a <tt>float</tt>:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; minute/60.0

0.98333333333333328

<a name="toc21"></a>

<h2><font color="black"><a name="htoc22">2.6</a>&nbsp;&nbsp;Expressions</font></h2>

<font color="black">An <b>expression</b> 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 <tt>x</tt> has been assigned a value):</font><br />

<br />

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<pre><font size="4" color="blue">

17

x

x + 17

</font></pre><font color="black">If you type an expression in interactive mode, the interpreter

<b>evaluates</b> it and displays the result:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; 1 + 1

2

</font></pre><font color="black">But in a script, an expression all by itself doesn't

do anything! This is a common

source of confusion for beginners.<br />

</font><div align="left"><font color="black"><b>Exercise&nbsp;1</b>&nbsp;&nbsp;<em>

Type the following statements in the Python interpreter to see

what they do:

</em></font><pre><font size="4" color="blue"><em>

5

x = 5

x + 1

</em></font></pre></div><br />

<a name="toc22"></a>

<h2><font color="black"><a name="htoc23">2.7</a>&nbsp;&nbsp;Order of operations</font></h2>

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<font color="black">When more than one operator appears in an expression, the order of

evaluation depends on the <b>rules of precedence</b>. For

mathematical operators, Python follows mathematical convention.

The acronym <b>PEMDAS</b> is a useful way to

remember the rules:</font><br />

<br />

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<ul><li><font color="black"><b>P</b>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, <tt>2 * (3-1)</tt> is 4,

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

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

if it doesn't change the result.</font><br />

<br />

</li><li><font color="black"><b>E</b>xponentiation has the next highest precedence, so

<tt>2**1+1</tt> is 3, not 4, and <tt>3*1**3</tt> is 3, not 27.</font><br />

<br />

</li><li><font color="black"><b>M</b>ultiplication and <b>D</b>ivision have the same precedence,

which is higher than <b>A</b>ddition and <b>S</b>ubtraction, which also

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

<tt>6+4/2</tt> is 8, not 5.</font><br />

<br />

</li><li><font color="black">Operators with the same precedence are evaluated from left to

right. So in the expression <tt>5-3-1</tt> is 1, not 3 because the

<tt>5-3</tt> happens first and then <tt>1</tt> is subtracted from <tt>2</tt>.</font></li></ul>

<font color="black">When in doubt always put parentheses in your expressions to make sure

the computations are performed in the order you intend.</font><br />

<br />

<a name="toc23"></a>

<h2><font color="black"><a name="htoc24">2.8</a>&nbsp;&nbsp;Modulus operator</font></h2>

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<font color="black">The <b>modulus operator</b> 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:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; quotient = 7 / 3

&gt;&gt;&gt; print quotient

2

&gt;&gt;&gt; remainder = 7 % 3

&gt;&gt;&gt; print remainder

1

</font></pre><font color="black">So 7 divided by 3 is 2 with 1 left over.<br />

<br />

The modulus operator turns out to be surprisingly useful. For

example, you can check whether one number is divisible by another---if

<tt>x % y</tt> is zero, then <tt>x</tt> is divisible by <tt>y</tt>.<br />

<br />

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<br />

Also, you can extract the right-most digit

or digits from a number. For example, <tt>x % 10</tt> yields the

right-most digit of <tt>x</tt> (in base 10). Similarly <tt>x % 100</tt>

yields the last two digits.</font><br />

<br />

<a name="toc24"></a>

<h2><font color="black"><a name="htoc25">2.9</a>&nbsp;&nbsp;String operations</font></h2>

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<font color="black">The <tt>+</tt> operator works with strings, but it

is not addition in the mathematical sense. Instead it performs

<b>concatenation</b>, which means joining the strings by

linking them end-to-end. For example:</font><br />

<br />

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<pre><font size="4" color="blue">

&gt;&gt;&gt; first = 10

&gt;&gt;&gt; second = 15

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

25

&gt;&gt;&gt; first = '100'

&gt;&gt;&gt; second = '150'

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

100150

</font></pre><font color="black">The output of this program is <tt>throatwarbler</tt>.</font><br />

<br />

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<h2><font color="black"><a name="htoc26">2.10</a>&nbsp;&nbsp;Asking the user for input</font></h2>

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<font color="black">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 name="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 <font color="purple">Return</font> or <font color="purple">Enter</font>, the program resumes and <code>raw_input</code>

returns what the user typed as a string.</font><br />

<br />

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<pre><font size="4" color="blue">

&gt;&gt;&gt; input = raw_input()

Some silly stuff

&gt;&gt;&gt; print input

Some silly stuff

</font></pre><font color="black">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:</font><br />

<br />

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<pre><font size="4" color="blue">

&gt;&gt;&gt; name = raw_input('What is your name?\n')

What is your name?

Chuck

&gt;&gt;&gt; print name

Chuck

</font></pre><font color="black">The sequence <code>\n</code> at the end of the prompt represents a <b>newline</b>,

which is a special character that causes a line break.

That's why the user's input appears below the prompt.<br />

<br />

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<br />

If you expect the user to type an integer, you can try to convert

the return value to <tt>int</tt> using the <tt>int()</tt> function:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; prompt = 'What...is the airspeed velocity of an unladen swallow?\n'

&gt;&gt;&gt; speed = raw_input(prompt)

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

17

&gt;&gt;&gt; int(speed)

17

&gt;&gt;&gt; int(speed) + 5

22

</font></pre><font color="black">But if the user types something other than a string of digits,

you get an error:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; speed = raw_input(prompt)

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

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

&gt;&gt;&gt; int(speed)

ValueError: invalid literal for int()

</font></pre><font color="black">We will see how to handle this kind of error later.</font><br />

<br />

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<br />

<a name="toc26"></a>

<h2><font color="black"><a name="htoc27">2.11</a>&nbsp;&nbsp;Comments</font></h2>

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<font color="black">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.<br />

<br />

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

<b>comments</b>, and they start with the <code>#</code> symbol:

</font><pre><font size="4" color="blue">

# compute the percentage of the hour that has elapsed

percentage = (minute * 100) / 60

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

comments at the end of a line:

</font><pre><font size="4" color="blue">

percentage = (minute * 100) / 60 # percentage of an hour

</font></pre><font color="black">Everything from the <tt>#</tt> to the end of the line is ignored---it

has no effect on the program.<br />

<br />

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 much more useful to explain <em>why</em>.<br />

<br />

This comment is redundant with the code and useless:

</font><pre><font size="4" color="blue">

v = 5 # assign 5 to v

</font></pre><font color="black">This comment contains useful information that is not in the code:

</font><pre><font size="4" color="blue">

v = 5 # velocity in meters/second.

</font></pre><font color="black">Good variable names can reduce the need for comments, but

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

a tradeoff.</font><br />

<br />

<a name="toc27"></a>

<h2><font color="black"><a name="htoc28">2.12</a>&nbsp;&nbsp;Choosing mnemonic variable names</font></h2>

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<font color="black">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.

</font><pre><font size="4" color="blue">

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

</font></pre><font color="black">The Python interpreter sees all three of these programs as <em>exactly the

same</em> but humans see and understand these programs quite differently.

Humans will most quickly understand the <b>intent</b>

of the second program because the

programmer has chosen variable names that reflect the intent of the programmer

regarding what data will be stored in each variable.<br />

<br />

We call these wisely-chosen variable names "mnemonic variable names". The

word <em>mnemonic</em>^{<a name="text6" href="#note6">4</a>}

means "memory aid".

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

in the first place.<br />

<br />

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'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 which have names that are too descriptive start to look like

part of the language and not just well-chosen variable names.<br />

<br />

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:

</font><pre><font size="4" color="blue">

for word in words:

print word

</font></pre><font color="black">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 <em>must</em> be the same as this example and what parts of the code are simply

choices made by the programmer.<br />

<br />

The following code is equivalent to the above code:

</font><pre><font size="4" color="blue">

for slice in pizza:

print slice

</font></pre><font color="black">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.<br />

<br />

But if our program is truly about reading data and looking for words in the data,

<tt>pizza</tt> and <tt>slice</tt> are very un-mnemonic variable names. Choosing them

as variable names distracts from the meaning of the program.<br />

<br />

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:<tt><br />

<br />

<b>for</b> word <b>in</b> words<b>:<br />

<code> </code>print</b> word </tt><br />

<br />

The parts of the code that are defined by

Python (<tt>for</tt>, <tt>in</tt>, <tt>print</tt>, and <tt>:</tt>) are in bold

and the programmer chosen variables (<tt>word</tt> and <tt>words</tt>) 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.</font><br />

<br />

<a name="toc28"></a>

<h2><font color="black"><a name="htoc29">2.13</a>&nbsp;&nbsp;Debugging</font></h2>

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<font color="black">At this point the syntax error you are most likely to make is

an illegal variable name, like <tt>class</tt> and <tt>yield</tt>, which

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

illegal characters.<br />

<br />

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<a name="@default93"></a><br />

<br />

If you put a space in a variable name, Python thinks it is two

operands without an operator:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; bad name = 5

SyntaxError: invalid syntax

</font></pre><font color="black">For syntax errors, the error messages don't help much.

The most common messages are <tt>SyntaxError: invalid syntax</tt> and

<tt>SyntaxError: invalid token</tt>, neither of which is very informative.<br />

<br />

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<a name="@default95"></a>

<a name="@default96"></a>

<a name="@default97"></a>

<a name="@default98"></a><br />

<br />

The runtime error you are most likely to make is a "use before

def;" that is, trying to use a variable before you have assigned

a value. This can happen if you spell a variable name wrong:

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; principal = 327.68

&gt;&gt;&gt; interest = principle * rate

NameError: name 'principle' is not defined

</font></pre><font color="black">Variables names are case sensitive, so <tt>LaTeX</tt> is not the

same as <tt>latex</tt>.<br />

<br />

<a name="@default99"></a>

<a name="@default100"></a>

<a name="@default101"></a><br />

<br />

At this point the most likely cause of a semantic error is

the order of operations. For example, to evaluate 1/2 <font face="symbol">p</font>,

you might be tempted to write

</font><pre><font size="4" color="blue">

&gt;&gt;&gt; 1.0 / 2.0 * pi

</font></pre><font color="black">But the division happens first, so you would get <font face="symbol">p</font> / 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't

get an error message; you just get the wrong answer.</font><br />

<br />

<a name="@default102"></a><br />

<br />

<a name="toc29"></a>

<h2><font color="black"><a name="htoc30">2.14</a>&nbsp;&nbsp;Glossary</font></h2>

<dl compact="compact"><dt><font color="black"><b>assignment:</b></font></dt><dd><font color="black"> A statement that assigns a value to a variable.

</font><a name="@default103"></a><br />

<br />

</dd><dt><font color="black"><b>concatenate:</b></font></dt><dd><font color="black"> To join two operands end-to-end.

</font><a name="@default104"></a><br />

<br />

</dd><dt><font color="black"><b>comment:</b></font></dt><dd><font color="black"> 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.

</font><a name="@default105"></a><br />

<br />

</dd><dt><font color="black"><b>evaluate:</b></font></dt><dd><font color="black"> To simplify an expression by performing the operations

in order to yield a single value.</font><br />

<br />

</dd><dt><font color="black"><b>expression:</b></font></dt><dd><font color="black"> A combination of variables, operators, and values that

represents a single result value.

</font><a name="@default106"></a><br />

<br />

</dd><dt><font color="black"><b>floating-point:</b></font></dt><dd><font color="black"> A type that represents numbers with fractional

parts.

</font><a name="@default107"></a><br />

<br />

</dd><dt><font color="black"><b>floor division:</b></font></dt><dd><font color="black"> The operation that divides two numbers and chops off

the fraction part.

</font><a name="@default108"></a><br />

<br />

</dd><dt><font color="black"><b>integer:</b></font></dt><dd><font color="black"> A type that represents whole numbers.

</font><a name="@default109"></a><br />

<br />

</dd><dt><font color="black"><b>keyword:</b></font></dt><dd><font color="black"> A reserved word that is used by the compiler to parse a

program; you cannot use keywords like <tt>if</tt>, <tt>def</tt>, and <tt>while</tt> as

variable names.

</font><a name="@default110"></a><br />

<br />

</dd><dt><font color="black"><b>mnemonic:</b></font></dt><dd><font color="black"> A memory aid. We often give variables mnemonic names

to help us remember what is stored in the variable.

</font><a name="@default111"></a><br />

<br />

</dd><dt><font color="black"><b>modulus operator:</b></font></dt><dd><font color="black"> An operator, denoted with a percent sign

(<tt>%</tt>), that works on integers and yields the remainder when one

number is divided by another.

</font><a name="@default112"></a>

<a name="@default113"></a><br />

<br />

</dd><dt><font color="black"><b>operand:</b></font></dt><dd><font color="black"> One of the values on which an operator operates.

</font><a name="@default114"></a><br />

<br />

</dd><dt><font color="black"><b>operator:</b></font></dt><dd><font color="black"> A special symbol that represents a simple computation like

addition, multiplication, or string concatenation.

</font><a name="@default115"></a><br />

<br />

</dd><dt><font color="black"><b>rules of precedence:</b></font></dt><dd><font color="black"> The set of rules governing the order in which

expressions involving multiple operators and operands are evaluated.

</font><a name="@default116"></a>

<a name="@default117"></a><br />

<br />

</dd><dt><font color="black"><b>statement:</b></font></dt><dd><font color="black"> A section of code that represents a command or action. So

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

</font><a name="@default118"></a><br />

<br />

</dd><dt><font color="black"><b>string:</b></font></dt><dd><font color="black"> A type that represents sequences of characters.

</font><a name="@default119"></a><br />

<br />

</dd><dt><font color="black"><b>type:</b></font></dt><dd><font color="black"> A category of values. The types we have seen so far

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

</font><a name="@default120"></a><br />

<br />

</dd><dt><font color="black"><b>value:</b></font></dt><dd><font color="black"> One of the basic units of data, like a number or string,

that a program manipulates.

</font><a name="@default121"></a><br />

<br />

</dd><dt><font color="black"><b>variable:</b></font></dt><dd><font color="black"> A name that refers to a value.

</font><a name="@default122"></a></dd></dl>

<a name="toc30"></a>

<h2><font color="black"><a name="htoc31">2.15</a>&nbsp;&nbsp;Exercises</font></h2><br />

<div align="left"><font color="black"><b>Exercise&nbsp;2</b>&nbsp;&nbsp;<em>

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

and then welcomes them.

</em></font><pre><font color="black"><em>

Enter your name: Chuck

Hello Chuck

</em></font></pre></div><br />

<div align="left"><font color="black"><b>Exercise&nbsp;3</b>&nbsp;&nbsp;<em>

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

gross pay.

</em></font><pre><font color="black"><em>

Enter Hours: 35

Enter Rate: 2.75

Pay: 96.25

</em></font></pre></div><font color="black">

We won'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 <tt>round</tt> function to properly round the resulting pay

to two decimal places.<br />

</font><div align="left"><font color="black"><b>Exercise&nbsp;4</b>&nbsp;&nbsp;<em>

Assume that we execute the following assignment statements:

</em></font><pre><font color="black"><em>

width = 17

height = 12.0

<font color="black"><em>For each of the following expressions, write the value of the

expression and the type (of the value of the expression).

</em></font><ol type="1"><li><font color="black"><tt><em>width/2</em></tt></font><br />

<br />

</li><li><font color="black"><tt><em>width/2.0</em></tt></font><br />

<br />

</li><li><font color="black"><tt><em>height/3</em></tt></font><br />

<br />

</li><li><font color="black"><tt><em>1 + 2 * 5</em></tt></font></li></ol>

<font color="black"><em>Use the Python interpreter to check your answers.

</em></font></div><br />

<div align="left"><font color="black"><b>Exercise&nbsp;5</b>&nbsp;&nbsp;<em>

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

convert the temperature to Fahrenheit and print out the converted

temperature.

</em></font></div><br />

<hr width="50%" size="1" /><dl><dt><font size="5" color="black"><a name="note3" href="#text3">1</a></font></dt><dd><font color="black">In Python 3.0, <tt>exec</tt> is no

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

</font></dd><dt><font size="5" color="black"><a name="note4" href="#text4">2</a></font></dt><dd><font color="black">In Python 3.0,

the result of this division is a <tt>float</tt>.

In Python 3.0, the new operator

<tt>//</tt> performs integer division.

</font></dd><dt><font size="5" color="black"><a name="note5" href="#text5">3</a></font></dt><dd><font color="black">In Python 3.0, this function is named

<tt>input</tt>.

</font></dd><dt><font size="5" color="black"><a name="note6" href="#text6">4</a></font></dt><dd><font color="black">See

<tt>http://en.wikipedia.org/wiki/Mnemonic</tt>

for an extended description of the word "mnemonic".

<hr />

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