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<h1><font color="black"><a name="htoc72">Chapter&nbsp;6</a>&nbsp;&nbsp;Strings</font></h1>

<a name="strings"></a>

<a name="toc66"></a>

<h2><font color="black"><a name="htoc73">6.1</a>&nbsp;&nbsp;A string is a sequence</font></h2>

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<font color="black">A string is a <b>sequence</b> of characters.

You can access the characters one at a time with the

bracket operator:

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

&gt;&gt;&gt; fruit = 'banana'

&gt;&gt;&gt; letter = fruit[1]

</font></pre><a name="@default327"></a><font color="black">

The second statement extracts the character at index position 1 from the

<tt>fruit</tt> variable and assigns it to <tt>letter</tt> variable. <br />

<br />

The expression in brackets is called an <b>index</b>.

The index indicates which character in the sequence you

want (hence the name).<br />

<br />

But you might not get what you expect:

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

&gt;&gt;&gt; print letter

a

</font></pre><font color="black">For most people, the first letter of <code>'banana'</code> is <tt>b</tt>, not

<tt>a</tt>. But in Python, the index is an offset from the

beginning of the string, and the offset of the first letter is zero.

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

&gt;&gt;&gt; letter = fruit[0]

&gt;&gt;&gt; print letter

b

</font></pre><font color="black">So <tt>b</tt> is the 0th letter ("zero-eth") of <code>'banana'</code>, <tt>a</tt>

is the 1th letter ("one-eth"), and <tt>n</tt> is the 2th ("two-eth")

letter.<br />

</font><div align="center"><font color="black"><img src="cfbook009.png" /></font></div>

<br />

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<a name="@default329"></a><br />

<font color="black">You can use any expression, including variables and operators, as an

index, but the value of the index has to be an integer. Otherwise you

get:</font><br />

<br />

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<a name="@default332"></a>

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

&gt;&gt;&gt; letter = fruit[1.5]

TypeError: string indices must be integers

<a name="toc67"></a>

<h2><font color="black"><a name="htoc74">6.2</a>&nbsp;&nbsp;Getting the length of a string using <tt>len</tt></font></h2>

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<a name="@default334"></a>

<font color="black"><tt>len</tt> is a built-in function that returns the number of characters

in a string:

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

&gt;&gt;&gt; fruit = 'banana'

&gt;&gt;&gt; len(fruit)

6

</font></pre><font color="black">To get the last letter of a string, you might be tempted to try something

like this:</font><br />

<br />

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<a name="@default336"></a>

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

&gt;&gt;&gt; length = len(fruit)

&gt;&gt;&gt; last = fruit[length]

IndexError: string index out of range

</font></pre><font color="black">The reason for the <tt>IndexError</tt> is that there is no letter in <tt>'banana'</tt> with the index 6. Since we started counting at zero, the

six letters are numbered 0 to 5. To get the last character, you have

to subtract 1 from <tt>length</tt>:

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

&gt;&gt;&gt; last = fruit[length-1]

&gt;&gt;&gt; print last

a

</font></pre><font color="black">Alternatively, you can use negative indices, which count backward from

the end of the string. The expression <tt>fruit[-1]</tt> yields the last

letter, <tt>fruit[-2]</tt> yields the second to last, and so on.</font><br />

<br />

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<br />

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<h2><font color="black"><a name="htoc75">6.3</a>&nbsp;&nbsp;Traversal through a string with a loop</font></h2>

<a name="for"></a>

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<font color="black">A lot of computations involve processing a string one character at a

time. Often they start at the beginning, select each character in

turn, do something to it, and continue until the end. This pattern of

processing is called a <b>traversal</b>. One way to write a traversal

is with a <tt>while</tt> loop:

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

index = 0

while index &lt; len(fruit):

letter = fruit[index]

print letter

index = index + 1

</font></pre><font color="black">This loop traverses the string and displays each letter on a line by

itself. The loop condition is <tt>index &lt; len(fruit)</tt>, so

when <tt>index</tt> is equal to the length of the string, the

condition is false, and the body of the loop is not executed. The

last character accessed is the one with the index <tt>len(fruit)-1</tt>,

which is the last character in the string.<br />

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

Write a <tt>while</tt> loop that starts at the last character in the string

and works its way backwards to the first character in the string,

printing each letter on a separate line, except backwards.

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

<font color="black">Another way to write a traversal is with a <tt>for</tt> loop:

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

for char in fruit:

print char

</font></pre><font color="black">Each time through the loop, the next character in the string is assigned

to the variable <tt>char</tt>. The loop continues until no characters are

left.</font><br />

<br />

<a name="toc69"></a>

<h2><font color="black"><a name="htoc76">6.4</a>&nbsp;&nbsp;String slices</font></h2>

<a name="slice"></a>

<a name="@default345"></a>

<a name="@default346"></a>

<a name="@default347"></a>

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<font color="black">A segment of a string is called a <b>slice</b>. Selecting a slice is

similar to selecting a character:

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

&gt;&gt;&gt; s = 'Monty Python'

&gt;&gt;&gt; print s[0:5]

Monty

&gt;&gt;&gt; print s[6:13]

Python

</font></pre><font color="black">The operator <tt>[n:m]</tt> returns the part of the string from the

"n-eth" character to the "m-eth" character, including the first but

excluding the last. <br />

<br />

If you omit the first index (before the colon), the slice starts at

the beginning of the string. If you omit the second index, the slice

goes to the end of the string:

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

&gt;&gt;&gt; fruit = 'banana'

&gt;&gt;&gt; fruit[:3]

'ban'

&gt;&gt;&gt; fruit[3:]

'ana'

</font></pre><font color="black">If the first index is greater than or equal to the second the result

is an <b>empty string</b>, represented by two quotation marks:</font><br />

<br />

<a name="@default350"></a>

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

&gt;&gt;&gt; fruit = 'banana'

&gt;&gt;&gt; fruit[3:3]

"

</font></pre><font color="black">An empty string contains no characters and has length 0, but other

than that, it is the same as any other string.<br />

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

Given that <tt>fruit</tt> is a string, what does

<tt>fruit[:]</tt> mean?</em></font><br />

<br />

<a name="@default351"></a>

<a name="@default352"></a></div><br />

<a name="toc70"></a>

<h2><font color="black"><a name="htoc77">6.5</a>&nbsp;&nbsp;Strings are immutable</font></h2>

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<a name="@default354"></a>

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<font color="black">It is tempting to use the <tt>[]</tt> operator on the left side of an

assignment, with the intention of changing a character in a string.

For example:</font><br />

<br />

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<a name="@default357"></a>

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

&gt;&gt;&gt; greeting = 'Hello, world!'

&gt;&gt;&gt; greeting[0] = 'J'

TypeError: object does not support item assignment

</font></pre><font color="black">The "object" in this case is the string and the "item" is

the character you tried to assign. For now, an <b>object</b> is

the same thing as a value, but we will refine that definition

later. An <b>item</b> is one of the values in a sequence.<br />

<br />

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<a name="@default359"></a>

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<br />

The reason for the error is that

strings are <b>immutable</b>, which means you can't change an

existing string. The best you can do is create a new string

that is a variation on the original:

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

&gt;&gt;&gt; greeting = 'Hello, world!'

&gt;&gt;&gt; new_greeting = 'J' + greeting[1:]

&gt;&gt;&gt; print new_greeting

Jello, world!

</font></pre><font color="black">This example concatenates a new first letter onto

a slice of <tt>greeting</tt>. It has no effect on

the original string.</font><br />

<br />

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<br />

<a name="toc71"></a>

<h2><font color="black"><a name="htoc78">6.6</a>&nbsp;&nbsp;Looping and counting</font></h2>

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<font color="black">The following program counts the number of times the letter <tt>a</tt>

appears in a string:

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

word = 'banana'

count = 0

for letter in word:

if letter == 'a':

count = count + 1

print count

</font></pre><font color="black">This program demonstrates another pattern of computation called a <b>counter</b>. The variable <tt>count</tt> is initialized to 0 and then

incremented each time an <tt>a</tt> is found.

When the loop exits, <tt>count</tt>

contains the result---the total number of <tt>a</tt>'s.<br />

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

<a name="@default367"></a><br />

<br />

<em>Encapsulate this code in a function named <tt>count</tt>, and generalize it so that it accepts the string and the

letter as arguments.

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

<a name="toc72"></a>

<h2><font color="black"><a name="htoc79">6.7</a>&nbsp;&nbsp;The <tt>in</tt> operator</font></h2>

<a name="inboth"></a>

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<font color="black">The word <tt>in</tt> is a boolean operator that takes two strings and

returns <tt>True</tt> if the first appears as a substring in the second:

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

&gt;&gt;&gt; 'a' in 'banana'

True

&gt;&gt;&gt; 'seed' in 'banana'

False

<a name="toc73"></a>

<h2><font color="black"><a name="htoc80">6.8</a>&nbsp;&nbsp;String comparison</font></h2>

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<font color="black">The comparison operators work on strings. To see if two strings are equal:

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

if word == 'banana':

print 'All right, bananas.'

</font></pre><font color="black">Other comparison operations are useful for putting words in alphabetical

order:

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

if word &lt; 'banana':

print 'Your word,' + word + ', comes before banana.'

elif word &gt; 'banana':

print 'Your word,' + word + ', comes after banana.'

else:

print 'All right, bananas.'

</font></pre><font color="black">Python does not handle uppercase and lowercase letters the same way

that people do. All the uppercase letters come before all the

lowercase letters, so:

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

Your word, Pineapple, comes before banana.

</font></pre><font color="black">A common way to address this problem is to convert strings to a

standard format, such as all lowercase, before performing the

comparison. Keep that in mind in case you have to defend yourself

against a man armed with a Pineapple.</font><br />

<br />

<a name="toc74"></a>

<h2><font color="black"><a name="htoc81">6.9</a>&nbsp;&nbsp;<tt>string</tt> methods</font></h2>

<font color="black">Strings are an example of Python <b>objects</b>. An object contains

both data (the actual string itself) as well as <b>methods</b>, which

are effectively functions which that are built into the object and

are available to any <b>instance</b> of the object.<br />

<br />

Python has a function called <tt>dir</tt> that lists the methods available

for an object. The <tt>type</tt> function shows the type of an object

and the <tt>dir</tt> function shows the available methods.

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

&gt;&gt;&gt; stuff = 'Hello world'

&gt;&gt;&gt; type(stuff)

&lt;type 'str'&gt;

&gt;&gt;&gt; dir(stuff)

['capitalize', 'center', 'count', 'decode', 'encode',

'endswith', 'expandtabs', 'find', 'format', 'index',

'isalnum', 'isalpha', 'isdigit', 'islower', 'isspace',

'istitle', 'isupper', 'join', 'ljust', 'lower', 'lstrip',

'partition', 'replace', 'rfind', 'rindex', 'rjust',

'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines',

'startswith', 'strip', 'swapcase', 'title', 'translate',

'upper', 'zfill']

&gt;&gt;&gt; help(str.capitalize)

Help on method_descriptor:

capitalize(...)

S.capitalize() -&gt; string

Return a copy of the string S with only its first character

capitalized.

&gt;&gt;&gt;

<font color="black">While the <tt>dir</tt> function lists the methods, and you

can use <tt>help</tt> to get some simple documentation on a method,

a better source of documentation for string methods would be

<tt>docs.python.org/library/string.html</tt>.<br />

<br />

Calling a <b>method</b> is similar to calling a function---it

takes arguments and

returns a value---but the syntax is different. We call a method

by appending the method name to the variable name using the period

as a delimiter.<br />

<br />

For example, the

method <tt>upper</tt> takes a string and returns a new string with

all uppercase letters:<br />

<br />

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<a name="@default375"></a><br />

<br />

Instead of the function syntax <tt>upper(word)</tt>, it uses

the method syntax <tt>word.upper()</tt>.</font><br />

<br />

<a name="@default376"></a>

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

&gt;&gt;&gt; word = 'banana'

&gt;&gt;&gt; new_word = word.upper()

&gt;&gt;&gt; print new_word

BANANA

</font></pre><font color="black">This form of dot notation specifies the name of the method, <tt>upper</tt>, and the name of the string to apply the method to, <tt>word</tt>. The empty parentheses indicate that this method takes no

argument.<br />

<br />

<a name="@default377"></a><br />

<br />

A method call is called an <b>invocation</b>; in this case, we would

say that we are invoking <tt>upper</tt> on the <tt>word</tt>.<br />

<br />

<a name="@default378"></a><br />

<br />

As it turns out, there is a string method named <tt>find</tt> that

is remarkably similar to the function we wrote:

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

&gt;&gt;&gt; word = 'banana'

&gt;&gt;&gt; index = word.find('a')

&gt;&gt;&gt; print index

1

</font></pre><font color="black">In this example, we invoke <tt>find</tt> on <tt>word</tt> and pass

the letter we are looking for as a parameter.<br />

<br />

Actually, the <tt>find</tt> method is more general than our function;

it can find substrings, not just characters:

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

&gt;&gt;&gt; word.find('na')

2

</font></pre><font color="black">It can take as a second argument the index where it should start:</font><br />

<br />

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<a name="@default380"></a>

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

&gt;&gt;&gt; word.find('na', 3)

4

</font></pre><font color="black">One common task is to remove white space (spaces, tabs, or newlines) from

the beginning and end of a string using the <tt>strip</tt> method:

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

&gt;&gt;&gt; line = ' Here we go '

&gt;&gt;&gt; line.strip()

'Here we go'

</font></pre><font color="black">Some methods such as <b>startswith</b> return boolean values.

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

&gt;&gt;&gt; line = 'Please have a nice day'

&gt;&gt;&gt; line.startswith('Please')

True

&gt;&gt;&gt; line.startswith('p')

False

</font></pre><font color="black">You will note that <tt>startswith</tt> requires case to match so sometimes

we take a line and map it all to lowercase before we do any checking

using the <tt>lower</tt> method.

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

&gt;&gt;&gt; line = 'Please have a nice day'

&gt;&gt;&gt; line.startswith('p')

False

&gt;&gt;&gt; line.lower()

'please have a nice day'

&gt;&gt;&gt; line.lower().startswith('p')

True

</font></pre><font color="black">In the last example, the method <tt>lower</tt> is called

and then we use <tt>startswith</tt>

to check to see if the resulting lowercase string

starts with the letter "p". As long as we are careful

with the order, we can make multiple method calls in a

single expression.<br />

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

<a name="@default381"></a>

<a name="@default382"></a><br />

<br />

<em>There is a string method called <tt>count</tt> that is similar

to the function in the previous exercise. Read the documentation

of this method at

<tt>docs.python.org/library/string.html</tt>

and write an invocation that counts the number of times the

letter a occurs

in <code>'banana'</code>.

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

<a name="toc75"></a>

<h2><font color="black"><a name="htoc82">6.10</a>&nbsp;&nbsp;Parsing strings</font></h2>

<font color="black">Often, we want to look into a string and find a substring. For example

if we were presented a series of lines formatted as follows:

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

From stephen.marquard@<b> uct.ac.za</b> Sat Jan 5 09:14:16 2008

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

</font><font size="3" color="black">And we wanted to pull out only the second half of the address (i.e.

<tt>uct.ac.za</tt>) from each line. We can do this by using the <tt>find</tt>

method and string slicing. <br />

<br />

First, we will find the position of the at-sign in the string. Then we will

find the position of the first space <em>after</em> the at-sign. And then we

will use string slicing to extract the portion of the string which we

are looking for.

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

&gt;&gt;&gt; data = 'From stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008'

&gt;&gt;&gt; atpos = data.find('@')

&gt;&gt;&gt; print atpos

21

&gt;&gt;&gt; sppos = data.find(' ',atpos)

&gt;&gt;&gt; print sppos

31

&gt;&gt;&gt; host = data[atpos+1:sppos]

&gt;&gt;&gt; print host

uct.ac.za

&gt;&gt;&gt;

</font></pre><font color="black">We use a version of the <tt>find</tt> method which allows us to specify

a position in the string where we want <tt>find</tt> to start looking.

When we slice, we extract the characters

from "one beyond the at-sign through up to <em>but not including</em> the

space character". <br />

<br />

The documentation for the <tt>find</tt> method is available at

<tt>docs.python.org/library/string.html</tt>.</font><br />

<br />

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<h2><font color="black"><a name="htoc83">6.11</a>&nbsp;&nbsp;Format operator</font></h2>

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<a name="@default384"></a>

<font color="black">The <b>format operator</b>, <tt>%</tt>

allows us to construct strings, replacing parts of the strings

with the data stored in variables.

When applied to integers, <tt>%</tt> is the modulus operator. But

when the first operand is a string, <tt>%</tt> is the format operator.<br />

<br />

<a name="@default385"></a><br />

<br />

The first operand is the <b>format string</b>, which contains

one or more <b>format sequences</b> that

specify how

the second operand is formatted. The result is a string.<br />

<br />

<a name="@default386"></a><br />

<br />

For example, the format sequence <code>'%d'</code> means that

the second operand should be formatted as an

integer (<tt>d</tt> stands for "decimal"):

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

&gt;&gt;&gt; camels = 42

&gt;&gt;&gt; '%d' % camels

'42'

</font></pre><font color="black">The result is the string <code>'42'</code>, which is not to be confused

with the integer value <tt>42</tt>.<br />

<br />

A format sequence can appear anywhere in the string,

so you can embed a value in a sentence:

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

&gt;&gt;&gt; camels = 42

&gt;&gt;&gt; 'I have spotted %d camels.' % camels

'I have spotted 42 camels.'

</font></pre><font color="black">If there is more than one format sequence in the string,

the second argument has to be a tuple. Each format sequence is

matched with an element of the tuple, in order.<br />

<br />

The following example uses <code>'%d'</code> to format an integer,

<code>'%g'</code> to format

a floating-point number (don't ask why), and <code>'%s'</code> to format

a string:

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

&gt;&gt;&gt; 'In %d years I have spotted %g %s.' % (3, 0.1, 'camels')

'In 3 years I have spotted 0.1 camels.'

</font></pre><font color="black">The number of elements in the tuple has to match the number

of format sequences in the string. Also, the types of the

elements have to match the format sequences:</font><br />

<br />

<a name="@default387"></a>

<a name="@default388"></a>

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

&gt;&gt;&gt; '%d %d %d' % (1, 2)

TypeError: not enough arguments for format string

&gt;&gt;&gt; '%d' % 'dollars'

TypeError: illegal argument type for built-in operation

</font></pre><font color="black">In the first example, there aren't enough elements; in the

second, the element is the wrong type.<br />

<br />

The format operator is powerful, but it can be difficult to use. You

can read more about it at

<tt>docs.python.org/lib/typesseq-strings.html</tt>.</font><br />

<br />

<a name="toc77"></a>

<h2><font color="black"><a name="htoc84">6.12</a>&nbsp;&nbsp;Debugging</font></h2>

<a name="@default389"></a>

<font color="black">A skill that you should cultivate as you program is always

asking yourself, "What could go wrong here?" or alternatively,

"What crazy thing might our user do to crash our (seemingly)

perfect program?".<br />

<br />

For example, look at the program which we used to demonstrate

the <tt>while</tt> loop in the chapter on iteration:

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

while True:

line = raw_input('&gt; ')

if line[0] == '#' :

continue

if line == 'done':

break

print line

print 'Done!'

</font></pre><font color="black">Look what happens when the user enters an empty line of input:

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

&gt; hello there

hello there

&gt; # don't print this

&gt; print this!

print this!

&gt;

Traceback (most recent call last):

File "copytildone.py", line 3, in &lt;module&gt;

if line[0] == '#' :

</font></pre><font color="black">The code works fine until it is presented an empty line. Then

there is no zeroth character so we get a traceback. There are two

solutions to this to make line three "safe" even if the line is

empty.<br />

<br />

One possibility is to simply use the <tt>startswith</tt> method

which returns <tt>False</tt> if the string is empty.

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

if line.startswith('#') :

</font></pre><a name="@default390"></a>

<a name="@default391"></a><font color="black">

Another way to safely write the <tt>if</tt> statement using the <b>guardian</b>

pattern and make sure the second logical expression is evaluated

only where there is at least one character in the string.:

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

if len(line) &gt; 0 and line[0] == '#' :

<a name="toc78"></a>

<h2><font color="black"><a name="htoc85">6.13</a>&nbsp;&nbsp;Glossary</font></h2>

<dl compact="compact"><dt><font color="black"><b>counter:</b></font></dt><dd><font color="black"> A variable used to count something, usually initialized

to zero and then incremented.

</font><a name="@default392"></a><br />

<br />

</dd><dt><font color="black"><b>empty string:</b></font></dt><dd><font color="black"> A string with no characters and length 0, represented

by two quotation marks.

</font><a name="@default393"></a><br />

<br />

</dd><dt><font color="black"><b>format operator:</b></font></dt><dd><font color="black"> An operator, <tt>%</tt>, that takes a format

string and a tuple and generates a string that includes

the elements of the tuple formatted as specified by the format string.

</font><a name="@default394"></a>

<a name="@default395"></a><br />

<br />

</dd><dt><font color="black"><b>format sequence:</b></font></dt><dd><font color="black"> A sequence of characters in a format string,

like <tt>%d</tt>, that specifies how a value should be formatted.

</font><a name="@default396"></a><br />

<br />

</dd><dt><font color="black"><b>format string:</b></font></dt><dd><font color="black"> A string, used with the format operator, that

contains format sequences.

</font><a name="@default397"></a><br />

<br />

</dd><dt><font color="black"><b>flag:</b></font></dt><dd><font color="black"> A boolean variable used to indicate whether a condition

is true.

</font><a name="@default398"></a><br />

<br />

</dd><dt><font color="black"><b>invocation:</b></font></dt><dd><font color="black"> A statement that calls a method.

</font><a name="@default399"></a><br />

<br />

</dd><dt><font color="black"><b>immutable:</b></font></dt><dd><font color="black"> The property of a sequence whose items cannot

be assigned.

</font><a name="@default400"></a><br />

<br />

</dd><dt><font color="black"><b>index:</b></font></dt><dd><font color="black"> An integer value used to select an item in

a sequence, such as a character in a string.

</font><a name="@default401"></a><br />

<br />

</dd><dt><font color="black"><b>item:</b></font></dt><dd><font color="black"> One of the values in a sequence.

</font><a name="@default402"></a><br />

<br />

</dd><dt><font color="black"><b>method:</b></font></dt><dd><font color="black"> A function that is associated with an object and called

using dot notation.

</font><a name="@default403"></a><br />

<br />

</dd><dt><font color="black"><b>object:</b></font></dt><dd><font color="black"> Something a variable can refer to. For now,

you can use "object" and "value" interchangeably.

</font><a name="@default404"></a><br />

<br />

</dd><dt><font color="black"><b>search:</b></font></dt><dd><font color="black"> A pattern of traversal that stops

when it finds what it is looking for.

</font><a name="@default405"></a>

<a name="@default406"></a><br />

<br />

</dd><dt><font color="black"><b>sequence:</b></font></dt><dd><font color="black"> An ordered set; that is, a set of

values where each value is identified by an integer index.

</font><a name="@default407"></a><br />

<br />

</dd><dt><font color="black"><b>slice:</b></font></dt><dd><font color="black"> A part of a string specified by a range of indices.

</font><a name="@default408"></a><br />

<br />

</dd><dt><font color="black"><b>traverse:</b></font></dt><dd><font color="black"> To iterate through the items in a sequence,

performing a similar operation on each.

</font><a name="@default409"></a></dd></dl>

<a name="toc79"></a>

<h2><font color="black"><a name="htoc86">6.14</a>&nbsp;&nbsp;Exercises</font></h2><br />

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

Take the following Python code that stores a string:`

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

str = 'X-DSPAM-Confidence: <b> 0.8475</b>'

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

</font><font size="3" color="black"><em>Use <tt>find</tt> and string slicing to extract the portion

of the string after the colon character and then use the

<tt>float</tt> function to convert the extracted string

into a floating point number.</em></font></div><br />

<div align="left"><font color="black"><b>Exercise&nbsp;6</b>&nbsp;&nbsp;

<a name="@default410"></a>

<a name="@default411"></a><br />

<br />

<em>Read the documentation of the string methods at

<tt>docs.python.org/lib/string-methods.html</tt>. You

might want to experiment with some of them to make sure

you understand how they work. <tt>strip</tt> and

<tt>replace</tt> are particularly useful.<br />

<br />

The documentation uses a syntax that might be confusing.

For example, in <code>find(sub[, start[, end]])</code>, the brackets

indicate optional arguments. So <tt>sub</tt> is required, but

<tt>start</tt> is optional, and if you include <tt>start</tt>,

then <tt>end</tt> is optional.

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

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