<h5 id="81-introduction">8.1 INTRODUCTION</h5> <ul> <li>A sequence is an orderly collection of items, each indexed by an integer.</li> <li>Python has several sequence data types: strings, lists, and tuples.</li> <li>A dictionary is another data type that falls under the category of mapping.</li> <li>In this chapter, strings will be studied in detail.</li> <li>Lists will be covered in Chapter 9, and tuples and dictionaries will be discussed in Chapter 10.</li> </ul> <p>======</p> <h5 id="82-strings">8.2 STRINGS</h5> <ul> <li>A string in Python is a sequence made up of one or more UNICODE characters.</li> <li>It can be created by enclosing characters in single, double, or triple quotes.</li> <li>Examples of string creation: str1 = ‘Hello World!’, str2 = “Hello World!”, str3 = ‘‘‘Hello World!’’’, str4 = “““Hello World!”””.</li> <li>The value of str3 and str4 can be extended to multiple lines using triple quotes.</li> <li>Python does not have a character data type; a string of length one is considered as a character.</li> </ul> <p>======</p> <h5 id="821-accessing-characters-in-a-string">8.2.1 Accessing Characters in a String</h5> <ul> <li>The text introduces the concept of using special characters and negative indices in Python’s strings.</li> <li>The <code>len()</code> function returns the length of a string. For example, <code>len('Hello World!')</code> returns 12.</li> <li>Strings in Python can be indexed using square brackets; negative indices can be used to index from the end of the string. For instance, <code>'Hello World!'[-1]</code> returns ‘!’, and <code>'Hello World!'[-12]</code> returns ‘H’.</li> <li>The first character of a string can be accessed using <code>string[-len(string)]</code>, and the last character can be accessed using <code>string[len(string)-1]</code> or <code>string[-1]</code>.</li> <li>It is important to note that Python string indices start from 0 and end at <code>len(string)-1</code>.</li> </ul> <p>======</p> <h5 id="822-string-is-immutable">8.2.2 String is Immutable</h5> <ul> <li>A string is an immutable data type, meaning its contents cannot be changed once created.</li> <li>Trying to modify an existing string will result in an error, such as “TypeError: ‘str’ object does not support item assignment.”</li> <li>For example, attempting to replace the character ’e’ with ‘a’ in the string “Hello World!” will not be successful.</li> <li>This behavior is due to the immutable nature of strings in Python.</li> <li>To manipulate or change strings, you need to create new strings based on the original string.</li> </ul> <p>======</p> <h5 id="83-string-operations">8.3 STRING OPERATIONS</h5> <ul> <li>Strings in Python are sequences of characters.</li> <li>Concatenation: This operation is performed using the ‘+’ operator to combine two strings. For example: “Hello” + “World” results in “HelloWorld”.</li> <li>Repetition: This operation is performed using the ‘*’ operator to repeat a string. For example: “Hello” * 3 results in “HelloHelloHello”.</li> <li>Membership: This operation is performed using the ‘in’ keyword to check if a character exists in the string. For example: ‘a’ in ‘Hello’ results in True.</li> <li>Slicing: This operation is performed to get a part of the string. For example: ‘Hello’[0:2] results in ‘He’.</li> </ul> <p>======</p> <h5 id="831-concatenation">8.3.1 Concatenation</h5> <ul> <li>Concatenation in Python is the process of joining two strings.</li> <li>The concatenation operator is the plus symbol (+).</li> <li>For example, if str1 = “Hello” and str2 = “World!”, then str1 + str2 results in “HelloWorld!”.</li> <li>The original strings remain unchanged after the concatenation operation.</li> <li>This is different from assignment in Python, where the right-hand side value is assigned to the left-hand side variable.</li> </ul> <p>======</p> <h5 id="832-repetition">8.3.2 Repetition</h5> <ul> <li>Python allows repetition of a string using the repetition operator, denoted by <code>*</code>.</li> <li>For example, if <code>str1 = 'Hello'</code>, then <code>str1 * 2</code> will result in <code>'HelloHello'</code>.</li> <li>The string <code>str1</code> remains unchanged after the use of the repetition operator.</li> <li>This operator can be used to repeat a string multiple times, as in <code>str1 * 5</code>, which will result in <code>'HelloHelloHelloHelloHello'</code>.</li> <li>The number of repetitions is determined by the integer value following the <code>*</code> symbol.</li> </ul> <p>======</p> <h5 id="833-membership">8.3.3 Membership</h5> <ul> <li>Python uses two membership operators, ‘in’ and ’not in’, to check if a value is present or not in a sequence such as strings, lists, tuples, etc.</li> <li>The ‘in’ operator returns True if the first string is a substring of the second string. For example, ‘W’ is in ‘Hello World!’ and ‘Wor’ is in ‘Hello World!’, so it returns True.</li> <li>The ’not in’ operator returns True if the first string is not a substring of the second string. For example, ‘My’ is not in ‘Hello World!’, so it returns True.</li> <li>The ‘in’ and ’not in’ operators can be used with other sequences like lists, tuples, etc. to check if a value is present or not in those sequences.</li> <li>The ‘in’ and ’not in’ operators can be used in conditional statements to perform specific actions based on whether a value is present or not in a sequence.</li> </ul> <p>======</p> <h5 id="834-slicing">8.3.4 Slicing</h5> <ul> <li>In Python, string slicing is a method to access a part of a string or substring by specifying an index range.</li> <li>The slice operation str1[n: m] returns the part of the string str1 starting from index n (inclusive) and ending at m (exclusive).</li> <li>The slice operation can also take a third index, k, that specifies the step size. For example, str1[n : m : k] means every kth character has to be extracted from the string str1 starting from n and ending at m-1.</li> <li>Negative indexes can also be used for slicing. For example, str1[-6:-1] returns the characters at index -6, -5, -4, -3, and -2.</li> <li>If both indexes are ignored and step size as -1 is given, the string is obtained in the reverse order. For example, str1[::-1] returns the reverse of the string str1.</li> </ul> <p>======</p> <h5 id="84-traversing-a-string">8.4 TRAVERSING A STRING</h5> <ul> <li>Accessing characters of a string or traversing a string can be done using for loop and while loop.</li> <li>In string traversal using for loop, a loop is initiated over the string and each character is accessed using the loop variable.</li> <li>In string traversal using while loop, a loop is initiated with an index variable which is incremented in each iteration till the length of the string.</li> <li>The for loop starts from the first character of the string and ends automatically at the last character.</li> <li>The while loop runs till the condition index < len(str) is True, where index varies from 0 to len(str1)-1.</li> <li>Both the loops print all the characters of the string one by one, resulting in the original string.</li> </ul> <p>======</p> <h5 id="85-string-methods-and-built-in-functions">8.5 STRING METHODS AND BUILT-IN FUNCTIONS</h5> <ul> <li>The <code>len()</code> function returns the length of a given string.</li> <li>The <code>title()</code> function returns a string with the first letter of every word in uppercase and the rest in lowercase.</li> <li>The <code>lower()</code> and <code>upper()</code> functions return a string with all lowercase and uppercase letters, respectively.</li> <li>The <code>count(str, start, end)</code> function returns the number of times a substring occurs in a given string.</li> <li>The <code>find(str, start, end)</code> function returns the first occurrence index of a substring in a given string, and <code>index(str, start, end)</code> raises an exception if the substring is not present.</li> <li>The <code>endswith()</code> and <code>startswith()</code> functions return True if a string ends or starts with a specified substring, respectively.</li> <li>The <code>isalnum()</code>, <code>islower()</code>, <code>isupper()</code>, <code>isspace()</code>, and <code>istitle()</code> functions return True if a string meets certain conditions related to alphanumeric characters, case, whitespace, and title casing.</li> <li>The <code>lstrip()</code>, <code>rstrip()</code>, and <code>strip()</code> functions return a string with spaces removed from the left, right, or both sides, respectively.</li> <li>The <code>replace(oldstr, newstr)</code> function replaces all occurrences of an old substring with a new one.</li> <li>The <code>join()</code> function concatenates elements in a list or iterable into a single string with a specified separator.</li> <li>The <code>partition(separator)</code> function partitions a string into three parts at the first occurrence of a separator.</li> <li>The <code>split(delimiter)</code> function splits a string into a list of words delimited by a specified substring.</li> </ul> <p>======</p> <h5 id="86-handling-strings">8.6 HANDLING STRINGS</h5> <ul> <li>Program 8-1 is a user defined function <code>charCount(ch,st)</code> that counts the number of times a character occurs in a string.</li> <li>Program 8-2 is a user defined function <code>replaceVowel(st)</code> that replaces all vowels in a string with ‘*’.</li> <li>Program 8-3 displays a string in reverse order without creating a new string.</li> <li>Program 8-4 reverses a string passed as a parameter and stores the reversed string in a new string using the <code>reverseString(st)</code> function.</li> <li>Program 8-5 checks if a string is a palindrome or not using the <code>checkPalin(st)</code> function. A palindrome is a string that reads the same backwards as forwards.</li> </ul> <p>======</p>