1st PUC Computer Science Chapter 8 Strings Notes | Class 11 Python Strings

📚 Chapter Overview

This chapter explains Strings in Python. A string is a sequence made up of one or more UNICODE characters. The chapter covers creation of strings, accessing characters, indexing, negative indexing, string length, immutability, string operations, slicing, traversal, string methods, built-in functions and handling strings using user-defined functions.

8.2 Strings

8.3 String Operations

8.4 String Traversal

8.5 String Methods

8.6 Handling Strings

5 Textbook Programs

2 Exercises

5 Programming Problems

📑 Table of Contents

8.1 Introduction
8.2 Strings
8.2.1 Accessing Characters in a String
8.2.2 String is Immutable
8.3 String Operations
8.3.1 Concatenation
8.3.2 Repetition
8.3.3 Membership
8.3.4 Slicing
8.4 Traversing a String
8.5 String Methods and Built-in Functions
8.6 Handling Strings
Textbook Programs
Chapter Summary
Textbook Exercise & Programming Problems
Important Questions
Quick Revision

8.1 Introduction

A sequence is an orderly collection of items and each item is indexed by an integer. The chapter introduces strings in detail.

The sequence data types introduced earlier include:

Strings
Lists
Tuples

Dictionary is another data type introduced earlier and it belongs to the category of mapping.

Exam Point: Strings are covered in detail in this chapter. Lists, tuples and dictionaries are covered in later chapters of the textbook.

8.2 Strings

Definition: A string is a sequence made up of one or more UNICODE characters. A character can be a letter, digit, whitespace or any other symbol.

A string can be created by enclosing one or more characters in single quotes, double quotes or triple quotes.

String Creation

str1 = 'Hello World!'
str2 = "Hello World!"
str3 = """Hello World!"""
str4 = '''Hello World!'''

These variables represent the same string value Hello World!.

Multi-line Strings

Triple quotes can be used to extend strings over multiple lines.

str3 = """Hello World!
welcome to the world of Python"""

str4 = '''Hello World!
welcome to the world of Python'''

Remember: Python does not have a separate character data type. A string of length one is considered as a character.

8.2.1 Accessing Characters in a String

Indexing: Each individual character in a string can be accessed using indexing. The index is written inside square brackets [ ].

The first character from the left has index 0. If the length of the string is n, the last character has index n-1.

Indexing of Hello World!

0

1

2

3

4

5

6

7

8

9

10

11

H

e

l

o

W

o

r

l

d

!

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

Positive indices move from left to right; negative indices move from right to left.

verakworld.com

Example: Positive Indexing

str1 = 'Hello World!'

str1[0]
'H'

str1[6]
'W'

str1[11]
'!'

Index Out of Range

str1[15]

IndexError: string index out of range

If the index is outside the valid range, Python gives an IndexError.

Index Expression

str1[2+4]
'W'

An index can be an expression, but the expression must evaluate to an integer.

str1[1.5]

TypeError: string indices must be integers

Negative Indexing

Python also supports negative indexes. The first character from the right has index -1 and the last character from the right has index -n, where n is the length of the string.

str1[-1]
'!'

str1[-12]
'H'

Exam Point: Positive indexing ranges from 0 to n-1. Negative indexing ranges from -n to -1.

Length of a String – len()

len(): The built-in function len() returns the length of the string passed as its parameter.

str1 = 'Hello World!'

len(str1)
12

n = len(str1)
print(n)
12

For str1 = 'Hello World!', the length is 12.

str1[n-1]
'!'

str1[-n]
'H'

8.2.2 String is Immutable

Immutable: A string is immutable, which means its contents cannot be changed after it has been created.

An attempt to modify an individual character of a string results in an error.

str1 = "Hello World!"

str1[1] = 'a'

TypeError: 'str' object does not support item assignment

Important: String characters cannot be directly replaced because strings are immutable.

8.3 String Operations

Since a string is a sequence of characters, Python provides operations for working with strings.

Operation	Operator / Technique	Purpose
Concatenation	`+`	Joins two strings.
Repetition	`*`	Repeats a string.
Membership	`in`, `not in`	Checks whether a string or substring is present.
Slicing	`[n:m]`	Extracts part of a string.

8.3.1 Concatenation

Concatenation: To concatenate means to join strings. Python uses the + operator for concatenation.

str1 = 'Hello'
str2 = 'World!'

str1 + str2
'HelloWorld!'

The original strings remain unchanged after concatenation.

str1
'Hello'

str2
'World!'

8.3.2 Repetition

Repetition: Python uses the * operator to repeat a string.

str1 = 'Hello'

str1 * 2
'HelloHello'

str1 * 5
'HelloHelloHelloHelloHello'

Remember: The repetition operation does not change the original string.

8.3.3 Membership

Python provides two membership operators: in and not in.

in Operator

The in operator returns True if the first string appears as a substring in the second string. Otherwise, it returns False.

str1 = 'Hello World!'

'W' in str1
True

'Wor' in str1
True

'My' in str1
False

not in Operator

The not in operator returns True when the first string does not occur in the second string.

'My' not in str1
True

'Hello' not in str1
False

Operator	Returns True when
`in`	The first string occurs in the second string.
`not in`	The first string does not occur in the second string.

8.3.4 Slicing

Slicing: Slicing is used to access a part of a string or substring by specifying an index range.

For a string str1, the operation str1[n:m] starts at index n (inclusive) and ends before index m (exclusive).

Important Formula: The number of characters in str1[n:m] is m − n.

Basic Slicing

str1 = 'Hello World!'

str1[1:5]
'ello'

str1[7:10]
'orl'

Index Larger Than String Length

str1[3:20]
'lo World!'

An index that is too large is truncated down to the end of the string.

First Index Greater Than Second Index

str1[7:2]
''

If the first index is greater than the second index in this form, the result is an empty string.

Omitting the First Index

str1[:5]
'Hello'

If the first index is not specified, slicing starts from index 0.

Omitting the Second Index

str1[6:]
'World!'

If the second index is not specified, slicing continues to the end of the string.

Step Size

Slicing can also contain a third value representing the step size: str1[n:m:k].

It extracts every kth character starting from n and ending at m-1. The default step size is 1.

str1[0:10:2]
'HloWr'

str1[0:10:3]
'HlWl'

Negative Slicing

str1[-6:-1]
'World'

Reverse a String Using Slicing

str1[::-1]
'!dlroW olleH'

Exam Point: str1[n:m] includes index n but excludes index m.

8.4 Traversing a String

String Traversal: Traversing a string means accessing its characters one by one.

The textbook demonstrates string traversal using:

for loop
while loop

A. String Traversal Using for Loop

str1 = 'Hello World!'

for ch in str1:
    print(ch,end = '')

Output

Hello World!

The loop starts with the first character of the string and automatically stops after accessing the last character.

B. String Traversal Using while Loop

str1 = 'Hello World!'
index = 0

while index < len(str1):
    print(str1[index],end = '')
    index += 1

Output

Hello World!

The while loop continues while index < len(str1) is true. The value of index varies from 0 to len(str1)-1.

String Traversal Concept

First Character → Next Character → Next Character → Last Character

Characters are accessed one by one during traversal.

verakworld.com

8.5 String Methods and Built-in Functions

Python provides several built-in functions and methods for working with strings.

Method / Function	Description	Textbook Example
`len()`	Returns the length of the given string.	`len(str1)` → `12`
`title()`	Converts the first letter of every word to uppercase and the remaining letters to lowercase.	`str1.title()` → `'Hello World!'`
`lower()`	Converts uppercase letters to lowercase.	`str1.lower()` → `'hello world!'`
`upper()`	Converts lowercase letters to uppercase.	`str1.upper()` → `'HELLO WORLD!'`
`count(str,start,end)`	Returns the number of times a substring occurs in a string.	`str1.count('Hello')` → `3`
`find(str,start,end)`	Returns the index of the first occurrence of a substring. Returns -1 when the substring is not present.	`str1.find('Hee')` → `-1`
`index(str,start,end)`	Similar to find(), but raises an exception if the substring is not present.	`str1.index('Hee')` → ValueError
`endswith()`	Returns True if the string ends with the supplied substring.	`str1.endswith('!')` → `True`
`startswith()`	Returns True if the string starts with the supplied substring.	`str1.startswith('He')` → `True`
`isalnum()`	Returns True when the characters are alphabets or numbers and there is no whitespace or special symbol.	`'HelloWorld2'.isalnum()` → `True`
`islower()`	Returns True for a non-empty string with lowercase alphabetic characters according to the conditions shown in the textbook.	`'hello world!'.islower()` → `True`
`isupper()`	Returns True for a non-empty string with uppercase alphabetic characters according to the conditions shown in the textbook.	`'HELLO WORLD!'.isupper()` → `True`
`isspace()`	Returns True when all characters are whitespace.	`' \n \t \r'.isspace()` → `True`
`istitle()`	Returns True if the non-empty string is in title case.	`'Hello World!'.istitle()` → `True`
`lstrip()`	Removes spaces from the left side of the string.	`' Hello World! '.lstrip()`
`rstrip()`	Removes spaces from the right side of the string.	`' Hello World! '.rstrip()`
`strip()`	Removes spaces from both left and right sides.	`' Hello World! '.strip()`
`replace(oldstr,newstr)`	Replaces occurrences of the old string with the new string.	`str1.replace('World','Country')`
`join()`	Joins the characters of a string using another string as a separator.	`'-'.join('HelloWorld!')`
`partition()`	Partitions a string at the first occurrence of a separator and returns three parts.	`str1.partition('is')`
`split()`	Returns a list of words separated by the specified delimiter.	`str1.split()`

Exam Focus: Learn the purpose, syntax/form, and output of the important string methods, especially len(), title(), lower(), upper(), count(), find(), index(), startswith(), endswith(), replace(), split() and partition().

Selected Method Examples

str1 = 'hello WORLD!'
str1.title()
'Hello World!'

str1.lower()
'hello world!'

str1.upper()
'HELLO WORLD!'

str1 = 'Hello World! Hello Hello'

str1.count('Hello')
3

str1.find('Hello')
0

str1.find('Hee')
-1

str1 = 'Hello World!'

str1.endswith('World!')
True

str1.endswith('!')
True

str1.startswith('He')
True

str1.startswith('Hee')
False

str1 = 'HelloWorld2'
str1.isalnum()
True

str1 = 'HelloWorld!!'
str1.isalnum()
False

str1 = ' Hello World! '

str1.lstrip()
'Hello World! '

str1.rstrip()
' Hello World!'

str1.strip()
'Hello World!'

str1 = 'Hello World!'

str1.replace('o','*')
'Hell* W*rld!'

str1.replace('World','Country')
'Hello Country!'

str1 = 'HelloWorld!'
str2 = '-'

str2.join(str1)
'H-e-l-l-o-W-o-r-l-d-!'

str1 = 'India is a Great Country'

str1.partition('is')
('India ', 'is', ' a Great Country')

str1 = 'India is a Great Country'

str1.split()
['India', 'is', 'a', 'Great', 'Country']

8.6 Handling Strings

This section explains how user-defined functions can be used to perform different operations on strings.

Exam Focus: The textbook gives five programs for handling strings using user-defined functions and string operations.

Program 8-1 – Count Character Occurrences

Write a program with a user-defined function to count the number of times a character passed as an argument occurs in a string.

# Program 8-1
# Function to count the number of times a character occurs in a string

def charCount(ch,st):
    count = 0
    for character in st:
        if character == ch:
            count += 1
    return count
# end of function

st = input("Enter a string: ")
ch = input("Enter the character to be searched: ")

count = charCount(ch,st)

print("Number of times character",ch,
      "occurs in the string is:",count)

Output

Enter a string: Today is a Holiday
Enter the character to be searched: a
Number of times character a occurs in the string is: 3

verakworld.com

Program 8-2 – Replace Vowels with *

Write a user-defined function with a string as a parameter that replaces all vowels in the string with *.

# Program 8-2
# Function to replace all vowels in the string with '*'

def replaceVowel(st):
    # create an empty string
    newstr = ''

    for character in st:
        # check if next character is a vowel
        if character in 'aeiouAEIOU':
            # Replace vowel with *
            newstr += '*'
        else:
            newstr += character

    return newstr
# end of function

st = input("Enter a String: ")
st1 = replaceVowel(st)

print("The original String is:",st)
print("The modified String is:",st1)

Output

Enter a String: Hello World
The original String is: Hello World
The modified String is: H*ll* W*rld

verakworld.com

Program 8-3 – Reverse String Without New String

Write a program to input a string and print it in reverse order without creating a new string.

# Program 8-3
# Program to display string in reverse order

st = input("Enter a string: ")

for i in range(-1,-len(st)-1,-1):
    print(st[i],end='')

Output

Enter a string: Hello World
dlroW olleH

verakworld.com

Program 8-4 – Reverse String Using Function

Write a user-defined function that reverses a string passed as a parameter and stores the reversed string in a new string.

# Program 8-4
# Function to reverse a string

def reverseString(st):
    newstr = ''       # create a new string
    length = len(st)

    for i in range(-1,-length-1,-1):
        newstr += st[i]

    return newstr
# end of function

st = input("Enter a String: ")
st1 = reverseString(st)

print("The original String is:",st)
print("The reversed String is:",st1)

Output

Enter a String: Hello World
The original String is: Hello World
The reversed String is: dlroW olleH

verakworld.com

Program 8-5 – Check Palindrome

Palindrome: A string is called a palindrome if it reads the same backwards as forwards. The textbook gives Kanak as an example.

# Program 8-5
# Function to check if string is palindrome or not

def checkPalin(st):
    i = 0
    j = len(st) - 1

    while(i <= j):
        if(st[i] != st[j]):
            return False

        i += 1
        j -= 1

    return True
# end of function

st = input("Enter a String: ")
result = checkPalin(st)

if result == True:
    print("The given string",st,"is a palindrome")
else:
    print("The given string",st,"is not a palindrome")

Output 1

Enter a String: kanak
The given string kanak is a palindrome

Output 2

Enter a String: computer
The given string computer is not a palindrome

Palindrome Concept

k a n a k

Same sequence when read forwards and backwards.

verakworld.com

📌 Chapter Summary

A string is a sequence of characters enclosed in single, double or triple quotes.
Indexing is used to access individual characters within a string.
The first character has index 0 and the last character has index n-1.
Negative indexing ranges from -n to -1.
Strings in Python are immutable.
The in operator checks whether a string occurs as a substring.
The not in operator performs the reverse membership check.
Slicing retrieves a portion of a string using an index range.
str1[n:m] starts from n and ends before m.
A string can be traversed using a for loop or while loop.
Python provides many built-in functions and methods for strings.
User-defined functions can be used to perform string operations.

📖 Textbook Exercise

Exercise 1

Consider the following string:

mySubject = "Computer Science"

Find the output of:

print(mySubject[0:len(mySubject)])
print(mySubject[-7:-1])
print(mySubject[::2])
print(mySubject[len(mySubject)-1])
print(2*mySubject)
print(mySubject[::-2])
print(mySubject[:3] + mySubject[3:])
print(mySubject.swapcase())
print(mySubject.startswith('Comp'))
print(mySubject.isalpha())

Exercise 2

Consider:

myAddress = "WZ-1,New Ganga Nagar,New Delhi"

Find the output of:

print(myAddress.lower())
print(myAddress.upper())
print(myAddress.count('New'))
print(myAddress.find('New'))
print(myAddress.rfind('New'))
print(myAddress.split(','))
print(myAddress.split(' '))
print(myAddress.replace('New','Old'))
print(myAddress.partition(','))
print(myAddress.index('Agra'))

💻 Textbook Programming Problems

Write a program to input line(s) of text from the user until Enter is pressed. Count the total number of characters including white spaces, alphabets, digits, special symbols and words.
Write a user-defined function to convert a string containing more than one word into title case.
Write a function deleteChar() that takes a string and a character and creates a new string after deleting all occurrences of that character.
Input a string having some digits and write a function to return the sum of digits present in the string.
Write a function that takes a sentence where each word is separated by a space, replaces each blank with a hyphen and returns the modified sentence.

📝 Important Questions

1-Mark / Very Short Answer

1. What is a string?

A string is a sequence made up of one or more UNICODE characters.

2. How can a string be created in Python?

By enclosing characters in single, double or triple quotes.

3. What is the index of the first character?

The first character has index 0.

4. What is negative indexing?

It accesses characters from the right side of the string.

5. What is the first negative index?

-1.

6. What does len() return?

It returns the length of a string.

7. Are strings mutable?

No. Strings are immutable.

8. Which operator is used for concatenation?

The + operator.

9. Which operator is used for repetition?

The * operator.

10. Name the membership operators.

in and not in.

11. What is slicing?

Slicing is retrieving a part of a string using an index range.

12. Which loops are used for string traversal?

for loop and while loop.

2-Mark / Short Answer

1. Explain positive and negative indexing.

Positive indexing starts from 0 at the left side and ends at n-1. Negative indexing starts from -1 at the right side and ends at -n.

2. Explain string immutability.

A string cannot be changed after it is created. Trying to assign a new value to an individual character produces an error.

3. Differentiate between in and not in.

in	not in
Returns True when the first string occurs in the second.	Returns True when the first string does not occur in the second.

4. Explain slicing syntax.

str1[n:m] returns characters starting from index n and ending before index m. The number of characters is m-n.

5. Explain string traversal using a while loop.

Start index at 0 and continue while index is less than the string length. Access the character using str1[index] and increment index after each iteration.

Important Programming Questions

Write a program to count occurrences of a character in a string.
Write a program to replace vowels with *.
Write a program to display a string in reverse order.
Write a function to reverse a string.
Write a function to check whether a string is palindrome or not.

⚡ Quick Revision

Important Definitions

String: A sequence of one or more UNICODE characters.
Indexing: Accessing individual characters using indexes.
Immutable: A string whose contents cannot be changed after creation.
Slicing: Accessing a portion of a string using an index range.
Palindrome: A string that reads the same forwards and backwards.

Important Index Rules

Rule	Value / Form
First positive index	`0`
Last positive index	`n-1`
First negative index	`-1`
Last negative index	`-n`
Basic slicing	`str1[n:m]`
Slicing with step	`str1[n:m:k]`
Reverse using slicing	`str1[::-1]`

Important Operators

Operator	Purpose
`+`	Concatenation
`*`	Repetition
`in`	Membership check
`not in`	Reverse membership check

Important String Methods

len(), title(), lower(), upper(), count(), find(), index(), endswith(), startswith(), isalnum(), islower(), isupper(), isspace(), istitle(), lstrip(), rstrip(), strip(), replace(), join(), partition(), split()

Exam-Focused Points

Remember that string indexing starts from 0.
Remember negative indexing starts from -1.
Strings are immutable.
+ joins strings.
* repeats strings.
in and not in are membership operators.
Slicing uses start, stop and optional step.
str1[n:m] excludes index m.
String traversal can be performed with for and while loops.
Practice all five textbook programs.
Pay special attention to string method outputs in programming questions.

🎯 Final Revision

Before the examination, revise the chapter in this order:

String definition and creation
Positive and negative indexing
len()
String immutability
Concatenation and repetition
Membership operators
Slicing and step size
String traversal using for and while loops
String methods and built-in functions
All five textbook programs
Textbook exercises
Programming problems

Last-Minute Exam Tip: Focus especially on definitions, indexing rules, slicing, string methods, outputs and the five programs given in the chapter.

1st PUC Computer Science Chapter 8 Strings Notes | Class 11 Python Strings

📚 Chapter Overview

📑 Table of Contents

8.1 Introduction

8.2 Strings

String Creation

Multi-line Strings

8.2.1 Accessing Characters in a String

Example: Positive Indexing

Index Out of Range

Index Expression

Negative Indexing

Length of a String – len()

8.2.2 String is Immutable

8.3 String Operations

8.3.1 Concatenation

8.3.2 Repetition

8.3.3 Membership

in Operator

not in Operator

8.3.4 Slicing

Basic Slicing

Index Larger Than String Length

First Index Greater Than Second Index

Omitting the First Index

Omitting the Second Index

Step Size

Negative Slicing

Reverse a String Using Slicing

8.4 Traversing a String

A. String Traversal Using for Loop

B. String Traversal Using while Loop

8.5 String Methods and Built-in Functions

Selected Method Examples

8.6 Handling Strings

Program 8-1 – Count Character Occurrences

Program 8-2 – Replace Vowels with *

Program 8-3 – Reverse String Without New String

Program 8-4 – Reverse String Using Function

Program 8-5 – Check Palindrome

📌 Chapter Summary

📖 Textbook Exercise

Exercise 1

Exercise 2

💻 Textbook Programming Problems

📝 Important Questions

1-Mark / Very Short Answer

2-Mark / Short Answer

Important Programming Questions

⚡ Quick Revision

Important Definitions

Important Index Rules

Important Operators

Important String Methods

Exam-Focused Points

🎯 Final Revision

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