Strings in Python Notes – Class 11 CS (083)

Strings in Python Notes Class 11 CS Notes covering string concepts, operations, slicing, built-in functions, methods, and solved examples. Perfect for CBSE exam preparation.

Strings in Python

• A string is a sequence of one or more Unicode characters.
• These characters can be: Letters (A, B, C), Digits (1, 2, 3), Spaces ( ), Special symbols (@, #, $, %, etc.)
• Strings are created by enclosing characters inside: Single quotes ‘ ‘, Double quotes ” “, Triple single quotes ”’ ”’, Triple double quotes “”” “””

Example of Strings

str1 = ‘Hello World!’
str2 = “Hello World!”
str3 = “””Hello World!”””
str4 = ”’Hello World!”’

All the above variables contain the same string value: Hello World!

Multiline Strings

Triple quotes allow a string to span across multiple lines.

Example

str3 = “””Hello World!
Welcome to the world of Python”””

str4 = ”’Hello World!
Welcome to the world of Python”’

Output

Hello World!
Welcome to the world of Python

Accessing Characters in a String

• Each character in a string can be accessed using indexing.
• The position of a character is represented by an index number written inside square brackets [ ].
• Positive Indexing: Indexing starts from 0.  The first character has index 0 and last character has in-1, where n is the length of the string.
• Index Out of Range: If the index value is outside the valid range, Python gives an error.
• Index Can Be an Expression: The index can also be an expression, but it must evaluate to an integer.
• Negative Indexing: Python also allows negative indexing.
  • Negative indexing starts from the right side.The last character has index -1.The first character has index -n.
• len(): Python provides the built-in len() function to find the number of characters in a string.

Example of String Indexing, Length Function and Errors

# Initializing a string
str1 = “Hello World!”

# Accessing characters using positive indexing
print(“First character :”, str1[0])
print(“Seventh character :”, str1[6])
print(“Last character :”, str1[11])

# Index as an expression
print(“Character at index 2 + 4 :”, str1[2 + 4])

# Accessing characters using negative indexing
print(“Last character using negative index :”, str1[-1])
print(“First character using negative index :”, str1[-12])

# Finding length of the string
print(“Length of string :”, len(str1))

# Using length to access characters
n = len(str1)

print(“Last character using length :”, str1[n – 1])
print(“First character using length :”, str1[-n])

# Index out of range
print(str1[15])

# Invalid index type
print(str1[1.5])

Output

First character : H
Seventh character : W
Last character : !
Character at index 2 + 4 : W
Last character using negative index : !
First character using negative index : H
Length of string : 12
Last character using length : !
First character using length : H
IndexError: string index out of range
TypeError: string indices must be integers

Indexing of String “cbsesolutions”

String Operations

A string is a sequence of characters. Python provides several operations that can be performed on strings to manipulate and access data easily.

Some commonly used string operations are:

• Concatenation
• Repetition
• Membership
• Slicing

Concatenation of Strings

• Concatenation means joining two or more strings together.
• In Python, strings can be concatenated using the plus (+) operator.

Example

# First string
str1 = “CBSE “

# Second string
str2 = “Solutions”

# Concatenating strings
result = str1 + str2

print(“Concatenated String :”, result)

# Original strings remain unchanged
print(“str1 =”, str1)
print(“str2 =”, str2)

Output

Concatenated String : CBSE Solutions
str1 = CBSE
str2 = Solutions

Repetition of Strings

Python allows us to repeat a string using the repetition operator (*).

Example

# Assigning a string
str1 = “Python “
# Repeating the string 2 times
print(str1 * 2)
# Original string remains unchanged
print(“str1 =”, str1)

Output

Python Python
str1 = Python

Membership Operators

• Python provides two membership operators: in and not in
• These operators are used to check whether a substring is present in a string or not.
• The in operator returns True if the given substring exists in the string; otherwise, it returns False.
• The not in operator returns True if the given substring is not present in the string; otherwise, it returns False.

Example

str1 = “CBSE Solutions”

# Using in operator
print(“CBSE” in str1)
print(“Sol” in str1)
print(“Python” in str1)

# Using not in operator
print(“Notes” not in str1)
print(“CBSE” not in str1)

Output

True
True
False
True
False

Slicing in Strings

• Slicing is used to access a part of a string or extract a substring.
• In Python, slicing is done using the syntax: string[start : end : step]
  • start → Starting index (inclusive)
  • end → Ending index (exclusive)
  • step → Difference between consecutive characters (optional)
• The substring includes characters from index start to end – 1.
• The starting index is included, but the ending index is excluded.

Example of String Slicing

# Initializing a string
str1 = “CBSE Solutions”

# Slicing from index 0 to 3
print(str1[0:4])

# Slicing from index 5 to 8
print(str1[5:9])

# Ending index greater than string length
print(str1[3:20])

# First index greater than second index
print(str1[8:3])

# Slicing from beginning to index 5
print(str1[:6])

# Slicing from index 5 to end
print(str1[5:])

# Slicing with step size 2
print(str1[0:14:2])

# Slicing with step size 3
print(str1[0:14:3])

# Negative indexing in slicing
print(str1[-9:-1])

# Reversing the string
print(str1[::-1])

Output

CBSE
Solu
E Solutions

CBSE S
Solutions
CS oltos
CESuin
olution
snoituloS ESBC

Traversing a String

• String Traversal means accessing each character of a string one by one.
• In Python, a string can be traversed using: for loop or while loop

String Traversal Using for Loop

str1 = “CBSE Solutions”
for ch in str1:
    print(ch, end = “”)

Output

CBSE Solutions

String Traversal Using while Loop

In a while loop, indexing is used to access each character of the string.

Example

str1 = “CBSE Solutions”
index = 0
# len() returns the length of the string
while index < len(str1):
    print(str1[index], end = “”)
    index += 1

Output

CBSE Solutions

String Methods

Function / Method	Explanation	Example
len()	Returns the total number of characters in a string.	str1 = “CBSE Solutions” print(len(str1)) Output: 14
capitalize()	Converts the first character to uppercase and remaining characters to lowercase.	str1 = “cbse solutions” print(str1.capitalize()) Output: Cbse solutions
title()	Converts the first letter of each word to uppercase.	str1 = “cbse solutions” print(str1.title()) Output: Cbse Solutions
lower()	Converts all characters to lowercase.	str1 = “CBSE Solutions” print(str1.lower()) Output: cbse solutions
upper()	Converts all characters to uppercase.	str1 = “CBSE Solutions” print(str1.upper()) Output: CBSE SOLUTIONS
count()	Returns the number of occurrences of a substring.	str1 = “CBSE Solutions” print(str1.count(“o”)) Output: 2
find()	Returns the index of the first occurrence of a substring. Returns -1 if not found.	str1 = “CBSE Solutions” print(str1.find(“S”)) Output: 0
index()	Returns the index of the first occurrence of a substring. Gives error if not found.	str1 = “CBSE Solutions” print(str1.index(“l”)) Output: 7
endswith()	Returns True if the string ends with the given substring.	str1 = “CBSE Solutions” print(str1.endswith(“ions”)) Output: True
startswith()	Returns True if the string starts with the given substring.	str1 = “CBSE Solutions” print(str1.startswith(“CBSE”)) Output: True
isalnum()	Returns True if all characters are alphabets or digits.	str1 = “CBSE123” print(str1.isalnum()) Output: True
isalpha()	Returns True if all characters are alphabets only.	str1 = “CBSE” print(str1.isalpha()) Output: True
isdigit()	Returns True if all characters are digits only.	str1 = “2025” print(str1.isdigit()) Output: True
islower()	Returns True if all letters are lowercase.	str1 = “cbse solutions” print(str1.islower()) Output: True
isupper()	Returns True if all letters are uppercase.	str1 = “CBSE” print(str1.isupper()) Output: True
isspace()	Returns True if the string contains only spaces.	str1 = ” ” print(str1.isspace()) Output: True
lstrip()	Removes spaces from the left side of the string.	str1 = ” CBSE Solutions” print(str1.lstrip()) Output: CBSE Solutions
rstrip()	Removes spaces from the right side of the string.	str1 = “CBSE Solutions ” print(str1.rstrip()) Output: CBSE Solutions
strip()	Removes spaces from both sides of the string.	str1 = ” CBSE Solutions ” print(str1.strip()) Output: CBSE Solutions
replace()	Replaces a substring with another substring.	str1 = “CBSE Notes” print(str1.replace(“Notes”, “Solutions”)) Output: CBSE Solutions
join()	Joins elements of a sequence using a string separator.	str1 = ” ” print(str1.join([“CBSE”, “Solutions”])) Output: CBSE Solutions
partition()	Splits the string into three parts using the given separator.	str1 = “CBSE-Solutions” print(str1.partition(“-“)) Output: (‘CBSE’, ‘-‘, ‘Solutions’)
split()	Splits the string into a list using a separator.	str1 = “CBSE Solutions” print(str1.split()) Output: [‘CBSE’, ‘Solutions’]