Python Strings – Introduction, Indexing & Traversing | Notes with Examples | CBSE Class 11 Computer Science
Class 11 · Computer Science
Python Strings – Introduction, Indexing and Traversing
A string is one of the most commonly used data types in Python. It is a sequence of Unicode characters enclosed within single quotes (' '), double quotes (" "), or triple quotes (''' ''' or """ """). Strings are widely used for storing names, addresses, passwords, messages, email IDs, and other textual information.
Learning Objectives
- Understand the concept of strings.
- Create strings in different ways.
- Access characters using indexing.
- Differentiate between positive and negative indexing.
- Traverse strings using loops.
- Understand string immutability.
What is a String?
A string is an ordered collection (sequence) of characters. A character may be a letter, digit, space, punctuation mark, or any special symbol.
Examples
name="Rahul"
city='Jaipur'
course="Computer Science"
password="ABC@123"
message="Welcome to Python"Characteristics of Strings
- Strings are sequences of characters.
- Strings are enclosed within quotes.
- Strings are immutable.
- Each character has an index.
- Strings support indexing and slicing.
- Strings can be traversed using loops.
Creating Strings
Using Single Quotes
name='Python'Using Double Quotes
name="Python"Using Triple Quotes
message="""Welcome
to
Python"""String Indexing
Every character in a string has a unique position called an index. Python supports both positive indexing and negative indexing.
Example
word="PYTHON"| Character | P | Y | T | H | O | N |
|---|---|---|---|---|---|---|
| Positive Index | 0 | 1 | 2 | 3 | 4 | 5 |
| Negative Index | -6 | -5 | -4 | -3 | -2 | -1 |
Accessing Characters
word="PYTHON"
print(word[0])
print(word[3])
print(word[-1])
print(word[-4])Output
P
H
N
TAccessing First and Last Character
name="Computer"
print(name[0])
print(name[-1])Output
C
rTraversing a String Using for Loop
Traversing means accessing each character of the string one by one.
text="Python"
for ch in text:
print(ch)Output
P
y
t
h
o
nTraversing a String Using while Loop
text="Python"
i=0
while iOutput
P
y
t
h
o
nFinding the Length of a String
The len() function returns the total number of characters in a string, including spaces and special characters.
text="Computer Science"
print(len(text))Output
16String Immutability
Strings in Python are immutable, which means their characters cannot be modified after the string is created.
Wrong
name="Python"
name[0]="J"Result
TypeErrorCorrect Method
name="Python"
name="Jython"Real-Life Applications
- Storing names of students.
- Saving email addresses.
- Processing passwords.
- Reading text files.
- Displaying messages.
- Searching words in documents.
- Generating reports.
- Processing customer information.
Common Programming Mistakes
1. Accessing an Invalid Index
name="Python"
print(name[10])Result: IndexError
2. Trying to Modify a Character
name="Python"
name[1]="A"Result: TypeError because strings are immutable.
3. Forgetting Quotes
name=PythonResult: NameError because Python treats Python as a variable instead of a string.
CBSE Important Programs
- Print each character of a string using a
forloop. - Print each character using a
whileloop. - Display the first and last character of a string.
- Find the length of a string.
- Count the number of characters by traversing the string.
- Print characters at even index positions.
- Print characters at odd index positions.
- Reverse print a string using indexing.
Viva Questions
- What is a string?
- How are strings created in Python?
- What is indexing?
- Differentiate between positive and negative indexing.
- What is string traversal?
- Can strings be modified after creation?
- What is meant by string immutability?
- Which function returns the length of a string?
- Can spaces be stored in a string?
- Give two real-life applications of strings.
Exam Tips
- Remember that indexing starts from 0.
- Negative indexing starts from -1.
- Strings are immutable.
- Use
len()to determine the length of a string. - Practice traversing strings using both
forandwhileloops.
Quick Revision
- A string is a sequence of Unicode characters.
- Strings can be enclosed in single, double, or triple quotes.
- Each character has a positive and negative index.
- Strings can be traversed using loops.
- Strings are immutable and cannot be modified directly.
len()returns the total number of characters in a string.