Class 11 • Chapter 7

String Manipulation

Chapter 7 explores String Manipulation, emphasizing string immutability, index-based access, slicing techniques, and powerful built-in string methods.

17 note blocks4 exam topics

🎯 Exam Focus Areas

Define String Immutability.Calculate the exact output of complex string slicing expressions (e.g., `word[1:5:2]`).Use negative indexing to access characters from the end.Identify the output of standard string methods like `find()`, `count()`, and `replace()`.

In Python, a String is a sequence of characters enclosed in quotes. Strings are one of the most frequently used data types because almost all human-readable data (names, addresses, passwords) is processed as text. Python provides incredibly powerful, built-in tools to slice, search, and modify strings with minimal effort.

7.1 String Creation and Immutability

You can create strings using single quotes (`'...'`), double quotes (`"..."`), or triple quotes (`'''...'''` or `"""..."""`). Triple quotes are unique because they allow strings to span multiple lines. CRITICAL RULE: Strings in Python are Immutable. Once a string is created in memory, it cannot be changed. If you try to modify a string, Python destroys the old one and creates a brand new string in memory.

name = "Basant"
# name[0] = "P"  <-- THIS WILL CRASH! Strings are immutable.

# To 'modify', we must create a new string
name = "P" + name[1:]  # This is valid

paragraph = """
This is a 
multi-line 
string.
"""

7.2 String Indexing

Every character in a string is assigned an integer index indicating its position. Python supports both Forward Indexing (starting at 0) and Backward Indexing (starting at -1 from the very end).

text = "PYTHON"
# Forward: P=0, Y=1, T=2, H=3, O=4, N=5
# Backward: P=-6, Y=-5, T=-4, H=-3, O=-2, N=-1

print(text[0])   # Output: P
print(text[-1])  # Output: N (The fastest way to get the last character)

7.3 String Slicing

Slicing allows you to extract a specific substring from a larger string. The syntax is `string[start : stop : step]`.

1start: The index where the slice begins (inclusive). Defaults to 0.
2stop: The index where the slice ends (EXCLUSIVE). Defaults to the end of the string.
3step: The jump between characters. Defaults to 1.

word = "PROGRAMMING"

print(word[0:7])    # 'PROGRAM' (Extracts indices 0 to 6)
print(word[3:])     # 'GRAMMING' (From index 3 to the end)
print(word[:3])     # 'PRO' (From beginning to index 2)
print(word[::2])    # 'PORMIG' (Every 2nd character)
print(word[::-1])   # 'GNIMMARGORP' (The easiest way to reverse a string!)

7.4 String Operators

Python overloads mathematical operators to work with strings.

1Concatenation (+): Joins two strings end-to-end. (e.g., `'Ice' + 'cream'` -> `'Icecream'`)
2Replication (*): Repeats a string multiple times. (e.g., `'Hi' * 3` -> `'HiHiHi'`)
3Membership (in / not in): Checks if a substring exists inside a string, returning a Boolean.

7.5 Built-in String Methods

Methods are functions that belong to the string object. Because strings are immutable, methods NEVER modify the original string; they always return a brand new string.

1len(str): Returns the total number of characters. (Technically a built-in function, not a method).
2str.upper() / str.lower(): Converts entirely to upper or lowercase.
3str.capitalize(): Capitalizes only the very first letter.
4str.count(sub): Returns how many times a substring appears.
5str.find(sub): Returns the lowest index where the substring is found. Returns -1 if not found.
6str.replace(old, new): Replaces occurrences of 'old' with 'new'.
7str.isalnum() / str.isalpha() / str.isdigit(): Checks if the string consists solely of alphanumeric characters, letters, or numbers.

📝 Quick Revision Points

1Strings are Immutable (cannot be changed).
2Indexing: Left-to-right starts at 0. Right-to-left starts at -1.
3Slicing: `[start:stop:step]`. Stop is EXCLUSIVE.
4`'A' + 'B'` = Concatenation (`'AB'`).
5`'A' * 3` = Replication (`'AAA'`).
6`'x' in 'box'` = Membership (`True`).

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name = "Basant" # name[0] = "P" <-- THIS WILL CRASH! Strings are immutable. # To 'modify', we must create a new string name = "P" + name[1:] # This is valid paragraph = """ This is a multi-line string. """

text = "PYTHON" # Forward: P=0, Y=1, T=2, H=3, O=4, N=5 # Backward: P=-6, Y=-5, T=-4, H=-3, O=-2, N=-1 print(text[0]) # Output: P print(text[-1]) # Output: N (The fastest way to get the last character)

word = "PROGRAMMING" print(word[0:7]) # 'PROGRAM' (Extracts indices 0 to 6) print(word[3:]) # 'GRAMMING' (From index 3 to the end) print(word[:3]) # 'PRO' (From beginning to index 2) print(word[::2]) # 'PORMIG' (Every 2nd character) print(word[::-1]) # 'GNIMMARGORP' (The easiest way to reverse a string!)