Chapter 8: Strings

CBSE Unit: Unit 2, Python Programming (45 marks) Marks Weightage: ~6-8 marks Priority: CRITICAL, string operations and methods frequently asked

Key Concepts

8.1 Creating Strings

str1 = 'Hello'          # Single quotes
str2 = "Hello"           # Double quotes
str3 = '''Hello           # Triple quotes (multi-line)
World'''
str4 = """Hello World"""  # Triple double quotes

Strings are sequences of UNICODE characters
Strings are immutable (cannot change individual characters)

8.2 Accessing Characters (Indexing)

s = "HELLO"
# Positive index: 0  1  2  3  4
# Negative index: -5 -4 -3 -2 -1
s[0]    # 'H'
s[-1]   # 'O'
s[4]    # 'O'

8.3 String Slicing

s = "COMPUTER"
s[0:4]      # 'COMP'    (index 0 to 3)
s[2:6]      # 'MPUT'
s[:4]       # 'COMP'    (start defaults to 0)
s[4:]       # 'UTER'    (end defaults to len)
s[:]        # 'COMPUTER' (full string)
s[::2]      # 'CMUE'    (every 2nd character)
s[::-1]     # 'RETUPMOC' (reverse string)
s[-4:]      # 'UTER'

Syntax: string[start:stop:step], stop is excluded

8.4 String Operations

Concatenation (+)

"Hello" + " " + "World"    # "Hello World"

Repetition (*)

"Ha" * 3                   # "HaHaHa"

Membership (in, not in)

"llo" in "Hello"           # True
"xyz" not in "Hello"       # True

Comparison, Compared character by character using ASCII/Unicode values

"abc" < "abd" is True (compares at first different character)

8.5 Traversing a String

s = "Hello"
for ch in s:
    print(ch, end=" ")     # H e l l o

for i in range(len(s)):
    print(s[i], end=" ")   # H e l l o

8.6 Built-in Functions for Strings

Function	Description	Example
`len(s)`	Length of string	`len("Hello")` = 5
`max(s)`	Character with highest ASCII	`max("Hello")` = 'o'
`min(s)`	Character with lowest ASCII	`min("Hello")` = 'H'

8.7 String Methods

Method	Description	Example	Output
`upper()`	All uppercase	`"hello".upper()`	`"HELLO"`
`lower()`	All lowercase	`"HELLO".lower()`	`"hello"`
`title()`	Title case	`"hello world".title()`	`"Hello World"`
`capitalize()`	First char uppercase	`"hello".capitalize()`	`"Hello"`
`swapcase()`	Swap case	`"Hello".swapcase()`	`"hELLO"`
`count(sub)`	Count occurrences	`"hello".count("l")`	`2`
`find(sub)`	Index of first occurrence (-1 if not found)	`"hello".find("ll")`	`2`
`index(sub)`	Like find() but raises ValueError if not found	`"hello".index("ll")`	`2`
`startswith(s)`	Starts with given string	`"Hello".startswith("He")`	`True`
`endswith(s)`	Ends with given string	`"Hello".endswith("lo")`	`True`
`replace(old, new)`	Replace substring	`"Hello".replace("l", "L")`	`"HeLLo"`
`strip()`	Remove leading/trailing whitespace	`" hi ".strip()`	`"hi"`
`lstrip()`	Remove leading whitespace	`" hi ".lstrip()`	`"hi "`
`rstrip()`	Remove trailing whitespace	`" hi ".rstrip()`	`" hi"`
`split(sep)`	Split into list	`"a,b,c".split(",")`	`['a','b','c']`
`join(list)`	Join list into string	`",".join(['a','b'])`	`"a,b"`
`isalpha()`	All alphabets?	`"Hello".isalpha()`	`True`
`isdigit()`	All digits?	`"123".isdigit()`	`True`
`isalnum()`	All alphanumeric?	`"abc123".isalnum()`	`True`
`isspace()`	All whitespace?	`" ".isspace()`	`True`
`isupper()`	All uppercase?	`"HELLO".isupper()`	`True`
`islower()`	All lowercase?	`"hello".islower()`	`True`

Programs/Code Examples

Count Vowels in a String

s = input("Enter string: ")
count = 0
for ch in s:
    if ch in "aeiouAEIOU":
        count += 1
print("Vowels:", count)

Reverse a String

s = input("Enter string: ")
print("Reversed:", s[::-1])

Check Palindrome

s = input("Enter string: ")
if s == s[::-1]:
    print("Palindrome")
else:
    print("Not Palindrome")

Common Board Exam Question Patterns

Find output of string slicing operations (2-3 marks)
String methods output (2-3 marks)
Write a program to count characters/vowels/words (3-4 marks)
Difference between find() and index(), strip() variants (2 marks)
String traversal using for loop (2 marks)
Immutability question: Why can't we do s[0] = 'h'? (1-2 marks)

Key Points Students Miss

Strings are immutable: s[0] = 'h' raises TypeError
Slicing s[a:b] excludes index b (goes up to b-1)
s[::-1] reverses the string, most elegant way
find() returns -1 if not found; index() raises ValueError
All string methods return a new string (original unchanged because immutable)
split() without argument splits on whitespace (spaces, tabs, newlines)
Negative index: -1 is the last character, -2 is second-to-last
len() is a function, not a method (use len(s) not s.len())
String concatenation with + requires BOTH operands to be strings
"Hello" * 0 returns empty string ""