🔤 String Manipulation

Your name, your school's name, a WhatsApp message, a password, an email address — all of these are text. In Python, text is stored as a string, and almost every real program has to work with text: checking a password, counting words in an essay, or finding a name in a list. This chapter shows how to visit each character of a string, how to cut out part of a string with slicing, what string operators like +, * and in do, and which built-in string methods save us from writing long programs.

9.1 🏁 Introduction

A string is a sequence of characters enclosed in single quotes (' '), double quotes (" ") or triple quotes (''' ''' / """ """). A character can be a letter, digit, space, symbol or even an emoji.

s1 = 'Hello'
s2 = "Python 3.12"
s3 = """Line one
Line two"""
s4 = ""              # an empty string — length 0

You learnt in the Data Handling chapter that each character of a string has an index, and Python supports two-way indexing:

  String :    C     O     M     P     U     T     E     R
 Forward :    0     1     2     3     4     5     6     7
Backward :   -8    -7    -6    -5    -4    -3    -2    -1

word = "COMPUTER"
print(word[0], word[5], word[-1], word[-3])    # C T R T
print(len(word))                               # 8

9.1.1 Strings are Immutable 🔒

Once a string is created, its characters cannot be changed in place.

name = "Rahul"
# name[0] = "M"        # ❌ TypeError: 'str' object does not support item assignment
name = "M" + name[1:]  # ✅ create a NEW string instead
print(name)            # Mahul

s = "hello"
s.upper()          # result thrown away
print(s)           # hello   ← unchanged!
s = s.upper()      # ✅ store the new string
print(s)           # HELLO

9.2 🚶 Traversing a String

Traversing means visiting each character of a string one by one, usually with a loop.

Method 1 — Directly over the characters:

for ch in "INDIA":
    print(ch, end=" ")
# Output: I N D I A

Method 2 — Using the index with range(len(s)):

s = "INDIA"
for i in range(len(s)):
    print(i, s[i])

0 I
1 N
2 D
3 I
4 A

Method 3 — Printing a string in reverse:

s = "PYTHON"
for i in range(len(s) - 1, -1, -1):
    print(s[i], end="")
# Output: NOHTYP

9.3 ➕ String Operators

9.3.1 Concatenation Operator + 🔗

The + operator joins two strings end to end.

first = "Good"
second = "Morning"
print(first + " " + second)     # Good Morning
print("10" + "20")              # 1020   (text join, NOT addition!)

9.3.2 Replication Operator * 🔁

The * operator repeats a string a given number of times. One operand must be a string and the other an integer.

print("Ha" * 3)      # HaHaHa
print(3 * "-=")      # -=-=-=
print("abc" * 0)     # (empty string)

9.3.3 Membership Operators in and not in 🔍

They check whether a character or substring is present in a string, and return True or False.

s = "Informatics"
print("for" in s)        # True
print("For" in s)        # False  (case-sensitive!)
print("z" not in s)      # True

9.3.4 Comparison Operators ⚖️

Strings can be compared with ==, !=, <, >, <=, >=. Python compares them character by character using the characters' Unicode (ASCII) values.

print("apple" < "apricot")    # True   ('p' = 112 < 'r' = 114 at index 2)
print("Zebra" < "apple")      # True   ('Z' = 90 < 'a' = 97)
print("abc" < "abcd")         # True   (a shorter prefix is smaller)
print("10" < "9")             # True   ('1' = 49 < '9' = 57) — text, not numbers!

ord() and chr() convert between a character and its Unicode value:

print(ord("A"))     # 65
print(chr(97))      # a
print(chr(ord("a") - 32))   # A

9.4 ✂️ String Slicing

Slicing extracts a part (substring) of a string.

Syntax:  string[start : stop : step]

Let's slice s = "PROGRAMMING":

  P   R   O   G   R   A   M   M   I   N   G
  0   1   2   3   4   5   6   7   8   9   10
-11 -10  -9  -8  -7  -6  -5  -4  -3  -2  -1

9.5 🧰 Built-in String Functions and Methods

Python provides many ready-made methods for strings. They are called using the dot notation: string.method(). The only common one that is a function (not a method) is len().

9.5.1 Length, Case-Changing and Counting 🔠

s = "the quick brown fox"
print(s.capitalize())    # The quick brown fox
print(s.title())         # The Quick Brown Fox
print(s.count("o"))      # 2
print(s.count("the"))    # 1

9.5.2 Searching: find(), index(), startswith(), endswith() 🔎

s = "banana"
print(s.find("na"))       # 2
print(s.find("xyz"))      # -1
# print(s.index("xyz"))   # ❌ ValueError: substring not found

find() vs index() — The Critical Comparison:

9.5.3 Checking Methods: is...() ✅

These methods test the characters of a string and return True or False. They all return False for an empty string.

print("Hello World".isalpha())   # False  (the space is not a letter)
print("12345".isdigit())         # True
print("-5".isdigit())            # False  ('-' is not a digit)
print("abc123".islower())        # True   (digits are ignored; the letters are lowercase)
print("".isalpha())              # False  (empty string)

9.5.4 Stripping Whitespace: strip(), lstrip(), rstrip() 🧽

You can also pass the characters to remove:

print("xxHelloxx".strip("x"))        # Hello
print("www.cbse.gov.in".lstrip("w.")) # cbse.gov.in
print("Hello!!!".rstrip("!"))         # Hello

9.5.5 Replacing, Splitting, Joining and Partitioning 🔧

sentence = "Python is easy and Python is fun"
print(sentence.replace("Python", "Java"))
# Java is easy and Java is fun

words = sentence.split()
print(len(words))                 # 7  (number of words)

print("*".join("ABC"))            # A*B*C
print(" ".join(["Hello", "World"]))   # Hello World

print("user@gmail.com".partition("@"))
# ('user', '@', 'gmail.com')
print("hello".partition("@"))
# ('hello', '', '')   ← separator not found

split() vs partition() — The Critical Comparison:

9.6 💻 Important String Programs

Code Example: count_characters.py

# Count vowels, consonants, digits, spaces and uppercase letters
s = input("Enter a string: ")
vowels = consonants = digits = spaces = upper = 0
for ch in s:
    if ch.isalpha():
        if ch.lower() in "aeiou":
            vowels += 1
        else:
            consonants += 1
        if ch.isupper():
            upper += 1
    elif ch.isdigit():
        digits += 1
    elif ch.isspace():
        spaces += 1
print("Vowels:", vowels)
print("Consonants:", consonants)
print("Digits:", digits)
print("Spaces:", spaces)
print("Uppercase letters:", upper)

Input: CBSE Class 11 Exam
Vowels: 4
Consonants: 9
Digits: 2
Spaces: 3
Uppercase letters: 6

Code Example: palindrome_string.py

s = input("Enter a string: ")
if s.lower() == s.lower()[::-1]:
    print(s, "is a palindrome")
else:
    print(s, "is not a palindrome")
# Input Madam  → Madam is a palindrome
# Input Python → Python is not a palindrome

Code Example: palindrome_without_slicing.py

# The same check using a loop (asked when slicing is not allowed)
s = input("Enter a string: ")
is_palin = True
n = len(s)
for i in range(n // 2):
    if s[i] != s[n - 1 - i]:
        is_palin = False
        break
print("Palindrome" if is_palin else "Not a palindrome")
# Input level → Palindrome

Code Example: word_stats.py

# Count words and find the longest word in a sentence
sentence = input("Enter a sentence: ")
words = sentence.split()
print("Number of words:", len(words))
longest = ""
for w in words:
    if len(w) > len(longest):
        longest = w
print("Longest word:", longest)
# Input: Python makes programming simple
# Number of words: 4
# Longest word: programming

Code Example: toggle_case.py

# Convert uppercase to lowercase and vice versa without swapcase()
s = input("Enter a string: ")
result = ""
for ch in s:
    if ch.isupper():
        result += ch.lower()
    elif ch.islower():
        result += ch.upper()
    else:
        result += ch
print(result)
# Input: Hello World 2025 → hELLO wORLD 2025

Code Example: initials.py

# Print the initials of a full name
name = input("Enter full name: ")
initials = ""
for part in name.split():
    initials += part[0].upper() + "."
print(initials)
# Input: mohandas karamchand gandhi → M.K.G.

⚠️ Common Errors and Misconceptions

🔑 Quick Revision

String Basics:

• A sequence of characters in quotes; immutable
• Indices: 0 to n − 1 (forward), −1 to −n (backward)
• Traverse with for ch in s or for i in range(len(s))

Operators:

Slicing s[start:stop:step]: start included · stop excluded · s[::-1] reverses · never an IndexError

Methods at a Glance:

• Case: capitalize(), title(), lower(), upper(), swapcase()
• Search: count(), find() (−1), index() (ValueError), startswith(), endswith()
• Check: isalpha(), isdigit(), isalnum(), islower(), isupper(), isspace()
• Clean: strip(), lstrip(), rstrip()
• Transform: replace(), split() → list, join(), partition() → 3-tuple

ASCII values: '0' = 48 · 'A' = 65 · 'a' = 97 · space = 32

🎯 Sample Exam Questions

Q1: Very Short Answer [1 mark each]

a) What is the output of print("CBSE"[::-1])? → ESBC

b) What does "hello".find("z") return? → -1

c) What is the output of print("ab" * 2 + "c")? → ababc

d) Which method returns a tuple of three parts? → partition()

e) What is the output of print(len("Hi there!"))? → 9

Q2: Slicing Output [2 marks]

Q: If s = "INFORMATICS", write the output of:

a) s[2:6] b) s[-4:] c) s[::3] d) s[7:1:-2]

Answer:

  I   N   F   O   R   M   A   T   I   C   S
  0   1   2   3   4   5   6   7   8   9   10

a) 'FORM' b) 'TICS' c) 'IOAC' (indices 0, 3, 6, 9) d) 'TMO' (indices 7, 5, 3)

Q3: Output Based [2 marks]

Q: Write the output of the following code:

s = "Python Programming"
print(s.count("o"), s.find("P", 1), s.upper()[7:10])
print(s.split("o"))
print(s.partition(" "))

Answer:

2 7 PRO
['Pyth', 'n Pr', 'gramming']
('Python', ' ', 'Programming')

Q4: Output Based [3 marks]

Q: Write the output of the following code:

msg = "Board@2026"
new = ""
for ch in msg:
    if ch.isupper():
        new += ch.lower()
    elif ch.islower():
        new += ch.upper()
    elif ch.isdigit():
        new += str(int(ch) + 1)
    else:
        new += "#"
print(new)

Answer:

bOARD#3137

(Letters swap case, each digit increases by 1 → 2026 becomes 3137, and '@' becomes '#'.)

Q5: Program [3 marks]

Q: Write a program to input a string and print how many times each vowel occurs in it.

s = input("Enter a string: ").lower()
for v in "aeiou":
    print(v, ":", s.count(v))

Input: Education is important
a : 2
e : 1
i : 3
o : 2
u : 1

✏️ Practice Problems

1. Write the output of the following, given s = "COMPUTER SCIENCE": (a) s[3:8] (b) s[-7:] (c) s[::4] (d) s[10:2:-3]

2. Differentiate between isalpha(), isdigit() and isalnum() with one example each.

3. Write a program to input a string and print it with every word's first letter in uppercase, WITHOUT using title().

4. Write a program to count the number of times the word "the" appears in a sentence (as a whole word, ignoring case).

5. Write a program to remove all the vowels from a string entered by the user.

6. Write a program that inputs an email address and prints the username and the domain name separately using partition().

7. Write the output of: (a) " Hello ".strip() (b) "a,b,,c".split(",") (c) "-".join("2026") (d) "banana".replace("an", "AN", 1)

8. Write a program to check whether a password is valid: at least 8 characters, with at least one uppercase letter, one lowercase letter and one digit.

9. Explain with an example why s[20] gives an error but s[5:20] does not, for s = "Python".

10. Write a program to print the following pattern for the word "PYTHON":

    P
    PY
    PYT
    PYTH
    PYTHO
    PYTHON