8.1 Introduction

In programming, data is not always numeric.

Real-world programs deal with:

• names of students
• addresses
• messages
• passwords
• search terms
• sentences and paragraphs

Such data consists of characters, not numbers.

To represent and manipulate textual data, Python provides the string data type.

A string is one of the most commonly used data types in Python, and understanding it well is essential for:

• input handling
• data validation
• text processing
• file handling (later chapters)

8.2 Strings

What Is a String?

A string is a sequence of characters enclosed within quotes.

In Python, a character itself is treated as a string of length 1.

Strings can be created using:

• single quotes ' '
• double quotes " "
• triple quotes ''' ''' or """ """

Creating Strings

name = "Amit"
subject = 'Computer Science'

Both are valid and equivalent.

Why Python Allows Multiple Quotes

This flexibility allows us to include quotes inside strings without confusion.

msg1 = "It's a sunny day"
msg2 = 'He said, "Hello"'

Multi-line Strings

Triple quotes allow strings to span multiple lines.

address = """NCERT
New Delhi
India"""

These are commonly used for:

• long messages
• documentation strings
• formatted output

Strings Are Immutable (Very Important Concept)

Once a string is created, it cannot be changed.

s = "Python"

You cannot modify individual characters:

s[0] = "J"   # ❌ TypeError

Why?

Because strings are immutable objects. Any operation that appears to “change” a string actually creates a new string.

This concept explains many behaviors students find confusing later.

Length of a String

The length of a string is the number of characters it contains.

s = "Python"
print(len(s))   # 6

Spaces are also characters:

s = "Hello World"
print(len(s))   # 11

8.3 String Operations

Strings support several operations that allow us to combine, compare, and repeat them.

8.3.1 Concatenation (+)

Concatenation joins two strings end to end.

first = "Computer"
second = "Science"
result = first + " " + second
print(result)

Output:

Computer Science

Important:

• Both operands must be strings

• Python does not auto-convert numbers to strings

  # print("Age: " + 15)  ❌ TypeError
  print("Age: " + str(15))
  

8.3.2 Repetition (*)

Repeats a string multiple times.

s = "Hi "
print(s * 3)

Output:

Hi Hi Hi 

8.3.3 Comparison of Strings

Strings can be compared using relational operators.

print("apple" == "apple")   # True
print("Apple" == "apple")   # False

Comparison is:

• case-sensitive

• based on Unicode values

  print("Zoo" > "apple")   # False
  

This explains why sorting words sometimes gives unexpected results.

8.3.4 Membership Operators (in, not in)

Used to check whether a substring exists in a string.

s = "Computer Science"

print("Science" in s)     # True
print("Math" not in s)    # True

This is widely used in:

• searching
• validation
• filtering text

8.4 Traversing a String

What Does Traversing Mean?

Traversing a string means accessing its characters one by one.

Since strings are sequences, each character has an index.

Indexing

• Index starts from 0

• Negative indexing starts from -1

  s = "Python"
  
  print(s[0])    # P
  print(s[3])    # h
  print(s[-1])   # n
  

Traversing Using for Loop

s = "Python"

for ch in s:
    print(ch)

Explanation:

• Python automatically gives one character at a time
• No index handling needed

Traversing Using Index and range()

s = "Python"

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

This method is useful when:

• index position matters
• you need both index and character

Reverse Traversal

s = "Python"

for i in range(len(s) - 1, -1, -1):
    print(s[i])

8.5 String Methods and Built-in Functions

Python provides many built-in methods for string processing.

Because strings are immutable, these methods return new strings.

Case Conversion Methods

s = "Computer Science"

print(s.upper())
print(s.lower())
print(s.title())
print(s.capitalize())

Searching Methods

s = "Computer Science"

print(s.find("Science"))   # starting index
print(s.find("Math"))      # -1 (not found)

Checking Content (Very Important)

print("abc".isalpha())     # True
print("123".isdigit())    # True
print("abc123".isalnum()) # True
print(" ".isspace())      # True

Used heavily in input validation.

Splitting and Joining

line = "Python is easy"
words = line.split()
print(words)

joined = "-".join(words)
print(joined)

Stripping Extra Spaces

s = "   Hello   "

print(s.strip())
print(s.lstrip())
print(s.rstrip())

Built-in Functions on Strings

s = "Python"

print(len(s))
print(max(s))
print(min(s))
print(sorted(s))

These functions work because strings are iterable sequences.

8.6 Handling Strings

This section focuses on practical string handling, combining multiple concepts.

Example 1: Counting Vowels

s = input("Enter a string: ")
count = 0

for ch in s:
    if ch.lower() in "aeiou":
        count += 1

print("Number of vowels:", count)

Concepts used:

• traversal
• membership operator
• case conversion

Example 2: Palindrome Check

s = input("Enter a string: ")
rev = ""

for ch in s:
    rev = ch + rev

if s == rev:
    print("Palindrome")
else:
    print("Not a palindrome")

Example 3: Removing Spaces

s = input("Enter a string: ")
result = ""

for ch in s:
    if ch != " ":
        result += ch

print(result)

Example 4: Frequency of Characters

s = input("Enter a string: ")

for ch in s:
    print(ch, ":", s.count(ch))

(Improved versions using dictionaries come later.)

Key Conceptual Takeaways

1. A string is a sequence of characters enclosed in single, double or triple quotes.
2. Indexing is used for accessing individual characters within a string.
3. The first character has the index 0 and the last character has the index n-1 where n is the length of the string. The negative indexing ranges from -n to -1.
4. Strings in Python are immutable, i.e., a string cannot be changed after it is created.
5. Membership operator in takes two strings and returns True if the first string appears as a substring in the second else returns False. Membership operator ‘not in’ does the reverse.
6. Retrieving a portion of a string is called slicing. This can be done by specifying an index range. The slice operation str1[n:m] returns the part of the string str1 starting from index n (inclusive) and ending at m (exclusive).
7. Each character of a string can be accessed either using a for loop or while loop.
8. There are many built-in functions for working with strings in Python.