Introduction To Python (Chapter: 3)

Notes

Introduction to String

A string is one of the most commonly used data types.

A string is an ordered and immutable sequence of characters enclosed within quotes.

Strings are used to store text data such as names, messages, addresses, passwords, etc.

Example:


name = "Rajat"
Here, "Rajat" is a string.

Characteristics of String

✔ Ordered (characters have fixed positions)
✔ Indexed (we can access characters using index)
✔ Immutable (cannot be changed after creation)
✔ Allows duplicate characters
✔ Supports slicing

Creating a String

Python allows three ways to create strings:


1. Single Quotes
a = 'Hello'

2. Double Quotes
b = "Python"

3. Triple Quotes (Multiline String)
c = """This is
a multiline
string"""

Indexing in String

Each character in a string has a position number called index.

Index always starts from 0.


a = "Python"

Character   P   y   t   h   o   n
Index       0   1   2   3   4   5

Accessing characters:
print(a[0])   # P
print(a[3])   # h

Negative Indexing


Negative index starts from -1 (from right side).

print(a[-1])  # n
print(a[-2])  # o

String Slicing

Slicing is used to extract part of a string.

Syntax:


string[start : end : step]

Example:


a = "Python"

print(a[0:4])   # Pyth
print(a[2:])    # thon
print(a[:4])    # Pyth
print(a[::-1])  # nohtyP

Immutability of String


Strings cannot be changed after creation.

Wrong:
a = "Hello"
a[0] = "Y"   # Error

Correct:
a = "Hello"
a = "Y" + a[1:]
print(a)

String Operators


1. Concatenation (+)
Used to join strings.

a = "Hello"
b = "World"

print(a + " " + b)
Output:
Hello World

2. Repetition (*)
print("Hi " * 3)
Output:
Hi Hi Hi

Membership Operators


Used to check if character or word exists in string.

a = "Python"

print("P" in a)
print("z" not in a)

Built-in String Functions


len()
Returns length of string.

a = "Python"
print(len(a))

max() and min()
Return highest and lowest ASCII value character.

a = "Python"
print(max(a))
print(min(a))

Important String Methods


1. lower()
Converts to lowercase.
a = "HELLO"
print(a.lower())

2. upper()
a = "hello"
print(a.upper())

3. strip()
Removes spaces from both sides.
a = "  hello  "
print(a.strip())

4. replace()
a = "Hello World"
print(a.replace("World", "Python"))

5. split()
Converts string into list.
a = "apple,banana,orange"
print(a.split(","))

6. join()
Joins list into string.
a = ["apple", "banana", "orange"]
print(", ".join(a))

7. find()
Returns index of substring.
a = "Python Programming"
print(a.find("Pro"))
If not found → returns -1.

8. count()
a = "banana"
print(a.count("a"))

String Formatting


Using format()
name = "Rajat"
age = 25

print("My name is {} and I am {}".format(name, age))

Using f-string (Modern Method)
name = "Rajat"
age = 25

print(f"My name is {name} and I am {age}")

Escape Characters


Escape  Meaning
\n      New line
\t      Tab
\       Backslash
'       Single quote
"       Double quote

Example:
print("Hello\nWorld")

Looping Through String


a = "Python"

for i in a:
    print(i)

Practical Programs


1. Reverse a String
a = "Python"
print(a[::-1])

2. Check Palindrome
a = "madam"

if a == a[::-1]:
    print("Palindrome")
else:
    print("Not Palindrome")

3. Count Vowels
a = "Python Programming"
vowels = "aeiouAEIOU"

count = 0
for i in a:
    if i in vowels:
        count += 1

print(count)

Advantages of String

Easy to use
Powerful built-in methods
Used in almost every application
Essential for text processing

Python - String