String Indexing and Slicing in Python

In Python, strings are sequences of characters that can be accessed using indexing and slicing operations. Here’s a comprehensive explanation with examples:

Indexing

Strings in Python are zero-indexed, meaning the first character has index 0.

python

text = "Python"

Index positions:

text

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

Positive Indexing (left to right)

python

print(text[0])   # 'P' - first character
print(text[1])   # 'y' - second character
print(text[5])   # 'n' - last character

Negative Indexing (right to left)

Negative indices count from the end of the string:

text

P  y  t  h  o  n
-6 -5 -4 -3 -2 -1

python

print(text[-1])  # 'n' - last character
print(text[-2])  # 'o' - second last character
print(text[-6])  # 'P' - first character

Slicing

Slicing allows you to extract a substring using the syntax: string[start:stop:step]

Basic Slicing

python

text = "Python Programming"

# Get characters from index 0 to 5 (not including 5)
print(text[0:5])   # 'Pytho'

# Get characters from index 7 to end
print(text[7:])     # 'Programming'

# Get characters from beginning to index 5
print(text[:5])     # 'Pytho'

# Get the entire string
print(text[:])      # 'Python Programming'

# Get every second character
print(text[::2])    # 'Pto rgamn'

Negative Slicing

python

# Get last 5 characters
print(text[-5:])    # 'mming'

# Get all characters except last 5
print(text[:-5])    # 'Python Progr'

# Get characters from -10 to -5
print(text[-10:-5]) # ' Progr'

Step Parameter

python

# Reverse the string
print(text[::-1])   # 'gnimmargorP nohtyP'

# Get every 3rd character
print(text[::3])    # 'Ph rn'

Practical Examples

1. Extracting file extensions:

python

filename = "document.pdf"
extension = filename[-3:]
print(extension)  # 'pdf'

2. Getting domain from email:

python

email = "user@example.com"
domain = email[email.index('@')+1:]
print(domain)  # 'example.com'

3. Checking if a string is a palindrome:

python

word = "madam"
is_palindrome = word == word[::-1]
print(is_palindrome)  # True

4. Extracting parts of a date:

python

date = "2023-04-15"
year = date[:4]
month = date[5:7]
day = date[8:]
print(year, month, day)  # '2023' '04' '15'

Remember that strings are immutable in Python, 

1. Basic String Operations

Concatenation (+)

python

str1 = "Hello"
str2 = "World"
result = str1 + " " + str2
print(result)  # "Hello World"

Repetition (*)

python

word = "Hi"
repeated = word * 3
print(repeated)  # "HiHiHi"

Length (len())

python

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

in and not in Operators in Python Strings

The in and not in operators are membership operators used to check if a substring exists within a string. They return boolean values (True or False).

in Operator

Checks if a substring exists in a string.

Syntax:

python

substring in string

Examples:

python

text = "Python programming is fun"

# Check for single character
print('P' in text)        # True
print('z' in text)        # False

# Check for multiple characters
print('pro' in text)      # True
print('gram' in text)     # True
print('java' in text)     # False

# Case-sensitive check
print('python' in text)   # False (because of case difference)

not in Operator

Checks if a substring does not exist in a string.

Syntax:

python

substring not in string

Examples:

python

text = "Python programming is fun"

print('java' not in text)     # True
print('fun' not in text)      # False
print('Fun' not in text)      # True (case-sensitive)

Practical Use Cases

1. Input validation:

python

email = input("Enter your email: ")
if "@" not in email or "." not in email:
    print("Invalid email address")

2. Search functionality:

python

products = ["Python book", "Java course", "JavaScript tutorial"]
search_term = "python"
results = [p for p in products if search_term.lower() in p.lower()]
print(results)  # ['Python book']

3. Conditional execution:

python

filename = "report.pdf"
if ".pdf" in filename:
    print("This is a PDF file")

4. Security check:

python

user_input = input("Enter comment: ")
if "script" not in user_input.lower():
    print("Comment accepted")
else:
    print("Potential XSS attack detected")

Important Notes

1. These operations are case-sensitive:

python

print('python' in 'Python')  # False

2. They work with empty strings:

python

print('' in 'Python')       # True (empty string is always considered present)
print('' not in 'Python')   # False

3. Performance: These operations are highly optimized in Python and execute very quickly even for large strings.
4. For exact matching rather than substring checking, use == instead:

python

print('python' == 'Python')  # False