## Strings in Python

A string in Python is a sequence of characters. Strings are immutable, meaning once they are created, they cannot be changed.

### Creating Strings

You can create strings using single quotes, double quotes, or triple quotes (for multi-line strings).

```python

# Single quotes

str1 = 'Hello'

# Double quotes

str2 = "World"

# Triple quotes

str3 = '''This is a

multi-line

string'''

```

### Accessing Characters in a String

You can access characters in a string using indexing. Python uses zero-based indexing, so the first character has index 0.

```python

s = "Hello"

# Accessing first character

print(s[0]) # Output: H

# Accessing last character

print(s[-1]) # Output: o

```

### Slicing Strings

You can get a substring using slicing.

```python

s = "Hello, World!"

# Slicing from index 0 to 4

print(s[0:5]) # Output: Hello

# Slicing from index 7 to end

print(s[7:]) # Output: World!

# Slicing with negative indices

print(s[-6:]) # Output: World!

```

### String Methods

Python provides a variety of built-in string methods for common operations.

#### Changing Case

```python

s = "Hello, World!"

# Convert to uppercase

print(s.upper()) # Output: HELLO, WORLD!

# Convert to lowercase

print(s.lower()) # Output: hello, world!

# Capitalize the first letter

print(s.capitalize()) # Output: Hello, world!

# Title case

print(s.title()) # Output: Hello, World!

```

#### Searching and Replacing

```python

s = "Hello, World!"

# Find the index of a substring

print(s.find('World')) # Output: 7

# Replace a substring

print(s.replace('World', 'Universe')) # Output: Hello, Universe!

```

#### Splitting and Joining

```python

s = "Hello, World!"

# Split string into a list of substrings

print(s.split(', ')) # Output: ['Hello', 'World!']

# Join a list of strings into a single string

words = ['Hello', 'World!']

print(' '.join(words)) # Output: Hello World!

```

### String Formatting

Python offers several ways to format strings.

#### Using the `%` Operator

```python

name = "Alice"

age = 30

formatted_string = "My name is %s and I am %d years old." % (name, age)

print(formatted_string) # Output: My name is Alice and I am 30 years old.

```

#### Using `str.format()`

```python

name = "Alice"

age = 30

formatted_string = "My name is {} and I am {} years old.".format(name, age)

print(formatted_string) # Output: My name is Alice and I am 30 years old.

```

#### Using f-Strings (Python 3.6+)

```python

name = "Alice"

age = 30

formatted_string = f"My name is {name} and I am {age} years old."

print(formatted_string) # Output: My name is Alice and I am 30 years old.

```

### String Immutability

Strings in Python are immutable, meaning you cannot change a string after it has been created. Instead, you create new strings.

```python

s = "Hello"

# Attempt to change the first character (will raise an error)

# s[0] = 'h' # TypeError: 'str' object does not support item assignment

# Correct way to change a string

s = 'h' + s[1:]

print(s) # Output: hello

```

### Useful String Methods

Here are a few more useful string methods:

- `strip()`: Removes whitespace from the beginning and end of a string.

- `startswith()`: Checks if a string starts with a specified substring.

- `endswith()`: Checks if a string ends with a specified substring.

- `isalpha()`: Checks if all characters in the string are alphabetic.

- `isdigit()`: Checks if all characters in the string are digits.

- `islower()`: Checks if all characters in the string are lowercase.

- `isupper()`: Checks if all characters in the string are uppercase.

### Examples

Here are some practical examples of using strings in Python:

```python

# Removing whitespace

s = " Hello, World! "

print(s.strip()) # Output: Hello, World!

# Checking start and end

print(s.startswith(" He")) # Output: True

print(s.endswith("ld! ")) # Output: True

# Checking character properties

print("abc".isalpha()) # Output: True

print("123".isdigit()) # Output: True

# Checking case

print("hello".islower()) # Output: True

print("HELLO".isupper()) # Output: True

```

5 practice questions on strings in Python:

1. **Reverse a String:**

Write a function `reverse_string(s)` that takes a string `s` and returns the string reversed.

```python

def reverse_string(s):

# Your code here

pass

# Example usage

print(reverse_string("hello")) # Output: "olle"

```

2. **Count Vowels:**

Write a function `count_vowels(s)` that takes a string `s` and returns the number of vowels (a, e, i, o, u) in the string.

```python

def count_vowels(s):

# Your code here

pass

# Example usage

print(count_vowels("hello")) # Output: 2

```

3. **Check Palindrome:**

Write a function `is_palindrome(s)` that takes a string `s` and returns `True` if the string is a palindrome (reads the same forwards and backwards), and `False` otherwise.

```python

def is_palindrome(s):

# Your code here

pass

# Example usage

print(is_palindrome("racecar")) # Output: True

print(is_palindrome("hello")) # Output: False

```

4. **Find the Longest Word:**

Write a function `longest_word(s)` that takes a string `s` and returns the longest word in the string. Assume that words are separated by spaces.

```python

def longest_word(s):

# Your code here

pass

# Example usage

print(longest_word("The quick brown fox jumps over the lazy dog")) # Output: "jumps"

```

5. **Character Frequency:**

Write a function `char_frequency(s)` that takes a string `s` and returns a dictionary where the keys are characters and the values are the number of times each character appears in the string.

```python

def char_frequency(s):

# Your code here

pass

# Example usage

print(char_frequency("hello")) # Output: {'h': 1, 'e': 1, 'l': 2, 'o': 1}

```