Python Lists

1. What is a List?

• A list is a collection of items that are ordered, mutable (changeable), and allow duplicate elements.
• Lists are one of the most versatile data structures in Python.
• Lists are defined using square brackets [].

Example:

fruits = ["apple", "banana", "cherry"]

2. List Operations

1. Accessing Elements

• Use indexing to access individual elements in a list.
• Python uses zero-based indexing (the first element has index 0).

Example:

fruits = ["apple", "banana", "cherry"]
print(fruits[0])  # Output: apple
print(fruits[-1])  # Output: cherry (negative indexing starts from the end)

2. Slicing

• Extract a sublist using a range of indices.
• Syntax: list[start:stop:step]

Example:

numbers = [1, 2, 3, 4, 5]
print(numbers[1:4])  # Output: [2, 3, 4] (from index 1 to 3)
print(numbers[::2])  # Output: [1, 3, 5] (every second element)

3. Modifying Elements

• Lists are mutable, so you can change their elements.

Example:

fruits = ["apple", "banana", "cherry"]
fruits[1] = "blueberry"
print(fruits)  # Output: ['apple', 'blueberry', 'cherry']

4. Adding Elements

• Use append() to add an element to the end of the list.
• Use insert() to add an element at a specific position.
• Use extend() to add multiple elements from another list.

Examples:

fruits = ["apple", "banana"]
fruits.append("cherry")  # Add to the end
print(fruits)  # Output: ['apple', 'banana', 'cherry']

fruits.insert(1, "blueberry")  # Insert at index 1
print(fruits)  # Output: ['apple', 'blueberry', 'banana', 'cherry']

fruits.extend(["mango", "orange"])  # Add multiple elements
print(fruits)  # Output: ['apple', 'blueberry', 'banana', 'cherry', 'mango', 'orange']

5. Removing Elements

• Use remove() to remove the first occurrence of a value.
• Use pop() to remove an element at a specific index (or the last element if no index is provided).
• Use del to delete an element or a slice of elements.
• Use clear() to remove all elements from the list.

Examples:

fruits = ["apple", "banana", "cherry", "banana"]
fruits.remove("banana")  # Remove the first "banana"
print(fruits)  # Output: ['apple', 'cherry', 'banana']

fruits.pop(1)  # Remove element at index 1
print(fruits)  # Output: ['apple', 'banana']

del fruits[0]  # Delete element at index 0
print(fruits)  # Output: ['banana']

fruits.clear()  # Remove all elements
print(fruits)  # Output: []

6. Finding Elements

• Use index() to find the index of the first occurrence of a value.
• Use in to check if an element exists in the list.

Examples:

fruits = ["apple", "banana", "cherry"]
print(fruits.index("banana"))  # Output: 1
print("cherry" in fruits)  # Output: True

7. Sorting

• Use sort() to sort the list in place (modifies the original list).
• Use sorted() to return a new sorted list (does not modify the original list).

Examples:

numbers = [3, 1, 4, 1, 5, 9]
numbers.sort()
print(numbers)  # Output: [1, 1, 3, 4, 5, 9]

sorted_numbers = sorted(numbers, reverse=True)  # Sort in descending order
print(sorted_numbers)  # Output: [9, 5, 4, 3, 1, 1]

8. Reversing

• Use reverse() to reverse the list in place.

Example:

fruits = ["apple", "banana", "cherry"]
fruits.reverse()
print(fruits)  # Output: ['cherry', 'banana', 'apple']

9. Length

• Use len() to find the number of elements in a list.

Example:

fruits = ["apple", "banana", "cherry"]
print(len(fruits))  # Output: 3

3. List Comprehensions

• A concise way to create lists using a single line of code.
• Syntax: [expression for item in iterable if condition]

Examples:

squares = [x ** 2 for x in range(5)]
print(squares)  # Output: [0, 1, 4, 9, 16]

even_numbers = [x for x in range(10) if x % 2 == 0]
print(even_numbers)  # Output: [0, 2, 4, 6, 8]

4. Nested Lists

• A list can contain other lists, creating a multi-dimensional structure.

Example:

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(matrix[1][2])  # Output: 6 (access element in the second row, third column)

5. Additional Examples

• Creating a List:

  names = ["Raj", "Ram", "Anand", "Bala"]
  print(names)  # Output: ['Raj', 'Ram', 'Anand', 'Bala']
  

• Adding Elements:

  names.append("Karthik")
  print(names)  # Output: ['Raj', 'Ram', 'Anand', 'Bala', 'Karthik']
  

• Removing Elements:

  names.remove("Ram")
  print(names)  # Output: ['Raj', 'Anand', 'Bala', 'Karthik']
  

• Sorting:

  names.sort()
  print(names)  # Output: ['Anand', 'Bala', 'Karthik', 'Raj']
  

• List Comprehension:

  name_lengths = [len(name) for name in names]
  print(name_lengths)  # Output: [5, 4, 7, 3]
  

6. Best Practices

• Use list comprehensions for concise and readable code.
• Avoid modifying a list while iterating over it.
• Use append() instead of + for adding elements to a list (more efficient).