List

1. What is a List?

• A list in Python is a collection of items (also called elements or values) that are ordered and mutable (i.e., you can change their contents after creation).

• Lists can contain different data types such as integers, floats, strings, or even other lists.

• Lists are written using square brackets [], and the elements are separated by commas.

fruits = ["apple", "banana", "cherry"]
mixed_list = [1, "Hello", 3.5]
 
2. Creating and Accessing Lists

You can create a list by enclosing elements in square brackets, and the elements can be of any type (integer, string, etc.).
numbers = [1, 2, 3, 4, 5]
names = ["Alice", "Bob", "Charlie"]

Accessing list elements: You can access the elements of a list using indexing (starting from 0 for the first element).

fruits = ["apple", "banana", "cherry"]
print(fruits[0]) 
Output: "apple"
print(fruits[2]) 
Output: "cherry"


Negative indexing allows you to access elements from the end of the list:

print(fruits[-1]) 
Output: "cherry" (last element)

print(fruits[-2]) 
Output: "banana" (second last element)


3. Indexing and Slicing Lists

Indexing: Just like strings, list elements can be accessed using their index positions.

my_list = [10, 20, 30, 40, 50]
print(my_list[1]) 
Output: 20

Slicing: Slicing allows you to retrieve a subset of the list by specifying a range [start:end:step].

• start: the index where the slice begins (inclusive).

• end: the index where the slice ends (exclusive).

• step: how many elements to skip between each element.

my_list = [10, 20, 30, 40, 50, 60]
print(my_list[1:4]) 
Output: [20, 30, 40] (indexes 1 to 3)

print(my_list[:3])  
Output: [10, 20, 30] (start from index 0)

print(my_list[3:])  
Output: [40, 50, 60] (from index 3 to end)

print(my_list[::2]) 
Output: [10, 30, 50] (every second element)


Negative slicing: You can also use negative numbers for slicing, starting from the end of the list.

print(my_list[-4:-1]) 
Output: [30, 40, 50]

4. List Methods

Python provides several built-in methods to manipulate lists:

(a) .append(): Adds an element to the end of the list.
fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits) 
Output: ['apple', 'banana', 'cherry']


(b) .insert(): Inserts an element at a specific position.
fruits.insert(1, "orange")
print(fruits) 
Output: ['apple', 'orange', 'banana', 'cherry']


(c) .remove(): Removes the first occurrence of the specified value.
fruits.remove("banana")
print(fruits) 
Output: ['apple', 'orange', 'cherry']


(d) .pop(): Removes and returns the element at the specified position (default is the last element if no index is given).
last_fruit = fruits.pop()
print(fruits)    
Output: ['apple', 'orange']
print(last_fruit) 
Output: 'cherry'


(e) .extend(): Extends the list by adding elements from another list or iterable.
vegetables = ["carrot", "broccoli"]
fruits.extend(vegetables)
print(fruits) 
Output: ['apple', 'orange', 'carrot', 'broccoli']


(f) .index(): Returns the index of the first occurrence of the specified value.
print(fruits.index("apple")) 
Output: 0


(g) .count(): Returns the number of times a specified value occurs in the list.
print(fruits.count("apple")) 
Output: 1


(h) .reverse(): Reverses the elements of the list in place.
fruits.reverse()
print(fruits) 
Output: ['broccoli', 'carrot', 'orange', 'apple']


(i) .sort(): Sorts the list in ascending order by default.
numbers = [3, 1, 4, 1, 5]
numbers.sort()
print(numbers) 
Output: [1, 1, 3, 4, 5]


(j) .clear(): Removes all elements from the list.
fruits.clear()
print(fruits) 
Output: []


5. Nested Lists

A nested list is a list that contains other lists as its elements.
nested_list = [[1, 2, 3], ["apple", "banana", "cherry"], [10.5, 20.75]]
 
You can access elements of nested lists by using multiple indices.
print(nested_list[0][1])
Output: 2 (second element of the first list)

print(nested_list[1][2]) 
Output: "cherry" (third element of the second list)


Nested lists can be useful for representing matrices or tables.
matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]
print(matrix[1][1]) 
Output: 5 (element in the second row and second column)


6. List Comprehension

List comprehension is a concise way to create lists using a for-loop within square brackets. It's more compact than traditional loops.

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

You can also include conditional statements in list comprehension.
even_squares = [x**2 for x in range(10) if x % 2 == 0]
print(even_squares) 
Output: [0, 4, 16, 36, 64]

Nested list comprehension: You can generate complex lists using nested comprehensions.

matrix = [[j for j in range(3)] for i in range(3)]
print(matrix)
Output:
[[0, 1, 2],
[0, 1, 2],
[0, 1, 2]]