Lists and Tuples in Python – List and Tuple Comprehension, Usecases

Python Lists: A Comprehensive Guide

What is a List?

Lists are a fundamental data structure in Python used to store collections of items. They are:

• Ordered: Elements maintain a defined sequence.
• Mutable: Elements can be modified after creation.
• Defined by: Square brackets [].

Example:

fruits = ['apple', 'banana', 'orange']
print(fruits)  # Output: ['apple', 'banana', 'orange']

Accessing Elements in a List

Positive Indexing

a = [10, 20, 30, 40, 50]
print(a[0])  # Output: 10
print(a[4])  # Output: 50

Negative Indexing (Access elements from the end)

a = [1, 2, 3, 4, 5]
print(a[-1])  # Output: 5
print(a[-3])  # Output: 3

Slicing

my_list = [10, 20, 30, 40, 50]
print(my_list[0:3])  # Output: [10, 20, 30]
print(my_list[::2])  # Output: [10, 30, 50]

List Operations

Modifying Elements

numbers = [1, 2, 3, 4, 5]
numbers[2] = 10
print(numbers)  # Output: [1, 2, 10, 4, 5]

Adding Elements

numbers.append(6)  # Adds at the end
numbers.insert(1, 9)  # Insert at index 1
numbers.extend([7, 8])  # Merge another list
print(numbers)  # Output: [1, 9, 2, 10, 4, 5, 6, 7, 8]

Removing Elements

numbers.remove(10)  # Removes first occurrence
popped = numbers.pop(2)  # Removes by index
del numbers[0]  # Delete by index
numbers.clear()  # Clears entire list

Sorting and Reversing

numbers = [3, 1, 4, 1, 5, 9, 2]
numbers.sort()  # Ascending order
numbers.reverse()  # Reverse order
print(numbers)  # Output: [9, 5, 4, 3, 2, 1, 1]

List Comprehensions

Basic Example (Square of Numbers)

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

With Condition (Filtering)

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

With If-Else

labels = ["Even" if x % 2 == 0 else "Odd" for x in range(5)]
print(labels)  # Output: ['Even', 'Odd', 'Even', 'Odd', 'Even']

Flatten a List of Lists

matrix = [[1, 2, 3], [4, 5, 6]]
flattened = [num for row in matrix for num in row]
print(flattened)  # Output: [1, 2, 3, 4, 5, 6]

Advanced Examples

# Squares for even numbers, cubes for odd numbers
numbers = range(1, 11)
result = [x**2 if x % 2 == 0 else x**3 for x in numbers]
print(result)

# Filtering odd numbers and multiples of 3, adding 1 to odd numbers
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
result = [x + 1 if x % 2 != 0 else x for x in numbers if x % 3 == 0]
print(result)  # Output: [4, 7, 10]

Taking User Input for Lists

List of Integers from User Input

user_input = input("Enter numbers separated by spaces: ")
numbers = [int(num) for num in user_input.split()]
print("List of numbers:", numbers)

List of Strings from User Input

user_input = input("Enter words separated by spaces: ")
words = user_input.split()
print("List of words:", words)

Error Handling for Input

def get_int_list():
    while True:
        try:
            input_string = input("Enter integers separated by spaces: ")
            return list(map(int, input_string.split()))
        except ValueError:
            print("Invalid input. Please enter integers only.")

int_list = get_int_list()
print("The list of integers is:", int_list)

Summary

This guide provides a structured overview of lists, including indexing, slicing, comprehensions, and user input handling. Mastering these concepts will enhance your Python programming efficiency!