Python – List Comprehension

Python List Comprehension

List comprehension is a concise and efficient way to create new lists by applying an expression to each item in an existing iterable (like a list, tuple, etc.). It can also include conditions for filtering items.

The basic syntax of a list comprehension is:

[expression for item in iterable]

You can also add conditions to the comprehension:

[expression for item in iterable if condition]

Basic Examples:

1. Creating a new list by applying an operation to each element:

my_list = [1, 2, 3, 4, 5]

# Create a new list with each item doubled
new_list = [x * 2 for x in my_list]

print(new_list)  # Output: [2, 4, 6, 8, 10]

2. List comprehension with a condition (filtering):

my_list = [1, 2, 3, 4, 5]

# Create a new list with only even numbers
new_list = [x for x in my_list if x % 2 == 0]

print(new_list)  # Output: [2, 4]

3. List comprehension with an else statement:

You can also use an else statement to include an alternative expression when the condition is not met.

my_list = [1, 2, 3, 4, 5]

# Create a new list where even numbers are squared, and odd numbers are multiplied by 2
new_list = [x**2 if x % 2 == 0 else x * 2 for x in my_list]

print(new_list)  # Output: [2, 4, 6, 16, 10]

More Complex Examples:

4. List comprehension with multiple conditions:

You can add multiple conditions in the if statement to filter the items more precisely.

my_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Create a new list with numbers divisible by both 2 and 3
new_list = [x for x in my_list if x % 2 == 0 and x % 3 == 0]

print(new_list)  # Output: [6]

5. List comprehension for nested loops:

You can also use list comprehension for nested loops to handle more complex cases like iterating over a list of lists.

nested_list = [[1, 2], [3, 4], [5, 6]]

# Create a new list containing all items from the sublists
new_list = [item for sublist in nested_list for item in sublist]

print(new_list)  # Output: [1, 2, 3, 4, 5, 6]

6. List comprehension with a function:

You can apply a function to each item while creating the new list.

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

# Create a new list with the uppercase version of each string
new_list = [fruit.upper() for fruit in my_list]

print(new_list)  # Output: ['APPLE', 'BANANA', 'CHERRY']

7. Nested list comprehension:

You can also use list comprehension to create a list of lists.

# Create a 2D matrix using list comprehension
matrix = [[row * col for col in range(1, 4)] for row in range(1, 4)]

print(matrix)
# Output: 
# [
#    [1, 2, 3],
#    [2, 4, 6],
#    [3, 6, 9]
# ]

Benefits of List Comprehension:

• Compact: It allows you to perform operations in a single line of code, reducing the need for loops and making the code cleaner and more readable.
• Faster: List comprehensions are typically faster than using regular loops because they are optimized for creating lists.

Summary:

• Basic Syntax: [expression for item in iterable]
• With Condition: [expression for item in iterable if condition]
• With else: [expression if condition else alternate for item in iterable]
• Multiple Conditions: You can combine multiple conditions in the if statement.
• Nested Loops: You can loop through multiple lists with nested comprehensions.
• Functions: You can apply a function to each item in the list comprehension.

List comprehensions are a powerful tool for generating new lists in a concise and efficient manner.