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Lesson Content

List Comprehensions: Concise List Creation

List comprehensions provide a concise way to create lists. They're more Pythonic and often faster than traditional loops.

Basic List Comprehension

# Traditional way
squares = []
for x in range(5):
    squares.append(x ** 2)
# Result: [0, 1, 4, 9, 16]

# List comprehension way
squares = [x ** 2 for x in range(5)]
# Result: [0, 1, 4, 9, 16]

List Comprehension with Condition

# Even numbers
 evens = [x for x in range(10) if x % 2 == 0]
# Result: [0, 2, 4, 6, 8]

# Filter and transform
words = ["hello", "world", "python"]
lengths = [len(word) for word in words if len(word) > 5]
# Result: [6] (only 'python' has length > 5)

Nested List Comprehensions

# Matrix (list of lists)
matrix = [[i * j for j in range(3)] for i in range(3)]
# Result: [[0, 0, 0], [0, 1, 2], [0, 2, 4]]

Lambda Functions

Anonymous functions defined with lambda:

# Regular function
def add(x, y):
    return x + y

# Lambda function
add = lambda x, y: x + y

# Common use: with map(), filter(), sorted()
numbers = [1, 2, 3, 4, 5]

# Map: apply function to each item
squared = list(map(lambda x: x ** 2, numbers))
# Result: [1, 4, 9, 16, 25]

# Filter: keep items that meet condition
evens = list(filter(lambda x: x % 2 == 0, numbers))
# Result: [2, 4]

# Sorted: sort with custom key
students = [("Alice", 20), ("Bob", 18)]
sorted_by_age = sorted(students, key=lambda x: x[1])
# Result: [('Bob', 18), ('Alice', 20)]

Dictionary and Set Comprehensions

# Dictionary comprehension
squares_dict = {x: x ** 2 for x in range(5)}
# Result: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# Set comprehension
unique_lengths = {len(word) for word in ["hi", "hello", "hi"]}
# Result: {2, 5}

When to Use Comprehensions

✅ Simple transformations and filters
✅ Creating lists from other iterables
✅ More readable and Pythonic
❌ Complex logic (use regular loops)
❌ When you need side effects

Example Code

Use list comprehensions to create filtered and transformed lists.

# Create a list of squares for numbers 0-9
squares = [x ** 2 for x in range(10)]
print(squares)

# Create a list of even numbers from 0-20
evens = [x for x in range(21) if x % 2 == 0]
print(evens)

# Convert strings to uppercase using list comprehension
words = ["hello", "world", "python"]
uppercase = [word.upper() for word in words]
print(uppercase)

Expected Output:

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
['HELLO', 'WORLD', 'PYTHON']

Study Tips

•Read the theory content thoroughly before practicing
•Review the example code to understand key concepts
•Proceed to the Practice tab when you're ready to code