Python Basics — Day 18 Lambda Functions in Python

01. What is a Lambda Function?

• A one-line anonymous function (no name).

• Defined using the syntax:
  lambda parameters: expression

add = lambda x, y: x + y
print(add(3, 5))   # 8

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02. Regular Function vs Lambda Function

# Regular function
def square(x):
    return x * x

# Lambda function
square2 = lambda x: x * x

print(square(4))   # 16
print(square2(4))  # 16

👉 Both give the same result, but lambdas are useful for short, inline code.

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03. Common Uses of Lambda Functions

List Sorting with key

nums = [3, 1, 5, 2, 4]
nums.sort(key=lambda x: -x)   # Sort in descending order
print(nums)  # [5, 4, 3, 2, 1]

map() – Apply Function to All Elements

numbers = [1, 2, 3, 4]
squares = list(map(lambda x: x**2, numbers))
print(squares)  # [1, 4, 9, 16]

filter() – Select Elements That Meet a Condition

numbers = [10, 15, 20, 25]
even = list(filter(lambda x: x % 2 == 0, numbers))
print(even)  # [10, 20]

reduce() – Accumulate Values

from functools import reduce

numbers = [1, 2, 3, 4]
product = reduce(lambda x, y: x * y, numbers)
print(product)  # 24

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04. Lambda with Conditional Expression

max_num = lambda a, b: a if a > b else b
print(max_num(10, 20))  # 20

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05. Practice Examples

Example 1: Sort Strings by Length

words = ["apple", "banana", "kiwi"]
words.sort(key=lambda w: len(w))
print(words)   # ['kiwi', 'apple', 'banana']

Example 2: Filter Odd Numbers

nums = [1, 2, 3, 4, 5]
odds = list(filter(lambda n: n % 2 == 1, nums))
print(odds)   # [1, 3, 5]