Lesson 17: Lambda, Map, Filter, Reduce in Python

🎯 Lesson Objective

To learn anonymous functions with lambda, and functional programming tools like map, filter, and reduce for concise and efficient data processing.

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🧩 1. Lambda Functions

A lambda function is a small anonymous function defined with the lambda keyword.

Syntax:

lambda arguments: expression

✅ Example 1: Square of a Number

square = lambda x: x**2
print(square(5))

Output:

25

✅ Example 2: Sum of Two Numbers

add = lambda a, b: a + b
print(add(10, 15))

Output:

25

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🔄 2. Map Function

map() applies a function to each element of an iterable.

Syntax:

map(function, iterable)

✅ Example: Square a List of Numbers

nums = [1, 2, 3, 4, 5]
squares = list(map(lambda x: x**2, nums))
print(squares)

Output:

[1, 4, 9, 16, 25]

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🧰 3. Filter Function

filter() returns elements of an iterable that satisfy a condition.

Syntax:

filter(function, iterable)

✅ Example: Even Numbers from a List

nums = [1, 2, 3, 4, 5, 6]
evens = list(filter(lambda x: x % 2 == 0, nums))
print(evens)

Output:

[2, 4, 6]

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🧱 4. Reduce Function

reduce() applies a function cumulatively to the items of an iterable to reduce it to a single value.

Requires functools module.

from functools import reduce

nums = [1, 2, 3, 4, 5]
sum_all = reduce(lambda x, y: x + y, nums)
print(sum_all)

Output:

15

✅ Example 2: Product of Numbers

product = reduce(lambda x, y: x * y, nums)
print(product)

Output:

120

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🔄 5. Combining Map, Filter, Reduce

✅ Example: Sum of Squares of Even Numbers

from functools import reduce

nums = [1, 2, 3, 4, 5, 6]
evens = filter(lambda x: x % 2 == 0, nums)
squares = map(lambda x: x**2, evens)
sum_squares = reduce(lambda x, y: x + y, squares)
print(sum_squares)

Output:

56

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🌍 6. Real-Life Example – Processing Grades

grades = [80, 90, 75, 88, 95]

# Increase each grade by 5
grades_inc = list(map(lambda x: x + 5, grades))

# Keep only grades >= 90
top_grades = list(filter(lambda x: x >= 90, grades_inc))

# Sum of top grades
total_top = reduce(lambda x, y: x + y, top_grades)

print("Grades +5:", grades_inc)
print("Top Grades:", top_grades)
print("Sum of Top Grades:", total_top)

Output:

Grades +5: [85, 95, 80, 93, 100]
Top Grades: [95, 93, 100]
Sum of Top Grades: 288