Python Learning Series · @onlysamarrr · Day 22 / 30

Day 22: lambda, map, filter

Day 22 teaches lambda, map and filter. lambda is a small one-line function without a name, map() runs a function on every item, and filter() keeps only items that pass a condition.

Phase: Intermediate · Reels: 106/450 – 110/450

Quick Answer

Day 22 teaches lambda, map and filter. lambda is a small one-line function without a name, map() runs a function on every item, and filter() keeps only items that pass a condition.

What You Will Learn

lambda x: x * 2 is a small anonymous function: x is the input, x * 2 is the output.
map(lambda p: p * 0.9, prices) applies a 10% discount to every price.
filter(lambda n: n % 2 == 0, ...) keeps only even numbers.
sorted(names, key=len) sorts names by their length, using len as the key function.
If a function needs a name or is long, use def instead of lambda: do not overuse lambda.

Code

double = lambda x: x * 2
print(double(5))
prices = [100, 250, 400]
off = list(map(lambda p: p * 0.9, prices))
print(off)
ev = list(filter(lambda n: n % 2 == 0, range(7)))
print(ev)
names = ["Al", "Riya", "Arhan"]
print(sorted(names, key=len))

Output

10
[90.0, 225.0, 360.0]
[0, 2, 4, 6]
['Al', 'Riya', 'Arhan']

Code Explained, Line by Line

lambda x: x * 2 is a small anonymous function: x is the input, x * 2 is the output.
map(lambda p: p * 0.9, prices) applies a 10% discount to every price.
filter(lambda n: n % 2 == 0, ...) keeps only even numbers.
sorted(names, key=len) sorts names by their length, using len as the key function.
If a function needs a name or is long, use def instead of lambda: do not overuse lambda.

Common Mistake: Do not overuse lambda. For longer logic, write a named function with def so the code stays readable.

Real-Life Use

Discounts, data cleaning and sorting by a custom rule use map, filter and key functions.

Practice Task: Apply a discount to prices with map, and filter out even numbers.

Test Yourself — MCQs

Select (tick) an option, then tap Check answer to see instantly whether your answer is right or wrong, with an explanation. You can change your answer and check again.

Q1. What is lambda x: x * 2?

A. A named function B. A small anonymous function C. A loop D. A class

Q2. What does map() do?

A. Filters items B. Runs a function on every item C. Sorts items D. Deletes items

Q3. What does filter() keep?

A. All items B. Only items that pass the condition C. Only numbers D. Only the first item

Today's 5 Reels (from the 90-Day Plan Excel)

106/450	Concept	lambda function
107/450	Live demo	map aur filter demo
108/450	Common mistake	lambda ko overuse mat karo
109/450	Quiz	Quiz: even numbers filter
110/450	Interview / real life	Real life: prices pe discount

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