Lesson 4.04: List Comprehensions

Microsoft Philanthropies TEALS Program

Introduction to Computer Science

Semester 2

Nested Lists

After this lesson, you will be able to...

• Define and identify list comprehensions
• Use list comprehensions to map and filter lists

Today’s Plan

Day 1

Lesson

Lab

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How does a list comprehension work?

It is really just a new type of for loop that makes a list.

Brackets make a new list.

Each element is created by an expression.

A for loop does the iterating.

In the last lesson we saw this:

from random import randint

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def make_random_row(columns):

return [randint(0, 9) for i in range(columns)]

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def make_random_matrix(rows, columns):

return [make_random_row(columns) for i in range(rows)]

This new for loop is called a list comprehension. We can use it to make a new list from another list.

Remember map? Our traversal to transform a list?

Do you remember filter? Selecting a new list from a list?

Your Lab for today. Create a list comprehension

• Multiply every number in a list by another number.
• Return a new list.
• Starter code!

Starter code in the assignment.