List Comprehensions

Create a List of Squares

How would I create a list of numbers that are the square root of 0 through 9? →

""" don't just use the list literal """
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

squares = []
for i in range(10):
	squares.append(i**2)

Create a List of Squares

How would I create a list of numbers that are the square root of 0 through 9, but only include squares that are divisible by 2? →

""" don't just use the list literal """
[0, 4, 16, 36, 64]

squares = []
for i in range(10):
	if i % 2 == 0:
		squares.append(i**2)

Quick Summary

• in each instance, we created a list by starting with an empty list
• …using a for loop
• …and appending on each iteration

List Comprehensions

List comprehensions are another, more concise way of creating lists. A list comprehension is syntactic sugar (syntax within a programming language that is designed to make things easier to read or to express) for the code that we created previously.

List comprehensions make new lists

List Comprehension Syntax

• consists of square brackets to make a list
• a for loop-like expression within the brackets
• before the for loop-like expression, an expression that calculates each element of the new list
• after the for loop-like expression, an optional expression that determines if an element should be in the new list

Two Examples: Squares, and Squares Divisible by 2

[x * x for x in range(10)]
[x * x for x in range(10) if x % 2 == 0]

Another List Comprehension

Write a list comprehension that creates a new list by adding exclamation points to every element in an existing list. →

items = ['foo', 'bar', 'baz', 'busy']

[s + '!' for s in items]

And Another List Comprehension!

Limit the previous list to strings of length 3. →

"""
filter the list called items (below), so that the resulting list is:
['foo', 'bar', 'baz']
"""
items = ['foo', 'bar', 'baz', 'busy']

[s for s in items if len(s) == 3]