List Comprehension
List comprehensions are a faster and more elegant way to create a list based on an existing iterable (lists, strings, etc.). It’s unique to Python, so it's often seen as a Pythonic way of coding.
For example, you can very easily use a
for loop for creating a list:listL = [] for i in range(10): listL.append(i * i) print(listL) # prints [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
But, python allows you to create it in one line using list comprehension. Here’s the general formula for it:
[expression for item in iterable if condition]. Here is an example of list comprehension:
squares = [i * i for i in range(10)] print(squares) # prints [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
Let’s figure out each part of the formula. In the example.
i * i is the expression. i is the item. range(10) is the iterable. Nothing is used for the if condition since it isn’t necessary for this case.Here's an example of using list comprehension with an if statement:
a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] squares = [i * i for i in a if i > 5] print(squares) # prints [36, 49, 64, 81, 100]
Because the formula for list comprehension is very flexible, you can make lists of almost anything. In this case, the if condition is
if I > 5.Here is an example of using list comprehension to capitalize all the letters from a word and put them into a list:
word = " Hey how are you" asks = [i.upper() for i in word] print(asks) # prints [' ', 'H', 'E', 'Y', ' ', 'H', 'O', 'W', # ' ', 'A', 'R', 'E', ' ', 'Y', 'O', 'U']
View code on GitHub.
Practice
Square Root List
Take a user-given list of numbers and make a list of all the square roots of the numbers using list comprehension. Use a list comprehension to solve it.
Hint: The square root of a number is the same as taking the ½ power of a number.
Odd Squares
Given a list of integers, create a list of all the squares of the odd integers within the list. Use a list comprehension to solve it.
Even Words
Given a string that the user inputs create a list that contains the square of the lengths of words with an even amount of characters. For example, if the string is “I am cool”, the list would be
[4, 16]. Use a list comprehension to solve it.Copyright © 2021 Code 4 Tomorrow. All rights reserved.
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