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Introduction to Python list comprehensions

In programming, you often need to transform elements of a list and returns a new list.

For example, suppose that you have a list of five numbers like this:

Code language: Python (python)

And you want to get a list of squares based on this numbers list

The straight forward way is to use a for loop:

Code language: Python (python)

In this example, the for loop iterates over the elements of the numbers list, squares each number and adds the result to the squares list.

Note that a square number is the product of the number multiplied by itself. For example, square number 2 is 2*2 = 4, square number of 3 is 3*3 = 9, and so on.

To make the code more concise, you can use the built-in map() function with a lambda expression:

Code language: Python (python)

Since the map() function returns an iterator, you need to use the list() function to convert the iterator to a list.

Both the for loop and map() function can help you create a new list based on an existing one. But the code isn’t really concise and beautiful.

To help you create a list based on the transformation of elements of an existing list, Python provides a feature called list comprehensions.

The following shows how to use the list comprehension to make a list of squares from the numbers list:

Code language: Python (python)

And here’s the list comprehension part:

Code language: Python (python)

A list comprehension consists of the following parts:

• An input list (numbers)
• A variable that represents the elements of the list (number)
• An output expression (number**2) that returns the elements of the output list from the elements of the input list.

The following shows the basic syntax of the Python list comprehension:

Code language: Python (python)

It’s equivalent to the following:

Code language: Python (python)

Printing all perfect squares from a list in Python using list comprehension and math module

In this tutorial, you will learn how to check whether the elements in a Python list entered by the user are perfect squares or not using list comprehension and the math module.

List comprehensions are a very cool trick where we can make a new list based on the values of an existing list in only one line, hence making the code in question look short and concise since we are not writing an entire loop committing to the problem.

The math module in python is a very useful tool as it contains a multitude of mathematical functions that we can use in our code.

In this example, we shall be taking a list input from the user and make a new list that consists of only perfect squares that were present in the previous list by checking if the square root of the number in question yields an integer value or a float value.

Introduction

In Python, the syntax of list comprehension is:

list_variable = [x for x in iterable]

Assume we want to create a list containing 100 numbers. Manually that would be a lot of typing work. So we would use a for loop, right?

We can define a for loop to fill the list.

numbers = [] for i in range(0,100): numbers.append(i) print(numbers)

That works, but it’s to much work. Instead you can use list comprehension.

You can replace all the code above with a one liner, which is how we obtain the same result:

numbers = [ x for x in range(100) ]

yes, inside the list we use a loop and the range() function

This is also useful if you want to create large lists.

Make Your Python Code Compact and Faster

List comprehension is a concise way of writing a for loop on a list or a string. Performing an action of each element of a list or string and generation of a new list is very easy, short, and compact using list comprehension. Normally we use a for loop for that. Doing a list comprehension is a comparatively faster and shorter way of writing a ‘for loop’. I am assuming you already know python programming at list beginner level.

In this article, I will explain list comprehension with a lot of examples starting from the simplest example and keep moving towards the more complex problems.

Let’s start by creating a simple list of digits:

digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Let’s make a new list that includes the square of each element of the digits list. I will do it using a regular for loop first and then using a list comprehension.

Here is the regular for loop:

Example 1:

numbers = []for i in digits:
numbers.append(i**2)

print(numbers)

Output:

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Now the same thing using a list comprehension:

[i**2 for i in digits]

Output:

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

As you can see instead of 4 lines of code I had to write just one simple line of code.

Example 2:

I will divide all the even numbers of the ‘digits’ list above and make a new list.

numbers = []for i in digits:
if i%2 ==0:
numbers.append(i/2)

print(numbers)

Output:

[0.0, 1.0, 2.0, 3.0, 4.0]

Here, we had to use an extra ‘if statement’ to check if the digit is even. Here is how to do the same thing in a list comprehension:

[i/2 for i in digits if i%2 ==0]

Output:

[0.0, 1.0, 2.0, 3.0, 4.0]

Example 3:

In this example, I will multiply the even elements of the ‘digits’ list by three and I will multiply the odd elements with three and add 1 to it to make it even. Here is the code:

numbers = []
for i in digits:
if i%2 ==0:
numbers.append(3*i)
else:
numbers.append(3*i + 1)
print(numbers)

Output:

[0, 4, 6, 10, 12, 16, 18, 22, 24, 28]

In this example, we have a for loop and an if-else statement. An implementation of the same code with list comprehension:

[3*i if i%2 ==0 else 3*i + 1 for i in digits]

Output:

[0, 4, 6, 10, 12, 16, 18, 22, 24, 28]

Example 4:

We can use the in-built functions in a list comprehension. I will import the math library and use the square root function in all the elements of the ‘digits’ list:

import math
[math.sqrt(i) for i in digits]

Output:

[0.0,
1.0,
1.4142135623730951,
1.7320508075688772,
2.0,
2.23606797749979,
2.449489742783178,
2.6457513110645907,
2.8284271247461903,
3.0]

Example 5:

This example will show you how to use a custom function use it in each element of a list using list comprehension. First, let’s define a function and then use it on the ‘digits’ list:

def form(n):
return 2**n - n**2[form(i) for i in digits]

Output:

[1, 1, 0, -1, 0, 7, 28, 79, 192, 431]

Look, how the formula in the function transforms each digit. I find it helpful to have the digit itself and the transformed number together. We can generate the tuples using the list comprehension where each tuple will include the digit itself and the transformation:

[(i, form(i)) for i in digits]

Output:

[(0, 1),
(1, 1),
(2, 0),
(3, -1),
(4, 0),
(5, 7),
(6, 28),
(7, 79),
(8, 192),
(9, 431)]

Instead of tuples, we could use the sets as well:

[{i, form(i)} for i in digits]

Output:

[{0, 1},
{1},
{0, 2},
{-1, 3},
{0, 4},
{5, 7},
{6, 28},
{7, 79},
{8, 192},
{9, 431}]

Example 6:

This example will work on a nested for loop. Let’s write a simple nested for loop and then we will see how to do that using list comprehension.

com = []for i in [3, 1, 4]:
for j in [9, 0, 2]:
com.append((i, j))
print(com)

Output:

[(3, 9), (3, 0), (3, 2), (1, 9), (1, 0), (1, 2), (4, 9), (4, 0), (4, 2)]

Here is the one-liner implementation using list comprehension of this nested for loop:

[(i, j) for i in [3, 1, 4] for j in [9, 0, 2]]

Output:

[(3, 9), (3, 0), (3, 2), (1, 9), (1, 0), (1, 2), (4, 9), (4, 0), (4, 2)]

Example 7:

I wanted to show an implementation of the if-else statement with nested for loop as well. This time I will check if the first list of Example 6 is an odd number. If it is odd, we will add 1 to it otherwise it will stay as it is.

com = []for i in [3, 1, 4]:
for j in [9, 0, 2]:
if i%2 != 0:
com.append((i+1, j))
else:
com.append((i, j))

print(com)

Output:

[(4, 9), (4, 0), (4, 2), (2, 9), (2, 0), (2, 2), (4, 9), (4, 0), (4, 2)]

Below is the list comprehension implementation of the above code block:

[(i+1, j) if i%2 !=0 else (i, j) for i in [3, 1, 4] for j in [9, 0, 2]]

Output:

[(4, 9), (4, 0), (4, 2), (2, 9), (2, 0), (2, 2), (4, 9), (4, 0), (4, 2)]

Example 8:

In this example, I will use a 2-dimensional list and flatten it using a list comprehension approach.

v = [[1, 3, 4], [2, 5, 6], [7, 5, 4]][i for elements in v for i in elements]

Output:

[1, 3, 4, 2, 5, 6, 7, 5, 4]

Example 9:

We will take a 2-dimensional list for this example as well and rotate it to 90 degrees. This example is pretty popular in coding interviews. Let’s implement it with regular for loop first as it can be slightly complicated for new programmers:

td = [[1, 3, 4],
[2, 5, 6],
[7, 5, 4]]res = []for i in range(len(td)):
el = []
for j in td:
el.append(j[i])
res.append(el)

print(res)

Output:

[[1, 2, 7], [3, 5, 5], [4, 6, 4]]

Here is how to do it using list comprehension:

[[j[i] for j in td] for i in range(len(td))]

Output:

[[1, 2, 7], [3, 5, 5], [4, 6, 4]]