How to use List and Dict Comprehension in Python

What is List Comprehension in Python? You will learn how to make List Comprehension and show how this also works with dictionaries, called Dict Comprehension. Then show how Dict Comprehension can be used for frequency count.

In this tutorial, you will:

• Understand List Comprehension in Python: Learn the concept and syntax of List Comprehension, a concise way to create lists based on existing lists or iterables.
• Explore Dict Comprehension: Discover how List Comprehension extends to dictionaries, known as Dict Comprehension, enabling the creation of dictionaries in a similar concise manner.
• Utilize Dict Comprehension for Frequency Count: Learn how Dict Comprehension can be used to efficiently perform frequency counting on elements within a list or iterable.

By the end of the tutorial, you will have a solid understanding of List Comprehension and Dict Comprehension in Python. You will be able to leverage these powerful techniques to create lists and dictionaries more efficiently and apply them to perform frequency counting tasks effectively.

Step 1: What is List Comprehension in Python?

A List Comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists.

Wikipedia.org

It is easiest to demonstrate how to create it in Python (see more about lists and about foo-loops).

my_list = [i for i in range(10)]

print(my_list)

Will result in.

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

Here you use the range(10) construct, which gives a sequence from 0 to 9 that can be used in the Comprehension construct.

A more direct example is given here.

my_new_list = [i*i for i in my_list]

print(my_new_list)

Which results in the following.

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

Step 2: List Comprehension with if statements

Let’s get straight to it (see more about if-statements).

my_list = [i for i in range(10) if i % 2 == 0]

print(my_list)

This will result in.

[0, 2, 4, 6, 8]

Also, you can make List Comprehension with if-else-statements.

my_list = [i if i % 2 else -i for i in range(10)]

print(my_list)

This results in.

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

Step 3: Dict Comprehension in Python

Let’s dive straight into it (see more about dict).

my_list = [i for i in range(10)]

my_dict = {i: i*i for i in my_list}
print(my_dict)

Which results in.

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}

Notice, that the lists do not need to contain integers or floats. The constructs also work with any other values.

Step 4: Frequency Count using Dict Comprehension

This is amazing.

text = 'aabbccccc'

freq = {c: text.count(c) for c in text}
print(freq)

Which results in.

{'a': 2, 'b': 2, 'c': 5}

Notice, that here we will recount each instance of the letters. To avoid that you can do as follows.

text = 'aabbccccc'

freq = {c: text.count(c) for c in set(text)}
print(freq)

Which results in the same output. The difference is that you only count for each letter once.

Step 5: Want more?

I am happy you asked. Now you have learned about List and Dict Comprehension in Python.

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