Python - List Frequency of Elements

We are given a list we need to count frequencies of all elements in given list. For example, n = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4] we need to count frequencies so that output should be {4: 4, 3: 3, 2: 2, 1: 1}.

Using collections.Counter

collections.Counter class provides a dictionary-like structure that counts occurrences of elements in a list. It returns a dictionary where keys are elements and values are their frequencies.

from collections import Counter

n = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
# Creates a dictionary with element counts
f = Counter(n)  

print(f)  

Counter({4: 4, 3: 3, 2: 2, 1: 1})

Explanation:

• Counter(n) creates a frequency dictionary where each key is a unique element, and its value represents count of occurrences.
• This method is efficient and concise for counting elements in a list making it useful for frequency analysis.

Using a Dictionary with get()

A dictionary with the get() method can be used to count element frequencies by initializing counts to zero and updating them dynamically. The get() method helps retrieve values safely avoiding key errors when encountering new elements.

n = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
f = {}

for num in n:
    # Increment count
    f[num] = f.get(num, 0) + 1  

print(f) 

{1: 1, 2: 2, 3: 3, 4: 4}

Explanation:

• get(num, 0) method initializes missing keys to 0 before incrementing, avoiding key errors.
• This approach manually builds a frequency dictionary by iterating over the list and counting occurrences.

Using collections.defaultdict

collections.defaultdict provides a dictionary with a default value type eliminating need to check for missing keys. It simplifies frequency counting by automatically initializing values when a new key is encountered.

from collections import defaultdict

n = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
f = defaultdict(int)

for num in n:
    # Increment count
    f[num] += 1  

print(dict(f)) 

{1: 1, 2: 2, 3: 3, 4: 4}

Explanation:

• defaultdict(int) initializes missing keys with 0 automatically, avoiding the need for get().
• This method ensures efficient frequency counting with minimal code by leveraging defaultdict's auto-initialization.