Element Occurrence in Dictionary of List Values - Python

Last Updated : 08 Feb, 2025

We are having a dictionary of list we need to find occurrence of all elements. For example, d = {'a': [1, 2, 3, 1], 'b': [3, 4, 1], 'c': [1, 5, 6]} we need to count the occurrence of all elements in dictionary list so that resultant output should be {'a': 2, 'b': 1, 'c': 1}.

Using a Dictionary Comprehension

We can use a dictionary comprehension to count occurrences of an element in each list value of dictionary. This approach iterates over dictionary items and applies count() method to determine frequency of target element in each list

Python

d = {'a': [1, 2, 3, 1], 'b': [3, 4, 1], 'c': [1, 5, 6]} 
ele = 1  # Target element to count  

# Dictionary comprehension to count occurrences of 'ele' in each list  
o = {k: v.count(ele) for k, v in d.items()}  
print(o)  # Output: {'a': 2, 'b': 1, 'c': 1}

Output

{'a': 2, 'b': 1, 'c': 1}

Explanation:

Dictionary comprehension iterates over each key-value pair in d and uses v.count(ele) to count occurrences of ele in list.
Result is a new dictionary where each key maps to count of ele in its corresponding list.

Using `collections.Counter`

We can use collections.Counter to count element occurrences in each list of dictionary efficiently. By applying Counter to each list we retrieve frequency counts and access target element’s count using .get() defaulting to zero if absent.

Python

from collections import Counter 
d = {'a': [1, 2, 3, 1], 'b': [3, 4, 1], 'c': [1, 5, 6]}  
ele = 1  # Target element to count  

# Use Counter to count occurrences of 'ele' in each list  
o = {k: Counter(v)[ele] for k, v in d.items()}  
print(o)

Output

{'a': 2, 'b': 1, 'c': 1}

Explanation:

Counter(v) creates a frequency dictionary for each list mapping elements to their counts.
Counter(v)[ele] retrieves count of ele defaulting to zero if ele is not present.

Using a Loop

We iterate over dictionary using a for loop and count target element in each list using the count() method results are stored in a new dictionary mapping each key to its corresponding count.

Python

d = {'a': [1, 2, 3, 1], 'b': [3, 4, 1], 'c': [1, 5, 6]}
ele = 1  
o = {}  
for key, values in d.items():  
    # Count occurrences of 'ele' in the list and store in 'o'  
    o[key] = values.count(ele)  

print(o)

Output

{'a': 2, 'b': 1, 'c': 1}

Explanation:

For loop iterates over each dictionary item using count(ele) to determine occurrences of ele in list.
Resulting counts are stored in a new dictionary mapping each key to frequency of ele in its corresponding list.