5 Best Ways to Find Which Element Occurs Exactly Once in Python

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πŸ’‘ Problem Formulation: In Python, it is a common task to process collections of elements and identify which elements are unique. Specifically, the challenge is to find elements that appear exactly once in a given list. For instance, given the input [3, 5, 3, 4, 6, 5, 5], the desired output should be [4, 6], as these numbers appear uniquely within the collection.

Method 1: Using a Dictionary

This method leverages a dictionary to count the occurrence of each element. By iterating over the list, the method populates the dictionary with elements as keys and their counts as values. After populating the dictionary, we can find the elements that occur exactly once.

Here’s an example:

def find_unique_numbers(numbers):
    frequency = {}
    for number in numbers:
        frequency[number] = frequency.get(number, 0) + 1
    return [number for number, count in frequency.items() if count == 1]

result = find_unique_numbers([3, 5, 3, 4, 6, 5, 5])


[4, 6]

This code snippet defines a function called find_unique_numbers that takes a list of numbers. It creates an empty dictionary called frequency to store the count of each number. It then iterates over the list and updates the count for each number. Finally, it returns a list of numbers whose count is exactly one.

Method 2: Using Collections.Counter

The Collections.Counter class from Python’s standard library simplifies the counting process by automatically creating a dictionary of elements and their counts. This method is clean, efficient, and requires significantly less code.

Here’s an example:

from collections import Counter

def unique_elements(data):
    counts = Counter(data)
    return [elem for elem, count in counts.items() if count == 1]

print(unique_elements([3, 5, 3, 4, 6, 5, 5]))


[4, 6]

This snippet imports Counter from the collections module and defines the function unique_elements. This function uses Counter to count occurrences of each element and then creates a list comprehension to extract the elements with a count of one.

Method 3: Using List Comprehension and count()

We can also use list comprehension with the count() method for each element to directly find unique elements. While simpler, this method can be less efficient for larger lists due to its O(n^2) time complexity.

Here’s an example:

def find_single_occurrences(seq):
    return [x for x in seq if seq.count(x) == 1]

print(find_single_occurrences([3, 5, 3, 4, 6, 5, 5]))


[4, 6]

In this approach, the function find_single_occurrences uses a list comprehension where it includes an element x only if the count() of x in the original list seq is exactly one. This straightforward code can become significantly slower when dealing with large datasets.

Method 4: Using NumPy Unique Function

For those who work with numeric data and are familiar with the NumPy library, the numpy.unique() function can be another effective way to find unique elements. This method is fast and works very well with large numeric datasets.

Here’s an example:

import numpy as np

def find_unique_with_numpy(arr):
    unique, counts = np.unique(arr, return_counts=True)
    return unique[counts == 1]

print(find_unique_with_numpy(np.array([3, 5, 3, 4, 6, 5, 5])))


[4 6]

This code utilizes the NumPy library for its unique() function, which returns a tuple containing the unique elements and their respective counts when return_counts is set to True. The result is then filtered to include only elements with a count of one.

Bonus One-Liner Method 5: Using set() and list comprehension

This is a condensed one-liner approach that uses set operations to find the unique elements within a list. It’s an elegant solution, but can be less readable for beginners.

Here’s an example:

nums = [3, 5, 3, 4, 6, 5, 5]
unique = [x for x in nums if nums.count(x) == 1]



[4, 6]

The one-liner defines a variable unique, which uses a list comprehension to go through the list nums and includes only those elements that appear exactly once, as determined by the count() method.


  • Method 1: Using a Dictionary. This traditional method is very versatile and can be applied to any type of elements. However, the manual implementation can be more error-prone and involves more lines of code than some other methods.
  • Method 2: Using Collections.Counter. It’s a clean and efficient approach. However, it requires importing an additional class from a Python module, which might be unnecessary for simple tasks.
  • Method 3: Using List Comprehension and count(). Simple to implement and easy to read, but not efficient for large lists due to its O(n^2) time complexity.
  • Method 4: Using NumPy Unique Function. This method is highly efficient for numeric datasets and benefits from the power of NumPy, but is not suitable for non-numeric data and requires the NumPy library.
  • Method 5: Condensed One-Liner. A concise one-liner that is elegant and quick to write, but it can be less readable, especially for those new to Python programming.