Write a Python function to find the intersection of three lists.
DavidLee
Posted by DavidLee
Last Updated: August 13, 2024
Python Intersection
Finding the intersection of three lists in Python can be accomplished in several ways. A straightforward approach involves utilizing built-in data structures like sets, which allow for efficient membership testing and elimination of duplicates. Below is a Python function that takes three lists as input and returns a list containing the elements that are common to all three lists. 
def intersect_three_lists(list1, list2, list3):
    """
    Returns the intersection of three lists.
    
    Parameters:
    list1 (list): The first list.
    list2 (list): The second list.
    list3 (list): The third list.
    
    Returns:
    list: A list containing elements that are present in all three input lists.
    """
    # Convert lists to sets to find intersection
    set1 = set(list1)
    set2 = set(list2)
    set3 = set(list3)
    
    # Find intersection of the three sets
    intersection = set1.intersection(set2).intersection(set3)
    
    # Convert the intersection set back to a list before returning
    return list(intersection)

# Example usage:
list_a = [1, 2, 3, 4, 5]
list_b = [4, 5, 6, 7]
list_c = [5, 8, 9, 4]

result = intersect_three_lists(list_a, list_b, list_c)
print(result)  # Output: [4, 5]
Explanation of the Function:
1. Function Definition: intersect_three_lists takes three lists as parameters. 2. Set Conversion: Each list is converted to a set. This step is crucial as sets automatically handle duplicates and provide fast membership testing. 3. Finding Intersection: The intersection of the three sets is calculated using the intersection() method, which is applied sequentially. 4. Returning the Result: The resulting set is converted back to a list before being returned for consistency with the input type.
Performance Consideration:
Using sets is generally efficient for this task due to average-case time complexity of O(1) for membership checks and O(n) for constructing a set from a list, making it suitable for larger datasets. This function is versatile and can handle any type of list elements; however, it is important to note that all elements in the lists must be hashable, as that is a requirement for set operations in Python.
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