Find maximum length sub-list in a nested list in Python

Finding the maximum length sub-list in a nested list is a common task when working with data structures in Python. This article explores different approaches to identify the longest sub-list and return both the sub-list and its length.

Using lambda and max()

The most efficient approach combines lambda with the max() function. The max() function uses a key parameter to determine which sub-list has the maximum length ?

Example

def longest(nested_list):
    longest_list = max(nested_list, key=lambda i: len(i))
    max_length = max(map(len, nested_list))
    return longest_list, max_length

# Driver Code
nested_data = [[1, 2], [2, 45, 6, 7], [11, 65, 2]]
print("Longest List and its length:")
print(longest(nested_data))

Longest List and its length:
([2, 45, 6, 7], 4)

Using len() and max() with Loop

This approach first calculates the maximum length, then iterates through the nested list to find the sub-list that matches this length ?

Example

def longest(nested_list):
    longest_list = []
    max_length = max(len(x) for x in nested_list)
    
    for sublist in nested_list:
        if len(sublist) == max_length:
            longest_list = sublist
            break  # Return first match
    
    return longest_list, max_length

# Driver Code
nested_data = [[1, 2], [2, 45, 6, 7], [11, 6, 2]]
print("Longest List and its length:")
print(longest(nested_data))

Longest List and its length:
([2, 45, 6, 7], 4)

Using map() Function

Similar to the previous approach, but uses map() to apply the len() function to all sub-lists at once ?

Example

def longest(nested_list):
    longest_list = []
    max_length = max(map(len, nested_list))
    
    for sublist in nested_list:
        if len(sublist) == max_length:
            longest_list = sublist
            break  # Return first match
    
    return longest_list, max_length

# Driver Code
nested_data = [[1, 2], [2, 45, 6, 7], [11, 6, 2]]
print("Longest List and its length:")
print(longest(nested_data))

Longest List and its length:
([2, 45, 6, 7], 4)

Comparison

Conclusion

The lambda with max() approach is the most efficient and readable solution. Use the loop-based methods when you need to handle multiple sub-lists of the same maximum length or require additional processing logic.