💡 Problem Formulation: When working with lists in Python, understanding how to effectively count unique sublists is a common challenge. Suppose we have a list of sublists, like [['apple', 'banana'], ['banana', 'apple'], ['apple', 'banana'], ['orange']], and we want to count how many distinct sublists exist, disregarding the order of elements within sublists. Our desired output for this example would be 2 unique sublists: one for [‘apple’, ‘banana’] and one for [‘orange’].
Method 1: Using a Custom Hash Function
This method involves creating a hash function that interprets each sublist as an immutable set, which allows for the identification of unique sublists where the order is irrelevant. The uniqueness is determined by a hash value that the custom function generates for each sublist.
Here’s an example:
def hashable_sublist(sublist):
return tuple(sorted(sublist))
list_of_sublists = [['apple', 'banana'], ['banana', 'apple'], ['apple', 'banana'], ['orange']]
unique_sublists = set(map(hashable_sublist, list_of_sublists))
unique_count = len(unique_sublists)
print(unique_count)Output:
2
This snippet creates a function hashable_sublist() that sorts and converts each sublist into a tuple, which is hashable. It then maps this function over the list of sublists, converting them into a set of unique elements (in this case, hashable tuples). Lastly, it prints the length of the set to find the number of unique sublists.
Method 2: Using frozenset
With this method, you utilize frozenset, a built-in immutable set-like collection in Python. It turns each sublist into a frozenset that can be directly used in a set for uniqueness determination.
Here’s an example:
list_of_sublists = [['apple', 'banana'], ['banana', 'apple'], ['apple', 'banana'], ['orange']] unique_sublists = set(frozenset(sublist) for sublist in list_of_sublists) unique_count = len(unique_sublists) print(unique_count)
Output:
2
This code converts each sublist into a frozenset, which is then stored in a set to automatically discard duplicates. The number of unique sublists is acquired by measuring the size of this set.
Method 3: Using Collections Counter
The collections module contains a Counter class that can count hashable objects. When combined with a function to convert sublists to an immutable form, it can be used to count unique sublists.
Here’s an example:
from collections import Counter
def hashable_sublist(sublist):
return frozenset(sublist)
list_of_sublists = [['apple', 'banana'], ['banana', 'apple'], ['apple', 'banana'], ['orange']]
sublists_counter = Counter(map(hashable_sublist, list_of_sublists))
unique_count = len(sublists_counter)
print(unique_count)Output:
2
In this example, we use the Counter class from the collections module in combination with a function that makes each sublist hashable by converting it to a frozenset. The Counter then easily counts unique items, providing the count of unique sublists.
Method 4: Custom Function with List Comprehension
Implementing a custom function that leverages list comprehensions can yield a straightforward counting solution. It requires more lines but can be transparent in terms of the operation it performs on the sublists.
Here’s an example:
def count_unique_sublists(lst):
unique_sublists = []
for sublist in lst:
fs = frozenset(sublist)
if fs not in unique_sublists:
unique_sublists.append(fs)
return len(unique_sublists)
list_of_sublists = [['apple', 'banana'], ['banana', 'apple'], ['apple', 'banana'], ['orange']]
unique_count = count_unique_sublists(list_of_sublists)
print(unique_count)Output:
2
Here, we’ve defined a function count_unique_sublists() that iterates over each sublist, converting it to a frozenset and then appending it to a unique_sublists list if it’s not already present. The final tally of unique sublists is the length of this list.
Bonus One-Liner Method 5: Using Set Comprehension
A set comprehension provides a concise one-liner solution to the problem by combining elements of the previous methods.
Here’s an example:
list_of_sublists = [['apple', 'banana'], ['banana', 'apple'], ['apple', 'banana'], ['orange']]
unique_count = len({frozenset(sublist) for sublist in list_of_sublists})
print(unique_count)Output:
2
This one-liner creates a set comprehension by transforming each sublist into a frozenset, hence counting unique sublists directly by checking the length of the set created.
Summary/Discussion
- Method 1: Custom Hash Function. Flexible and clear. Requires a custom function which can be a bit verbose.
- Method 2: Using frozenset. Simple and Pythonic. Works best for smaller lists due to potential performance issues with larger datasets.
- Method 3: Using Collections Counter. Useful for additional statistics beyond just counting. Slightly more complex due to dependency on the collections module.
- Method 4: Custom Function with List Comprehension. Very explicit, but potentially inefficient with large lists as it iteratively checks for membership.
- Method 5: Using Set Comprehension. The most succinct. Perfect for one-off calculations where code brevity is preferred.