Python program to get all subsets of given size of a set
Last Updated :
09 Aug, 2021
Given a set, write a Python program to generate all possible subset of size n of given set within a list.
Examples:
Input : {1, 2, 3}, n = 2
Output : [{1, 2}, {1, 3}, {2, 3}]
Input : {1, 2, 3, 4}, n = 3
Output : [{1, 2, 3}, {1, 2, 4}, {1, 3, 4}, {2, 3, 4}]
We have already discussed the same problem using the Naive approach in this article. This article focuses on the Pythonic approaches to Print all subsets of a given size of a set.
Python has itertools.combinations(iterable, n) which Return n length subsequences of elements from the input iterable. This can be used to Print all subsets of a given size of a set. Now, we have various alternatives to use this function.
Code #1 :
Simply pass the set as iterable and the size as arguments in the itertools.combinations() to directly fetch the combination list.
Python3
import itertools
def findsubsets(s, n):
return list(itertools.combinations(s, n))
s = {1, 2, 3}
n = 2
print(findsubsets(s, n))
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Output:
[(1, 2), (1, 3), (2, 3)]
Code #2 :
We can also use an alternative to the above-discussed method which is mapping set to itertools.combinations() function.
Python3
import itertools
from itertools import combinations, chain
def findsubsets(s, n):
return list(map(set, itertools.combinations(s, n)))
s = {1, 2, 3}
n = 2
print(findsubsets(s, n))
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Output:
[{1, 2}, {1, 3}, {2, 3}]
Code #3 :
Another method is to use for loop in itertools.combinations() function and append the combination sets to the list.
Python3
import itertools
def findsubsets(s, n):
return [set(i) for i in itertools.combinations(s, n)]
s = {1, 2, 3, 4}
n = 3
print(findsubsets(s, n))
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Output:
[{1, 2, 3}, {1, 2, 4}, {1, 3, 4}, {2, 3, 4}]
Code #4:
Many a time when this question is asked in interviews, it’s better to answer without using any module. So, here is the solution that does not use itertools module:
Python3
def subsets(numbers):
if numbers == []:
return [[]]
x = subsets(numbers[1:])
return x + [[numbers[0]] + y for y in x]
def subsets_of_given_size(numbers, n):
return [x for x in subsets(numbers) if len(x)==n]
if __name__ == '__main__':
numbers = [1, 2, 3, 4]
n = 3
print(subsets_of_given_size(numbers, n))
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Output:
[[2, 3, 4], [1, 3, 4], [1, 2, 4], [1, 2, 3]]
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