CODEWITHSUNDEEP
pythonlistrecursiontuples
dontknowwhatimdoing
dontknowwhatimdoing

Reputation: 13

Python recursive powerset function for tuples

I am trying to create a function that returns a powerset (all subsets of a given tuple of numbers) in the form of a tuple of tuples.

I have followed code that I have found previously, but this returns the output as a list of lists rather than a tuple of tuples.

def subs(S):
    if S == []:
        return [[]]

    x = subs(S[1:])

    return x + [[S[0]] + y for y in x]

This correctly returns: [[], [3], [2], [2, 3], [1], [1, 3], [1, 2], [1, 2, 3]] for an input of subs([1,2,3])

However I am looking for the function to input a tuple and return a tuple of tuples, such that the output would be:

>>> print (subs((1,2,3))
((), (3), (2), (2, 3), (1), (1, 3), (1, 2), (1, 2, 3))

Any help would be appreciated, thanks.

Upvotes: 0

Views: 293

Answers (3)

alani
alani

Reputation: 13069

The function for lists can be converted to use tuples instead. There is no direct equivalent of a list comprehension because the object needs to be mutable during the loop, but the result can be converted using tuple(). Care is needed with the syntax for 1-element tuples, so some extra commas have to be inserted in a couple of places.

def subs(S):
    if S == ():
        return ((),)

    x = subs(S[1:])

    return x + tuple([(S[0],) + y for y in x])

Upvotes: 0

AirSquid
AirSquid

Reputation: 11893

mashing tuples together or making new tuples from old ones is a bit tricky because they are immutable. Here is a hint that may put you in the right direction, along with the tip about indexing in the other solution. You can use the "scatter" operator to unpack an existing tuple into pieces when making a new one as such:

In [12]: c = ((2,3),(4,),(99,0,8))                                              

In [13]: d = (*c, (7,6,5))                                                      

In [14]: d                                                                      
Out[14]: ((2, 3), (4,), (99, 0, 8), (7, 6, 5))

Upvotes: 0

Balaji Ambresh
Balaji Ambresh

Reputation: 5037

Here you go:

pip install more-itertools

from more_itertools import powerset
print(tuple([x for x in powerset( (1, 2, 3) )]))

Output:

((), (1,), (2,), (3,), (1, 2), (1, 3), (2, 3), (1, 2, 3))

Upvotes: 1

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