I am stuck with recursion in Power Set using Python

Question

I am writing a program to return a set of subset of number. For example: list = [1,2,3] my desired function will return [[], [1], [2], [2,1], [3], [3,1], [3,2], [3,2,1]] However, it returns [[], [1], [2], [3]] Here is my code

# Write a powerSet function
def powerSet(list):
    if len(list) == 0:
        return [[]];
    else:
        temp = list.pop()
    return powerSet(list) + [[temp] + x for x in powerSet(list)];

list = [1,2,3];
print(powerSet(list));

list is a built-in, hence a poor choice for a variable name. Also -- pop() destructively mutates the passed list -- you should be modifying copies of it. — John Coleman
– John Coleman, Commented Feb 8, 2017 at 4:56

hsfzxjy · Accepted Answer · 2017-02-08 05:01:35Z

2

The problem is that you mutate the list object, so the two powerSet in the last line will not receive the same list (for example, in the first call, the first powerSet will get [1,2] but the second one will get [1]).

The solution is to copy the list when passing it down:

powerSet(list[:]) + [[temp] + x for x in powerSet(list[:])]

answered Feb 8, 2017 at 5:01

hsfzxjy

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Zealseeker · Accepted Answer · 2017-02-08 05:09:29Z

1

There is a built-in package for "subset"

import itertools

def subset(x):
    all_list = []
    for i in range(len(x)+1):
        all_list+=list(itertools.combinations(x,i))
    return all_list

print subset([1,2,3])
[output>>>] [(), (1,), (2,), (3,), (1, 2), (1, 3), (2, 3), (1, 2, 3)]

edited Feb 8, 2017 at 5:09

answered Feb 8, 2017 at 5:02

Zealseeker

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3 Comments

Dung Lemanh Over a year ago

Thank you; however, I think there is another way to solve this problem, but I will have to go from the beginning of the set instead of the end

Dung Lemanh Over a year ago

def p(l):if len(l) == 0: return [[]] #print(p(l[1:]) + [[l[0]] + x for x in p(l[1:])]) return p(l[1:]) + [[l[0]] + x for x in p(l[1:])] s = [1,2,3] print(p(s));

Zealseeker Over a year ago

Recursion is not easy to understand I think.. So thank you for the opportunity to study it. By the way, it is interesting to use "yield" and we can learn how python implement permutation and combination from docs.python.org/2/library/itertools.html#itertools.combinations

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