CODEWITHSUNDEEP
pythonalgorithmlist-comprehensionpowersetloop-unrolling
Jeff Benister
Jeff Benister

Reputation: 468

How to unroll a list comprehension

I have the following code I am trying to understand as I am new to Python. I understand that the code computes the powerset but the line subsetlist = [ subset + [item] for subset in result] is a little hard to understand. How can I break this compounded line to simple for loop for understanding.

def powerset(x):
    result = [[]]
    for item in x:
        subsetlist = [ subset + [item] for subset in result]
        result.extend(subsetlist)
    return result

This is what I have tried to make it simpler but it does not seem to work. My IDLE just gets stuck and does not print anything.

def powerset(x):
    result = [[]]
    for item in x:
        for subset in result:
            result.append(item)
    print(result)

Upvotes: 1

Views: 861

Answers (1)

Raymond Hettinger
Raymond Hettinger

Reputation: 226256

Hint:

You were very close. Just move the empty list creation inside the second loop.

Working code:

def powerset(x):
    result = [[]]
    for item in x:
        subsetlist = []
        for subset in result:
            subsetlist.append(subset + [item])
        result.extend(subsetlist)
    return result

Output from IDLE:

>>> powerset('abc')
[[], ['a'], ['b'], ['a', 'b'], ['c'], ['a', 'c'], ['b', 'c'], ['a', 'b', 'c']]

Upvotes: 2

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