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12282010, 10:27 PM

#1

LQ Newbie
Registered: Jun 2010
Location: boston & sf bay, depending on the day
Distribution: ubuntu, rhel, debian
Posts: 20
Rep:

Discussion  Python List Comprehensions; power set?
Hi,
I teach an introductory programming class in Python and the staff and I are trying to stump each other with some "challenge" list comprehension problems to give to our students as a fun optional problem.
I'd love your take  can you think of any tricky ones?
Additionally, we've been debating if you can generate a power set from a single list comprehension. I've come kind of close but always miss a few. Can anyone prove either side of the debate? (it would suffice to be able to generate a nearcomplete powerset, eg one that omits the empty set [] or generally gives 2**n  1 elements)
Thanks all, I look forward to reading your responses.
Sarina



12302010, 02:15 AM

#2

Senior Member
Registered: Nov 2005
Distribution: Debian
Posts: 2,569

It kind of depends on how you define a "set":
So gives a power set. If you want the sets explicitly represented as lists a nested list comprehension is required (which probabably doesn't count as a "single list comprehension"):
Code:
[ [ x for x in range(int(floor(log(max(n,1)) / log(2) + 1))) if (1 << x) & n ]
for n in range(2**5) ]
or more readably:
Code:
from math import log, floor
def bits(n):
return int(floor(log(max(n,1)) / log(2) + 1))
def set_from_int(n):
return [ x for x in range(bits(n)) if (1 << x) & n ]
def power_set(s):
return [ set_from_int(n) for n in range(2**s) ]



12312010, 12:54 AM

#3

LQ Newbie
Registered: Jun 2010
Location: boston & sf bay, depending on the day
Distribution: ubuntu, rhel, debian
Posts: 20
Original Poster
Rep:

Agreed I phrased the problem incorrectly.
By "power set" we mean to define the problem as follows:
Given a list of numbers my_list = [1, 2, 3, 4], can you give a list comprehension that generates all possible subsets of my_list?
So the list comprehension you gave is the basic idea. We have not visited this problem again since I posted the question but will update if one of us thinks of an elegant solution, or comes up with another fun challenge problem.



01022011, 04:46 AM

#4

Senior Member
Registered: Sep 2006
Location: Liverpool  England
Distribution: slackware64 13.37 and current, Dragonfly BSD
Posts: 1,810

Quote:
Given a list of numbers my_list = [1, 2, 3, 4], can you give a list comprehension that generates all possible subsets of my_list?

I'm not sure if I understand your concept of subsets of the list. The following provides a list of all possible combinations of length between 1 and the length of the list stored as tuples. I use the itertools module to make finding combinations easy and run a nested list comprehension.
Code:
import itertools
my_list=[1,2,3,4,5]
power_set=[result for number in range(1,len(my_list)+1) for result in itertools.combinations(my_list,number)]
print power_set
Hope that's useful. Personally I'd tend to use loops when list comprehensions become a bit tricky like this but each to his own.



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