I am a bit new to Python and am working through programming exercises. I have written the following recursive method to generate the power set based on an input list in Python. It should return a generator that generates the power set of a given list passed in as s. Each element in the power set should be a set.
def gps(s, res=set()):
if s:
elem = s.pop()
gps(s, res)
res.add(elem)
gps(s, res)
else:
yield res
When I call it using list(gps([1,2])) however it gives me []. The correct result should be something like [set(), {1}, {2}, {1, 2}].
I removed the yield statement, added two lines of code and played around with print statements to get this code, which prints the right results and seems like a step closer:
def gps(s, res=set()):
if s:
elem = s.pop()
gps(s, res)
res.add(elem)
gps(s, res)
s.append(elem)
res.remove(elem)
else:
print(res)
After reading another Stack Overflow answer I modified my function to use yield from but the modified code below still gives me incorrect results:
def gps(s, res=set()):
if s:
elem = s.pop()
yield from gps(s, res)
res.add(elem)
yield from gps(s, res)
s.append(elem)
res.remove(elem)
else:
yield res
Where am I going wrong with my approach? Any tips and clarifications would be appreciated.
Maybe you could try this code, without any built-in methods.
def subset(nums):
ans = [[]]
for n in nums:
ans += [a+[n] for a in ans]
return ans
nums = [1, 2, 3]
print(subset([1,2,3])) # [[], [1], [2], [1, 2], [3], [1, 3], [2, 3], [1, 2, 3]]
Or you still prefer the generator version:
def powerset(nums):
"""
This is a generator version
"""
if len(nums) <= 1:
yield nums
yield []
else:
for x in powerset(nums[1:]):
yield [nums[0]]+ x
yield x
if __name__ == '__main__':
nums = [1, 2, 3]
print(list(powerset(nums)))