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+--- Thread: Help understanding code (/thread-1776.html)
Help understanding code - Willi - Jan-25-2017
Hi,
I have started learning Python a week ago (I have previous experience with C++ and Java) and stumbled over piece of code I have trouble to understand.
def power_set(l): if not l: return [[]] return power_set(l[1:]) + [[l[0]] + x for x in power_set(l[1:])]The code calculates the power set. For power_set({1,2,3,4}) the output is {{}, {1}, {2}, {1,2}, {3}, {1,3}, {2,3}, {1,2,3}, {4}, {1,4}, {2,4}, {1,2,4}, {3,4}, {1,3,4}, {2,3,4}, {1,2,3,4}}. While the if statement is clear, the last is not .
I would be thankful if someone could explain me the last line.
RE: Help understanding code - Mekire - Jan-25-2017
So, nasty recursive code with part of it wrapped in a list comp... wonderful.
Let's try breaking that line up:
power_set(l[1:]) + [[l[0]] + x for x in power_set(l[1:])]We have:
power_set(l[1:])and
[[l[0]] + x for x in power_set(l[1:])]The first part is easy.
power_set(l[1:]) is the powerset of the current sequence without the first entry.So if you ask for the power set of [1,2,3,4] it gives the power set of [2,3,4].
The second part is the first character in the sequence + the previous expression.
So if you ask for the powerset of [1,2,3,4] it gives you [1] + each set in the powerset of [2,3,4].
So to try once again to put this into words:
""" The powerset of any sequence is equal to (the powerset of the sequence without the first entry) + (the first entry + (each set in the powerset of the sequence without the first entry))."""
Not sure if that is any clearer. I tried. =P
RE: Help understanding code - Willi - Jan-25-2017
Thank you very much. I have understood it now.
And yeah, it's the kind of recursive methods I wouldn't write myself voluntarily because of the hardness to read it (especially month later).
RE: Help understanding code - Skaperen - Jan-25-2017
sometimes, code like that can earn you elegance points from the Ph.D people ... even if the code is wrong.