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Groups > comp.lang.python > #5439

Re: Converting a set into list

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In article 
<34fc571c-f382-405d-94b1-0a673da5f46b@t16g2000vbi.googlegroups.com>,
 SigmundV <sigmundv@gmail.com> wrote:

> I think the OP wants to find the intersection of two lists.
> list(set(list1) & set(list2)) is indeed one way to achieve this. [i
> for i in list1 if i in list2] is another one.

Both ways work, but the first is O(n) and the second is O(n^2).

import time
n = 10000
list1 = range(n)
list2 = range(n)
t0 = time.time()
list(set(list1) & set(list2))
t1 = time.time()
print "list(set) method took %f seconds" % (t1 - t0)
t0 = time.time()
[i for i in list1 if i in list2]
t1 = time.time()
print "loop method took %f seconds" % (t1 - t0)


./intersect.py 100000
list(set) method took 0.004791 seconds
loop method took 1.437322 seconds

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Converting a set into list TheSaint <nobody@nowhere.net.no> - 2011-05-14 17:02 +0800
  Re: Converting a set into list Peter Otten <__peter__@web.de> - 2011-05-14 11:33 +0200
    Re: Converting a set into list TheSaint <nobody@nowhere.net.no> - 2011-05-14 22:14 +0800
      Re: Converting a set into list Chris Angelico <rosuav@gmail.com> - 2011-05-15 04:22 +1000
  Re: Converting a set into list Ben Finney <ben+python@benfinney.id.au> - 2011-05-15 00:12 +1000
    Re: Converting a set into list TheSaint <nobody@nowhere.net.no> - 2011-05-14 22:51 +0800
      Re: Converting a set into list Ben Finney <ben+python@benfinney.id.au> - 2011-05-15 09:21 +1000
        Re: Converting a set into list Chris Torek <nospam@torek.net> - 2011-05-15 02:11 +0000
          Re: Converting a set into list SigmundV <sigmundv@gmail.com> - 2011-05-15 04:18 -0700
            Re: Converting a set into list SigmundV <sigmundv@gmail.com> - 2011-05-15 04:23 -0700
            Re: Converting a set into list TheSaint <nobody@nowhere.net.no> - 2011-05-15 23:56 +0800
              Re: Converting a set into list Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2011-05-15 22:27 +0200
                Re: Converting a set into list Daniel Kluev <dan.kluev@gmail.com> - 2011-05-16 13:37 +1100
                Re: Converting a set into list Peter Otten <__peter__@web.de> - 2011-05-16 08:34 +0200
                Re: Converting a set into list TheSaint <nobody@nowhere.net.no> - 2011-05-16 22:23 +0800
                Re: Converting a set into list Ben Finney <ben+python@benfinney.id.au> - 2011-05-17 10:33 +1000
            Re: Converting a set into list Roy Smith <roy@panix.com> - 2011-05-15 13:07 -0400
          Re: Converting a set into list TheSaint <nobody@nowhere.net.no> - 2011-05-16 00:05 +0800
            Re: Converting a set into list Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-05-15 16:28 +0000
              Re: Converting a set into list TheSaint <nobody@nowhere.net.no> - 2011-05-16 00:35 +0800
          Re: Converting a set into list Duncan Booth <duncan.booth@invalid.invalid> - 2011-05-16 10:24 +0000
            Re: Converting a set into list Chris Torek <nospam@torek.net> - 2011-05-17 01:07 +0000

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