an element from a set

[Chris Rebert]

On Fri, May 14, 2010 at 11:23 PM, Carl Banks <pavlovevidence at gmail.com> wrote:
> On May 14, 9:39 am, Terry Reedy <tjre... at udel.edu> wrote:
>> On 5/14/2010 11:24 AM, gerardob wrote:
>> > Hello, let S be a python set which is not empty
>> > (http://docs.python.org/library/sets.html)
>>
>> > i would like to obtain one element (anyone, it doesn't matter which one) and
>> > assign it to a variable.
>>
>> > How can i do this?
>>
>> Depends on whether or not you want the element removed from the set
>>
>> #3.1
>>  >>> s=set(range(10))
>>  >>> s
>> {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
>>  >>> x=next(iter(s))
>>  >>> x
>> 0
>>  >>> s
>> {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} # x not removed
>>  >>> x = s.pop()
>>  >>> x
>> 0
>>  >>> s
>> {1, 2, 3, 4, 5, 6, 7, 8, 9} # x has been removed
>>
>> The choice of 0 is an implementation artifact. It could have been any
>> member.
>
> Which brings up an interesting question: how do you get a random
> element from a set?
>
> random.choice(list(s))
>
> is the most straightforward way and will work a lot of the time, but
> how would you avoid creating the list?  I can't think of a way off
> hand.

def random_set_elem(s):
    iterator = iter(s)
    n = random.randint(0, len(s)-1)
    for i in xrange(n):
        next(iterator)
    return next(iterator)

Avoids creating the list, but it's still O(M). If you're gonna pick
multiple times from the same set, you're better off just converting it
to a list.

Cheers,
Chris
--
http://blog.rebertia.com