[Python-Dev] Retrieve an arbitrary element from a set withoutremoving it (original) (raw)

Guido van Rossum guido at python.org
Wed Nov 4 03:11:02 CET 2009

On Tue, Nov 3, 2009 at 5:10 PM, Antoine Pitrou <solipsis at pitrou.net> wrote:

Greg Ewing <greg.ewing canterbury.ac.nz> writes:

> >>Picking a random element can be done in O(1) only if the data >>structure supports access by index, which Python's hash tables don't. > > Well, at the implementation level, they can. You'd just have to pick a new > random index until it points to a non-empty slot.

But that's hardly O(1). It is, assuming that the set has a built-in minimum fill level (e.g. it always keeps at least x% of its entries filled), and the random generator is good. (of course, it is "statistically O(1)", like lookups in a hash table actually)

Clever. But I don't think the set implementation should have a dependency on random(), so it would have to expose an "indexing" operation, which smells like it would expose too much of the implementation for comfort.

-- --Guido van Rossum (python.org/~guido)

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