Dictionary Keys question
FireNWater
khoard at gmail.com
Thu Jan 31 02:02:18 CET 2008
On Jan 30, 3:09 pm, Berteun Damman <berteun at NO_SPAMdds.nl> wrote:
> On Wed, 30 Jan 2008 14:47:36 -0800 (PST), FireNWater <kho... at gmail.com> wrote:
> > I'm curious why the different outputs of this code. If I make the
> > dictionary with letters as the keys, they are not listed in the
> > dictionary in alphabetical order, but if I use the integers then the
> > keys are in numerical order.
>
> > I know that the order of the keys is not important in a dictionary,
> > but I was just curious about what causes the differences. Thanks!!
>
> I don't know the exact way Python's hash function works, but I can take
> a guess. I'm sorry if I explain something you already know.
>
> A hash is for quickly looking up data. Yet, you don't want to waste too
> much memory. So there is a limit number of spaces allocated, in which to
> store objects. This number of spaces can be thought of as a list. Then,
> if you put something into the dict, Python computes the 'hash' of this
> object, which basically forms the index in the list where to store it.
> So every object should be mapped onto some index within the list. (If
> you retrieve it, the hash is computed again, and the value on that index
> is looked up, like list indexing, these are fast operations.)
>
> Say, if you have 100 spaces, and someone puts in integer in the list,
> the hashfunction used might be % 100. So the first 100 integers would
> always be placed at consecutive places. For strings however, a more
> complicated hash-function would be used, which takes into account more
> characters, so strings don't end up in order.
>
> For integers, if you put in integers that are spread very widely apart,
> they won't end up in order either (see the mod 100 example, 104 will
> come before 10).
>
> If you replace the list2 in your example by:
> list2 = [10000 * x for x in range(1,9)]
>
> You will see that this one doesn't end up in order either. So, there's
> no exception for integers when it comes to the order, yet the particular
> properties of the hash function will cause sequential integers to end up
> in order under some circumstances.
>
> Berteun
>
> PS:
> What happens if two values map onto the same space is of course an
> obvious question, and the trick is choosing your hashfunction so this
> occurs not very often on average. If it happens there are several
> strategies. Wikipedia probably has an explanation of how hash-functions
> can work in such a case.
Thank you for the explanation. . . I think I now have a (foggy)
understanding of hash tables. It seems to be a way to create order
(an index) out of disorder (random numbers or characters) behind the
scenes. .
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