[Haskell-cafe] Re: Data.Map: Enumerating ordered subset of keys

[Jared Updike]

On 2/8/09, Anish Muttreja <anishmuttreja at gmail.com> wrote:
> Maybe you wantData.Map.partition (\key -> key >= key1 && key <= key2) map

This will return what I want, but partition is O(n) and touches all
the keys, so if I had a million keys and only 2 of them matched the
key1..key2 range, it would still visit all of them before it
enumerated the ones that satisify the predicate. I don't believe
laziness would help here.

>  HTH,
>  Anish
>
>
>  On Sun, 08 Feb 2009 23:02:37 -0800, Jared Updike <jupdike at gmail.com> wrote:
>
>
> >
> > It looks like two "Map.split"s will do what I need except for allowing
> > more exact testing of <= vs. < (since == elements are left out of both
> > maps...?)
> >
> >  Jared.
> >
> > On 2/8/09, Jared Updike <jupdike at gmail.com> wrote:
> >
> > > I would like to enumerate a subset of keys in a Map satisfying \ key
> > >  >= key1 && key <= key2 but in the expected, reasonable amount of time
> > >  (e.g. < O(log(n)) + O(m) for n total keys and m keys in the subset).
> > >  (Or key > key1 and/or key < key2 or some such combination).
> > >
> > >  Is there an appropriate idiom or combination of library functions to
> > >  accomplish this, short of digging into the code for Data.Map and
> > >  writing such a function for a forked version of Data.Map?
> > >
> > >  For example I could try something like a Set.split of a Set.split of
> > >  Map.keysSet of my original map, but will laziness magically do what I
> > >  really want? which is to walk down the tree to key1 (or the nearest
> > >  key > key1) and enumerate keys in order until key2 is reached?
> > >
> > >  Data.Map almost looks like what I need if I can do this.
> > >
> > >
> > >  Jared.
> > >
> > >
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