# Re: hash_map

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 14 Jun 2007 01:41:25 -0700
Message-ID:
On Jun 14, 8:34 am, Erik Wikstr=F6m <eri...@student.chalmers.se> wrote:

On 14 Juni, 02:16, aaragon <alejandro.ara...@gmail.com> wrote:

I have a VERY BIG set of double values that I want to map to intervals
so I thought a clever way to do this was using a hash table. Let's say
that I want to map all double values in the range 0-0.5 to a single
std::pair<double,double>.

[snip]

Now, the thing is that I can't map the value of 0.05 to the same pair
because my hashing function doesn't to this. Any ideas???

I don't know how big your VERY BIG set of doubles is, but unless you
have performed some profiling using "standard solutions" I'd suggest
you do that before trying to optimize. In this case that means to use
std::map. Sure a good hash table is O(1), but if you don't have a good
hash-function you might end up with O(n) instead, a map is guaranteed
to be logarithmic, always.

There's of course the problem with comparing doubles like Bob R
pointed out, but a custom comparison functor would take care of that.

There are two problems: finding a hash function, and comparing.
For the first, I've yet to find a good solution to generate a
hash code from a double. It's far from trivial. For the
second... there's no guarantee that std::map won't have it as
well. There are two possible problems:

-- He has two double values which really aren't equal. In such
a case, std::map will not find the first using the second as
a key. If they're not equal, they're not equal.

-- He is calculating two values dynamically which should be
equal; the comparison function does something like:

bool operator()( Obj const& lhs, Obj const& rhs ) const
{
return lhs.f() < rhs.f() ;
}

Under certain conditions, with certain compilers, on certain
systems, this results in an unstable comparison functions,
because the intermediate results may be in extended
precision, and because the compiler spills one to memory
(thus reducing it to double precision), but keeps the other
in registers (in extended precision).

I'm very sceptical of the possibilities of using double for an
index, unless the doubles have a known source which ensures an
application correct rounding before they are used as an index.

--
James Kanze (GABI Software, from CAI) email:james.kanze@gmail.com
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