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Re: Vector Union
*To*: mathgroup at smc.vnet.net
*Subject*: [mg23141] Re: Vector Union
*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
*Date*: Thu, 20 Apr 2000 03:20:59 -0400 (EDT)
*Organization*: Universitaet Leipzig
*References*: <8djl4o$7jh@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
Hi Russel,
I overlooked you question, so my answer will be a bit late.
I can't give you a n-dimensional version but the extension is
easy if you know the dimension. The fastest version to find the
unique $n$-dimensional points in a set is to use a tree of $2^n$
In three dimensions one can use
$SamePrecision = 0.1*^-6;
pointCount = 0;
getIndex[p_List, p1_List] :=
Module[{index},
index = 1;
If[p[[1]] < p1[[1]], index += 1];
If[p[[2]] < p1[[2]], index += 2];
If[p[[3]] < p1[[3]], index += 4];
index
]
(* pointCount is fluid from MakeOctTree[] *)
OctTreeInsert[p_List, {}] := {p, pointCount++, Table[{}, {8}], {}}
OctTreeInsert[p_List, {p1_, i_Integer, l_, poly_List}] :=
Module[{index, delta, l1},
delta = p - p1;
If[Abs[Dot[delta, delta]] < $SamePrecision,
Return[{p1, i, l, poly}]
];
index = getIndex[p, p1];
l1 = OctTreeInsert[p, l[[index]]];
{p1, i, ReplacePart[l, l1, index], poly}
]
and for the points pnts
pointCount = 0;
uniquePoints=Cases[Fold[OctTreeInsert[#2, #1] &, {},
pnts], {_?NumericQ, _?NumericQ, _?NumericQ}, Infinity]
The speed gain depends on the number of points. MathGL3d uses an octtree
to make a indexed point set form the vertex data. Typical for 10^4
points the
usual search is to slow. A second advantage may be, that the octtree is
incremental. You can add a k+1 point to your set and find quickly if it
is
already in the set. The Union/Sort[] versions need always the full set
of
points and can't be incremented without a new Sort[] call that consume
(k+1)*Log[k+1]
operations where the octtree need only Log[k+1] operations.
For small point sets the overhead of the tree creation is to huge and
the other versions maye work well. Also the memory for the tree is a
constrain, because for a surface a maximum of 4 links may be used and
four links are useless memory. In higher dimensions this becomes more
dramatical.
How ever for more than 10^4 points (with MathGL3d I have tryed 10^7)
the octtree works excelent.
Regards
Jens
Russell Towle wrote:
>
> The problem of finding a function to return the union of a list of
> n-vectors which is at once *fast* and yet *unfailing* has beguiled me for
> several years. I contrived a slow but unfailing 'vunion' function about
> five years ago; made a not-so-slow and unfailing version a year ago; and
> then found it to occasionally fail when Chop was applied to the vectors
> without a second argument.
>
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