       Re: Using Nearest on a group of points

• To: mathgroup at smc.vnet.net
• Subject: [mg117606] Re: Using Nearest on a group of points
• From: DrMajorBob <btreat1 at austin.rr.com>
• Date: Thu, 24 Mar 2011 06:29:29 -0500 (EST)

```Run this a few times:

loop = Append[#, First@#] &;
pts = RandomInteger[{1, 15}, {10, 2}];
ch = pts[[loop@Quiet@ConvexHull@pts]];
sorted = loop@sortPoints@pts;
Show[ListPlot[{ch, sorted}, Joined -> True,
PlotStyle -> {{Red, Thickness[.01]}, {Blue, Thickness[.015]}}],
Graphics at {PointSize[.02], Line@pts}]

Darned if I can tell that any "shape" is really being captured.

Bobby

On Wed, 23 Mar 2011 02:55:09 -0500, Sseziwa Mukasa <mukasa at jeol.com> wrote:

>
> On Mar 22, 2011, at 6:10 AM, Martin Vavpoti=C4=8D wrote:
>
>> Hello. I need some help with the function Nearest.
>>
>> I have a groups of points, each with two coordinates (x,y), describing
>> a connected shape (the last point is a neighbor to the first one). The
>> initial order of these points is completely scrambled but I need them
>> to follow one another so their sequence describes a combined shape. I
>> thought of using the Nearest function but there is a problem.
>> Somewhere in the middle of my shape is a large gap where no points
>> reside. I fear that if I use Nearest, the function will find the wrong
>> point.
>>
>> What I need is for function Nearest to ignore points already sorted
>> and search only for points that have not been used yet.
>
>
> If your list isn't too long you can do it recursively:
>
> sortPoints[{x_}] := {x}
>
> sortPoints[points_] :=
>  With[{index =
>     First[Nearest[Rest[points] -> Automatic, First[points]]]},
> {First[points],
>     Sequence @@
>     sortPoints[Prepend[Drop[Rest[points], {index}], points[[index +
> 1]]]]}]
>
> (Debug) In:= sortPoints[RandomInteger[{1, 10}, {10, 2}]]
> (Debug) Out= {{10, 1}, {8, 3}, {9, 5}, {8, 8}, {8, 9}, {10, 10}, {4,
> 9}, {2, 10}, {4,
>   4}, {2, 2}}
>
> Regards,
> 	Ssezi
>

--
DrMajorBob at yahoo.com

```

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