RE: Chop? Programming Challenge!!

• To: mathgroup at smc.vnet.net
• Subject: [mg23079] RE: [mg23007] Chop? Programming Challenge!!
• From: Russell Towle <rustybel at foothill.net>
• Date: Sat, 15 Apr 2000 03:00:36 -0400 (EDT)
• References: <v03102800b51b08904e68@[209.77.117.91]>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi David,

I'm afraid I was a little premature in my celebration of a new vector union
function. I tried it in my notebook which takes solid sections of
four-dimensional polytopes. When the polytopes are convex I use a
ConvexHull function to make the sections in the 3-space. This function is
sensitive to duplicate points, so I always run the points of the section
through vunion before passing them to ConvexHull3D.

In my test, most point-sets worked fine, but in one case, out of 20, the
new function returned 81 points, the old, 80. And there were only 80
distinct points.

I was curious to see how the new function would work on the 4-vectors
themselves, before projecting into the 3-space. Here too my vunion function
returned a list of 80 distinct 4-vectors. But the new function returned a
list of 92. So it failed badly with the 4-vectors.

I suspect the problem is the same as before: the ordering of the vectors.
In the spurious list of 81 3-vectors, the two duplicates were separated by
half-dozen other vectors, and neither contained a 0.

It seems that we must discover an iron-clad way to order a Chopped list of
n-vectors.

Thanks again!

Russell Towle
Box 141
Dutch Flat, CA 95714
(530) 389-2872

```

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