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RE: [mg23007] Chop? Programming Challenge!!
Posted:
Apr 15, 2000 3:43 AM


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 fourdimensional polytopes. When the polytopes are convex I use a ConvexHull function to make the sections in the 3space. 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 pointsets 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 4vectors themselves, before projecting into the 3space. Here too my vunion function returned a list of 80 distinct 4vectors. But the new function returned a list of 92. So it failed badly with the 4vectors.
I suspect the problem is the same as before: the ordering of the vectors. In the spurious list of 81 3vectors, the two duplicates were separated by halfdozen other vectors, and neither contained a 0.
It seems that we must discover an ironclad way to order a Chopped list of nvectors.
Thanks again!
Russell Towle Box 141 Dutch Flat, CA 95714 (530) 3892872



