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MathGroup Archive 1997

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lists of pairs

  • To: mathgroup at smc.vnet.net
  • Subject: [mg8528] lists of pairs
  • From: Russell Towle <rustybel at foothill.net>
  • Date: Thu, 4 Sep 1997 02:20:30 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi all, many thanks to all who responded to my question about how to remove
"reversed" pairs of integers from a list of pairs.  As it happens I found
the answer immediately after posting the query, in one of Jeffrey Adams'
packages, entitled "PolytopeObjects."  The solution I found there was
identical to that proposed by most of you.

For a list of integer pairs named "edges," Union [ Sort /@ edges ] does the
trick quite nicely and quickly.

Also, much to my distress, Will Self alerted me to the fact that my example
list of edges was not composed entirely of double-pairs {a, b} and {b, a},
a horrible horrible mistake on my part.  Sorry about that!

The reason why the list always arises as reversed pairs is that it is
derived from a list of polygons bounding a polyhedron.  Each polygon shares
an edge, and if all the polygons have their vertices ordered consistently
(counter-clockwise, say, as seen from outsode the polyhedron), then each
edgee occurs twice in the list, as {a, b} and {b, a}. --Russell Towle




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