Re: Repeats and Triangulation

[Craig Markwardt <craigmnet(at)cow.physics.wisc.edu>]

Ben Tupper <pemaquidriver@tidewater.net> writes:
> The REPEATS keyword to TRIANGULATION returns a 2,n element
> array of pairs indices of repeated values.  If no values are
> repeated then REPEATS = [-1,-1].  My question is 'How do I
> pull out the repeated values efficiently?'
> 
...
>  ;make the polygon descriptor (see IDLgrPolyLine)
> List = Conn[Conn[0]:Conn[1]-1L]
> Ptr = Ptr_NEW([N_elements(List),List])
> For  i = 1, n_elements(X) -1 DO Begin
>  List = Conn[conn[i]:Conn[i+1]-1]
>  *Ptr = [*Ptr, n_elements(List), List]
> EndFor

Umm, while the procedure does not document how it treats repeated
points -- which might be considered a documentation bug -- the format
of the adjacency list is very similar to the output from the
REVERSE_INDICES keyword of HISTOGRAM.  That is, there are bins that
have no entries, and there is a way to ignore them.  How about
inserting an IF clause which tests for this?

List = Conn[Conn[0]:Conn[1]-1L]
Ptr = Ptr_NEW([N_elements(List),List])
For  i = 1, n_elements(X) -1 DO IF CONN[i+1] GT CONN[i] then Begin
 List = Conn[conn[i]:Conn[i+1]-1]
 *Ptr = [*Ptr, n_elements(List), List]
EndIf

The above solution works for me.

Your other options are to:
 * pre-check your point list to remove repeats.  This might be done
   similar to UNIQ.
 * use the REPEATS array to remove duplicates.
   DELMASK = lonarr(n_elements(x))
   DELMASK (REP(1,*)) = 1
   and then check DELMASK(i) EQ 1 before adding the point to your list.

Good luck,
Craig


-- 
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Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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