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## sparse 2-D lookup

 From: Jeff Abrahamson Subject: sparse 2-D lookup Date: Tue, 25 Oct 2005 18:43:22 -0400 User-agent: Mutt/1.5.9i

```I want an array of (x,y,z) triples.  The problem is that x and y come
from the same very large set, but very few (x,y) pairs will actually
be used.  The z value is computed based on x and y, so this is really
a function f(x,y) = z.

Naively I'd create a sparse array, but sparse arrays seem (it's not
clear, though) to become regular arrays if I add elements to them.
Thus,

v = sparse(10000000,10000000)
v =

Compressed Column Sparse (rows=10000000, cols=10000000, nnz=0)

octave:3> v(1,1) = 1

(and memory usage jumps to near 2 GB and my machine slows to a crawl).

Any suggestions on how to do the bookkeeping?

The particular task is that I want to subdivide a triangulated surface
using midpoints of edges.  So I have a 3 x n array vertices and a 3 x n
of triangles.  I want to add midpoints to the vertex array, but I
don't want to add each midpoint twice, so I want an efficient lookup
mechanism so I can say "Is the midpoint of (x,y) already assigned?  If
yes, what is its index in the vertex array, if no add it for me."

Thanks much for any pointers.

--
Jeff

Jeff Abrahamson  <http://www.purple.com/jeff/>    +1 215/837-2287
GPG fingerprint: 1A1A BA95 D082 A558 A276  63C6 16BF 8C4C 0D1D AE4B

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