sparse 2-D lookup

[John W. Eaton]

On 25-Oct-2005, Jeff Abrahamson wrote:

| 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).

You must be using the sparse code from octave-forge.  The sparse code
that is part of Octave 2.9.x does not have this problem.

jwe



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