Ahh. You want something else then. Sparse is not a sparse array, it is only a sparse matrix. Sparse matrices have been well studied and there are many good storage mechanisms and algorithms using them.

Sparse arrays, on the other hand are usually just implemented as dictionaries. I know of no sparse array implementation for any array-oriented environment. You could make your own without too much difficulty, supporting just the operations you care about.

Which operations do you want to do on these "sparse arrays"?

Well, pretty much everything NumPy supplies --array arithmetic and inner products at least. I am looking for a quick way to a prototype, a cleaner solution will have to wait. I did find a pure-python implementation of just what you described: dict-based sparse array, but I doubt that a python dictionary could handle 1e9 members in a reasonable amount of time. I took a look at NumPy's arrayobject.c; as I said, my C is rusty but it looks simple enough to create a default-value array that "looks" like a NumPy array... Thanks again, Costas _________________________________________________________________ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail