Re: How to quickly find number of non-zero elements in sparse matrix rows?

*To*: mathgroup at smc.vnet.net*Subject*: [mg56847] Re: How to quickly find number of non-zero elements in sparse matrix rows?*From*: Paul Abbott <paul at physics.uwa.edu.au>*Date*: Sat, 7 May 2005 15:35:08 -0400 (EDT)*Organization*: The University of Western Australia*Sender*: owner-wri-mathgroup at wolfram.com

Paul Abbott wrote: > The 4-th part of the data structure records the cumulative sum of > non-zero entries. However, there seems to be no simple way to access > this information. Usually, for special formats you can use Part to > extract such information but Part is interpreted by SparseArray, > circumventing this. Of course one can use pattern-matching! For a sparse array, sa=SparseArray[{i_,i_} -> 1,{10,10}] extract the cumulative sum of non-zero entries, sa /. SparseArray[_, _, _, x_] :> x[[2,1]] and use ListConvolve to give the number of non-zero entries in each row. ListConvolve[{1, -1}, %] I think that this is about as fast as you can hope for. Cheers, Paul -- Paul Abbott Phone: +61 8 6488 2734 School of Physics, M013 Fax: +61 8 6488 1014 The University of Western Australia (CRICOS Provider No 00126G) AUSTRALIA http://physics.uwa.edu.au/~paul http://InternationalMathematicaSymposium.org/IMS2005/