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

*To*: mathgroup at smc.vnet.net*Subject*: [mg56841] How to quickly find number of non-zero elements in sparse matrix rows?*From*: Matt Pharr <matt at pharr.org>*Date*: Sat, 7 May 2005 04:17:07 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

I have a sparse matrix, roughly 200k by 200k, with about .01% of the entries non zero, represented with SparseArray. I'd like to reasonably efficiently generate a 200k-long list where each element is the number of non-zero entries in the corresponding row of the matrix. I haven't been able to figure out a quick way to do this. One approach I've tried is the following (using a made up SparseArray of an identity matrix to illustrate the point): In[76]:= sa = SparseArray[Table[{i,i}\[Rule]1, {i, 200000}]]; In[77]:= rowLen[sa_, r_] := Length[ArrayRules[Take[sa, {r}]]]-1 However, it's quite slow--about 1/10 of a second for each value computed (on a 1GHz Mac G4) In[80]:= Table[rowLen[sa,i], {i,100}] // Timing Out[80]= {12.4165 Second,{1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, 1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}} I've got to assume that there's an efficient way to do this--does anyone have any suggestions? thanks, -matt -- Matt Pharr matt at pharr.org <URL:http://graphics.stanford.edu/~mmp> ======================================================================= In a cruel and evil world, being cynical can allow you to get some entertainment out of it. --Daniel Waters

**Follow-Ups**:**Re: How to quickly find number of non-zero elements in sparse matrix rows?***From:*Pratik Desai <pdesai1@umbc.edu>

**Re: How to quickly find number of non-zero elements in sparse matrix rows?***From:*Chris Chiasson <chris.chiasson@gmail.com>