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

*To*: mathgroup at smc.vnet.net*Subject*: [mg56856] Re: [mg56841] How to quickly find number of non-zero elements in sparse matrix rows?*From*: Chris Chiasson <chris.chiasson at gmail.com>*Date*: Sat, 7 May 2005 15:35:18 -0400 (EDT)*References*: <200505070817.EAA20272@smc.vnet.net>*Reply-to*: Chris Chiasson <chris.chiasson at gmail.com>*Sender*: owner-wri-mathgroup at wolfram.com

Matt, Try this: <<LinearAlgebra`MatrixManipulation` MatrixPlot[sa=SparseArray[Table[{i+20,i+20}\[Rule]1,{i,200}]]]; arules=First@First@##&/@Most@ArrayRules[sa]; <<Statistics`DataManipulation` frules=Append[Rule@@@Reverse/@Frequencies@arules,_?NumericQ->0]; populationlist=(Range@Length@sa)/.frules Regards, On 5/7/05, Matt Pharr <matt at pharr.org> wrote: > > 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 > > -- Chris Chiasson http://chrischiasson.com/ 1 (810) 265-3161

**References**:**How to quickly find number of non-zero elements in sparse matrix rows?***From:*Matt Pharr <matt@pharr.org>