Consider the following little "benchmark" > require(Matrix) > tmp <- Matrix(c(rep(1,1000),rep(0,9000)),ncol=1) > ind <- sample(1:10000,10000) > system.time(tmp[ind,]) user system elapsed 0.004 0.001 0.005
> ind <- sample(1:1000,10000,replace=TRUE) > system.time(tmp[ind,]) user system elapsed 0.654 0.006 0.703 > system.time(Matrix(as(tmp,"matrix")[ind,])) user system elapsed 0.005 0.000 0.006 First I access all 10000 rows in a random order, which is fast, but when I access the first 1000 rows 10000 times there is a considerable slowdown. Last I convert back and forth between matrix and Matrix and get a serious speedup. Am I missing a point here? Should I not use indexing with "[" for the sparse matrices if I have repeated indices? I'm running Mac OS X, version 10.5.6, with Matrix package version 0.999375-21. I hope that somebody can enlighten me on this issue. Thanks, Niels -- Niels Richard Hansen Associate Professor Department of Mathematical Sciences University of Copenhagen Universitetsparken 5 2100 Copenhagen Ø Denmark +45 353 20783 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
