On Apr 26, 2011, at 14:36 , David Winsemius wrote: > > On Apr 26, 2011, at 8:01 AM, Lars Bishop wrote: > >> Hi, >> >> I need to extract the second largest element from each row of a >> matrix. Below is my solution, but I think there should be a more efficient >> way to accomplish the same, or not? >> >> >> set.seed(1) >> a <- matrix(rnorm(9), 3 ,3) >> sec.large <- as.vector(apply(a, 1, order, decreasing=T)[2,]) >> ans <- sapply(1:length(sec.large), function(i) a[i, sec.large[i]]) >> ans > > There are probably many but this one is reasonably compact, one-step, and > readable: > > > ans2 <- apply(a, 1, function(i) sort(i)[ dim(a)[2]-1]) > > ans2 > > Refreshing my mail client proves I was right about many solutions, but this > is the first (so far) to use the dim attribute.
Anything with sort() or order() will have complexity O(n*log(n)) or worse (n is the number of columns), whereas finding the k-th largest element has complexity O(k*n). For moderate n, this may be unimportant, but you could potentially find a speedup using sort.int(i, decreasing=TRUE, partial=2)[2] or max(i[-which.max(i)]) -- Peter Dalgaard Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ 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.
