Yes, use the drop argument;
apply(x[rows,,drop=F],2,mean) On Sat, May 9, 2009 at 2:33 PM, Peter Kharchenko < peter.kharche...@post.harvard.edu> wrote: > Dear fellow R users, > I can't figure out how to do a simple thing properly: apply an operation to > matrix columns on a selected subset of rows. Things go wrong when only one > row is being selected. I am sure there's a way to do this properly. > > Here's an example: > # define a 3-by-4 matrix x > > x <- matrix(runif(12),ncol=4) > > str(x) > num [1:3, 1:4] 0.568 0.217 0.309 0.859 0.651 ... > > # calculate column means for selected rows > > rows <- c(1,2) > > apply(x[rows,],2,mean) > [1] 0.3923531 0.7552746 0.3661532 0.1069531 > # now the same thing, but the "rows" vector is actually just one row > > rows <- c(2) > > apply(x[rows,],2,mean) > Error in apply(x[rows, ], 2, mean) : dim(X) must have a positive length > > The problem is that while x[rows,] in the first case returned a matrix, in > the second case, when only one row was selected, it returned a vector (and > the apply obviously failed). Is there a general way to subset a matrix so > it still returns a matrix even if it's one row? > Unfortunately doing as.matrix(x[rows,]) doesn't work either, as it returns > a transposed matrix in the case of a single row. > > Is there a way to do this properly without writing out hideous if > statements accounting for single row exception? > > thanks, > -peter. > > ______________________________________________ > 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. > -- Henrique Dallazuanna Curitiba-Paraná-Brasil 25° 25' 40" S 49° 16' 22" O [[alternative HTML version deleted]]
______________________________________________ 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.
