Stavros Macrakis <macra...@alum.mit.edu> writes: > Assuming your matrix is: > > mm <- matrix(runif(6*6),6,6) > > And your blocks are defined by integers or factors: > > cfact <- c(1,1,1,2,3,3) > rfact <- c(1,1,1,2,2,3) > > Then the following should do the trick: > > matrix(tapply(mm, outer(rfact,cfact,paste), mean), > length(unique(rfact)))
or the variant idx <- outer(rfact, (cfact - 1) * max(rfact), "+") matrix(tapply(m, idx, mean), max(rfact)) The assumption is that cfact, rfact are integer valued with max(rfact) <= nrow(m), max(cfact) <= ncol(m). I think Stavros' solution will run in to trouble when there are more than 9 row blocks, and '10 1' sorts before '2 1', for instance. Martin > The 'outer' calculates a joint factor for each element of the matrix; the > 'tapply' treats the matrix as a vector, grouping by factor and calculating > means; the 'matrix' rearranges them as a matrix corresponding to the > original block structure. > > Is that what you had in mind? > > -s > > > On Mon, Feb 16, 2009 at 12:43 PM, Titus von der Malsburg <malsb...@gmail.com >> wrote: > >> >> Hi list! I have a large matrix which I'd like to partition into blocks >> and for each block I'd like to compute the mean. Following a example >> where each letter marks a block of the partition: >> >> a a a d g g >> a a a d g g >> a a a d g g >> b b b e h h >> b b b e h h >> c c c f i i >> >> I'm only interested in the resulting matrix of means. How can this be >> done efficiently? >> >> Thanks! Titus >> >> ______________________________________________ >> 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. >> > > [[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. -- Martin Morgan Computational Biology / Fred Hutchinson Cancer Research Center 1100 Fairview Ave. N. PO Box 19024 Seattle, WA 98109 Location: Arnold Building M2 B169 Phone: (206) 667-2793 ______________________________________________ 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.
