On Sun, Jul 20, 2008 at 4:47 PM, hadley wickham <[EMAIL PROTECTED]> wrote: > On Sun, Jul 20, 2008 at 4:16 PM, Ralph S. <[EMAIL PROTECTED]> wrote: >> >> Hi, >> >> I am trying to calculate the sum for each occurrence of the level of a >> factor in a very large matrix. In addition, I want to save that sum together >> with the information of the level of the factor and the level of a second >> factor. >> >> My matrix looks like this: >> >> x<-matrix(c(1,1,1,2,2,3,3,1,1,7,7,7,4,4,2,2,7,7,1,1,1,1,1,1,2,5,5),9,3) >> >> I want to sum according to the levels in the first column and save the sum >> with the information of the level in the first and the second column in a >> new matrix. >> >> That is, I want output in the matrix of form: >> >> 1 7 3 >> 2 4 2 >> 3 2 3 >> 1 7 10 >> > > Why that and not: > > 1 7 13 > 2 4 2 > 3 2 3 > > ?
Here's a solution for that case: index <- x[, 2] + x[, 1] * max(x[, 2]) cbind(x[!duplicated(index), 1:2], tapply(x[, 3], index, sum)) It takes about half a second for a million row matrix. Hadley -- http://had.co.nz/ ______________________________________________ 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.
