Rory Winston wrote: > Hi > > Does anyone know how I might pick out diagonal elements of a matrix using a > vector? > > If I create a matrix a: > > a <- matrix(c(1:16), 4, byrow=TRUE) > > and I want to pick out the elements (1,1),(2,2),(3,3), or another arbitrary > diagonal (upper or lower), is there any way I can use a vector to do this? > So if I want a diagonal of size 3, I could create a vector like x <- c(0:2) > and then pick out a[1+x,1+x]? Currently, using a vector in this way gives me > a sub-matrix, as I get the cartesian product of the indexes (which is to be > expected, as I guess I am misusing the vector notion here). > > Anyone know how to do this? > diag() is what you want, as others have said.
For more general cases, you can also consider matrix indexing using two columns of indices: > diag(a) [1] 1 6 11 16 > x <- 0:2 > a[cbind(1 + x, 1 + x)] [1] 1 6 11 Duncan Murdoch > Cheers > Rory > > [[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. > ______________________________________________ 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.
