On Wed, 17 Nov 2010, Edzer Pebesma wrote:
List,given an arbitrary n x m boolean matrix, say [,1] [,2] [,3] [,4] [,5] [1,] FALSE FALSE FALSE FALSE FALSE [2,] FALSE FALSE TRUE FALSE FALSE [3,] FALSE FALSE FALSE FALSE FALSE [4,] FALSE FALSE FALSE TRUE TRUE I have the TRUE entries (indexes) of each column, in a list [[1]] integer(0) [[2]] integer(0) [[3]] [1] 2 [[4]] [1] 4 [[5]] [1] 4 How can I, from this list, efficiently obtain the TRUE entries of each row, in this case [[1]] integer(0) [[2]] [1] 3 [[3]] integer(0) [[4]] [1] 4 5 without constructing the n x m boolean matrix?
Use sparse matrices? library(Matrix) i <- c(2, 4, 4) j <- c(3, 4, 5) x <- c(TRUE, TRUE, TRUE) M <- sparseMatrix(i, j, x=x) here <- which(M, arr.ind=TRUE) split(here[,2], here[,1]) is almost there, what remains is to insert the empty list elements. Roger
-- Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: roger.biv...@nhh.no _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
