On Apr 10, 2012, at 8:01 PM, Worik R wrote: > Thank you. > > That was exactly what I need. > > Looking at '?[' I see... > > drop: For matrices and arrays. If TRUE the result is coerced to > the lowest possible dimension (see the examples). This only > works for extracting elements, not for the replacement. See > drop for further details. > > > But that implies that in the case where 0 rows are returned it > should be coerced into zero dimensions. I am not quite sure what it > would mean to be coerced into zero dimensions, but if I read that > without having seen the actual behavior (impossible now) I would > assume... > > M[M[,"a"]==1000,] (from my example below) > > would return NULL, which has class "NULL" rather than a matrix with > zero rows.
Cue music: http://www.youtube.com/watch?v=NzlG28B-R8Y : "That signpost up ahead ... You're entering into a land of shadow and substance. You just crossedoer into the R Zen Zone. ... where the sounds of one hand clapping is the test question: > dim(M[0, ]) [1] 0 4 > str(M[0, ]) int[0 , 1:4] - attr(*, "dimnames")=List of 2 ..$ : NULL ..$ : chr [1:4] "a" "b" "c" "d" > dim(M[1, ]) NULL > > thanks > Worik > > On Wed, Apr 11, 2012 at 11:54 AM, David Winsemius <dwinsem...@comcast.net > > wrote: > > On Apr 10, 2012, at 7:33 PM, Worik R wrote: > > Friends > > I am extracting sub-sets of the rows of a matrix. Generally the > result is > a matrix. But there is a special case. When the result returned is a > single row it is returned as a vector (in the example below an integer > vector). If there are 0, or more than 1 rows returned the result is a > matrix. > > I am doing this in a function and I cannot be sure how many rows I am > removing. How can I do this in a general way that always returns a > matrix? > > > ?"[" > > > M[1, , drop=FALSE] > > a b c d > a1 0 3 2 1 > > class( M[1, , drop=FALSE] ) > [1] "matrix" > > > M <- matrix(0:3, nrow=3, ncol=4) > colnames(M) <- c('a','b','c','d') > rownames(M) <- c('a1','b2','c3') > N <- M[M[,"a"]==0,] > O <- M[M[,"a"]!=0,] > P <- M[M[,"a"]==100,] > c(class(M), class(N), class(O), class(P)) > [1] "matrix" "integer" "matrix" "matrix" > M > a b c d > a1 0 3 2 1 > b2 1 0 3 2 > c3 2 1 0 3 > N > a b c d > 0 3 2 1 > O > a b c d > b2 1 0 3 2 > c3 2 1 0 3 > P > a b c d > > > cheers > Worik > > [[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. > > David Winsemius, MD > West Hartford, CT > > David Winsemius, MD West Hartford, CT [[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.
