Check out `drop' ?drop
a <- matrix(c(1, 2, 3, 4), nrow = 2) rmask <- c(TRUE, FALSE) b <- a[rmask, , drop=FALSE] colSums(b) This will maintain the matrix structure. Ravi. ---------------------------------------------------------------------------- ------- Ravi Varadhan, Ph.D. Assistant Professor, The Center on Aging and Health Division of Geriatric Medicine and Gerontology Johns Hopkins University Ph: (410) 502-2619 Fax: (410) 614-9625 Email: rvarad...@jhmi.edu Webpage: http://www.jhsph.edu/agingandhealth/People/Faculty_personal_pages/Varadhan.h tml ---------------------------------------------------------------------------- -------- -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of Austin Huang Sent: Friday, December 04, 2009 3:52 PM To: R-help@r-project.org Subject: [R] logical masking of a matrix converts it to a vector One problem I've been having is the special case in which only one row/column remains and the variable gets converted into a vector when entries are removed by logical masking. This is a problem because subsequent code may rely on matrix operations (apply, colsums, dim, etc) For example: > a <- matrix(c(1, 2, 3, 4), nrow = 2) > a [,1] [,2] [1,] 1 3 [2,] 2 4 > rmask <- c(TRUE, FALSE) > b <- a[rmask, ] > colSums(b) Error in colSums(b) : 'x' must be an array of at least two dimensions To ensure the code works regardless of how the matrix gets modified, I need to explicitly recast results to a matrix, for example: b <- matrix(a[rmask, ], ncol = 2) This can get messy, requiring extra work to maintain column/row names or additional commands to determine the correct dimensions of the matrix (for example, if I'm masking both rows and columns simultaneously). Is there a more elegant way to deal with this in R? ~A [[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.
