Dear fellow R users, I can't figure out how to do a simple thing properly: apply an operation to matrix columns on a selected subset of rows. Things go wrong when only one row is being selected. I am sure there's a way to do this properly. Here's an example: # define a 3-by-4 matrix x > x <- matrix(runif(12),ncol=4) > str(x) num [1:3, 1:4] 0.568 0.217 0.309 0.859 0.651 ... # calculate column means for selected rows > rows <- c(1,2) > apply(x[rows,],2,mean) [1] 0.3923531 0.7552746 0.3661532 0.1069531 # now the same thing, but the "rows" vector is actually just one row > rows <- c(2) > apply(x[rows,],2,mean) Error in apply(x[rows, ], 2, mean) : dim(X) must have a positive length The problem is that while x[rows,] in the first case returned a matrix, in the second case, when only one row was selected, it returned a vector (and the apply obviously failed). Is there a general way to subset a matrix so it still returns a matrix even if it's one row? Unfortunately doing as.matrix(x[rows,]) doesn't work either, as it returns a transposed matrix in the case of a single row. Is there a way to do this properly without writing out hideous if statements accounting for single row exception? thanks, -peter.
a general way to select a subset of matrix rows?
3 messages · Peter Kharchenko, Henrique Dallazuanna, Stephanie Kovalchik
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One simple adjustment is the following: apply(matrix(x[rows,],nr=length(rows)),2,mean) Quoting Peter Kharchenko <peter.kharchenko at post.harvard.edu>:
Dear fellow R users, I can't figure out how to do a simple thing properly: apply an operation to matrix columns on a selected subset of rows. Things go wrong when only one row is being selected. I am sure there's a way to do this properly. Here's an example: # define a 3-by-4 matrix x
x <- matrix(runif(12),ncol=4) str(x)
num [1:3, 1:4] 0.568 0.217 0.309 0.859 0.651 ... # calculate column means for selected rows
rows <- c(1,2) apply(x[rows,],2,mean)
[1] 0.3923531 0.7552746 0.3661532 0.1069531 # now the same thing, but the "rows" vector is actually just one row
rows <- c(2) apply(x[rows,],2,mean)
Error in apply(x[rows, ], 2, mean) : dim(X) must have a positive length The problem is that while x[rows,] in the first case returned a matrix, in the second case, when only one row was selected, it returned a vector (and the apply obviously failed). Is there a general way to subset a matrix so it still returns a matrix even if it's one row? Unfortunately doing as.matrix(x[rows,]) doesn't work either, as it returns a transposed matrix in the case of a single row. Is there a way to do this properly without writing out hideous if statements accounting for single row exception? thanks, -peter.
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