How to subset matrix to one column, maintain matrix data type, maintain row/column names?

问题

When I subset a matrix to a single column, the result is of class numeric, not matrix (i.e. myMatrix[ , 5 ] to subset to the fifth column). Is there a compact way to subset to a single column, maintain the matrix format, and maintain the row/column names without doing something complicated like:

matrix( myMatrix[ , 5 ] , dimnames = list( rownames( myMatrix ) , colnames( myMatrix )[ 5 ] )

回答1:

Use the drop=FALSE argument to [.

m <- matrix(1:10,5,2)
rownames(m) <- 1:5
colnames(m) <- 1:2
m[,1]             # vector
m[,1,drop=FALSE]  # matrix

来源：https://stackoverflow.com/questions/7352254/how-to-subset-matrix-to-one-column-maintain-matrix-data-type-maintain-row-colu