r - Select columns by class (e.g. numeric) from a data.table

Question

Why doesn't this work with data.table?

It works with data.frame. Is there a way to do this with a data table?

x <- data.table(v1=1:20,v2=1:20,v3=1:20,v4=letters[1:20])
y <- x[ , sapply(x, is.numeric)]

This returns:

v1    v2    v3    v4
TRUE  TRUE  TRUE FALSE

Answer 1

From data.table 1.13.0 ".SDcols accepts a function which is used to select the columns of .SD". Thus, simply .SDcols = is.numeric:

x[ , .SD, .SDcols = is.numeric]