R data.table：.SD 的名称不可用于分配

Question

Often, I want to manipulate several variables in a DT and I need to select the column names based on their names or class.

d <- data.table(x = 1:10, y= letters[1:10])

# My usual approach
col <- str_subset(names(d), '^x')
d[, (col) := 2:11]

However, it would be very useful and less verbose to do this:

 d[, (names(.SD)) := 2:11, .SDcols = patterns('^x')]

But this throws an error:

Error in `[.data.table`(d, , `:=`((names(.SD)), 2:11), .SDcols = patterns("^x")) : 
  LHS of := isn't column names ('character') or positions ('integer' or 'numeric')
> 

The column names of.SD are available, though:

> d[, names(.SD), .SDcols = patterns('^x')]
[1] "x"

Why aren't the names of.SD available for assignment on the LHS of:=?

Answer 1

As noted this is not yet possible. The workaround only adds one line of code:

cols = grep('^x', names(d))
d[ , (cols) := 2:11, .SDcols = cols]