R data.table - new column with ':=' and keep existing column

Question

Is it possible to create a new column and keep (few) existing columns in the statement ? eg creation of "x" column and then keeping "x" and "mpg" column

dt <- data.table(mtcars)

dt[,x:=mpg]

dt[,.(x,mpg)]

Answer 1

If we need it in a single step, instead of doing the := to modify the original dataset, specify it with = inside list or .(

dt[,.(x = mpg, mpg)]

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Or if it necessary to create the column in original dataset, it can be piped

dt[, x := mpg][, .(x, mpg)]

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If we want to update the columns in the original object, another option is set

set(dt[, x:= mpg], i = NULL, j = names(dt)[!names(dt) %in% c('x', 'mpg')], value = NULL)

Answer 2

如果你想通过引用进行替换，使用:=那么你可以做

dt[, x:=mpg][, setdiff(colnames(dt), c('x', 'mpg')) := NULL]