data.table Adding and modifying columns Editing values

Example

# example data
DT = as.data.table(mtcars, keep.rownames = TRUE)

Editing a column

Use the := operator inside j to create new columns or modify existing ones:

DT[, mpg_sq := mpg^2]

Editing on a subset of rows

Use the i argument to subset to rows "where" edits should be made:

DT[1:3, newvar := "Hello"]

As in a data.frame, we can subset using row numbers or logical tests. It is also possible to use [a "join" in i when modifying][need_a_link].

Removing a column

Remove columns by setting to NULL:

DT[, mpg_sq := NULL]

Note that we do not <- assign the result, since DT has been modified in-place.

Editing multiple columns

Add multiple columns by using the := operator's multivariate format:

DT[, `:=`(mpg_sq = mpg^2, wt_sqrt = sqrt(wt))]
# or 
DT[, c("mpg_sq", "wt_sqrt") := .(mpg^2, sqrt(wt))]

The .() syntax is used when the right-hand side of LHS := RHS is a list of columns.

Editing multiple sequentially-dependent columns

If the columns are dependent and must be defined in sequence, some ways to do that are:

DT[, c("mpg_sq", "mpg2_hp") := .(temp1 <- mpg^2, temp1/hp)]
# or
DT[, c("mpg_sq", "mpg2_hp") := {temp1 = mpg^2; .(temp1, temp1/hp)}]

Editing columns by dynamically-determined names

For dynamically-determined column names, use parentheses:

vn = "mpg_sq"
DT[, (vn) := mpg^2]

Using set

Columns can also be modified with set for a small reduction in overhead, though this is rarely necessary:

set(DT, j = "hp_over_wt", v = mtcars$hp/mtcars$wt)