How to one-hot-encode factor variables with data.table?

Question

For those unfamiliar, one-hot encoding simply refers to converting a column of categories (i.e. a factor) into multiple columns of binary indicator variables where each new

Answer 1

If no one posts a clean way to write this out by hand each time, you can always make a function/macro:

OHE <- function(dt, grp, encodeCols) {
        grpSymb = as.symbol(grp)
        for (col in encodeCols) {
                colSymb = as.symbol(col)
                eval(bquote(
                            dt[, .SD
                               ][, V1 := 1
                               ][, dcast(.SD, .(grpSymb) ~ .(colSymb), fun=sum, value.var='V1')
                               ][, setnames(.SD, setdiff(colnames(.SD), grp), sprintf("%s_%s", col, setdiff(colnames(.SD), grp)))
                               ][, dt <<- dt[.SD, on=grp]
                               ]
                            ))
        }
        dt
}

dtOHE = OHE(dt, 'ID', c('Color', 'Shape'))
dtOHE

   ID Color    Shape Color_blue Color_green Color_red Shape_cirlce Shape_square Shape_triangle
1:  1 green   square          0           1         0            0            1              0
2:  2   red triangle          0           0         1            0            0              1
3:  3   red   square          0           0         1            0            1              0
4:  4  blue triangle          1           0         0            0            0              1
5:  5 green   cirlce          0           1         0            1            0              0