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

Here you go:

dcast(melt(dt, id.vars='ID'), ID ~ variable + value, fun = length)
#   ID Color_blue Color_green Color_red Shape_cirlce Shape_square Shape_triangle
#1:  1          0           1         0            0            1              0
#2:  2          0           0         1            0            0              1
#3:  3          0           0         1            0            1              0
#4:  4          1           0         0            0            0              1
#5:  5          0           1         0            1            0              0

To get the missing factors you can do the following:

res = dcast(melt(dt, id = 'ID', value.factor = T), ID ~ value, drop = F, fun = length)
setnames(res, c("ID", unlist(lapply(2:ncol(dt),
                             function(i) paste(names(dt)[i], levels(dt[[i]]), sep = "_")))))
res
#   ID Color_blue Color_green Color_red Color_purple Shape_cirlce Shape_square Shape_triangle
#1:  1          0           1         0            0            0            1              0
#2:  2          0           0         1            0            0            0              1
#3:  3          0           0         1            0            0            1              0
#4:  4          1           0         0            0            0            0              1
#5:  5          0           1         0            0            1            0              0