Update class of a subset of columns in data.table

Question

I want to change a couple of data.table columns from factor to character

library(data.table)

ir <- as.data.table(iris)
ir[, Species2 := Species]

I can identify which columns I need to change

facs <- which(sapply(ir, is.factor))
facs

And I can update the columns by name:

ir[, c("Species", "Species2") := lapply(.SD, as.character), .SDcols = facs]
sapply(ir, class)

Is there a way to update the columns without referencing them by name?

Answer 1

You're very close. As @akrun mentioned in a comment, you can reference the columns by index, which you've obtained using which .

ir[, which(sapply(ir, is.factor)) := lapply(.SD, as.character), .SDcols = facs]

Or even better, as @Frank mentioned in a comment, you can use parentheses.

ir[, (fac) := lapply(.SD, as.character), .SDcols = fac]

Now if you look at str(ir) , you'll see that Species and Species2 are now chr rather than factor .