How to drop unused levels in table with data.table?

Question

Consider the following data.table:

x <- data.table(
          x=sample(letters[1:5],10,rep=T), 
          y=factor(sample(letters[1:5],10,rep         


        

Answer 1

You can use droplevel as follows

x[,y:=droplevels(y)]

this overwrites y by reference with droplevels(y)

Results in

> table(x)
   y
x   b c d e
  a 1 1 1 2
  b 0 1 0 0
  c 1 0 0 0
  d 1 0 0 0
  e 0 0 2 0