How can I drop unused levels from a data frame?

Question

Given the following mock data:

set.seed(123)
x <- data.frame(let = sample(letters[1:5], 100, replace = T), 
                num = sample(1:10, 100, replac         


        

Answer 1

Just do y$let <- factor(y$let). Running factor on an existing factor variable will reset the levels to only those that are present.