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.

Answer 2

There's a recently added function in R for this:

y <- droplevels(y)

Answer 3

Adding to Hong Ooi's answer, here is an example I found from R-Bloggers.

# Create some fake data
x <- as.factor(sample(head(colors()),100,replace=TRUE))
levels(x)
x <- x[x!="aliceblue"]
levels(x) # still the same levels
table(x) # even though one level has 0 entries!

The solution is simple: run factor() again:
x <- factor(x)
levels(x)