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

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)