Add value with no occurrences to table in R

Question

Say I have a range of exam grades in a data frame column:

grades <- c("B", "C", "C", "C", "D", "D", "E", "F", "F")
grades.df <- data.frame(grades)

When illustrating this, the illustration would be a bit misleading, since it doesn't show 0 for a grade that everyone would expect to be there: "A":

barplot(table(grades))

How can I add "A" with 0 occurrences to this table, such that it would appear in the bar plot with zero height?

Answer 1

Use a factor with appropriate levels:

grades <- factor(c("B", "C", "C", "C", "D", "D", "E", "F", "F"),levels=LETTERS[1:6])
table(grades)
grades
A B C D E F 
0 1 3 2 1 2 

barplot(table(grades))