向R中的表添加没有出现的值

Question

假设我在数据框列中有一系列考试成绩:

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

在说明这一点时,插图会有点误导,因为它没有显示每个人都期望在那里的等级为0:"A":

barplot(table(grades))

如何将0次出现的"A"添加到此表中,以使其显示在零高度的条形图中？

Answer 1

使用factor适当级别的a:

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))