對于單個因子變量

# 創(chuàng)建額外的因子水平 (gamma)

x <- factor(c("alpha","beta","alpha"), levels=c("alpha","beta","gamma"))

x

#> [1] alpha beta alpha

#> Levels: alpha beta gamma

# 移除額外的因子水平

x <- factor(x)

x

#> [1] alpha beta alpha

#> Levels: alpha beta

當(dāng)導(dǎo)入數(shù)據(jù)之后，可能有一個混合因子變量和其他向量的數(shù)據(jù)框，然后想要重新計(jì)算所有因子的水平?？梢允褂胐roplevels()函數(shù)實(shí)現(xiàn)這一點(diǎn)。

# 創(chuàng)建一些因子的數(shù)據(jù)框 (有額外的因子水平)

df <- data.frame(

x = factor(c("alpha","beta","alpha"), levels=c("alpha","beta","gamma")),

y = c(5,8,2),

z = factor(c("red","green","green"), levels=c("red","green","blue"))

)

# 顯示因子水平 (with extra levels)

df$x

#> [1] alpha beta alpha

#> Levels: alpha beta gamma

df$z

#> [1] red green green

#> Levels: red green blue

# 丟掉額外因子水平

df <- droplevels(df)

# 再次顯示因子，現(xiàn)在沒有額外的因子水平了

df$x

#> [1] alpha beta alpha

#> Levels: alpha beta

df$z

#> [1] red green green

#> Levels: red green