对于单个因子变量

# 创建额外的因子水平 (gamma)

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

x

#> [1] alpha beta alpha

#> Levels: alpha beta gamma

# 移除额外的因子水平

x <- factor(x)

x

#> [1] alpha beta alpha

#> Levels: alpha beta

当导入数据之后，可能有一个混合因子变量和其他向量的数据框，然后想要重新计算所有因子的水平。可以使用droplevels()函数实现这一点。

# 创建一些因子的数据框 (有额外的因子水平)

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"))

)

# 显示因子水平 (with extra levels)

df$x

#> [1] alpha beta alpha

#> Levels: alpha beta gamma

df$z

#> [1] red green green

#> Levels: red green blue

# 丢掉额外因子水平

df <- droplevels(df)

# 再次显示因子，现在没有额外的因子水平了

df$x

#> [1] alpha beta alpha

#> Levels: alpha beta

df$z

#> [1] red green green

#> Levels: red green