如何操作 R 中数据框的因子级别？

Question

I have in a data frame (Vitamin_C) one column (size) which only has two factor levels : "(1,20]" and "(20,35]". I want to change "(1,20]" to "reduce" and "(20,35]" to "normal".

I tried as follow:

Vitamin_C$size[Vitamin_C$size== "(1,20]"]<-c("Reduced")

Vitamin_C$size[Vitamin_C$size== "(20,35]"]<-c("Normal")

Error message:

Warning message:
In `[<-.factor`(`*tmp*`, Vitamin_C$size == "(1,20]", value = c(NA,  :
  invalid factor level, NA generated

Answer 1

With base R, use factor() :

Vitamin_C$size = factor(
  Vitamin_C$size,
  levels = c("(1,20]", "(20, 35]"),
  labels = c("Reduced", "Normal")
)

You could also directly modify the levels (make sure they start in the order you think they are!)

levels(Vitamin_C$size) = c("Reduced", "Normal")

As mentioned in comments, if you are creating this factor with cut , you can specify the labels you want directly in the cut command, which is best of all.