在 R dataframe 中创建一个新的 class

Question

I am in need of creating a new class in R dataframe. Example,

asd <- data.frame(a = c("A", "B"), b = c("D","S"))
class(asd$b) <- "New" 

As you see, I have a created a new class "New". But when I do below operation, I do not get a

asd %>% select_if(is.New) 
Error in is_logical(.predicate) : object 'is.New' not found

Expected output

   b
1  D
2  S

Answer 1

Just because you create a class of "New" doesn't mean a function with the name is.New was also created. The generic form of is() takes a class name as a character value. You would use it like

asd %>% select_if(~is(., "New"))

And if you wanted to create is.New you could do

is.New <- function(x) is(x, "New")

asd %>% select_if(is.New) 

Answer 2

We may use select with where

library(dplyr)
asd %>%
    select(where(~ is(., "New")))

-output

 b
1 D
2 S