How to check if an R object has a certain attribute?

Question

How can I check if an R object has a certain attribute? For example, I would like to check if a vector has a "labels" attribute. How can I do this? Exists already a function that does that?

my_vector <- c(1, 2, 3)
my_vector_labelled <- `attr<-`(my_vector, "labels", c(a = 1, b = 2, c = 3)) 

let's assume there is a function named has_attribute(x, attr). The the expected result would be:

> has_attribute(my_vector, "labels")
 FALSE
> has_attribute(my_vector_labelled, "labels")
 TRUE

Answer 1

Two ways:

• %in% names(attributes(..):

  "labels" %in% names(attributes(my_vector))
  # [1] FALSE
  "labels" %in% names(attributes(my_vector_labelled))
  # [1] TRUE
  

• is.null(attr(..,"")):

  is.null(attr(my_vector, "labels"))
  # [1] TRUE                                   # NOT present
  is.null(attr(my_vector_labelled, "labels"))
  # [1] FALSE                                  # present
  

  (Perhaps !is.null(attr(..)) is preferred?)