CODEWITHSUNDEEP
roop
d.putto
d.putto

Reputation: 7575

how to determine class of an object

Note- The related question does not explain why attr(myVar, "class") returns NULL but not list (and why class(myVar) returns list). What is the stander way in R to check the class of an object?

How do I find out if an object is "my custom class" or is a "list" of "my custom class" objects?

foo <- function(x) {
  a=list(x=x)
  attr(a, "class") <- "myclass"
  return(a)
}

newVar = list(foo(10),foo(20))

Now I want to find out what class of newVar .

attr(newVar, "class")  # NULL, but not list!
#NULL

##however this works fine
attr(newVar[[1]], "class")
#[1] "myclass"

Why is it so? What is the correct way to determine class in R?

Upvotes: 2

Views: 21462

Answers (1)

Roland
Roland

Reputation: 132706

The "correct way" to determine the S3 class of an object is using function class.

Implicit classes:

class(list(1))
#[1] "list"
class(1:5)
#[1] "integer"

Explicit classes:

class(list(1))
class(lm(Sepal.Length ~ Sepal.Width, data = iris))
#[1] "lm"

x <- 1:5
class(x) <- "myclass"
class(x)
#[1] "myclass"

Since a list can contain anything, you have to loop through it to find out the classes of the objects inside it, e.g., sapply(yourlist, class).

The class ID is stored as an attribute (as are names, dimensions and some other stuff), but usually you don't need to worry about such internals, since R offers accessor functions.

Upvotes: 4

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