In R, objects are allowed to have attributes, which is a way for users to tag additional information to an R object.

There are a few reasons why one might want to use attributes. One reason that I encountered recently was to ensure that the type of object returned from a function remains consistent across a range of function options.

For example, imagine that you have a function that does a lot of complicated work to get an intermediate result res1, and just a little bit more work to get the final result res2. In some cases you might want to just return res2, while in other cases you might want to return res1 as well to save you computation in the future.

Without attributes, you might do something like this:

f <- function(keep_intermediate) {
    ...
    if (keep_intermediate) {
        return(list(res1 = res1, res2 = res2))
    } else {
        return(res2)
    }
}

The tricky thing here is that the return value is a list if keep_intermediate = TRUE, but may not be if keep_intermediate = FALSE. With attributes, you can avoid this issue:

f <- function(keep_intermediate) {
    ...
    if (keep_intermediate) {
        attr(res2, "res1") <- res1
    }
    return(res2)
}

Working with attributes in R

Use the attributes() function to look at all the attributes an object has (returned as a list). The code below shows that by default a matrix will have a dim attribute, and the output of the lm() function will have names and class attributes.

x <- matrix(rnorm(10), ncol = 2)
attributes(x)
# $dim
# [1] 5 2

fit <- lm(rnorm(5) ~ x)
attributes(fit)
# $names
# [1] "coefficients" "residuals" "effects" 
# [4] "rank" "fitted.values" "assign" 
# [7] "qr" "df.residual" "xlevels" 
# [10] "call" "terms" "model" 
# 
# $class
# [1] "lm"

Use the attr() function to set an attribute for an object:

x <- 1:3
attr(x, "test") <- "this is a test"
x
# [1] 1 2 3
# attr(,"test")
# [1] "this is a test"

Note that the method above will not work when the object is NULL:

y <- NULL
attr(y, "attr1") <- "test"
# Error in attr(y, "attr1") <- "test" : attempt to set an attribute on NULL

If the object is NULL, we can do the following instead:

attributes(y)$attr1 <- "this works"
attributes(y)["attr2"] <- "this also works"
str(y)
# list()
# - attr(*, "attr1")= chr "this works"
# - attr(*, "attr2")= chr "this also works"

Checking if an object has a particular attribute doesn’t seem that easy, the code below is what I have (perhaps there is an easier way!). Note that

attributes(attributes(y))
# $names
# [1] "attr1" "attr2"

Hence, the code below checks if y has the "attr1" and "attr3" attribute:

"attr1" %in% attributes(attributes(y))$names
# [1] TRUE
"attr3" %in% attributes(attributes(y))$names
# [1] FALSE

You can use the following code to remove an attribute:

attr(y, "attr1") <- NULL
str(y)
# list()
# - attr(*, "attr2")= chr "this also works"

References:

1. Stack Overflow. How to set attributes for a variable in R?
2. de Vries, A., and Meys, J. How to Play With Attributes in R.