Let's say I have a vector where I've set a few attributes:
vec <- sample(50:100,1000, replace=TRUE)
attr(vec, "someattr") <- "Hello World"
When I subset the vector, the attributes are dropped. For example:
tmp.vec <- vec[which(vec > 80)]
attributes(tmp.vec) # Now NULL
Is there a way to, subset and persist attributes without having to save them to another temporary object?
Bonus: Where would one find documentation of this behaviour?
I would write a method for [ or subset() (depending on how you are subsetting) and arrange for that to preserve the attributes. That would need a "class" attribute also adding to your vector so that dispatch occurs.
vec <- 1:10
attr(vec, "someattr") <- "Hello World"
class(vec) <- "foo"
At this point, subsetting removes attributes:
> vec[1:5]
[1] 1 2 3 4 5
If we add a method [.foo we can preserve the attributes:
`[.foo` <- function(x, i, ...) {
attrs <- attributes(x)
out <- unclass(x)
out <- out[i]
attributes(out) <- attrs
out
}
Now the desired behaviour is preserved
> vec[1:5]
[1] 1 2 3 4 5
attr(,"someattr")
[1] "Hello World"
attr(,"class")
[1] "foo"
And the answer to the bonus question:
From ?"[" in the details section:
Subsetting (except by an empty index) will drop all attributes except names, dim and dimnames.