Attributes
Attributes are pieces of added information that do not directly influence the normal behavior of an object, but can help extend their functionality and identifiability.
Attached attributes can be queried with the attributes() function.
# sequence of values
a <- 1:6
attributes(a)
NULL
Normally R objects do not get instantiated with attributes - but there are ways how we can make this happen.
Names
The names attributes can be added to vectors and lists. They are very helpful in identifying elements.
A names attribute itself is always a character vector, and can be assigned with using the names function on the left-hand side of the normal assignment operation (technically, this is the names<- function).
a <- 1:6
names(a) <- c("a", "b", "c", "d", "e", "f")
a b c d e f
1 2 3 4 5 6
The assigned names can be queried with the names() function:
b <- names(a)
b
[1] "a" "b" "c" "d" "e" "f"
Which will return character vector.
Also if you now query the attributes of the vector you will get a response that is not NULL.
att <- attributes(a)
att
$names
[1] "a" "b" "c" "d" "e" "f"
This is the first time when we encounter the list type (try typeof(), class() and str() with the att object). We will look into lists later.
Correct number of elements
It is logical to assume that the character vector assigned to be the names attribute should have the same length as the number of elements in the vector.
If you try to assign a longer character vector to be the names than the number of elements that you have in the vector, you will get an error message:
d <- 1:4
names(d) <- c("a", "b", "c", "d", "e", "f")
Error in names(d) <- c("a", "b", "c", "d", "e", "f") :
'names' attribute [6] must be the same length as the vector [4]
If, however, you have less elements in the vector that was supposed to become the names, the final values will be filled with missing values (NA):
f <- 1:5
names(f) <- c("a", "b", "c", "d")
f
a b c d <NA>
1 2 3 4 5
Duplicates
Normal names attributes can have the same value repeated multiple times.
g <- 1:5
names(g) <- c("a", "b", "c", "d", "a")
g
a b c d a
1 2 3 4 5