R in a Nutshell, 2nd Edition

Special Values

There are a few special values that are used in R.

NA

In R, the NA values are used to represent missing values. (NA stands for “not available.”) You may encounter NA values in text loaded into R (to represent missing values) or in data loaded from databases (to replace NULL values).

If you expand the size of a vector (or matrix or array) beyond the size where values were defined, the new spaces will have the value NA:

> v <- c(1,2,3)
> v
[1] 1 2 3
> length(v) <- 4
> v
[1]  1  2  3 NA

Inf and -Inf

If a computation results in a number that is too big, R will return Inf for a positive number and -Inf for a negative number (meaning positive and negative infinity, respectively):

> 2 ^ 1024
[1] Inf
> - 2 ^ 1024
[1] -Inf

This is also the value returned when you divide by 0:

> 1 / 0
[1] Inf

NaN

Sometimes, a computation will produce a result that makes little sense. In these cases, R will often return NaN (meaning “not a number”):

> Inf - Inf
[1] NaN
> 0 / 0
[1] NaN

NULL

Additionally, there is a null object in R, represented by the symbol NULL. (The symbol NULL always points to the same object.) NULL is often used as an argument in functions to mean that no value was assigned to the argument. Additionally, some functions may return NULL. Note that NULL is not the same as NA, Inf, -Inf, or NaN.