The Art of R Programming

Writing Upstairs

As mentioned earlier, code that exists at a certain level of the environment hierarchy has at least read access to all the variables at the levels above it. On the other hand, direct write access to variables at higher levels via the standard <- operator is not possible.

If you wish to write to a global variable—or more generally, to any variable higher in the environment hierarchy than the level at which your write statement exists—you can use the superassignment operator, <<-, or the assign() function. Let’s discuss the superassignment operator first.

Writing to Nonlocals with the Superassignment Operator

Consider the following code:

> two <- function(u) { + u <<- 2*u + z <- 2*z + } > x <- 1 > z <- 3 > u Error: object "u" not ...