[R] What does the "<<-" operator mean?

[Marc Schwartz]

On Apr 21, 2011, at 2:08 PM, Cliff Clive wrote:

> I've been reading some code from an example in a blog post (
> http://www.maxdama.com/ here ) and I came across an operator that I hadn't
> seen before.  The author used a <<- operator to update a variable, like so:
> 
> ecov_xy <<- ecov_xy+decay*(x[t]*y[t]-ecov_xy)
> 
> At first I thought it was a mistake and tried replacing it with the usual <-
> assignment operator, but I didn't get the same results.  So what does the
> double arrow <<- operator do?
> 

It is a "super assignment" operator, which means that the assignment is done in the global environment, rather than to a variable that is within a function and therefore may only have local scope.

This is covered a bit in "An Introduction To R":

  http://cran.r-project.org/doc/manuals/R-intro.html#Assignment-within-functions

and a bit more in:

 ?"<-"

or

 ?"<<-"

It can be a bit hazardous to use, given the potential confusion over variable scoping issues, which is why <- and <<- may not be interchangeable as you are observing.

HTH,

Marc Schwartz