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

Marc Schwartz marc_schwartz at me.com
Thu Apr 21 21:59:09 CEST 2011

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":


and a bit more in:




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.


Marc Schwartz

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