Assignment operators in R: '<-' and '<<-'

Question

What are the differences in the assignment operators <- and <<- in R?

And when should <<- be used?

this was asked before: http://stackoverflow.com/questions/5785290/what-is-the-difference-between-assign-and-in-r — joekz, Feb 21 '14 at 20:54
See also [this](http://stackoverflow.com/q/9851655/324364) and [this](http://stackoverflow.com/questions/10904124/global-and-local-variables-in-r). The latter in particular is a very close duplicate. — joran, Feb 21 '14 at 20:57
@PaulHiemstra, you don't consider closures a valid use? I'm thinking particularly of the case where `< — BrodieG, Feb 21 '14 at 21:45
@BrodieG you mentioned in the vast majority of situations `< — csiu, Feb 24 '14 at 07:43
@csiu, see **[this discussion](http://stackoverflow.com/questions/10659133/local-variables-within-aes/10659410#10659410)** in another SO question — BrodieG, Feb 24 '14 at 13:17

score 7 · Accepted Answer · answered Feb 21 '14 at 20:53

<- assigns an object to the environment in which it is evaluated (local scope). <<- assigns an object to the next highest environment that the name is found in or the global namespace if no name is found. See the documentation here.

<<- is usually only used in functions, but be careful. <<- can be much harder to debug because it is harder to trace the evaluation of the assignment. It is better to write functions with return statements instead.

Hadley Wickham has a good explination in his Advanced R Programming Book.

Assignment operators in R: '<-' and '<<-'

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