What is the difference between assign() and <<- in R?

Question

The normal approach to writing functions in R (as I understand) is to avoid side-effects and return a value from a function.

contained <- function(x) {
  x_squared <- x^2
  return(x_squared)
}

In this case, the value computed from the input into the function is returned. But the variable x_squared is not available.

But if you need to violate this basic functional programming tenet (and I'm not sure how serious R is about this issue) and return an object from a function, you have two choices.

escape <- function(x){
  x_squared  <<- x^2
  assign("x_times_x", x*x, envir = .GlobalEnv)
}

Both objects x_squared and x_times_x are returned. Is one method preferable to the other and why so?

score 20 · Accepted Answer · edited Dec 02 '22 at 00:46

20

Thomas Lumley answers this in a superb post on r-help the other day. <<- is about the enclosing environment so you can do thing like this (and again, I quote his post from April 22 in this thread):

make.accumulator<-function(){
    a <- 0
    function(x) {
        a <<- a + x
        a
    }
}

> f<-make.accumulator()
> f(1)
[1] 1
> f(1)
[1] 2
> f(11)
[1] 13
> f(11)
[1] 24

This is a legitimate use of <<- as "super-assignment" with lexical scope. And not simply to assign in the global environment. For that, Thomas has these choice words:

The Evil and Wrong use is to modify variables in the global environment.

Very good advice.

edited Dec 02 '22 at 00:46

Gregor Thomas

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answered Apr 26 '11 at 03:59

Dirk Eddelbuettel

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Perhaps you should [link to it](http://r.789695.n4.nabble.com/What-does-the-quot-lt-lt-quot-operator-mean-tt3466657.html#a3466737) – Henry Apr 26 '11 at 08:11
thanks Dirk, I was concerned there was an evil application to `<<-` and this example of correct-ness and good-ness helps clarify the issue. Plus One for Mr Lumley – Milktrader Apr 26 '11 at 13:01

John McDonnell · Answer 2 · 2011-04-26T04:05:04.497

2

According to the manual page here,

The operators <<- and ->> cause a search to made through the environment for an existing definition of the variable being assigned.

I've never had to do this in practice, but to my mind, assign wins a lot of points for specifying the environment exactly, without even having to think about R's scoping rules. The <<- performs a search through environments and is therefore a little bit harder to interpret.

EDIT: In deference to @Dirk and @Hadley, it sounds like assign is the appropriate way to actually assign to the global environment (when that's what you know you want), while <<- is the appropriate way to "bump up" to a broader scope.

edited Apr 26 '11 at 04:05

answered Apr 26 '11 at 03:06

John McDonnell

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Yes, but often you don't know exactly where the environment you want assign in is. For most purposes `<<-` is the right choice. – hadley Apr 26 '11 at 03:12
@hadley I seem to remember reading that there was some consideration given to removing `<<-` from R as an operator. – Milktrader Apr 26 '11 at 03:28
Not a chance if you ask me as it would a ton of existing code. R doesn't usually strive to do that. – Dirk Eddelbuettel Apr 26 '11 at 03:59

score 0 · Answer 3 · answered Jun 11 '15 at 02:56

As pointed out by @John in his answer, assign lets you specify the environment specifically. A specific application would be in the following:

testfn <- function(x){

    x_squared <- NULL

    escape <- function(x){
        x_squared  <<- x^2
        assign("x_times_x", x*x, envir = parent.frame(n = 1))
    }

    escape(x)

    print(x_squared)
    print(x_times_x)
}

where we use both <<- and assign. Notice that if you want to use <<- to assign to the environment of the top level function, you need to declare/initialise the variable. However, with assign you can use parent.frame(1) to specify the encapsulating environment.

What is the difference between assign() and <<- in R?

3 Answers3

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