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

functional-programming, global-variables, r

Solution

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.

Problem

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?

Original source