Global and local variables in R

Question

I am a newbie for R, and I am quite confused with the usage of local and global variables in R.

I read some posts on the internet that say if I use = or <- I will assign the variable in the current environment, and with <<- I can access a global variable when inside a function.

However, as I remember in C++ local variables arise whenever you declare a variable inside brackets {}, so I'm wondering if this is the same for R? Or is it just for functions in R that we have the concept of local variables.

I did a little experiment, which seems to suggest that only brackets are not enough, am I getting anything wrong?

{
   x=matrix(1:10,2,5)
}
print(x[2,2])
[1] 4

Answer 1

Variables declared inside a function are local to that function. For instance:

foo <- function() {
    bar <- 1
}
foo()
bar

gives the following error: Error: object 'bar' not found.

If you want to make bar a global variable, you should do:

foo <- function() {
    bar <<- 1
}
foo()
bar

In this case bar is accessible from outside the function.

However, unlike C, C++ or many other languages, brackets do not determine the scope of variables. For instance, in the following code snippet:

if (x > 10) {
    y <- 0
}
else {
    y <- 1
}

y remains accessible after the if-else statement.

As you well say, you can also create nested environments. You can have a look at these two links for understanding how to use them:

1. http://stat.ethz.ch/R-manual/R-devel/library/base/html/environment.html
2. http://stat.ethz.ch/R-manual/R-devel/library/base/html/get.html

Here you have a small example:

test.env <- new.env()

assign('var', 100, envir=test.env)
# or simply
test.env$var <- 100

get('var') # var cannot be found since it is not defined in this environment
get('var', envir=test.env) # now it can be found