R in a Nutshell, 2nd Edition

Side Effects

All functions in R return a value. Some functions also do other things: change variables in the current environment (or in other environments), plot graphics, load or save files, or access the network. These operations are called side effects.

Changes to Other Environments

We have already seen some examples of functions with side effects. In Chapter 8, we showed how to directly access symbols and objects in an environment (or in parent environments). We also showed how to access objects on the call stack.

An important function that causes side effects is the <<- operator. This operator takes the following form: var <<- value. This operator will cause the interpreter to first search through the current environment to find the symbol var. If the interpreter does not find the symbol var in the current environment, then the interpreter will next search through the parent environment. The interpreter will recursively search through environments until it either finds the symbol var or reaches the global environment. If it reaches the global environment before the symbol var is found, then R will assign value to var in the global environment.

Here is an example that compares the behavior of the <- assignment operator and the <<- operator:

> x
Error: object "x" not found
> doesnt.assign.x <- function(i) {x <- i}
> doesnt.assign.x(4)
> x
Error: object "x" not found
> assigns.x <- function(i) {x <<- i}
> assigns.x(4)
> x
[1] 4

Input/Output

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