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:
. This operator will cause
the interpreter to first search through the current environment to
find the symbol var
<<-
value
. 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 var
to var
in the
global environment.value
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
R does a lot of stuff, but it’s not completely ...
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