Question

Program R

Please write about the difference between the assignment operators <- and <<- ? You need to refer to help(assignOps) for this answer.

Answer 1

There are three different assignment operators: two of them have leftwards and rightwards forms.

The operators <- assign into the environment in which they are evaluated. The operator <- can be used anywhere.

The operators <-  can be used, almost interchangeably, to assign to variable in the same environment.

The operators <<- is normally only used in functions, and cause a search to be made through parent environments for an existing definition of the variable being assigned. If such a variable is found (and its binding is not locked) then its value is redefined, otherwise assignment takes place in the global environment. Note that their semantics differ from that in the S language, but are useful in conjunction with the scoping rules of R. See ‘The R Language Definition’ manual for further details and examples.

The <<- operator is used for assigning to variables in the parent environments (more like global assignments). The rightward assignments, although available are rarely used.

First, let’s look at an example.

x <- rnorm(100)
y <- 2*x + rnorm(100)
lm(formula=y~x)

The above code uses both <- and = symbols, but the work they do are different. <- in the first two lines are used as assignment operator while = in the third line does not serves as assignment operator but an operator that specifies a named parameter formula for lm function.

In other words, <- evaluates the the expression on its right side (rnorm(100)) and assign the evaluated value to the symbol (variable) on the left side (x) in the current environment. = evaluates the expression on its right side (y~x) and set the evaluated value to the parameter of the name specified on the left side (formula) for a certain function.

We know that <- and = are perfectly equivalent when they are used as assignment operators.