R get local variable from formula

Question

Consider the R code :

foo <- function(formula){
    Y <- get(formula[[2]])
    print(Y)
}
main <- function(){
    Y <- 1
    X <- 2
    foo(Y ~ X)
}
main()

The outcome says that in get(formula[[2]]), it cannot find object Y. How can I make a function read formula in a local setting? How do I change the code so that it runs and prints Y?

Answer 1

By default get will look for Y in the the current environment of the call to get, which is just what is available in foo itself (i.e. pos = -1). One of the nice things about R formulas is that they have an associated environment as an attribute, so we can specify to get that it should look in that environment instead:

foo <- function(formula){
  Y <- get(formula[[2]], environment(formula))
  print(Y)
}

main()

[1] 1

Specifying the environment is a good idea generally, so that e.g. this will still get the Y from the original context:

foo <- function(formula){
  Y <- 3
  bar <- get(formula[[2]], environment(formula))
  print(bar)
}