Search code examples
rclassformula

Get the right hand side variables of an R formula

I'm writing my first S3 class and associated methods and I would like to know how to subset my input data set in order to keep only the variables specified in the formula?

data(iris)
f <- Species~Petal.Length + Petal.Width

With model.frame(f,iris) I get a subset with all the variables in the formula. How to automatically keep only the right hand side variables (in the example Petal.Length and Petal.Width)?

Solution

  • You want labels and terms; see ?labels, ?terms, and ?terms.object.

    labels(terms(f))
# [1] "Petal.Length" "Petal.Width"

    In particular, labels.terms returns the "term.labels" attribute of a terms object, which excludes the LHS variable.