How to reliably get dependent variable name from formula object?

Question

Let\'s say I have the following formula:

myformula<-formula(\"depVar ~ Var1 + Var2\")

How to reliably get dependent variable name from formu

Answer 1

I suppose you could also cook your own function to work with terms():

getResponse <- function(formula) {
    tt <- terms(formula)
    vars <- as.character(attr(tt, "variables"))[-1] ## [1] is the list call
    response <- attr(tt, "response") # index of response var
    vars[response] 
}

R> myformula <- formula("depVar ~ Var1 + Var2")
R> getResponse(myformula)
[1] "depVar"

It is just as hacky as as.character(myformyula)[[2]] but you have the assurance that you get the correct variable as the ordering of the call parse tree isn't going to change any time soon.

This isn't so good with multiple dependent variables:

R> myformula <- formula("depVar1 + depVar2 ~ Var1 + Var2")
R> getResponse(myformula)
[1] "depVar1 + depVar2"

as they'll need further processing.