Formula Input Error in R

Question

I am trying to do something relatively simple. I am trying to come up with a regression function using the lm() function and get the diagnostics.
Now the input for the lm() function is one which has a formula class.

How can a function be designed so as to test if the input is formula, stop and throw an error if it is not?

Answer 1

You can use class function to check whether an object is a formula or not

> fo <- y ~ x1*x2   # this is formula class
> stopifnot(class(fo)=="formula")
> 
> fo <- 1
> stopifnot(class(fo)=="formula")  # this not a formula
Error: class(fo) == "formula" is not TRUE

You can also define a function to test whether an object is a formula

> is.formula <- function(x){
    class(x)=="formula"
  }
> 
> is.formula( y ~ x1*x2)
[1] TRUE
> is.formula(2)
[1] FALSE

If you want to write a custom error message you can proceed as follows (thanks to nico)

formula.test <- function(x){
    ifelse( class(x)=="formula",
          "This is a formula, you can go ahead!",
          stop("This is not a formula, we must stop here."))
}

formula.test(y ~ x1*x2)  # this is OK
formula.test("a")        # stops execution and throws an error
formula.test(1)          # stops execution and throws an error