Undefined string argument on R function

Question

I have an r function in a package that I am developing, of the following format which downloads and returns a dataframe based on the argument arg1. arg1 can only be either of the options "datatype1" or "datatype2". Currently the function returns a NULL when any other value is put in for arg1. What is the best way to inform the user that it is an invalid argument and handle the else block?

output_data <- function(arg1 = "datatype1"){
    if(arg1 == "datatype1){
        return(dt1)
    } else if (arg1 == "datatype2"){
        return(dt2)
    } else {
    }
}

Depends how much you want to punish the user, `warning()` is a gentle way. — user2974951, Oct 20 '20 at 12:42
@user2974951 I think “punish” is a very unfortunate term here. The user isn’t punished by error messages — on the contrary: The user is punished by silently getting wrong results! — Konrad Rudolph, Oct 20 '20 at 12:47

Konrad Rudolph · Accepted Answer · 2020-10-20T12:49:52.273

“Best” is debatable but in most situations the best course of action is indeed to fail fast. Therefore a good option is to throw an error via stop or its relative, stopifnot:

output_data <- function(arg1 = "datatype1"){
    stopifnot(arg1 %in% paste0("datatype", 1 : 2))
    if (arg1 == "datatype1") {
        dt1
    } else if (arg1 == "datatype2") {
        dt2
    }
}

(Note that I’ve removed the extraneous return function calls from your code.)

That said, this attempt contains a lot of redundant code that gets more as you have more datasets. Why not use list lookup:

datasets = list(datatype1 = dt1, datatype2 = dt2)

output_data = function (which) {
    stopifnot(which %in% names(datasets))
    datasets[[which]]
}

Undefined string argument on R function

1 Answers1