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R - set several factor levels as default argument in a function

I'm trying to create a function that looks up price and car type from a data set. Both will have default arguments. For price, this is easy enough. But for the car type (which I have as factors), I can't find a way to set all factors as default.

The goal is that if you don't set anything in car_type, it will return all possible car types.

search <- function(start_price = 0, end_price = 1000, car_type = ???){
  subset_data <- auto[price <= end_price   &
                      price >  start_price &
                      vehicleType == car_type]
  return(subset_data)
}
search()

So that the "search()" returns all cars between the prices of 0 and 1000 and of all possible car types. I've tried using vectors and lists, without any luck.

Solution

  • The usual way to approach this is to use NULL as a default and handle that in the function.

    search <- function(start_price = 0, end_price = 1000, car_type = NULL){
  if (is.null(car_type) {
    car_type <- levels(auto$vehicleType)
  }
  subset_data <- auto[price <= end_price   &
                      price >  start_price &
                      vehicleType %in% car_type]
  return(subset_data)
}