Creating your own condition

This lesson covers creating your own condition in R, with the aim of expressing kinds of error more specifically.

structure() lets you build a condition carrying your own class. Naming a class per kind of error makes it much easier to tell what went wrong.

In the sample code, check_age() builds a condition of class validationError with structure() when the age is below zero, and raises it with stop(). R's condition system plays much the same role as exception classes elsewhere.

A common early stumble is the distinctive syntax of that condition system. Compared with a plain stop("message"), building a custom class is fiddly and takes a while to feel natural.

In professional work, being able to tell "invalid input" apart from "the network failed" is what lets you write a suitable response to each.

📖 Reference code

check_age <- function(age) {
  if (age < 0) {
    cond <- structure(
      class = c("validationError", "error", "condition"),
      list(message = "Age must be 0 or greater", call = sys.call())
    )
    stop(cond)
  }
  age
}

tryCatch({
  check_age(-5)
}, validationError = function(e) {
  cat("ValidationError:", conditionMessage(e), "\n")
})

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run R directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).