Exit in R Programming Language

Here’s an idiomatic R code example that demonstrates the concept of exiting a program:

# Function to demonstrate exit behavior
demonstrate_exit <- function() {
  # This will be executed
  print("Starting the function")
  
  # Register a cleanup handler
  on.exit(print("This cleanup won't run if we use quit()"))
  
  # Exit the R session
  quit(save = "no", status = 3, runLast = FALSE)
  
  # This line will never be reached
  print("This won't be printed")
}

# Run the function
demonstrate_exit()

This R code example demonstrates how to exit a program prematurely using the quit() function. Let’s break it down:

1. We define a function called demonstrate_exit() to encapsulate our example.

2. Inside the function, we first print a message to show that the function has started executing.

3. We use on.exit() to register a cleanup handler. This is similar to defer in some other languages, but it’s important to note that it won’t run if we use quit().

4. We call quit() with several arguments:

   • save = "no": This tells R not to save the workspace image.
   • status = 3: This sets the exit status to 3, similar to the original example.
   • runLast = FALSE: This prevents R from running .Last() or on.exit() code.

5. The last print() statement will never be executed because the program exits before reaching it.

To run this example:

1. Save the code in a file, e.g., exit_example.R.
2. Open a terminal and navigate to the directory containing the file.
3. Run the script using R:

$ Rscript exit_example.R
[1] "Starting the function"

You can check the exit status in the terminal:

$ echo $?
3

Note that the cleanup message (“This cleanup won’t run if we use quit()”) is not printed, demonstrating that on.exit() handlers are not executed when using quit().

In R, unlike some other languages, we typically don’t use exit codes in scripts. Instead, we usually return values from functions or use error handling mechanisms. However, quit() can be useful in certain scenarios, such as when you need to exit an R session programmatically or when writing R scripts that interact with shell scripts.

Remember that abruptly exiting a program is often not the best practice in R. It’s generally better to handle errors gracefully and allow proper cleanup of resources. Use quit() judiciously and consider alternatives like return() for normal function exits or stop() for error conditions that should be caught and handled by the caller.