Working with R Functions and Their Parameters Using the Three-Dot Syntax

Understanding the Basics of R Functions and Their Parameters

R is a popular programming language used extensively in data analysis, statistical computing, and data visualization. One of its key features is the ability to create functions that perform specific tasks or calculations. These functions can take various parameters, which are values passed into the function when it’s called.

In this article, we’ll delve into one of the lesser-known aspects of R functions: handling parameters using the three-dot syntax (...). We’ll explore why and how to use this feature to extract arguments from a function call.

Introduction to the Three-Dot Syntax in R

The three-dot syntax is used to pass multiple arguments to an R function. For example, consider the following simple function:

myfun <- function(x, y) {
  plot(x, y)
}

To call this function with two arguments, say x = c(1, 2, 3) and y = c(4, 5, 6), you would use the following syntax:

myfun(c(1, 2, 3), c(4, 5, 6))

However, what if we want to call this function with more than two arguments? That’s where the three-dot syntax comes in. We can pass multiple additional arguments using ..., like so:

myfun(c(1, 2, 3), c(4, 5, 6), ..., col = "blue")

This allows us to call our function with any number of arguments, from two up to an arbitrary number.

Working with the Three-Dot Syntax Inside R Functions

Now that we’ve covered how to use the three-dot syntax when calling functions, let’s discuss how to work with it inside R functions. This is where things get interesting.

By default, when you use ... in a function definition, R creates a formal argument called ..., but it doesn’t really “see” its value. However, we can manipulate this behavior using some clever tricks and techniques.

Creating an Unevaluated List of Arguments

One way to work with the three-dot syntax inside an R function is to create an unevaluated list of arguments using the match.call() function. Here’s how you can do it:

myfun <- function(...) {
  dots = match.call(expand.dots = FALSE)
  print(dots)
}

In this example, we define a function called myfun that takes any number of arguments (using the three-dot syntax). Inside the function, we use match.call() to create an unevaluated list of arguments. The expand.dots = FALSE argument ensures that R doesn’t evaluate any of the arguments; instead, it simply returns a named list containing those arguments.

Creating a Named List of Arguments

Another way to work with the three-dot syntax inside an R function is to create a named list of arguments using the list() function. Here’s how you can do it:

myfun <- function(...) {
  dots = list()
  for (i in seq_along(match.call(expand.dots = TRUE))) {
    name = match.call(expand.dots = TRUE)$[[i]]$name
    value = match.call(expand.dots = TRUE)$[[i]]$value
    dots[[name]] = value
  }
  print(dots)
}

In this example, we define a function called myfun that takes any number of arguments (using the three-dot syntax). Inside the function, we create an empty named list using list(). We then iterate over the formal arguments created by match.call(expand.dots = TRUE) and populate our list with each argument’s name-value pair.

Using Par(“usr”) to Get the Plotting Device’s Limits

If you need to extract the limits of a plotting device (like an x-axis or y-axis) from your function, you can use the par("usr") function. Here’s how:

myfun <- function(x, y, ...) {
  plot(x, y)
  usr = par("usr")
  print(usr)
}

In this example, we define a function called myfun that takes any number of arguments (using the three-dot syntax). Inside the function, we create a new plot using plot(). We then use par("usr") to get the limits of the plotting device. Note that par("usr") returns a named vector containing the x-axis limits and y-axis limits.

Using Par(“usr”) to Get the Plotting Device’s Limits for Both Sides

If you need to extract both the positive and negative limits of a plotting device (like an x-axis or y-axis) from your function, you can use par("usr") with two separate calls. Here’s how:

myfun <- function(x, y, ...) {
  plot(x, y)
  usr_x = par("usr")[1]
  usr_y = par("usr")[2]
  print(c(usr_x, usr_x, usr_y))
}

In this example, we define a function called myfun that takes any number of arguments (using the three-dot syntax). Inside the function, we create a new plot using plot(). We then use par("usr") to get both the positive and negative limits of the plotting device. Note that we index into the vector returned by par("usr") with two separate indices: [1] for the left edge and [2] for the right edge.

Conclusion

In this article, we’ve explored one of the lesser-known aspects of R functions: handling parameters using the three-dot syntax (...). We’ve learned how to create an unevaluated list of arguments using match.call() and a named list of arguments using list(). We’ve also discussed how to use par("usr") to extract the limits of a plotting device. By mastering these techniques, you’ll be better equipped to work with R functions and extract values from their arguments.

Additional Resources

If you’re interested in learning more about R functions and their parameters, here are some additional resources:

The official R documentation for match.call(): https://cran.r-project.org/src/mathlib/R/Matches.html
The official R documentation for list(): https://cran.r-project.org/src/mathlib/R/Vector.html
A tutorial on working with R functions and their arguments: https://www.datacamp.com/tutorial/r-function-parameters-r-tutorial

Common Questions

Q: What is the difference between expand.dots = TRUE and expand.dots = FALSE? A: When expand.dots = TRUE, R evaluates all arguments passed to a function. When expand.dots = FALSE, R doesn’t evaluate any arguments.

Q: Why do I need to use match.call() instead of just using the arguments directly in my function definition? A: You don’t always need to use match.call(). If you want to create an unevaluated list of arguments, it’s a good idea to use this function. However, if you’re simply passing multiple values to your function without any additional processing, you can omit the argument creation altogether.

Q: How do I get the x-axis limits and y-axis limits from a plotting device? A: You can use par("usr") to get both the positive and negative limits of the plotting device.

Last modified on 2023-06-22