How to create custom functions in R: A Step-by-Step Process

published on 05 April 2024

Creating custom functions in R simplifies your data analysis process, making it more efficient and error-free. Custom functions allow you to automate repetitive tasks, organize your work into modular pieces, enhance the reusability of your code, improve readability, and facilitate testing. Here's a step-by-step guide on how to craft your own functions in R:

  • Understanding Functions: Learn the difference between built-in and user-defined functions.
  • Define the Function: Start with the function() keyword, naming your function, and specifying any required parameters.
  • Passing Arguments: Learn how to pass arguments and set default values for them.
  • Writing the Function Body: The core logic of your function goes here.
  • Testing and Debugging: Ensure your function works correctly under various conditions.
  • Advanced Features: Explore loops, vectorization, and creating functions without arguments.

This guide also covers best practices for naming conventions, focusing your functions, handling inputs and outputs, and documenting your work effectively. Whether you're a beginner or looking to enhance your R programming skills, understanding how to create custom functions is invaluable for tackling complex data projects.

Step 1: Define the Function Structure

To start making your own function in R, you first set it up using the function() keyword. Here's how you do it in simple steps:

function_name <- function(argument1, argument2, ...) {
  # What your function does

Here's what each part means:

  • function_name is what you decide to call your function. Pick a name that makes it clear what the function does.
  • The items inside the parentheses are the inputs your function needs. You can have as many as you need, just separate them with commas.
  • The part inside the curly braces {} is where you put the code that tells your function what to do.
  • You use <- to tell R that this is your new function.

For instance, if we make a simple function called add_three() that takes any number, adds 3 to it, and gives back the result, it looks like this:

add_three <- function(x) {

  result <- x + 3

And here's how you use it:

#> [1] 8

In this example, x is the number you want to add 3 to. The function takes that number, adds 3, saves it as result, and then return(result) means it gives you back the new number.

Starting with this basic setup is the first step in creating your own custom function! Now, you can add more stuff inside the function to make it do exactly what you need.

Step 2: Passing Arguments and Setting Default Values

When you're creating a custom function in R, you can add arguments, which are like placeholders for the data your function will work with. Think of arguments as the ingredients a recipe needs. You define what goes into the function, and then you can use different ingredients (values) each time you use it.

Here's a simple function that takes two arguments:

add_numbers <- function(x, y) {
  sum <- x + y

To use this function, you just tell it what values to use for x and y:

add_numbers(3, 5) # This will give you 8

You can also set up your function with default values. This means if you don't give it specific values to work with, it will use these defaults instead. It's like having a backup plan.

Here's how you do it with our add_numbers function:

add_numbers <- function(x = 0, y = 0) {
  sum <- x + y

Now, if you call the function without giving it any numbers, it will just add 0 + 0:

add_numbers() # This will give you 0

But, you can still change the numbers if you want something different:

add_numbers(3, 5) # And this will give you 8 again

Setting default values is great because it makes your functions more flexible. You can use them in different situations without having to change the function itself. Here are some things to remember:

  • To set a default, just write = default_value after the argument's name.
  • If you don't provide a value for an argument with a default, the function will use the default.
  • You can decide which arguments get defaults and which don't. Just remember to list the ones without defaults first.

Default values are super handy. They make your functions easier to use and more versatile, letting you handle different situations without extra work.

Step 3: Writing the Function Body

The function body is basically the engine of your function - it's where all the action happens. Think of it as a recipe. Just like you follow steps to make a cake, your function follows the steps in the body to do its job.

Let's look at an example. Say we're making a function to find the area of a rectangle. You'd write something like this:

area <- length * width

Here, you're just multiplying the length by the width. Easy, right?

Your function can have more than one step, too. Here's a bit more complex one:

quadratic <- function(a, b, c, x) {

  part1 <- a * (x^2)
  part2 <- b * x
  full <- part1 + part2 + c


This function goes through a few steps to solve a quadratic equation.

Here are some tips:

  • Use names that make sense, like length, width, and full so people can follow along.
  • Break big tasks into smaller parts.
  • Explain tricky parts with comments (those lines that start with #).
  • Check your work by running your function to make sure it does what you expect.

Writing Concise Function Bodies

Sometimes, you might want your function to do something simple, like making a long text shorter. You can keep it straight to the point like this:

shorten <- function(text, num_chars) {
  new_text <- substr(text, 1, num_chars)

Here, we're using R's substr() to cut the text down to a certain size and then give it back.

To keep things concise:

  • Lean on other functions to avoid reinventing the wheel.
  • Skip steps that aren't needed.
  • Focus on what's really necessary.

The more you practice, the better you'll get at making functions that are easy to use and understand. Start simple, build up gradually, and always test your work. Soon, creating custom functions in R will feel like second nature!

Step 4: Testing and Debugging Your Function

Before you use your function in a big project, it's important to make sure it works just right. Think of this as a trial run to catch any issues early on. This step is like making sure everything is in order before a big trip.

Here's what you should look at:

  • Function structure - Did you set up and use your function the right way?

  • Arguments - Is your function getting the information it needs? Try it with different things.

  • Logic - Is your function doing what you expect? Check it in various situations.

  • Edge cases - How does your function react to unusual or extreme inputs?

  • Integration - Does your function work well with the rest of your code?

Testing Strategies

To make sure your function is solid, try these approaches:

  • Use print statements - Put print() in your function to see what's happening at key points.

  • Check outputs - Use different inputs and see what comes out.

  • Try expected cases - Use situations you know should work a certain way.

  • Try edge cases - Test your function with odd or extreme inputs to see its response.

  • Use test datasets - Create small datasets to check if your function is doing its job right.

  • Comment out sections - Isolate parts of your function by turning off sections to see how each part works.

Debugging Tips

If something's not working, here's how to figure it out:

  • Read error messages carefully - They often tell you exactly what's wrong.

  • Check for typos - A small typo can cause big problems.

  • Print key values - Use print() to take a closer look at what's happening inside your function.

  • Divide code into sections - Test different parts one at a time to find where the issue is.

  • Check expected output - Go through your function step by step to see where things might be off.

  • Search online - If you're stuck, someone else might have had the same problem and found a solution.

Testing and fixing your function might take some time, but it's worth it to make sure it works perfectly. Think of it as making sure your car is ready for a long journey. It's all about making your custom function in R reliable and ready for action!


Step 5: Advanced Function Features

Once you're comfortable making basic functions in R, you can start exploring some advanced features to make your functions even more powerful. Here are a few advanced tricks:

1. Using Loops

Loops are a way to do something repeatedly. For example, if you want to print numbers 1 to 5, instead of writing out each print command, you can use a loop like this:

for (i in 1:5) {

This loop will go through each number from 1 to 5 and print it. You can use loops inside functions too. For instance, if you have a list of items and you want to print each one, you could write a function like this:

print_vector <- function(vec) {
  for (item in vec) {

Just give this function a list (or vector), and it will print every item on the list. Loops are super handy for repetitive tasks.

2. Vectorization

Vectorization allows your functions to handle whole lists of items at once, rather than one at a time. Here's how:

add_ten <- function(x) {
  x + 10

my_vector <- c(1, 3, 5)


This function adds 10 to every number in a list. So, if you give it c(1, 3, 5), it returns 11, 13, 15. Vectorization makes functions more versatile and powerful when working with lists of numbers.

3. Argument-less Functions

Sometimes, you might want a function that doesn't need any input to run. These functions do the same thing every time you call them.

print_date <- function() {

This function prints today's date whenever you call it, without needing any arguments. It's a simple way to run the same code easily whenever you need it.

These advanced features can make your custom functions in R more powerful and flexible. The more you practice with them, the more you'll find creative ways to use functions to make your work easier.

Best Practices for Custom Functions in R

When you're making your own functions in R, it's good to follow some smart tips to make sure they work well and are easy to understand. Here's what to keep in mind:

Naming Conventions

  • Pick names that clearly tell you what the function does, like calculate_average() or plot_timeseries().

  • Try to use the same kind of names for all your functions, like action words for functions that do something and thing words for functions that make something like plots.

  • Aim for names that are short but still clear. You want them to be easy to remember but also to explain what the function does.


  • Your function should focus on doing one thing really well. Avoid trying to do too much in one go.

  • If you have a big job, break it down into smaller parts and make a function for each part.

  • Organize your code by splitting it into different functions based on what they do.

Inputs and Outputs

  • Make sure it's clear what kind of data your function needs.

  • It's a good idea to check that the data given to your function is what you expect.

  • Think about having default settings for your function so it's easier to use.

  • Make sure what your function gives back is easy to use later on.


  • Always write notes explaining what your function is for, what it needs, and what it gives back.

  • Keep the way you write these notes the same for all your functions.

  • Show examples of how to use your function so it's easier for others to understand.

  • If you change your function, remember to update your notes too.

Following these tips might take a bit more time at the start, but it really helps in the long run. Your functions will be easier to use and share, making your R work smoother and more efficient!


Learning how to make your own functions in R is super helpful for making your data work smoother, more organized, and less prone to mistakes. By making functions for tasks you do a lot, you stop repeating the same steps manually and make sure you're doing things the same way every time.

In this guide, we went through how to create functions in R from start to finish, covering:

  • The benefits of making your own functions like saving time, keeping things consistent, and making your code easier to read
  • How to start making a function with the function() keyword
  • How to set up inputs for your function and give them default values if you don't provide specific ones
  • How to write the main part of the function where you tell it what to do
  • How to check your function to make sure it works right
  • How to use cool features like loops, working with lists all at once, and making functions that don't need any inputs
  • The best ways to name your functions, keep them focused, write helpful notes, and set up inputs and outputs

Getting good at making your own functions lets you do more with R. You can make R do exactly what you need, which is great for tackling bigger data projects.

Here's why being able to make functions is great for your data science career:

  • It shows off your coding skills. Making neat, well-explained functions can impress during job interviews.

  • It lets you take on bigger projects. With your own functions, you can handle more complex data tasks.

  • It makes your work reusable. You can use the same functions in different projects, saving time.

  • It cuts down on mistakes. Using functions means less manual work and more consistency.

  • It helps when working in teams. Functions make it easier for others to understand and use your code.

So, spend some time getting better at making functions in R. It's worth it for making your own work easier and for helping out your team.

How do I create a custom function in R?

To make your own function in R, just follow these simple steps:

  • Pick a name for your function, like add_three().
  • Use the function() command and put any inputs you need in parentheses, like function(x).
  • Use curly braces {} to start and end the part of the function that does the work.
  • Inside the curly braces, write the steps your function should do.
  • Use return() to send back the result from your function.

Here's a quick example:

add_three <- function(x) {

  y <- x + 3


This function takes a number, adds three to it, saves this new number as y, and then gives y back.

How are the functions created in R?

To make a function in R:

  1. Choose a name for your function, like rescale01().
  2. Inside function(), list any inputs it needs, like function(x).
  3. Write the steps your function will do between {} curly braces.
  4. Use <- to save this as your new function.

For example:

rescale01 <- function(x) {

  y <- (x - min(x)) / (max(x) - min(x))


This function takes a list of numbers x, changes the values to be between 0 and 1, saves the new list as y, and gives y back.

How would you write a user-defined function in R give an example?

Here's a simple example of a function you might make in R:

shortList <- function(x) {

  if(length(x) < 10){


This function looks at a list x. If the list has less than 10 items, it calculates the average. If there are more, it uses another function for longer lists.

How do you create an input function in R?

To make an input function in R, you could do something like this:

train_input_fn <- function() {
  input_fn(df, features = c("x1", "x2"), response = "y")

This sets up an input function using your data df, picking out "x1" and "x2" as the features to look at, and "y" as the response. You can then use this function to help with model training.

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