Functions

A function is a named, reusable block of code that performs a task. You've already used many: mean, paste, round. Now you'll write your own.

Real-world analogy: a function is a recipe. You write the steps once, then just say "make pancakes" whenever you want them.

Syntax

name <- function(argument1, argument2 = default) {
  # code
  result   # the last value is returned
}

Example

celsius_to_fahrenheit <- function(celsius) {
  celsius * 9 / 5 + 32
}

celsius_to_fahrenheit(25)
celsius_to_fahrenheit(c(0, 37, 100))

Expected output

[1] 77
[1]  32.0  98.6 212.0

• function(celsius) creates a function with one argument.
• The last expression is the function's result — no return needed.
• Because the body uses vectorized arithmetic, the function automatically works on whole vectors too.

Default arguments and named calls

greet <- function(name, greeting = "Hello") {
  paste0(greeting, ", ", name, "!")
}

greet("Ada")
greet("Ada", greeting = "Welcome")
greet(greeting = "Hi", name = "Linus")

[1] "Hello, Ada!"
[1] "Welcome, Ada!"
[1] "Hi, Linus!"

return() for early exits

return() exits immediately — handy for checking inputs first:

safe_divide <- function(a, b) {
  if (b == 0) {
    return(NA)
  }
  a / b
}

safe_divide(10, 2)
safe_divide(10, 0)

[1] 5
[1] NA

Common mistakes

• Ending a function with cat() or print() instead of returning a value. Return values from functions, and print them where you call the function.
• Forgetting to call the function with parentheses: typing greet prints the function's code instead of running it.

Practice

Write bmi(weight_kg, height_m) that returns the body-mass index (weight / height^2) rounded to one decimal. Test it with 70 kg and 1.75 m.

bmi <- function(weight_kg, height_m) {
  round(weight_kg / height_m^2, 1)
}

bmi(70, 1.75)

[1] 22.9