To define a function in R

circumference <- function(r){
    2*pi*r
}

If the function body only contain one statement, we can drop the curly braces

sum_two_nums <- function(x, y) x + y

An R function automatically returns the last evaluated expression in the function body. We can use an return() statement for an early return:

is_leap_year <- function(year) {
  if (year %% 400 == 0) {
    return(TRUE)
  }
 
  if (year %% 100 == 0) {
    return(FALSE)
  }
 
  if (year %% 4 == 0) {
    return(TRUE)
  }
 
  FALSE
}