Functions, Defaults, Dots and Pipes

Reusable building blocks

Define functions with function(args) { body }; the last evaluated expression is returned automatically (use return() for early exits). Arguments can have defaults and can be matched by name, so calls such as round(x, digits = 1) are clear. The special argument ... (dots) passes extra arguments through to other functions, as in wrappers around mean() or ggplot(). R uses lazy evaluation: arguments are evaluated only when used. Since R 4.1, anonymous functions have a short form, \(x) x * 2, and the native pipe |> passes the left-hand value as the first argument of the next call, turning nested calls into readable steps (data |> filter(...) |> summarise(...)). The magrittr pipe %>% from the tidyverse predates it and works similarly, with a few extra features. Functions are objects: you can store them in lists and pass them to other functions. Validate inputs early with stopifnot() or stop("message"), and signal problems with warning() and message().

A validated function and a pipeline

Defaults, dots, input checks and the native pipe.

gst_total <- function(amount, rate = 0.18, digits = 2) {
  stopifnot(is.numeric(amount), all(amount >= 0, na.rm = TRUE))
  round(amount * (1 + rate), digits)
}

gst_total(c(100, 250))                  # 118.0 295.0
gst_total(100, rate = 0.05)             # named argument

safe_mean <- function(x, ...) {
  if (length(x) == 0) return(NA_real_)  # early return
  mean(x, ...)                          # pass extra arguments such as na.rm
}
safe_mean(c(1, NA, 3), na.rm = TRUE)    # 2

square <- \(x) x^2                       # short anonymous function (R 4.1+)

c(4, 9, 16) |>
  sqrt() |>
  sum() |>
  square()                              # (2 + 3 + 4)^2 = 81

An assembly line

Nested calls like square(sum(sqrt(x))) must be read inside out. A pipe is an assembly line: the data moves left to right through each station in the order the work happens.

Quick check: What does the native pipe `x |> f(y)` mean?

• f(y, x)
• x(f, y)
• It assigns f to x
• f(x, y)