Higher-Order Functions

Functions as Values

In R, functions are first-class objects. You can pass them as arguments, return them from other functions, and store them in variables.

Passing Functions

apply_twice <- function(f, x) {
  f(f(x))
}

double <- function(x) x * 2
cat(apply_twice(double, 3), "\n")  # 12

Anonymous Functions

You can create functions inline without naming them:

result <- sapply(1:5, function(x) x ^ 2)
cat(result, "\n")  # 1 4 9 16 25

R 4.1+ also supports a shorthand syntax with \():

result <- sapply(1:5, \(x) x ^ 2)

Closures

Functions in R capture their enclosing environment:

make_adder <- function(n) {
  function(x) x + n
}

add5 <- make_adder(5)
cat(add5(3), "\n")   # 8
cat(add5(10), "\n")  # 15

The inner function remembers the value of n from when make_adder was called.

Your Task

Write a function make_multiplier that takes a number n and returns a function that multiplies its argument by n. Use it to create triple (multiplies by 3) and print triple(4) and triple(7).