Closures with State

Part of the Logic & Flow section of Coddy's R journey. Lesson 30 of 64.

Inside a function, <- creates a local variable. The operator <<- assigns to the variable of the same name in the enclosing function instead, so a closure can update a value it captured:

make_counter <- function() {
  count <- 0
  function() {
    count <<- count + 1
    count
  }
}
next_id <- make_counter()
print(next_id())
print(next_id())
print(next_id())

Output:

[1] 1
[1] 2
[1] 3

Each counter made by the maker has its own count, so they never interfere:

make_counter <- function() {
  count <- 0
  function() {
    count <<- count + 1
    count
  }
}
a <- make_counter()
b <- make_counter()
for (i in 1:2) a()
print(a())
print(b())

Output:

[1] 3
[1] 1

With a plain <- inside the closure, the update creates a new local variable that disappears after each call, and the stored value never changes:

make_broken <- function() {
  count <- 0
  function() {
    count <- count + 1
    count
  }
}
f <- make_broken()
for (i in 1:2) f()
print(f())

Output:

[1] 1

A maker can return a list of closures that share the same variables. Together they behave like a small object with its own data:

make_wallet <- function(balance) {
  list(
    add = function(x) balance <<- balance + x,
    get = function() balance
  )
}
w <- make_wallet(10)
w$add(5)
print(w$get())

Output:

[1] 15

Challenge

Easy

Complete make_counter(). It returns a function that, every time it is called, adds 1 to its own count and returns the new count, starting from 1.

The supplied code makes two counters, a and b. It reads a comma-separated list of a and b, calls the matching counter for each entry and prints each returned value on its own line.

Try it yourself

make_counter <- function() {
  # Write your code here
  function() 0
}

# Supplied input/output code: keep it as it is
input <- suppressWarnings(readLines(file("stdin")))
counters <- list(a = make_counter(), b = make_counter())
for (k in strsplit(input[1], ",")[[1]]) cat(counters[[k]](), "\n", sep = "")