I recently read a great post about environments in R, which featured this little bit of code:

> createCounter <- function(value) { function(i) { value <<- value+i} }
> counter <- createCounter(0)
> counter(1)
> a <- counter(0)
> a
[1] 1
> counter(1)
> counter(1)
> a <- counter(1)
> a
[1] 4
> a <- counter(5)
> a
[1] 9

I found this particular snippet quite interesting, but also somewhat difficult to read. Here’s my own version, inspired by the code in demo(scoping):

CreateCounter <- function(curr.count) {
  list(
    increment = function(amount) {
      curr.count <<- curr.count + amount
    },
    value = function() {
      return(curr.count)
    }
  )
}

My function is a little longer, but it lets you do this:

> counter <- CreateCounter(10)
> counter$increment(9)
> counter$value()
[1] 19