[R] how to write a function that remembers its state across its calls

[Hao Cen]

Hi,

I wonder how to write a function that remembers its state across its
calls. For example, I would like to compute the average of the pass three
values the function has seen

 f(1)  # NA
 f(2)  # NA
 f(3)  # 2
 f(4)  # 3

This would require f to keep track of the values it has seen. In other
languages like c++ or java it is easy to do by having a member variable. I
am not sure how to do similar things in R because the variables declared
in functions are gone after the function exits.

It is possible do a quick-dirty function by declaring a variable outside
of f to keep track of what the function has seen. But then the logic of
keeping tracking that variable is not encapsulated in f. It would be messy
if I have lots of such functions.

Thanks for any advice in advance.

Jeff