## Here is a little example which shows a fundamental difference between
## R and S. It is a little example from Abelson and Sussman which models
## the way in which bank accounts work. It shows how R functions can
## encapsulate state information.
##
## When invoked, "open.account" defines and returns three functions
## in a list. Because the variable "total" exists in the environment
## where these functions are defined they have access to its value.
## This is even true when "open.account" has returned. The only way
## to access the value of "total" is through the accessor functions
## withdraw, deposit and balance. Separate accounts maintain their
## own balances.
##
## This is a very nifty way of creating "closures" and a little thought
## will show you that there are many ways of using this in statistics.
# Copyright (C) 1997-8 The R Core Team
open.account <- function(total) {
list(
deposit = function(amount) {
if(amount <= 0)
stop("Deposits must be positive!\n")
total <<- total + amount
cat(amount,"deposited. Your balance is", total, "\n\n")
},
withdraw = function(amount) {
if(amount > total)
stop("You don't have that much money!\n")
total <<- total - amount
cat(amount,"withdrawn. Your balance is", total, "\n\n")
},
balance = function() {
cat("Your balance is", total, "\n\n")
}
)
}
ross <- open.account(100)
robert <- open.account(200)
ross$withdraw(30)
ross$balance()
robert$balance()
ross$deposit(50)
ross$balance()
try(ross$withdraw(500)) # no way..
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