Create Scoping.R

Scoping.R

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+## 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++ 	     cat(amount,"withdrawn.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$withdraw(30)
+30 withdrawn.  Your balance is 70 
+
+
+> ross$balance()
+Your balance is 70 
+
+
+> robert$balance()
+Your balance is 200 
+
+
+> ross$deposit(50)
+50 deposited. Your balance is 120 
+
+
+> ross$balance()
+Your balance is 120 
+
+
+> try(ross$withdraw(500)) # no way..
+Error in ross$withdraw(500) : You don't have that much money!