The Big R-Book. Philippe J. S. De Brouwer
us which environment is the current one.
environment()
environment() # get the environment ## <environment: R_GlobalEnv> rm(list=ls()) # clear the environment ls() # list all objects ## character(0) a <- “a” f <- function (x) print(x) ls() # note that x is not part of.GlobalEnv ## [1] “a” “f”
When a function starts to be executed this will create a new environment that is subordonated to the environment that calls the function.
# f # Multiple actions and side effects to illustrate environments # Arguments: # x -- single type f <- function(x){ # define a local function g() within f() g <- function(y){ b <- “local variable in g()” print(” -- function g() -- “) print(environment()) print(ls()) print(paste(“b is”, b)) print(paste(“c is”, c)) } # actions in function f: a <<- ‘new value for a, also in Global_env’ x <- ‘new value for x’ b <- d # d is taken from the environment higher c <- “only defined in f(), not in g()” g(“parameter to g”) print(” -- function f() -- “) print(environment()) print(ls()) print(paste(“a is”, a)) print(paste(“b is”, b)) print(paste(“c is”, c)) print(paste(“x is”, x)) } # Test the function f(): b <- a <- c <- d <- pi rm(x) ## Warning in rm(x): object ‘x’ not found f(a) ## [1] “ -- function g() -- “ ## <environment: 0x557538bf9e28> ## [1] “b” “y” ## [1] “b is local variable in g()” ## [1] “c is only defined in f(), not in g()” ## [1] “ -- function f() -- “ ## <environment: 0x557536bcc808> ## [1] “b” “c” “g” “x” ## [1] “a is new value for a, also in Global_env” ## [1] “b is 3.14159265358979” ## [1] “c is only defined in f(), not in g()” ## [1] “x is new value for x” # Check the existence and values: a ## [1] “new value for a, also in Global_env” b ## [1] 0 c ## [1] 3.141593 x ## Error in eval(expr, envir, enclos): object ‘x’ not found
Each function within a function or environment will create a new environment that has its own variables. Variable names can be the same but the local environment will always take precedence. A few things stand out in the example above.
The variable a does not appear in the scope of the functions.
However, a function can access a variable defined the level higher if it is not re-defined in the function itself (see what happens with the variable c: it is not defined in g(), so automatically R will search the environment above.
The function g() can use the variable b without defining it or without it being passed as an argument. When it changes that variable, it can use that new value, but once we are back in the function f(), the old value is still there.
5.2 Lexical Scoping in R
Just as any programming language, R as rules for lexical scoping. R is extremely flexible and this can be quite intimidating when starting, but it is possible to amass this flexibility.
First, a variable does not necessarily need to be declared,R will silently create it or even change it is type.
x <- ‘Philippe’ rm(x) # make sure the definition is removed x # x is indeed not there (generates an error message) ## Error in eval(expr, envir, enclos): object ‘x’ not found x <- 2L # now x is created as a long integer x <- pi # coerced to double (real) x <- c(LETTERS[1:5]) # now it is a vector of strings
This can, of course, lead to mistakes in our code: we do not have to declare variables, so we cannot group those declarations so that these error become obvious. This means that if there is a mistake, one might expect to see strange results that are hard to explain. In such case, debugging is not easy. However, this is quite unlikely to come into your way. Follow the rule that one function is never longer than half an A4 page and most likely this feature of R will save time instead of increasing debugging time.
Next, one will expect that each variable has a scope.
# f # Demonstrates the scope of variables f <- function() { a <- pi # define local variable print(a) # print the local variable print(b) # b is not in the scope of the function } # Define two variables a and b a <- 1 b <- 2 # Run the function and note that it knows both a and b. # For b it cannot find a local definition and hence # uses the definition of the higher level of scope. f() ## [1] 3.141593 ## [1] 2 # f() did not change the value of a in the environment that called f(): print(a) ## [1] 1
This illustrates that the scoping model in R is dynamical scoping. This means that when a variable is used, that R in the first place will try to find it in the local scope, if that fails, it goes a step up and continues to do so until R ends up at the root level or finds a definition.
dynamical scoping
Dynamic scoping is possible, because R is an interpreted language. If R would be compiled, then the compiler would have a hard time to create all possible memory allocations at the time of compilation.
To take this a step further we will study how the scoping within S3 objects works.1 The example below is provided by the R Core Team.
# Citation from the R documentation: # Copyright (C) 1997-8 The R Core Team open.account <- function(total) { list( deposit = function(amount) { if(amount <= ) 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 do not 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 do not have that much money!
This is a prime example of how flexible R is. At first this is quite bizarre, until we notice the <<- operator. This operator indicates that the definition of a level higher is to be used. Also the variable passed to the function automatically becomes an attribute of the object. Or maybe it was there because the