The Art of R Programming

>t<-4

> f(t)

[1] 13

[1] 120

>w

[1] 12

>t

[1] 4

So,wat the top level did not change, even though it appeared to change

withinf(). Only a localcopyofw, withinf(), changed. Similarly, the top-level

variabletdidn’t change, even though its associated formal argumentydid

change.

NOTE More precisely, references to the localwactually go to the same memory location as the
global one, until the value of the local changes. In that case, a new memory location
is used.

An important exception to this read-only nature of globals arises with

the superassignment operator, which we’ll discuss later in Section 7.8.1.

7.6.5 Extended Example: A Function to Display the Contents of a Call Frame

In single-stepping through your code in a debugging setting, you often want

to know the values of the local variables in your current function. You may

also want to know the values of the locals in the parent function—that is,

the one from which the current function was called. Here, we will develop

code to display these values, thereby further demonstrating access to the

environment hierarchy. (The code is adapted from myedtdbgdebugging

tool in R’s CRAN code repository.)

For example, consider the following code:

f <- function() {

a<-1

return(g(a)+a)

}

g <- function(aa) {

b<-2

aab <- h(aa+b)

return(aab)

}

h <- function(aaa) {

c<-3

return(aaa+c)

}

R Programming Structures 157