The Art of R Programming

Here’s an example:

>f

function(y) {

d<-8

return(h(d,y))

}

>h

function(dee,yyy) {

print(ls())

print(ls(envir=parent.frame(n=1)))

return(dee*(w+yyy))

}

> f(2)

[1] "dee" "yyy"

[1] "d" "y"

[1] 112

Withparent.frame(), the argumentnspecifies how many frames to go

up in the call chain. Here, we were in the midst of executingh(), which had

been called fromf(), so specifyingn=1gives usf()’s frame, and thus we get

its locals.

7.6.4 Functions Have (Almost) No Side Effects..........................

Yet another influence of the functional programming philosophy is that

functions do not change nonlocal variables; that is, generally, there are no

side effects. Roughly speaking, the code in a function has read access to its

nonlocal variables, but it does not have write access to them. Our code can

appear to reassign those variables, but the action will affect only copies, not

the variables themselves. Let’s demonstrate this by adding some more code

to our previous example.

>w<-12

>f

function(y) {

d<-8

w<-w+1

y<-y-2

print(w)

h <- function() {

return(d*(w+y))

}

return(h())

}

156 Chapter 7