The Art of R Programming

Environments created by inheritance in this manner are generally

referred to by their memory locations. Here is what happened after adding

aprintstatement tof()(usingedit(), not shown here) and then running

the code:

>f

function(y) {

d<-8

h <- function() {

return(d*(w+y))

}

print(environment(h))

return(h())

}

> f(2)

<environment: 0x875753c>

[1] 112

Compare all this to the situation in which the functions are not nested:

>f

function(y) {

d<-8

return(h())

}

>h

function() {

return(d*(w+y))

}

The result is as follows:

> f(5)

Error in h() : object 'd' not found

This does not work, asdis no longer in the environment ofh(), because

h()is defined at the top level. Thus, an error is generated.

Worse, if by happenstance there had been some unrelated variabledin

the top-level environment, we would not get an error message but instead

would have incorrect results.

You might wonder why R didn’t complain about the lack ofyin the

alternate definition ofh()in the preceding example. As mentioned earlier,

R doesn’t evaluate a variable until it needs it under a policy called lazy evalu-

ation. In this case, R had already encountered an error withdand thus never

got to the point where it would try to evaluatey.

154 Chapter 7