The Art of R Programming

14.3 Functional Programming and Memory Issues..................................

Most R objects areimmutable, or unchangeable. Thus, R operations are

implemented as functions that reassign to the given object, a trait that can

have performance implications.

14.3.1 Vector Assignment Issues........................................

As an example of some of the issues that can arise, consider this simple-

looking statement:

z[3] <- 8

As noted in Chapter 7, this assignment is more complex than it seems. It

is actually implemented via the replacement function"[<-"through this call

and assignment:

z <- "[<-"(z,3,value=8)

An internal copy ofzis made, element 3 of the copy is changed to 8,

and then the resulting vector is reassigned toz. And recall that the latter

simply means thatzis pointed to the copy.

In other words, even though we are ostensibly changing just one ele-

ment of the vector, the semantics say thatthe entire vector is recomputed. For

a long vector, this would slow down the program considerably. The same

would be true for a shorter vector if it were assigned from within a loop of

our code.

In some situations, R does take some measures to mitigate this impact,

but it is a key point to consider when aiming for fast code. You should be

mindful of this when working with vectors (including arrays). If your code

seems to be running unexpectedly slowly, assignment of vectors should be a

prime area of suspicion.

14.3.2 Copy-on-Change Issues..........................................

A related issue is that R (usually) follows acopy-on-changepolicy. For instance,

if we execute the following in the previous setting:

>y<-z

then initiallyyshares the same memory area withz. But if either of

them changes, then a copy is made in a different area of memory, and the

changed variable will occupy the new area of memory. However, only the

firstchange is affected, as the relocating of the moved variable means there

are no longer any sharing issues. The functiontracemem()will report such

memory relocations.

314 Chapter 14