The Art of R Programming

14 print(commdata$countabsamecomm/nreps)

15 }

Because this simple example has just two levels, it’s not too bad. How-

ever, nestedifstatements can become confusing when you have more levels.

Theforconstruct works on any vector, regardless of mode. You can loop

over a vector of filenames, for instance. Say we have a file namedfile1with

the following contents:

1 2 3 4 5 6

We also have a file namedfile2with these contents:

5

12

13

The following loop reads and prints each of these files. We use thescan()

function here to read in a file of numbers and store those values in a vector.

We’ll talk more aboutscan()in Chapter 10.

> for (fn in c("file1","file2")) print(scan(fn))

Read 6 items

[1]123456

Read 3 items

[1] 51213

So,fnis first set tofile1, and the file of that name is read in and printed

out. Then the same thing happens forfile2.

7.1.2 Looping Over Nonvector Sets....................................

R does not directly support iteration over nonvector sets, but there are a

couple of indirect yet easy ways to accomplish it:

• Uselapply(), assuming that the iterations of the loop are independent
  of each other, thus allowing them to be performed in any order.


• Useget(). As its name implies, this function takes as an argument a
  character string representing the name of some object and returns
  the object of that name. It sounds simple, butget()is a very powerful
  function.

142 Chapter 7