The Art of R Programming

A key aspect is the argumentfixed=TRUE. Without it, the splitting argu-

ment.(calledsplitin the list ofstrsplit()’s formal arguments) would have

been treated as a regular expression. Without settingfixed=TRUE,strsplit()

would have just separated all the letters.

Of course, we could also escape the period, as follows:

1 testsuffix <- function(fn,suff) {

2 parts <- strsplit(fn,"\\.")

3 nparts <- length(parts[[1]])

4 return(parts[[1]][nparts] == suff)

5 }

Let’s check to see if it still works.

> testsuffix("x.y.abc","abc")

[1] TRUE

Here’s another way to do the suffix-test code that’s a bit more involved

but a good illustration:

1 testsuffix <- function(fn,suff) {

2 ncf <- nchar(fn) # nchar() gives the string length

3 # determine where the period would start if suff is the suffix in fn

4 dotpos <- ncf - nchar(suff) + 1

5 # now check that suff is there

6 return(substr(fn,dotpos,ncf)==suff)

7 }

Let’s look at the call tosubstr()here, again withfn = "x.ac"and

suff = "abc". In this case,dotposwill be 1, which means there should be a

period at the first character infnif there is anabcsuffix. The call tosubstr()

then becomessubstr("x.ac",1,4), which extracts the substring in character

positions 1 through 4 ofx.ac. That substring will bex.ac, which is notabc,

so the filename’s suffix is found not to be the latter.

11.2.2 Extended Example: Forming Filenames............................

Suppose we want to create five files,q1.pdfthroughq5.pdf, consisting of

histograms of 100 random N(0,i^2 ) variates. We could execute the follow-

ing code:

1 for (i in 1:5) {

2 fname <- paste("q",i,".pdf")

3 pdf(fname)

4 hist(rnorm(100,sd=i))

5 dev.off()

6 }

256 Chapter 11