The Art of R Programming

In line 5, we need to determine whether all of thekvalues starting

atx[i]—that is, all of the values inx[i],x[i+1],...,x[i+k-1]—are 1s. The

expressionx[i:(i+k-1)]gives us this range inx, and then applyingall()

tells us whether there is a run there.

Let’s test it.

> y <- c(1,0,0,1,1,1,0,1,1)

> findruns(y,3)

[1] 4

> findruns(y,2)

[1]458

> findruns(y,6)

NULL

Although the use ofall()is good in the preceding code, the buildup

of the vectorrunsis not so good. Vector allocation is time consuming. Each

execution of the following slows down our code, as it allocates a new vector

in the callc(runs,i). (The fact that new vector is assigned torunsis irrele-

vant; we still have done a vector memory space allocation.)

runs <- c(runs,i)

In a short loop, this probably will be no problem, but when application

performance is an issue, there are better ways.

One alternative is to preallocate the memory space, like this:

1 findruns1 <- function(x,k) {

2 n <- length(x)

3 runs <- vector(length=n)

4 count <- 0

5 for (i in 1:(n-k+1)) {

6 if (all(x[i:(i+k-1)]==1)) {

7 count <- count + 1

8 runs[count] <- i

9 }

10 }

11 if (count > 0) {

12 runs <- runs[1:count]

13 } else runs <- NULL

14 return(runs)

15 }

In line 3, we set up space of a vector of lengthn. This means we avoid

new allocations during execution of the loop. We merely fillruns, in line 8.

Just before exiting the function, we redefinerunsin line 12 to remove the

unused portion of the vector.

36 Chapter 2