The Art of R Programming

Ifxis empty, this loop should not have any iterations, but it actually

has two, since1:length(x)evaluates to(1,0). We could fix this by writing the

statement as follows:

for (i in seq(x))

To see why this works, let’s do a quick test ofseq():

> x <- c(5,12,13)

>x

[1] 51213

> seq(x)

[1]123

> x <- NULL

>x

NULL

> seq(x)

integer(0)

You can see thatseq(x)gives us the same result as1:length(x)ifxis not

empty, but it correctly evaluates to NULL ifxis empty, resulting in zero iter-

ations in the above loop.

2.4.5 Repeating Vector Constants with rep().............................

Therep()(orrepeat) function allows us to conveniently put the same con-

stant into long vectors. The call form isrep(x,times), which creates a vector

oftimes*length(x)elements—that is,timescopies ofx. Here is an example:

> x <- rep(8,4)

>x

[1]8888

> rep(c(5,12,13),3)

[1] 51213 51213 51213

> rep(1:3,2)

[1]123123

There is also a named argumenteach, with very different behavior, which

interleaves the copies ofx.

> rep(c(5,12,13),each=2)

[1] 5 5 12 12 13 13

34 Chapter 2