The Art of R Programming

Note that duplicates are allowed.

> x <- c(4,2,17,5)

> y <- x[c(1,1,3)]

>y

[1] 4 4 17

Negative subscripts mean that we want to exclude the given elements in

our output.

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

> z[-1] # exclude element 1

[1] 12 13

> z[-1:-2] # exclude elements 1 through 2

[1] 13

In such contexts, it is often useful to use thelength()function. For

instance, suppose we wish to pick up all elements of a vectorzexcept for

the last. The following code will do just that:

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

> z[1:(length(z)-1)]

[1] 5 12

Or more simply:

> z[-length(z)]

[1] 5 12

This is more general than usingz[1:2]. Our program may need to work

for more than just vectors of length 2, and the second approach would give

us that generality.

2.4.3 Generating Useful Vectors with the : Operator.....................

There are a few R operators that are especially useful for creating vectors.

Let’s start with the colon operator:, which was introduced in Chapter 1. It

produces a vector consisting of a range of numbers.

> 5:8

[1]5678

> 5:1

[1]54321

32 Chapter 2