The Art of R Programming

> x <- c(x[1:3],20,x[4:6]) # insert 20

>x

[1] 12 5 13 20 16 8 20

> x <- x[-2:-4] # delete elements 2 through 4

>x

[1] 12 16 8 20

In the first case,xis originally of length 5, which we extend to 6 via con-

catenation and then reassignment. We didn’t literally change the length of

xbut instead created a new vector fromxand then assignedxto that new

vector.

NOTE Reassignment occurs even when you don’t see it, as you’ll see in Chapter 14.

For instance, even the innocuous-looking assignmentx[2] <- 12is actually a

reassignment.

Analogous operations can be used to change the size of a matrix. For

instance, therbind()(row bind) andcbind()(column bind) functions let you

add rows or columns to a matrix.

> one

[1]1111

>z

[,1] [,2] [,3]

[1,] 1 1 1

[2,] 2 1 0

[3,] 3 0 1

[4,] 4 0 0

> cbind(one,z)

[1,] 1111

[2,] 1210

[3,] 1301

[4,] 1400

Here,cbind()creates a new matrix by combining a column of 1s with the

columns ofz. We choose to get a quick printout, but we could have assigned

the result toz(or another variable), as follows:

z <- cbind(one,z)

Note, too, that we could have relied on recycling:

> cbind(1,z)

[,1] [,2] [,3] [,4]

[1,]1111

[2,]1210

[3,]1301

[4,]1400

74 Chapter 3