Statistics and Data Analysis for Microarrays Using R and Bioconductor, 2nd Edition

Introduction to R 139

> x<-1:12

> x

[1] 1 2 3 4 5 6 7 8 9 10 11 12

then we can tell the system that it should treat this object as a matrix rather

than a one-dimensiona l array. We can do that by specifying the dimensions

of the matrix with the function dim:

> dim(x) <- c(3,4)

> x

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

[1,] 1 4 7 10

[2,] 2 5 8 11

[3,] 3 6 9 12

From now on, the system will treat x as a two-dimensional array, a matrix

with 3 r ows and 4 columns. However, in r eality, nothing has changed in x.

Its values continue to be stored in the same memory locations as before and

indeed, they can be accessed using either a single index, specifying the position

of the desired element from the b eginning o f the arr ay:

> x[6]

[1] 6

or using two indexes, the way one normally