Naming Matrix Rows and Columns

The natural way to refer to rows and columns in a matrix is via the row and column numbers. However, you can also give names to these entities. Here’s an example:

> z
     [,1] [,2]
[1,]    1    3
[2,]    2    4
> colnames(z)
NULL
> colnames(z)  <- c("a","b")

> z
     a b
[1,] 1 3
[2,] 2 4
> colnames(z)
[1] "a" "b"
> z[,"a"]
[1] 1 2

As you see here, these names can then be used to reference specific columns. The function rownames() works similarly.

Naming rows and columns is usually less important when writing R code for general applications, but it can be useful when analyzing a specific data set.