The Art of R Programming

Avoiding Unintended Dimension Reduction

In the world of statistics, dimension reduction is a good thing, with many statistical procedures aimed to do it well. If we are working with, say, 10 variables and can reduce that number to 3 that still capture the essence of our data, we’re happy.

However, in R, something else might merit the name dimension reduction that we may sometimes wish to avoid. Say we have a four-row matrix and extract a row from it:

> z
     [,1] [,2]
[1,]    1    5
[2,]    2    6
[3,]    3    7
[4,]    4    8
> r <- z[2,]
> r
[1] 2 6

This seems innocuous, but note the format in which R has displayed r. It’s a vector format, not a matrix format. In other words, r is a vector of length 2, rather than a 1-by-2 matrix. We can confirm this in a couple of ways: ...