The Art of R Programming

Filtering

Another feature reflecting the functional language nature of R is filtering. This allows us to extract a vector’s elements that satisfy certain conditions. Filtering is one of the most common operations in R, as statistical analyses often focus on data that satisfies conditions of interest.

Generating Filtering Indices

Let’s start with a simple example:

> z <- c(5,2,-3,8)
> w <- z[z*z > 8]
> w
[1] 5  −3  8

Looking at this code in an intuitive, “What is our intent?” manner, we see that we asked R to extract from z all its elements whose squares were greater than 8 and then assign that subvector to w.

But filtering is such a key operation in R that it’s worthwhile to examine the technical details of how R achieves our intent above. Let’s look ...