R for Data Science

Chapter 12. Factors with forcats

Introduction

In R, factors are used to work with categorical variables, variables that have a fixed and known set of possible values. They are also useful when you want to display character vectors in a non-alphabetical order.

Historically, factors were much easier to work with than characters. As a result, many of the functions in base R automatically convert characters to factors. This means that factors often crop up in places where they’re not actually helpful. Fortunately, you don’t need to worry about that in the tidyverse, and can focus on situations where factors are genuinely useful.

For more historical context on factors, I recommend stringsAsFactors: An unauthorized biography by Roger Peng, and stringsAsFactors = <sigh> by Thomas Lumley.

Prerequisites

To work with factors, we’ll use the forcats package, which provides tools for dealing with categorical variables (and it’s an anagram of factors!). It provides a wide range of helpers for working with factors. forcats is not part of the core tidyverse, so we need to load it explicitly.

library(tidyverse)
library(forcats)

Creating Factors

Imagine that you have a variable that records month:

x1 <- c("Dec", "Apr", "Jan", "Mar")

Using a string to record this variable has two problems:

1. There are only twelve possible months, and there’s nothing saving you from typos:

   x2 <- c("Dec", "Apr", "Jam", "Mar")

2. It doesn’t sort in a useful way:

   sort(x1)
   #> [1] "Apr" "Dec" "Jan" "Mar" ...