Joseph Walker
by Joseph Walker
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Defining Factors

The forcats package is part of the tidyverse and is useful for dealing with factors. Factors are simply categorical variables, useful for controlling the levels and order of a vector.

Categorical or discrete variables, as opposed to continuous variables, are often qualitiative and can take on a finite number of values.

Examples of categorical variables: types of fruit, locations, party preference, ethnicity.

#colors is a character vector of length nine consisting of colors: red, blue, & green

colors <- c("red","blue","green","green","red","blue","red","green","blue")
class(colors)
## [1] "character"
#The factor() function works by assigning integers (number values) to the categorical values (red, blue, green) of the vector or variable. 

colors <- factor(colors)
print(colors)
## [1] red   blue  green green red   blue  red   green blue 
## Levels: blue green red
class(colors)
## [1] "factor"
str(colors)
##  Factor w/ 3 levels "blue","green",..: 3 1 2 2 3 1 3 2 1
#Each color is a level, and underlying each level is an integer associated with that level; red = 3, green = 2, blue = 1. The order of the levels is assigned alphabetically to the integer: b = 1, g = 2, r = 3.

#We can reorder the levels if we want to:
colors <- factor(colors, levels = c("red","blue", "green"))
print(colors)
## [1] red   blue  green green red   blue  red   green blue 
## Levels: red blue green
#The order of the vector stays the same, but now the order of the levels has changed so that red = 1, blue = 2, green = 3
str(colors)
##  Factor w/ 3 levels "red","blue","green": 1 2 3 3 1 2 1 3 2

Factors are a useful data structure that allow you to have more control over how you analyze and visualize your data.

#Let's compare the summary function for colors as a character vector and as a factor.
colors <- c("red","blue","green","green","red","blue","red","green","blue")

summary(colors)
##    Length     Class      Mode 
##         9 character character
colors <- factor(colors, levels = c("red","blue", "green"))

summary(colors)
##   red  blue green 
##     3     3     3
#As factors, each color becomes a distinct group and summarized separately.

The Forcats Package

As useful as factors are, they can be quite a pain to work with at times. Luckily, forcats has functions that allow you to manipulate factors.

First, let’s install the package using: install.packages("forcats")

forcats also comes installed as part of the tidyverse suite of packages. It is not part of the core group of tidyverse packages so we must load it explicitly.

library(tidyverse)
library(forcats)

Function: Factor Recode

One useful function of forcats is fct_recode. This allows you to change the levels (or name/identity) of a factor. Here’s an example:

#Let's use the airquality data set that comes pre-installed in R
glimpse(airquality)
## Observations: 153
## Variables: 6
## $ Ozone   <int> 41, 36, 12, 18, NA, 28, 23, 19, 8, NA, 7, 16, 11, 14, ...
## $ Solar.R <int> 190, 118, 149, 313, NA, NA, 299, 99, 19, 194, NA, 256,...
## $ Wind    <dbl> 7.4, 8.0, 12.6, 11.5, 14.3, 14.9, 8.6, 13.8, 20.1, 8.6...
## $ Temp    <int> 67, 72, 74, 62, 56, 66, 65, 59, 61, 69, 74, 69, 66, 68...
## $ Month   <int> 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, ...
## $ Day     <int> 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,...
#The month column is data type integer, let's change it to a factor 
airquality$Month <- factor(airquality$Month)

levels(airquality$Month)
## [1] "5" "6" "7" "8" "9"
#Let's rename the months using the fct_recode function. 
airquality$Month <- fct_recode(airquality$Month, May = '5', June = '6', July = '7', Aug = '8', Sept = '9')
glimpse(airquality$Month)
##  Factor w/ 5 levels "May","June","July",..: 1 1 1 1 1 1 1 1 1 1 ...
ggplot(airquality, aes(Month, Temp)) +
  geom_boxplot(aes(fill = Month)) +
  ggtitle(label = "Daily Temperatures Aggregated by Month")

plot of chunk change level values

The Factor Reverse Function

If you just want to reverse the order, there’s the fct_rev function. You can even use it in line when defining your aesthetics in ggplot like so:

ggplot(airquality, aes(fct_rev(Month), Temp)) +
  geom_boxplot(aes(fill = Month)) +
  labs(x = "Month") +
  ggtitle(label = "Our plot now has the x-axis in reverse order")

plot of chunk reverse the order

The Factor Relevel Function

Another useful function is fct_relevel. This function allows us to change any number of levels to any position.

airquality$Month <- fct_relevel(airquality$Month, 'Sept', 'July', 'May', 'Aug', 'June')

levels(airquality$Month)
## [1] "Sept" "July" "May"  "Aug"  "June"
# This may not seem useful at first, but when you need to visualize your data in a particular way, the fct_relevel function is extremely useful...

ggplot(airquality, aes(Month, Temp)) +
  geom_boxplot(aes(fill = Month)) +
  ggtitle(label = "Notice how the order of the level 'Month' has changed")

plot of chunk change level position

The Factor Reorder Function

And finally, it is often useful to reorder the factor in a way that is useful for visualization. For this, we can use the fct_reorder function.

For this example, let’s use the mtcars data set:

mtcars$model <- row.names(mtcars)

glimpse(mtcars)
## Observations: 32
## Variables: 12
## $ mpg   <dbl> 21.0, 21.0, 22.8, 21.4, 18.7, 18.1, 14.3, 24.4, 22.8, 19...
## $ cyl   <dbl> 6, 6, 4, 6, 8, 6, 8, 4, 4, 6, 6, 8, 8, 8, 8, 8, 8, 4, 4,...
## $ disp  <dbl> 160.0, 160.0, 108.0, 258.0, 360.0, 225.0, 360.0, 146.7, ...
## $ hp    <dbl> 110, 110, 93, 110, 175, 105, 245, 62, 95, 123, 123, 180,...
## $ drat  <dbl> 3.90, 3.90, 3.85, 3.08, 3.15, 2.76, 3.21, 3.69, 3.92, 3....
## $ wt    <dbl> 2.620, 2.875, 2.320, 3.215, 3.440, 3.460, 3.570, 3.190, ...
## $ qsec  <dbl> 16.46, 17.02, 18.61, 19.44, 17.02, 20.22, 15.84, 20.00, ...
## $ vs    <dbl> 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1,...
## $ am    <dbl> 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1,...
## $ gear  <dbl> 4, 4, 4, 3, 3, 3, 3, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 4, 4,...
## $ carb  <dbl> 4, 4, 1, 1, 2, 1, 4, 2, 2, 4, 4, 3, 3, 3, 4, 4, 4, 1, 2,...
## $ model <chr> "Mazda RX4", "Mazda RX4 Wag", "Datsun 710", "Hornet 4 Dr...
mtcars$model <- factor(mtcars$model)

ggplot(mtcars, aes(mpg, model)) +
  geom_point() +
  ggtitle(label = "MPG vs. Car Model")

plot of chunk fct_reorder setup

It’s difficult to make comparisons when the data is scattered. But we’re in luck! We can use the fct_reorder function to clean it up.

#fct_reorder takes three arguments: f = factor you want to reorder, x = the variable in which the order will be based upon, and optionally fun (a function to  be used if there are multiple values of x for each value of f.) Here we focus on only the first two arguments.

ggplot(mtcars, aes(mpg, fct_reorder(f = model, x = mpg))) +
  geom_point() +
  labs(y = "model") +
  ggtitle(label = "We can make better comparison by reordering the levels based on the mpg values!") +
  theme(plot.title = element_text(size = 10, face = 'bold'))

plot of chunk fct_reorder