Get All Factor Levels of Vector & Data Frame Column in R (2 Examples)

In this article you’ll learn how to return all levels of a factor object in the R programming language.

Table of contents:

It’s time to dive into the examples!

Example 1: List All Factor Levels of Vector

The following code shows how to print all levels of a factor vector to the RStudio console.

First, we have to create an exemplifying vector in R:

vec <- factor(c("A", "x", "bb", "A", "bb", "Hey"))  # Create example vector
vec                                                 # Print example vector
# [1] A   x   bb  A   bb  Hey
# Levels: A bb Hey x

Our example vector contains six vector elements and several different factor levels.

We can now apply the levels function to return all unique factor levels of our vector:

levels(vec)                                         # Return all factor levels of vector
# [1] "A"   "bb"  "Hey" "x"

The previous output of the RStudio console shows the four different levels of our example vector.

Example 2: List All Factor Levels of Data Frame Columns

In this example, I’ll explain how to determine the factor levels contained in a data frame.

First, we have to create a data frame in R:

data <- data.frame(x1 = factor(letters[1:5]),       # Create example data
                   x2 = factor(c("x", "y", "x", "x", "y")),
                   x3 = factor(c("aaa", "lili", "aaa", "lolo", "aa")))
data                                                # Print example data

In Table 1 you can see that we have managed to create a data frame with three factor variables.

We can now apply the levels function to only one column to get the factor levels of this specific column:

levels(data$x1)                                     # Return all factor levels of column
# [1] "a" "b" "c" "d" "e"

If we want to know the factor levels of each column of our data frame, we can apply the levels function in combination with the sapply function:

sapply(data, levels)                                # Return all factor levels of each column
# $x1
# [1] "a" "b" "c" "d" "e"
# 
# $x2
# [1] "x" "y"
# 
# $x3
# [1] "aa"   "aaa"  "lili" "lolo"
#

As you can see, we have created a list where each list element contains the factor levels of one of our data frame columns.

Video & Further Resources

Do you need more explanations on the R programming syntax of this article? Then you may want to have a look at the following video of my YouTube channel. In the video, I explain the contents of the present tutorial:

Furthermore, you might have a look at the other tutorials of my homepage. I have released numerous articles on topics such as ggplot2, factors, and graphics in R.

• Keep Unused Factor Levels in ggplot2 Barplot
• droplevels R Example
• Reorder Levels of Factor without Changing Order of Values
• R Programming Examples

To summarize: In this article, I have illustrated how to return all factor levels of a vector or data frame in the R programming language. Don’t hesitate to let me know in the comments section, in case you have any further questions.