Factor in R: Categorical Variable & Continuous Variables

In a dataset, we can distinguish two types of variables: categorical and continuous.

• In descriptive statistics for categorical variables in R, the value is limited and usually based on a particular finite group. For example, a categorical variable in R can be countries, year, gender, occupation.
• A continuous variable, however, can take any values, from integer to decimal. For example, we can have the revenue, price of a share, etc..

Categorical Variables

Categorical variables in R are stored into a factor. Let's check the code below to convert a character variable into a factor variable in R. Characters are not supported in machine learning algorithm, and the only way is to convert a string to an integer.

Syntax

factor(x = character(), levels, labels = levels, ordered = is.ordered(x))

Arguments:

• x: A vector of categorical data in R. Need to be a string or integer, not decimal.
• Levels: A vector of possible values taken by x. This argument is optional. The default value is the unique list of items of the vector x.
• Labels: Add a label to the x categorical data in R. For example, 1 can take the label `male` while 0, the label `female`.
• ordered: Determine if the levels should be ordered in categorical data in R.

Example:

Let's create a factor data frame.

# Create gender vector
gender_vector <- c("Male", "Female", "Female", "Male", "Male")
class(gender_vector)
# Convert gender_vector to a factor
factor_gender_vector <-factor(gender_vector)
class(factor_gender_vector)

Output:

## [1] "character"
## [1] "factor"

It is important to transform a string into factor variable in R when we perform Machine Learning task.

A categorical variable in R can be divided into nominal categorical variable and ordinal categorical variable.

Nominal Categorical Variable

A categorical variable has several values but the order does not matter. For instance, male or female. Categorical variables in R does not have ordering.

# Create a color vector
color_vector <- c('blue', 'red', 'green', 'white', 'black', 'yellow')
# Convert the vector to factor
factor_color <- factor(color_vector)
factor_color

Output:

## [1] blue   red    green  white  black  yellow
## Levels: black blue green red white yellow

From the factor_color, we can't tell any order.

Ordinal Categorical Variable

Ordinal categorical variables do have a natural ordering. We can specify the order, from the lowest to the highest with order = TRUE and highest to lowest with order = FALSE.

Example:

We can use summary to count the values for each factor variable in R.

# Create Ordinal categorical vector 
day_vector <- c('evening', 'morning', 'afternoon', 'midday', 'midnight', 'evening')
# Convert `day_vector` to a factor with ordered level
factor_day <- factor(day_vector, order = TRUE, levels =c('morning', 'midday', 'afternoon', 'evening', 'midnight'))
# Print the new variable
factor_day

Output:

## [1] evening   morning   afternoon midday    
midnight  evening  

Example:

## Levels: morning < midday < afternoon < evening < midnight
# Append the line to above code
# Count the number of occurence of each level
summary(factor_day)

Output:

##   morning    midday afternoon   evening  midnight
##         1         1         1         2         1

R ordered the level from 'morning' to 'midnight' as specified in the levels parenthesis.

Continuous Variables

Continuous class variables are the default value in R. They are stored as numeric or integer. We can see it from the dataset below. mtcars is a built-in dataset. It gathers information on different types of car. We can import it by using mtcars and check the class of the variable mpg, mile per gallon. It returns a numeric value, indicating a continuous variable.

dataset <- mtcars
class(dataset$mpg)

Output

## [1] "numeric"