How to Create Categorical Variables in R (With Examples)


You can use the following syntax to create a categorical variable in R:

#create categorical variable from scratch
cat_variable <- factor(c('A', 'B', 'C', 'D'))

#create categorical variable (with two possible values) from existing variable
cat_variable <- as.factor(ifelse(existing_variable < 4, 1, 0))

#create categorical variable (with multiple possible values) from existing variable
cat_variable <- as.factor(ifelse(existing_variable < 3, 'A',
                          ifelse(existing_variable < 4, 'B', 
                          ifelse(existing_variable < 5, 'C', 
                          ifelse(existing_variable < 6, 'D',0)))))

The following examples show how to use this syntax in practice.

Example 1: Create a Categorical Variable from Scratch

The following code shows how to create a categorical variable from scratch:

#create data frame
df <- data.frame(var1=c(1, 3, 3, 4, 5),
                 var2=c(7, 7, 8, 3, 2),
                 var3=c(3, 3, 6, 10, 12),
                 var4=c(14, 16, 22, 19, 18))

#view data frame
df

  var1 var2 var3 var4
1    1    7    3   14
2    3    7    3   16
3    3    8    6   22
4    4    3   10   19
5    5    2   12   18

#add categorical variable named 'type' to data frame
df$type <- factor(c('A', 'B', 'B', 'C', 'D'))

#view updated data frame
df

  var1 var2 var3 var4 type
1    1    7    3   14    A
2    3    7    3   16    B
3    3    8    6   22    B
4    4    3   10   19    C
5    5    2   12   18    D

Example 2: Create a Categorical Variable (with Two Values) from Existing Variable

The following code shows how to create a categorical variable from an existing variable in a data frame:

#create data frame
df <- data.frame(var1=c(1, 3, 3, 4, 5),
                 var2=c(7, 7, 8, 3, 2),
                 var3=c(3, 3, 6, 10, 12),
                 var4=c(14, 16, 22, 19, 18))

#view data frame
df

  var1 var2 var3 var4
1    1    7    3   14
2    3    7    3   16
3    3    8    6   22
4    4    3   10   19
5    5    2   12   18

#add categorical variable named 'type' using values from 'var4' column
df$type <- as.factor(ifelse(df$var1 < 4, 1, 0))

#view updated data frame
df

  var1 var2 var3 var4 type
1    1    7    3   14    1
2    3    7    3   16    1
3    3    8    6   22    1
4    4    3   10   19    0
5    5    2   12   18    0

Using the ifelse() statement, we created a new categorical variable called “type” that takes the following values:

  • 1 if the value in the ‘var1’ column is less than 4.
  • 0 if the value in the ‘var1’ column is not less than 4.

Example 3: Create a Categorical Variable (with Multiple Values) from Existing Variable

The following code shows how to create a categorical variable (with multiple values) from an existing variable in a data frame:

#create data frame
df <- data.frame(var1=c(1, 3, 3, 4, 5),
                 var2=c(7, 7, 8, 3, 2),
                 var3=c(3, 3, 6, 10, 12),
                 var4=c(14, 16, 22, 19, 18))

#view data frame
df

  var1 var2 var3 var4
1    1    7    3   14
2    3    7    3   16
3    3    8    6   22
4    4    3   10   19
5    5    2   12   18

#add categorical variable named 'type' using values from 'var4' column
df$type <- as.factor(ifelse(df$var1 < 3, 'A',
                     ifelse(df$var1 < 4, 'B', 
                     ifelse(df$var1 < 5, 'C', 
                     ifelse(df$var1 < 6, 'D', 'E')))))

#view updated data frame
df

  var1 var2 var3 var4 type
1    1    7    3   14    A
2    3    7    3   16    B
3    3    8    6   22    B
4    4    3   10   19    C
5    5    2   12   18    D

Using the ifelse() statement, we created a new categorical variable called “type” that takes the following values:

  • A‘ if the value in the ‘var1’ column is less than 3.
  • Else, ‘B‘ if the value in the ‘var1’ column is less than 4.
  • Else, ‘C‘ if the value in the ‘var1’ column is less than 5.
  • Else, ‘D‘ if the value in the ‘var1’ column is less than 6.
  • Else, ‘E‘.

Additional Resources

How to Create Dummy Variables in R
How to Convert Factor to Character in R
How to Convert Character to Numeric in R

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