Extract a particular level from factor column in an R data frame.

To extract a particular level from factor column in an R data frame, we can use levels function with the number of the factor.

For Example, if we have a data frame called df that contains a factor column say F then we can find the third level in F by using the below given command −

levels(df$F)[2]

Example 1

Following snippet creates a sample data frame −

Grp<-factor(sample(LETTERS[1:4],20,replace=TRUE))
df1<-data.frame(Grp)
df1

The following dataframe is created

  Grp
 1 D
 2 C
 3 C
 4 D
 5 B
 6 C
 7 D
 8 A
 9 B
10 C
11 B
12 C
13 B
14 B
15 C
16 A
17 C
18 B
19 B
20 D

To find the second level in Grp column of df1 on the above created data frame, add the following code to the above snippet −

Grp<-factor(sample(LETTERS[1:4],20,replace=TRUE))
df1<-data.frame(Grp)
levels(df1$Grp)[2]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "B"

To find the fourth level in Grp column of df1 on the above created data frame, add the following code to the above snippet −

Grp<-factor(sample(LETTERS[1:4],20,replace=TRUE))
df1<-data.frame(Grp)
levels(df1$Grp)[4]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "D"

Example 2

Following snippet creates a sample data frame −

Class<-factor(sample(c("First","Second","Third"),20,replace=TRUE))
df2<-data.frame(Class)
df2

The following dataframe is created

   Class
 1 Third
 2 Second
 3 First
 4 Third
 5 Third
 6 Third
 7 First
 8 Third
 9 Third
10 Second
11 Third
12 First
13 Second
14 First
15 First
16 Third
17 First
18 First
19 Third
20 Second

To find the second level in Class column of df2 on the above created data frame, add the following code to the above snippet −

Class<-factor(sample(c("First","Second","Third"),20,replace=TRUE))
df2<-data.frame(Class)
levels(df2$Class)[2]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "Second"

To find the first level in Class column of df2 on the above created data frame, add the following code to the above snippet −

Class<-factor(sample(c("First","Second","Third"),20,replace=TRUE))
df2<-data.frame(Class)
levels(df2$Class)[1]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "First"

To find the third level in Class column of df2 on the above created data frame, add the following code to the above snippet −

Class<-factor(sample(c("First","Second","Third"),20,replace=TRUE))
df2<-data.frame(Class)
levels(df2$Class)[3]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "Third"

Example 3

Following snippet creates a sample data frame −

Category<-factor(sample(c("Low","Medium","High"),20,replace=TRUE))
df3<-data.frame(Category)
df3

The following dataframe is created

   Category
 1 Low
 2 Medium
 3 Medium
 4 Medium
 5 Low
 6 High
 7 Low
 8 Medium
 9 Low
10 High
11 High
12 Medium
13 High
14 Medium
15 Medium
16 High
17 Low
18 High
19 High
20 Low

The levels will be sorted alphabetically hence the levels will be extracted based on first alphabet.

To find the first level in Category column of df3 on the above created data frame, add the following code to the above snippet −

Category<-factor(sample(c("Low","Medium","High"),20,replace=TRUE))
df3<-data.frame(Category)
levels(df3$Category)[1]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "High"

To find the second level in Category column of df3 on the above created data frame, add the following code to the above snippet −

Category<-factor(sample(c("Low","Medium","High"),20,replace=TRUE))
df3<-data.frame(Category)
levels(df3$Category)[2]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "Low"

To find the third level in Category column of df3 on the above created data frame, add the following code to the above snippet −

Category<-factor(sample(c("Low","Medium","High"),20,replace=TRUE))
df3<-data.frame(Category)
levels(df3$Category)[3]

If you execute all the above given snippets as a single program, it generates the following Output −

[1] "Medium"