One warning message you may encounter when using R is:
Warning message: In `[
This warning occurs when you attempt to add a value to a factor variable in R that does not already exist as a defined level.
The following example shows how to address this warning in practice.
How to Reproduce the Warning
Suppose we have the following data frame in R:
#create data frame
df frame(team=factor(c('A', 'A', 'B', 'B', 'B')),
points=c(99, 90, 86, 88, 95))
#view data frame
df
team points
1 A 99
2 A 90
3 B 86
4 B 88
5 B 95
#view structure of data frame
str(df)
'data.frame': 5 obs. of 2 variables:
$ team : Factor w/ 2 levels "A","B": 1 1 2 2 2
$ points: num 99 90 86 88 95
We can see that the team variable is a factor with two levels: “A” and “B”
Now suppose we attempt to add a new row to the end of the data frame using a value of “C” for team:
#add new row to end of data frame
df[nrow(df) + 1,] = c('C', 100)
Warning message:
In `[ We receive a warning message because the value “C” does not already exist as a factor level for the team variable.
It’s important to note that this is simply a warning message and R will still add the new row to the end of the data frame, but it will use a value of NA instead of “C”:
#view updated data frame
df
team points
1 A 99
2 A 90
3 B 86
4 B 88
5 B 95
6 NA 100
How to Avoid the Warning
To avoid the invalid factor level warning, we must first convert the factor variable to a character variable and then we can convert it back to a factor variable after adding the new row:
#convert team variable to character df$team character(df$team) #add new row to end of data frame df[nrow(df) + 1,] = c('C', 100) #convert team variable back to factor df$team factor(df$team) #view updated data frame df team points 1 A 99 2 A 90 3 B 86 4 B 88 5 B 95 6 C 100
Notice that we’re able to successfully add a new row to the end of the data frame and we avoid a warning message.
We can also check that the value “C” has been added as a factor level to the team variable:
#view structure of updated data frame
str(df)
'data.frame': 6 obs. of 2 variables:
$ team : Factor w/ 3 levels "A","B","C": 1 1 2 2 2 3
$ points: chr "99" "90" "86" "88" ... Additional Resources
The following tutorials explain how to fix other common errors in R:
How to Fix in R: Arguments imply differing number of rows
How to Fix in R: error in select unused arguments
How to Fix in R: replacement has length zero