# R Help (2 Examples) – Warning: invalid factor level, NA generated

In this tutorial, I’ll illustrate how to handle the warning message invalid factor level, NA generated in R.

The tutorial contains the following:

It’s time to dive into the examples.

## Example 1: Invalid Factor Level in Vector

The warning message “invalid factor level, NA generated” can appear in a vector or a data frame. In the first example, I’ll show you how to handle this warning in vectors.

Consider the following factor vector in R:

```x <- factor(c("A", "B", "A", "C"))                        # Create example vector
x                                                         # Print example vector
#  A B A C
# Levels: A B C```

Our example vector contains four elements and the three factor levels A, B, and C. To illustrate the warning message, I’ll replicate our example vector first:

`x1 <- x                                                   # Replicate example vector`

Now, let’s assume that we want to add a new element to our vector.

Let’s see what happens in practice:

```x1 <- "D"                                              # Try to add element to vector
# Warning message:
# In `[<-.factor`(`*tmp*`, 5, value = "D") :
# invalid factor level, NA generated```

The RStudio console returns the warning “invalid factor level, NA generated”. Also, our new vector contains an NA value instead of the new value that we wanted to append:

```x1                                                        # Print example vector
#  A B A C <NA>
# Levels: A B C```

The reason for this is that we tried to append the value “D”, i.e. a value that is not a factor level of our example vector.

If we want to append a value to a factor, which is not a factor level of this factor, we need to convert the vector to the character class first. Let’s do this in practice.

First, we replicate our original vector…

`x2 <- x                                                   # Replicate example vector`

…then we convert this vector to the character class…

`x2 <- as.character(x2)                                    # Convert factor to character`

…then we concatenate our vector with the new value “D” without a warning message

`x2 <- "D"                                              # Add element to vector`

…and then we convert our combined vector back to the factor class:

`x2 <- as.factor(x2)                                       # Convert character to factor`

Our final output looks as follows:

```x2                                                        # New factor level was created
#  A B A C D
# Levels: A B C D```

A factor vector with a new factor level D.

## Example 2: Invalid Factor Level in Data Frame

The warning message “invalid factor level, NA generated” often appears in data frames. Consider the following example data:

```data1 <- data.frame(col1 = c("A", "B", "A", "C"),         # Create example data
col2 = "x",
stringsAsFactors = TRUE)
data1                                                     # Print example data
# col1 col2
#    A    x
#    B    x
#    A    x
#    C    x```

If we try to replace an element of this data frame with a new value, we receive the warning “invalid factor level, NA generated”:

```data1\$col1 <- "D"                                      # Try to replace column value
# Warning message:
# In `[<-.factor`(`*tmp*`, 4, value = c(1L, 2L, 1L, NA)) :
# invalid factor level, NA generated```

At this point we have two options: Either we convert our data frame column to the character class manually; Or we create our example data frame with the specification stringsAsFactors = FALSE:

```data2 <- data.frame(col1 = c("A", "B", "A", "C"),         # Specify stringsAsFactors = FALSE
col2 = "x",
stringsAsFactors = FALSE)```

The data.frame function automatically converts characters to factors. By specifying stringsAsFactors = FALSE, we retain the character class.

Now, we can easily replace the elements of our data frame with new values:

`data2\$col1 <- "D"                                      # Replace column value`

## Video & Further Resources

I have recently released a video on my YouTube channel, which shows the topics of this page. You can find the video below.

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In addition, you might want to read the other articles of this website. You can find a selection of interesting articles here:

In summary: On this page, I illustrated how to deal with the warning message invalid factor level, NA generated in the R programming language. If you have additional questions and/or comments, tell me about it in the comments.

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