Convert NA into a factor level

You can use addNA().

x <- c(1, 1, 2, 2, 3, NA)
addNA(x)
# [1] 1    1    2    2    3    <NA>
# Levels: 1 2 3 <NA>

This is basically a convenience function for factoring with exclude = NULL. From help(factor) -

addNA modifies a factor by turning NA into an extra level (so that NA values are counted in tables, for instance).

So another reason this is nice is because if you already have a factor f, you can use addNA() to quickly add NA as a factor level without changing f. As mentioned in the documentation, this is handy for tables. It also reads nicely.

Set the exclude argument to NULL to include NAs as levels (and use factor instead of as.factor. Does the same thing and has more arguments to set):

a = factor(as.character(c(1, 1, 2, 2, 3, NA)), exclude = NULL)

> a
[1] 1    1    2    2    3    <NA>
Levels: 1 2 3 <NA>

You can add the NA as a level and change the level name to something more explicit than <NA> using fct_explicit_na from package forcats.

library(forcats)

By default you get the new level as (Missing):

fct_explicit_na(a)

[1] 1         1         2         2         3         (Missing)
Levels: 1 2 3 (Missing)

You can set it to something else:

fct_explicit_na(a, "unknown")

[1] 1       1       2       2       3       unknown
Levels: 1 2 3 unknown