Convert NA into a factor level

问题

I have a vector with NA values that I would like to replace by a new factor level NA.

a = as.factor(as.character(c(1, 1, 2, 2, 3, NA)))
a
[1] 1    1    2    2    3    <NA>
Levels: 1 2 3

This works, but it seems like a strange way to do it.

a = as.factor(ifelse(is.na(a), "NA", a))
class(a)
[1] "factor"

This is the expected output:

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

回答1:

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.

回答2:

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>

回答3:

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

来源：https://stackoverflow.com/questions/27195956/convert-na-into-a-factor-level

标签

missing-data