将NA转换为因子水平 [英] Convert NA into a factor level

问题描述

我有一个具有NA值的向量，我想用一个新的因子水平NA代替.

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

这有效，但似乎是一种奇怪的方法.

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"

这是预期的输出:

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

推荐答案

您可以使用addNA().

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

这基本上是使用exclude = NULL分解的便捷功能.来自help(factor)-

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

addNA通过将NA转换为额外级别来修改因子(例如，NA值在表中进行计数).

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

所以这很好的另一个原因是，如果您已经具有因子f，则可以使用addNA()快速将NA作为因子级别添加而无需更改f.如文档中所述，这对于表很方便.它也很好看.

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.

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