< NA>有什么区别?和NA? [英] What is the difference between <NA> and NA?

问题描述

我有一个名为SMOKE的因子，其级别为"Y"和"N".缺失值被替换为NA(从初始级别"NULL"开始).但是，当我查看该因素时，会得到如下信息:

I have a factor named SMOKE with levels "Y" and "N". Missing values were replaced with NA (from the initial level "NULL"). However when I view the factor I get something like this:

head(SMOKE)
N N <NA> Y Y N
Levels: Y N

为什么R将NA显示为<NA>?有区别吗?

Why is R displaying NA as <NA>? And is there a difference?

推荐答案

在处理factors时，当NA用尖括号(<NA>)包裹时，表明它实际上是NA .

When you are dealing with factors, when the NA is wrapped in angled brackets ( <NA> ), that indicates thtat it is in fact NA.

当它是NA不带括号时，则它不是 NA NA，而是标签为"NA"

When it is NA without brackets, then it is not NA, but rather a proper factor whose label is "NA"

# Note a 'real' NA and a string with the word "NA"
x <- factor(c("hello", NA, "world", "NA"))

x
[1] hello <NA>  world NA   
Levels: hello NA world      <~~ The string appears as a level, the actual NA does not. 

as.numeric(x)              
[1]  1 NA  3  2            <~~ The string has a numeric value (here, 2, alphabetically)
                               The NA's numeric value is just NA

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编辑以回答@Arun的问题:

R只是试图区分其值为两个字母"NA"的字符串和实际的缺失值NA 因此，显示df与df$y时看到的差异.示例:

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Edit to answer @Arun's question:

R is simply trying to distinguish between a string whose value are the two letters "NA" and an actual missing value, NA Thus the difference you see when displaying df versus df$y. Example:

df <- data.frame(x=1:4, y=c("a", NA_character_, "c", "NA"), stringsAsFactors=FALSE)

请注意NA的两种不同样式:

Note the two different styles of NA:

> df
  x    y
1 1    a
2 2 <NA>
3 3    c
4 4   NA

但是，如果我们只看'df $ y'

However, if we look at just 'df$y'

[1] "a"  NA   "c"  "NA"

但是，如果我们删除引号(类似于在控制台上打印data.frame时看到的内容):

But, if we remove the quotation marks (similar to what we see when printing a data.frame to the console):

print(df$y, quote=FALSE)
[1] a    <NA> c    NA  

因此，我们通过尖括号再次区分了NA.

And thus, we once again have the distinction of NA via the angled brackets.

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