What is the difference between <NA> and NA?

missing-data, na, r

Solution

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

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)

Note the two different styles of NA:

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

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

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

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  

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

Problem

Original source