What is the difference between and NA?

Question

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 somethi

Answer 1

When you are dealing with factors, when the NA is wrapped in angled brackets ( ), that indicates thtat 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   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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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 
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     c    NA  

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