Question:

I am just starting out on learning R and came across a piece of code as follows

vec_1 <- c("a","b", NA, "c","d") # create a subet of all elements which equal "a" vec_1[vec_1 == "a"]

The result from this is

## [1] "a" NA

Im just curious, since I am subsetting vec_1 for the value "a", why does NA also show up in my results?

This is because the result of anything == NA is NA. Even NA == NA is NA.

Here's the output of vec_1 == "a" -

[1] TRUE FALSE NA FALSE FALSE

and NA is not TRUE or FALSE so when you subset anything by NA you get NA. Check this out -

vec_1[NA] [1] NA NA NA NA NA

When dealing with NA, R tries to provide the most informative answer i.e. T | NA returns TRUE because it doesn't matter what NA is. Here are some more examples -

T | NA [1] TRUE F | NA [1] NA T & NA [1] NA F & NA [1] FALSE

R has no way to test equality with NA. In your case you can use %in% operator -

5 %in% NA [1] FALSE "a" %in% NA [1] FALSE vec_1[vec_1 %in% "a"] [1] "a"