Question

How do I use grep() to get NAs from a vector?

i.e: when I try grep(NA, c(1,NA))

I get [1] NA NA

Answer 1

You want is.na():

> vec <- c(1,NA)
> is.na(vec)
[1] FALSE  TRUE

If you want the NA, try

> which(is.na(vec))
[1] 2
> vec[which(is.na(vec))]
[1] NA
> vec[is.na(vec)] # simpler, logical subscripting
[1] NA

If you don’t, negate the output from is.na():

> !is.na(vec)
[1]  TRUE FALSE
> which(!is.na(vec))
[1] 1
> vec[which(!is.na(vec))]
[1] 1
> vec[!is.na(vec)] ## simpler, logical subscripting
[1] 1

One reason your code doesn’t work is that you gave NA as the pattern. To R this means that the pattern is not defined, so whether either of the elements of the vector match this pattern is also undefined – hence both are NA in the output.