Can someone please answer in layman terms how indexing (subsetting) with NA works. Even though there are some answers from google, I would like to understand it better in simple terms.
When indexing a vector (of length > 1) using a single NA, why does it yield five missing values?
> x <- 1:5
> x[NA]
[1] NA NA NA NA NA
From help("["):
When extracting, a numerical, logical or character NA index picks an unknown element and so returns NA in the corresponding element of a logical, integer, numeric, complex or character result, and NULL for a list.
What does "corresponding element" mean? This can be understood if you know about recycling of vector elements. x[NA] (this is a logical NA per default) in your example is actually "interpreted" as x[c(NA, NA, NA, NA, NA)] since logical indices are recycled. So, each element of x has a corresponding NA during subsetting and thus (per the quote above) NA is returned for each element of x. In layman's language: For each element of x we don't know if we want it. Thus an unknown value is returned for each element.
As @r2evans points out: x[NA_integer_] returns only one NA because integer indices are not recycled. In layman's language: We want one value but don't know which one. Thus, one unknown value is returned.