Surprising behavior of letters[c(NA, NA)]
On 18/12/2010 9:12 AM, Radford Neal wrote:
Duncan Murdoch writes: The relevant quote is in the Language Definition, talking about indices by type of index: "Logical. The indexing i should generally have the same length as x. If it is shorter, then its elements will be recycled as discussed in Section 3.3 [Elementary arithmetic operations], page 14. If it is longer, then x is conceptually extended with NAs. The selected values of x are those for which i is TRUE." But this certainly does not justify the actual behaviour. It says that, for example, (1:3)[NA] should not be a vector of three NAs, but rather a vector of length zero - since NONE of the indexes are TRUE. The actual behaviour of NA in a logical index makes no sense. It makes sense that NA in an integer index produces an NA in the result, since this NA might correctly express the uncertainty in the value at this position that follows from the uncertainty in the index (and hence produce sensible results in subsequent operations). But NA in a logical index should lead to a result that is of uncertain length. However, R has no mechanism for expressing such uncertainty, so it makes more sense that NA in a logical index should produce an error.
I agree that the behaviour is not particularly obvious, but I'm not so sure it should produce an error. We should get an error when the input is likely to be accidental or due to a misconception and the output could be accepted and lead to wrong results later. I think using an NA in a logical index is probably due to a misconception (e.g. thinking it is an NA_integer_), but the results are so weird that they are unlikely to pass unnoticed. And presumably whoever chose this behaviour back in the ancient past thought there was some use in including NA in a logical index, and someone out there in the real world has made use of it. But I wouldn't object if R version 3 gave errors for logical index vectors that were the wrong length or that contained NAs. Duncan Murdoch