William, David, and Peter,
Thank you all so much for your help on this. Though I had read the help files
on 'subset' and '[', I had not been able to discern from that text what the
problem was. I could not have solved it without your help.
The help file on 'subset' mentions "For ordinary vectors, the result is
simply x[subset & !is.na(subset)]." However, since I was dealing with a data
frame at the time, I failed to appreciate the relevance (but I get it now).
I've gone back to read the help on '[' and I still don't see where this
functionality is described. The page has section titled "NAs in indexing",
whose content I interpret as being limited to NAs in the indices (and not in
the returned values). I can't find any text in the documentation describing
the subtle behavior you kindly pointed out.
Thanks again,
Mauricio
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