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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