JPM Miao: Why can't R understand if(num!=NA)? Peter Dalgaard: Because comparison with an unknown value yields an unknown result. David Winsemius: Anything else would violate the Second Law of Thermodynamics. We cannot have comparisons reducing entropy, now can we? Uncertainty cannot run uphill. JPM Miao, Peter Dalgaard, and David Winsemius on why is.na() is needed R-help May 2013