a<-ad[trim(ad$airpt_code)!=trim(ad$airpt_metro_code),]
Error in Ops.factor(trim(ad$airpt_code), trim(ad$airpt_metro_code)) :
level sets of factors are different
Error in Ops.factor(trim(ad$airpt_code), trim(ad$airpt_metro_code)) :
level sets of factors are different
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