NA is.na(x)
NA is a logical constant of length 1 which contains
a missing value indicator. NA can be freely coerced to any
other vector type.
is.na takes a vector argument and returns a logical vector
of the same length containing TRUE for those elements marked
NA or NaN (!) and FALSE otherwise.
dim, dimnames and names attributes are preserved.
NaN, is.nan, etc.
and the utility function complete.cases.
na.action, na.omit, na.fail
on how methods can be tuned to deal with missing values.
is.na(c(1,NA)) #> F TRUE is.na(paste(c(1,NA)))#> F FALSE