NA is.na(x) is.na.data.frame(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.
The generic function is.na returns a logical vector
of the same ``form'' as its argument x, containing TRUE
for those elements marked
NA or NaN (!) and FALSE otherwise.
dim, dimnames and names attributes are preserved.
The method dispatching is C-internal, rather than via UseMethod.
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