

   NNoott AAvvaaiillaabbllee // ````MMiissssiinngg'''' VVaalluueess

        NA
        is.na(x)
        is.na.data.frame(x)

   DDeessccrriippttiioonn::

        `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.

   DDeettaaiillss::

        `is.na(x)' works elementwise when `x' is a `list'.

        The method dispatching is C-internal, rather than via
        `UseMethod'.

   SSeeee AAllssoo::

        `NaN', `is.nan', etc.  and the utility function `com-
        plete.cases'.

        `na.action', `na.omit', `na.fail' on how methods can be
        tuned to deal with missing values.

   EExxaammpplleess::

        is.na(c(1,NA))       #> F TRUE
        is.na(paste(c(1,NA)))#> F FALSE

        is.na(c(1,NA))       #> F TRUE
        is.na(paste(c(1,NA)))#> F FALSE

