

   NNoott AAvvaaiillaabbllee VVaalluuee

        NA
        is.na(x)
        as.na(x)

   DDeessccrriippttiioonn::

        `NA' is a logical constant of length 1 which contains a
        missing value indicator.  `NA' can be freely coerced to
        any other vector type.

        `as.na' ignores its argument and returns the value
        `NA'.

        `is.na' takes a vector argument and returns a logical
        vector of the same length containing `TRUE' for those
        elements marked `NA' and `FALSE' otherwise.  `dim',
        `dimnames' and `names' attributes are preserved.

   SSeeee AAllssoo::

        `complete.cases'.

