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ismissing(x) where x is REAL, LOGICAL or CHARACTER or a structure all of
  whose components are REAL, LOGICAL, or CHARACTER

Keywords: macros, general, missing values, variables, null variables
ismissing(x) returns a LOGICAL variable (with the same shape as x) which
is True where x is MISSING and False where x is not MISSING.  x must be
a vector, matrix, or array.  If x is a CHARACTER variable, an empty
string ("") is considered to be a missing value.

If x is a NULL variable, ismissing(x) is NULL.  See topic 'NULL'.

It is also acceptable for x to be a structure, whose non-structure
components are vectors, matrices or arrays.  In that case, ismissing()
returns a structure of the same form, each of whose non-structure
components is the result of applying ismissing() to the corresponding
component of x.

ismissing(x) has the same labels as x, if any.

   ismissing(vector(1, 3, ?, 7)) and ismissing(vector("A","B","","Z"))
     both return vector(F, F, T, F)
   x[vector(ismissing(x))] <- -1 replaces all MISSING values in x by -1.
   sum(vector(ismissing(x))) returns the number of MISSING values in x,
     whether x is a vector, matrix, array, or structure.

See also anymissing(), sum().

Gary Oehlert 2003-01-15