The most often useful functions for missing values are those for testing whether a given value is missing, described here. However, using one of the corresponding missing value testing functions for variables can be even easier (see Variable Missing Values).
Tests whether value is in one of the categories of missing values given by class. Returns true if so, false otherwise.
mv determines the width of value and provides the set of user-missing values to test.
The only difference among these functions in the form in which value is provided, so you may use whichever function is most convenient.
The class argument determines the exact kinds of missing values that the functions test for:
Returns true if value is in the set of user-missing values given by mv.
Returns true if value is system-missing. (If mv represents a set of string values, then value is never system-missing.)
Returns true if value is user-missing or system-missing.
Always returns false, that is, value is never considered missing.