Missing Values

With SAS/IML software, a numeric element can have a special value called a missing value, which indicates that the value is unknown or unspecified. Such missing values are coded, for logical comparison purposes, in the bit pattern of very large negative numbers. A numeric matrix can have any mixture of missing and nonmissing values. A matrix with missing values should not be confused with an empty or unvalued matrix—that is, a matrix with zero rows and zero columns.

In matrix literals, a numeric missing value is specified as a single period (.). In data processing operations that involve a SAS data set, you can append or delete missing values. All operations that move values also move missing values.

However, for efficiency reasons, SAS/IML software does not support missing values in most matrix operations and functions. For example, matrix multiplication of a matrix with missing values is not supported. Furthermore, many linear algebraic operations are not mathematically defined for a matrix with missing values. For example, the inverse of a matrix with missing values is meaningless.

See Chapter 5, Working with Matrices, and Chapter 22, Further Notes, for more details about missing values.