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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 involving a SAS data set, you can append or delete missing values. All operations that move values also move missing values properly.

SAS/IML software supports missing values in a limited way, however. Most matrix operators and functions do not support missing values. For example, matrix multiplication involving a matrix with missing values is not meaningful. Also, the inverse of a matrix with missing values has no meaning. Performing matrix operations such as these on matrices that have missing values can result in inconsistencies, depending on the host environment.

See Chapter 4 and Chapter 19 for more details about missing values.