Understanding the Language

Missing Values

With SAS/IML software, a numeric element can have a special value called a missing value that 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 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, "Working with Matrices," and Chapter 16, "Further Notes," for more details on missing values.

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