Mathematical Expressions
Expressions share a common syntax each place they appear in PSPP commands. Expressions are made up of "operands", which can be numbers, strings, variable names, or invocations of functions, separated by "operators".
Boolean Values
Some PSPP operators and expressions work with Boolean values, which represent true/false conditions. Booleans have only three possible values: 0 (false), 1 (true), and system-missing (unknown). System-missing is neither true nor false and indicates that the true value is unknown.
Boolean-typed operands or function arguments must take on one of these three values. Other values are considered false, but provoke a warning when the expression is evaluated.
Strings and Booleans are not compatible, and neither may be used in place of the other.
Missing Values
Most numeric operators yield system-missing when given any system-missing operand. A string operator given any system-missing operand typically results in the empty string. Exceptions are listed under particular operator descriptions.
String user-missing values are not treated specially in expressions.
User-missing values for numeric variables are always transformed into
the system-missing value, except inside the arguments to the VALUE and
SYSMIS functions.
The missing-value functions can be used to precisely control how missing values are treated in expressions.
Order of Operations
The following table describes operator precedence. Smaller-numbered levels in the table have higher precedence. Within a level, operations are always performed from left to right.
()**- Unary
+and- * /- Binary
+and- = >= > <= < <>NOTANDOR