EMS Manual

Compiled Filters
EMS Manual—426909-005
5-25
Boolean Expressions
Comparisons With Missing Values
The values of one or both operands of a comparison might be missing due to any of
these causes:

The comparison refers to a token that is not in the current event message.

The comparison refers to an optional parameter that was omitted when the other
parameters, if any, were supplied.

The comparison refers to a field of an extensible structured token and either:

The structured token itself is not in the current event message.

The token is there, but the field is not defined in this version of the structured
token.
If the value of either operand of a comparison is missing, the value of the comparison
is FALSE. That is, the comparison itself is FALSE but not necessarily the value of the
expression in which the comparison occurs.
These expressions are all FALSE if the tokens TKN^M and TKN^N are not in the event
message:
TKN^M = 5
TKN^M <> 5
TKN^M = TKN^N
TKN^M <> TKN^N
With the same assumptions, this expression is TRUE:
NOT ( TKN^M = 5 )
Boolean Expressions
Table 5-3 shows the elements that combine to form Boolean expressions. Some
expression elements are defined in the table. Other elements are only mentioned there
but are described more fully in a later subsection.
The letters A and B in the table represent Boolean expressions, which are legal
combinations of elements in the table.
Table 5-3. Elements of Boolean Expressions (page1of2)
Element Description
TRUE TRUE, a Boolean expression, is the Boolean value of a successful
comparison.
FALSE FALSE, a Boolean expression, is the Boolean value of an
unsuccessful comparison.
Boolean-
variable
Boolean-variable, a Boolean expression, has the value given it
by an assignment statement or the value FALSE if not set.