A predicate expression is an expression that is evaluated as being “true” or “false” as part of a statement that controls program execution. IDL evaluates predicate expressions in the following contexts:

• IF...THEN...ELSE statements
• ? : inline conditional expressions
• WHILE...DO statements
• REPEAT...UNTIL statements

The definition of true and false for the different data types is as follows:

Data Type	True	False
Byte, integer, and long	Odd integers	Zero or even integers
Floating point and complex	Non-zero values	Zero
String	Any string with non-zero length	Empty string ("")
Heap variables (pointers and objects)	Non-null values	Null values
List and Hash	List or Hash has at least one member	List or Hash is empty

If the LOGICAL_PREDICATE compile option is set:

Data Type	True	False
Numerical values	Non-zero values	Zero
String or heap variables	Non-null values	Null values

See COMPILE_OPT for additional details on the LOGICAL_PREDICATE compilation option.

In the following example, the logical statement for the condition is a conjunction of two conditions:

IF (LON GT -40) AND (LON LE -20) THEN ...

If both conditions (LON being larger than –40 and less than or equal to –20) are true, the statement following the THEN is executed.