Definition of True and False

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:

Table 7-3: Default Definitions of True and False
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	Null string (" ")
Heap variables (pointers and object references)	Non-null values	Null values

If the LOGICAL_PREDICATE compile option is set:

Table 7-4: True and False Definitions with LOGICAL_PREDICATE
Data Type	True	False
Numerical values	Non-zero values	Zero
String or heap variables	Non-null values	Null values

See "COMPILE_OPT" (IDL Reference Guide) 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.