Definition of True and False

Application Programming: Program Control

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

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

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 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.

IDL Online Help (March 06, 2007)