Undefined arguments

When you invoke an SPL routine, you can specify all, some, or none of the defined arguments. If you do not specify an argument, and if its corresponding parameter does not have a default value, the argument, which is used as a variable within the SPL routine, is given a status of undefined.

Undefined is a special status used for SPL variables that have no value. The SPL routine executes without error, as long as you do not attempt to use the variable that has the status undefined in the body of the routine.

The undefined status is not the same as a NULL value. (The NULL value means that the value is not known, or does not exist, or is not applicable.)