programming-for-dummies

602
Creating Subprograms and Functions
Every subprogram must have a unique name, which usually describes the
purpose of that subprogram, such as Calculate_Velocity or Check-
Password. The parameter list declares variables to hold any data the subprogram
may need from another part of the program. For example, a simple
parameter list might look like this:
SUB Name (BYVAL Age AS Integer)
Command
END SUB
The BYVAL keyword stands for By Value and means that the subprogram
receives a copy of data sent to it from another part of the program. If the
subprogram changes the value of that data, the new value of that data only
appears in the subprogram and not in any other part of the program. The
BYVAL keyword is optional in REALbasic.
Instead of the BYVAL keyword, you could also use the BYREF keyword,
which stands for By Reference, such as
SUB Name (BYREF Age AS Integer)
Command
END SUB
When accepting data By Reference, a subprogram can change the value of
that data, which can affect the rest of the program.
A function is a special version of a subprogram that always returns a single
value. To create a function, use the FUNCTION keyword, such as
FUNCTION Name (Parameter list) AS Datatype
Command
RETURN value
END SUB
The two main differences between a function and a subprogram (procedure)
are that a function needs to include the RETURN keyword and needs to be
defined as a specific data type.
The RETURN keyword defines a variable that contains a specific value that
the function calculates. This value gets returned back to another part of the
program. Because a function represents a single value, you must define the
data type of this value, such as an Integer or a String.