Computer Programming Concepts and Visual Basic David I. Schneider

dure by just using the variable in the Sub procedure? How do Sub procedures pass values back
to an event procedure? The answers to these questions provide a deeper understanding of the
workings of Sub procedures and reveal their full capabilities.
■ PASSING VALUES BACK FROM SUB PROCEDURES
Suppose a variable, call it arg, appears as an argument in a Call statement, and its corresponding
parameter in the Sub statement is par. After the Sub procedure is executed, arg will
have whatever value par had in the Sub procedure. Hence, not only is the value of arg passed
to par, but the value of par is passed back to arg.
EXAMPLE 1
The following program illustrates the transfer of the value of a parameter to its calling argument.
Private Sub cmdDisplay_Click()
Dim amt As Single
‘Illustrate effect of value of parameter on value of argument
picResults.Cls
amt = 2
picResults.Print amt;
Call Triple(amt)
picResults.Print amt
End Sub
Private Sub
Triple(num As Single)
‘Triple a number picResults.Print num;
num = 3 * num
picResults.Print num;
End Sub
[Run and then click the command button. The following is displayed in the picture box.]
2 2 6 6
Although this feature may be surprising at first glance, it provides a vehicle for passing
values from a Sub procedure back to the place from which the Sub procedure was called. Different
names may be used for an argument and its corresponding parameter, but only one
memory location is involved. Initially, the cmdDisplay_Click( ) event procedure allocates a
memory location to hold the value of amt (Figure 3-4(a)). When the Sub procedure is called,
the parameter num becomes the Sub procedure’s name for this memory location
(Figure 3-4(b)). When the value of num is tripled, the value in the memory location becomes
6 (Figure 3-4(c)). After the completion of the procedure, the parameter name num is forgotten;
however, its value lives on in amt (Figure 3-4(d)). The variable amt is said to be passed
by reference.
FIGURE 3-4 Passing a Variable by Reference to a Sub Procedure
Passing by reference has a wide variety of uses. In the next example, it is used as a vehicle
to transport a value from a Sub procedure back to an event procedure.
Sub Procedures, Part II 81