Computer Programming Concepts and Visual Basic David I. Schneider

Call CalculateSum(x, y, s)
Call DisplayResult(x, y, s)
End Sub
Private Sub CalculateSum(num1 As Single, num2 As Single, sum As Single)
‘Add the values of num1 and num2
‘and assign the value to sum
sum = num1 + num2
End Sub
Private Sub DisplayResult(num1 As Single, num2 As Single, sum As Single)
‘Display a sentence giving the two numbers and their sum
picResult.Cls
picResult.Print “The sum of”; num1; “and”; num2; “is”; sum
End Sub
Private Sub GetNumbers(num1 As Single, num2 As Single)
‘Record the two numbers in the text boxes
num1 = Val(txtFirstNum.Text)
num2 = Val(txtSecondNum.Text)
End Sub
■ PASSING BY VALUE
Sometimes you want to pass a variable to a Sub procedure, but you want to ensure that the variable
will retain its original value after the Sub procedure terminates—regardless of what was
done to the corresponding parameter inside the Sub procedure. Such a variable is said to be
passed by value. There are two ways to pass a variable by value.
1. In the Call statement, enclose the variable in an extra pair of parentheses.
2. In the Private Sub statement, precede the corresponding parameter with the
word ByVal.
For instance, in Example 1, if you change the Call statement to
Call Triple((amt))
then the output will be
2 2 6 2
The same output results if you change the Private Sub statement to
Private Sub Triple(ByVal num As Single)
When a variable is passed by value, two memory locations are involved. At the time the
Sub procedure is called, a temporary second memory location for the parameter is set aside
for the Sub procedure’s use and the value of the argument is copied into that location. After
the completion of the Sub procedure, the temporary memory location is released and the
value in it is lost.
■ LOCAL VARIABLES
Sub Procedures, Part II 83
When the same variable name appears in two different Sub procedures or in a Sub procedure
and an event procedure, Visual Basic gives the variables separate identities and treats them as
two different variables. A value assigned to a variable in one part of the program will not affect
the value of the like-named variable in the other part of the program, unless, of course,
the values are passed by a Call statement. Also, each time a Sub procedure is called, all declared
variables that are not parameters assume their default values. (Numeric variables have