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Programming with Microsoft
Visual Basic 2012
Chapter 7: Sub and Function Procedures
Previewing the Cerruti Company
Application
Figure 7-1 Interface showing the payroll calculations
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Previewing the Cerruti Company
Application
(cont.)
Figure 7-2 Message box containing a confirmation message
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Lesson A Objectives
After studying Lesson A, you should be able to:
• Create and call an independent Sub procedure
• Explain the difference between a Sub procedure and a
Function procedure
• Create a procedure that receives information passed to
it
• Explain the difference between passing data by value
and passing data by reference
• Create a Function procedure
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Sub Procedures
• Procedure
– A block of program code that performs a specific task
• Two types of Sub procedures in Visual Basic:
– Event procedure
• A procedure associated with a specific object and event
– Independent Sub procedure
• Independent of any object and event
• Processed only when called (invoked) by a Call statement
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Sub Procedures
(cont.)
Figure 7-3 Syntax and
examples of an independent
Sub procedure and the Call
statement
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Passing Variables
• Passing by value
– Passes a copy of the
variable’s value
– Example: Disclosing your
bank account balance
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• Passing by reference
– Passes a variable’s
address
– Example: Disclosing your
bank account number
Figure 7-4 Illustrations of passing by value and passing
by reference
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Passing Variables
(cont.)
Passing Variables by Value
• Provides only the contents of the variable to the
receiving procedure
• How to pass by value:
– Include the keyword ByVal before the parameter
• Reasons to pass by value:
– The procedure needs to know the contents of the
variable
– The procedure does not need to change the original
value
• By default, Visual Basic passes by value
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Passing Variables
(cont.)
Passing Variables by Value (cont.)
Figure 7-5 Favorite Title
application’s interface
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Figure 7-6 Partially-coded
btnDisplay_Click event procedure
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Passing Variables
(cont.)
Passing Variables by Value (cont.)
Figure 7-7 DisplayMsg procedure
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Figure 7-8 Message shown in the interface
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Passing Variables
(cont.)
Passing Variables by Value (cont.)
Figure 7-9 DisplayMsg procedure header and Call statement
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Passing Variables
(cont.)
Passing Variables by Reference
• Provides the address (memory location) of the
variable to the procedure
– The receiving procedure can thus access the variable
• Reason to pass by reference:
– The procedure needs to change the variable’s contents
• How to pass by reference:
– Include the keyword ByRef before the parameter
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Passing Variables
(cont.)
Passing Variables by Reference (cont.)
Figure 7-11 btnCalc_Click procedure from Chapter 6
Figure 7-10 Gross Pay application’s interface
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Passing Variables
(cont.)
Passing Variables by Reference (cont.)
Figure 7-12 Call statement entered in the procedure
Figure 7-13 CalcGross procedure
Figure 7-14 Gross pay shown in the interface
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Passing Variables
(cont.)
Passing Variables by Reference (cont.)
Figure 7-15
CalcGross and
btnCalc_Click
procedures
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Passing Variables
(cont.)
Passing Variables by Reference (cont.)
Figure 7-16 Desk-check
table before the Call
statement is processed
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Figure 7-17 Desk-check table after the Call statement and
CalcGross procedure header are processed
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Passing Variables
(cont.)
Passing Variables by Reference (cont.)
Figure 7-18 Desk-check table after the statement in the CalcGross procedure is processed
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Function Procedures
• Function procedure
–
–
–
–
A block of code that performs a specific task
Returns a value after completing its task
Visual Basic provides built-in functions
You can also create your own functions
• As dataType in header indicates the return type of data
– The Return statement is typically the last statement in a
function
• The Return expression type must agree with the As dataType
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Function Procedures
(cont.)
Figure 7-22 Syntax and examples of functions
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Function Procedures
(cont.)
Figure 7-23 Examples of invoking the GetNewPrice function
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Function Procedures
(cont.)
Figure 7-24 CalcGross function and btnCalc_Click procedure
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Lesson A Summary
• There are two types of Sub procedures:
– Event
– Independent
• A function performs a task and returns a value
• Independent procedures and functions are called from
an application’s code using the Call statement
• Passing by value sends a copy of the variable’s contents
to a procedure or function
• Passing by reference sends a variable’s address to a
procedure or function
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Lesson B Objectives
After studying Lesson B, you should be able to:
• Include a combo box in an interface
• Add items to a combo box
• Select a combo box item from code
• Determine the number of items in the list portion of a
combo box
• Sort the items in the list portion of a combo box
• Determine the item either selected or entered in a
combo box
• Code a combo box’s TextChanged event procedure
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Including a Combo Box in an
Interface
• Combo box
– Allows the user to select from a number of choices
– Allows the user to type an entry not on the list
– Can save space on a form
• A list box only allows the user to select from a number
of choices
• DropDownStyle property
– Three values:
• Simple
• DropDown (default)
• DropDownList
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Including a Combo Box in an
Interface
(cont.)
Figure 7-27 Examples of the combo box styles
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Including a Combo Box in an
Interface
(cont.)
Figure 7-28 Code associated with the combo boxes in Figure 7-27
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Including a Combo Box in an
Interface
(cont.)
Figure 7-29 Correct TabIndex values
Figure 7-30 Gross pay amount shown in the interface
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Including a Combo Box in an
Interface
(cont.)
Figure 7-31 Modified code for the Gross Pay application
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Lesson B Summary
• To add a combo box to a form:
– Use the ComboBox tool in the toolbox
• To specify the style of a combo box:
– Set the combo box’s DropDownStyle property
• To add items to a combo box:
– Use the Items collection’s Add method
– The method’s syntax is object.Items.Add(item)
– In the syntax, object is the name of the combo box, and
item is the text you want added to the list portion of the
control
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Lesson B Summary
(cont.)
• To automatically sort the items in the list portion of a
combo box:
– Set the combo box’s Sorted property to True
• To determine the number of items in the list portion of
a combo box:
– Use the Items collection’s Count property
– Its syntax is object.Items.Count, in which object is the
name of the combo box
• To select a combo box item from code:
– Use any of the following properties: SelectedIndex,
SelectedItem, or Text
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Lesson B Summary
(cont.)
• To determine the item either selected in the list portion
of a combo box or entered in the text portion:
– Use the combo box’s Text property
– However, if the combo box is a DropDownList style, you
also can use the SelectedIndex or SelectedItem property
• To process code when the value in a combo box’s Text
property changes:
– Enter the code in the combo box’s TextChanged event
procedure
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Lesson C Objectives
After studying Lesson C, you should be able to:
• Prevent a form from closing
• Round a number
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Creating the Cerruti Company
Application
Figure 7-33 TOE
chart for the Cerruti
Company
application
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Creating the Cerruti Company
Application
(cont.)
Figure 7-34 User interface for the Cerruti Company application
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Coding the FormClosing Event
Procedure
• FormClosing event
– Occurs when a form is about to be closed because:
• The computer processes the Me.Close() statement
• The user clicks the Close button on the form’s title bar
• Requirements for the FormClosing event procedure:
– Verifying that the user wants to close the application
– Taking appropriate action based on the user’s
response
• To prevent closing, set the Cancel property of the
FormClosing procedure’s e parameter to True
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Coding the FormClosing Event
Procedure
(cont.)
Figure 7-35 Pseudocode for
the FormClosing event
procedure
Figure 7-36 Message box displayed by the code in the FormClosing event procedure
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Coding the btnCalc_Click Procedure
Figure 7-37 Pseudocode for the btnCalc_Click procedure
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Coding the btnCalc_Click Procedure
(cont.)
Figure 7-38 Listing of named constants and variables for the btnCalc_Click procedure
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Coding the btnCalc_Click Procedure
(cont.)
Figure 7-39 Selection
structure entered in the
procedure
Figure 7-40 Second
selection structure
entered in the procedure
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Coding the btnCalc_Click Procedure
(cont.)
Creating the GetFwt Function
• To calculate weekly taxable wages:
– Multiply the number of withholding allowances by $73.08
(2012 withholding allowance value)
– Subtract this result from the weekly gross pay
• To determine federal withholding tax (FWT):
– Evaluate the weekly taxable wages and filing status
– Use data to look up the FWT in special FWT tables
• The GetFwt function emulates the FWT table lookup
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Coding the btnCalc_Click Procedure
(cont.)
Creating the GetFwt Function (cont.)
Figure 7-41 Weekly FWT
tables for the year 2012
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Coding the btnCalc_Click Procedure
(cont.)
Creating the GetFwt Function (cont.)
Figure 7-42 Example of a FWT calculation
Figure 7-43 Another example of a FWT calculation
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Coding the btnCalc_Click Procedure
(cont.)
Creating the GetFwt Function (cont.)
Figure 7-44 Pseudocode for the GetFwt function
Figure 7-45 GetFwt
function header and
footer
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Completing the btnCalc_Click
Procedure
Rounding Numbers
• You must call the GetFwt function from the btnCalc’s
Click event procedure
• Math.Round function
– Rounds a value to a specific number of decimal places
– Syntax: Math.Round (value[, digits])
• value is the numeric expression to work on
• digits is the integer indicating the number of places to the
right of the decimal point
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Completing the btnCalc_Click
Procedure
(cont.)
Rounding Numbers (cont.)
Figure 7-46 Syntax and examples of the Math.Round function
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Completing the btnCalc_Click
Procedure
(cont.)
Rounding Numbers (cont.)
Figure 7-48 Payroll calculations using the first set of test data
Figure 7-47 Data for testing the Cerruti Company’s application (continues)
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Completing the btnCalc_Click
Procedure
(cont.)
Rounding Numbers (cont.)
(continued)
Figure 7-49 Payroll calculations using the second set of test data
Figure 7-47 Data for testing the Cerruti Company’s application
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Completing the btnCalc_Click
Procedure
(cont.)
Rounding Numbers (cont.)
Figure 7-50 Cerruti Company application’s code (continues)
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Completing the btnCalc_Click
Procedure
(cont.)
Rounding Numbers (cont.)
(continued)
Figure 7-50 Cerruti Company application’s code (continues)
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Completing the btnCalc_Click
Procedure
(cont.)
Rounding Numbers (cont.)
(continued)
Figure 7-50 Cerruti Company application’s code (continues)
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Completing the btnCalc_Click
Procedure
(cont.)
(continued)
Rounding Numbers (cont.)
Figure 7-50 Cerruti Company
application’s code
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Lesson C Summary
• To process code when a form is about to be closed:
– Enter the code in the form’s FormClosing event procedure
– The FormClosing event occurs when the user clicks the
Close button on a form’s title bar or when the computer
processes the Me.Close() statement
• To prevent a form from being closed:
– Set the Cancel property of the FormClosing event
procedure’s e parameter to True, like this:
e.Cancel = True
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Lesson C Summary
(cont.)
• To round a number to a specific number of decimal
places:
– Use the Math.Round function
• The function’s syntax is Math.Round(value[, digits])
– value is a numeric expression
– digits (which is optional) is an integer indicating how
many places to the right of the decimal point are
included in the rounding
• If the digits argument is omitted, the Math.Round function
returns an integer
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