Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Seven Sub and Function Procedures
Previewing the Harvey Industries Application • Open the Harvey Industries.exe file • The Harvey Industries application calculates payroll for an employee Programming with Microsoft Visual Basic 2008, Fourth Edition
Previewing the Harvey Industries Application (continued) Figure 7-1: Payroll calculations shown in the interface Programming with Microsoft Visual Basic 2008, Fourth Edition
Lesson A Objectives After studying Lesson A, you should be able to: • 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 Programming with Microsoft Visual Basic 2008, Fourth Edition
Procedures • Procedure: • Block of program code that performs specific task • Two types of procedures in Visual Basic: • Sub procedure: Does not return value • Function procedure: Does return value Programming with Microsoft Visual Basic 2008, Fourth Edition
Sub Procedures • Two types of Sub procedures in Visual Basic: • Event procedures • Independent Sub procedures • Event procedure: Associated with specific object and event • Processed automatically in response to event • Independent Sub procedure: Independent of any object and event • Invoked from code using Call statement • Parameter: Data passed to procedure when invoked Programming with Microsoft Visual Basic 2008, Fourth Edition
Sub Procedures (continued) • Call statement: Used to invoke procedure • Syntax: Call procedurename([argumentlist]) • argumentlist: Used to pass information (optional) • Argument: Data item in argumentlist • Parameter: Data item in parameterlist • Relationship between arguments and parameters • Should agree in number, position, and data type • Types of data that can be passed as arguments to a procedure: • Variable, literal constant, named constant, keyword Programming with Microsoft Visual Basic 2008, Fourth Edition
Sub Procedures (continued) Figure 7-2: Syntax of an independent Sub procedure and the Call statement Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing Variables • A variable has value and unique address in memory • You can pass either variable’s value or its address to procedure • Passing by value: • Passes copy of value stored in variable • Passing by reference: • Passes memory address of variable • Allows procedure to change contents of variable Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Value • Passing by value: Provides only contents of variable to receiving procedure • How to pass by value: • Include keyword ByVal before parameter • Reasons to pass by value: • Procedure needs to know contents of variable • Procedure does not need to change original value • By default, Visual Basic passes by value Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Value (continued) Figure 7-4: Additional lines of code entered in the ShowMsg procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Value (continued) Figure 7-5: ShowMsg procedure and btnDisplay Click event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Reference • Passing by reference: Provides address (memory location) of variable to procedure • Receiving procedure can thus access variable • Reason to pass by reference: • Procedure needs to change variable’s contents • How to pass by reference: • Include keyword ByRef before parameter Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Reference (continued) Figure 7-8: CalcGrossPay procedure and btnCalc contro’s Click event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Reference (continued) Figure 7-9: Desk-check table before the computer processes the Call statement Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Reference (continued) Figure 7-10: Desk-check table after the computer processes the Call statement and the CalcGrossPay procedure header Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Reference (continued) Figure 7-11: Desk-check table after the computer processes the first statement in the CalcGrossPay procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Reference (continued) Figure 7-12: Desk-check table after the computer processes the statement in the selection structure’s true path Programming with Microsoft Visual Basic 2008, Fourth Edition
Passing a Variable by Reference (continued) Figure 7-13: Desk-check table after the CalcGrossPay procedure ends Programming with Microsoft Visual Basic 2008, Fourth Edition
Function Procedures • Function procedure: • Block of code that performs specific task • Returns value after completing its task • Visual Basic provides built-in functions • Can also create your own functions • As datatype in header indicates return type of data • Returnexpression type must agree with As datatype Programming with Microsoft Visual Basic 2008, Fourth Edition
Function Procedures (continued) Figure 7-14: Syntax, example, and steps for creating a function Programming with Microsoft Visual Basic 2008, Fourth Edition
Function Procedures (continued) Figure 7-15: Examples of invoking the GetNewPrice function Programming with Microsoft Visual Basic 2008, Fourth Edition
Function Procedures (continued) Figure 7-16: CalcGrossPay function and btnCalc control’s Click event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Lesson A Summary • Two types of procedures: Event and independent • Function: Performs task and returns value • Independent procedures and functions are called from application’s code using Call statement • Pass by value: Send copy of variable’s contents to procedure or function • Pass by reference: Send variable’s address to procedure or function Programming with Microsoft Visual Basic 2008, Fourth Edition
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 item either selected or entered in a combo box • Code a combo box’s TextChanged event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Including a Combo Box in an Interface • Combo box: • Allows user to select from number of choices • Allows user to type entry not on list • Can save space on form • List box does not share features two and three • DropDownStyle property: • Values: Simple, DropDown (default), DropDownList Programming with Microsoft Visual Basic 2008, Fourth Edition
Including a Combo Box in an Interface (continued) Figure 7-19: Examples of the combo box styles Programming with Microsoft Visual Basic 2008, Fourth Edition
Including a Combo Box in an Interface (continued) • Change in item value causes TextChanged event • Use Item collection’s Add method to add item • Other properties of combo box • Sorted: Sorts items in dictionary order • SelectedIndex: Used to select item in list portion • SelectedItem: Determines which item is selected • Text: Used to get or set value in text portion Programming with Microsoft Visual Basic 2008, Fourth Edition
Including a Combo Box in an Interface (continued) Figure 7-20: Code used to add items to the combo boxes and also select a default item Programming with Microsoft Visual Basic 2008, Fourth Edition
Including a Combo Box in an Interface (continued) Figure 7-22: Interface showing the gross pay Programming with Microsoft Visual Basic 2008, Fourth Edition
Lesson B Summary • Combo box displays list of items for selection • Combo box allows user to type entry not on list • Specify style of combo box using DropDownStyle property • Use Items collection’s Add method to add items to Combo box • Use combo box’s Sorted property to sort items in combo’s list Programming with Microsoft Visual Basic 2008, Fourth Edition
Lesson B Summary (continued) • Use SelectedIndex, SelectedItem, or Text property to select combo box item from code • Use SelectedIndex, SelectedItem, or Text property to determine item that was selected Programming with Microsoft Visual Basic 2008, Fourth Edition
Lesson C Objectives After studying Lesson C, you should be able to: • Prevent a form from closing • Round a number Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the Harvey Industries Application • Objective: • Calculate employee’s weekly gross pay, federal withholding tax (FWT), Social Security and Medicare (FICA) tax, and net pay • Review TOE chart for requirements Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the Harvey Industries Application (continued) Figure 7-25: User interface for the Harvey Industries application Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the FormClosing Event Procedure • FormClosing event: Occurs when form is about to be closed because: • Computer processes Me.Close() statement • User clicks Close button on form’s title bar • Requirement for FormClosing event procedure: • Verifying that user wants to close application • Taking appropriate action based on user’s response • To prevent closing, set Cancel property of FormClosing procedure’s e parameter to true Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the FormClosing Event Procedure (continued) Figure 26: Pseudocode for the FormClosing event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the FormClosing Event Procedure (continued) Figure 27: Message box displayed by the FormClosing event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the FormClosing Event Procedure (continued) Figure 28: Pseudocode for the btnCalc control’s Click event procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the btnCalc Control’s Click Event Procedure Figure 29: Selection structure entered in the procedure Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the GetFwt Function • How to calculate weekly taxable wages: • Multiply number of withholding allowances by $67.31 • Subtract this result from weekly gross pay • Determining federal withholding tax (FWT): • Evaluate weekly taxable wages and filing status • Use data to look up FWT in special FWT tables • GetFwt function emulates FWT table lookup Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the GetFwt Function (continued) Figure 7-31: FWT calculations for a married taxpayer with taxable wages of $288.46 Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the GetFwt Function (continued) Figure 7-32: FWT calculations for a single taxpayer with taxable wages of $600.00 Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the GetFwt Function (continued) Figure 7-33: Pseudocode for the GetFwt function Programming with Microsoft Visual Basic 2008, Fourth Edition
Coding the GetFwt Function (continued) Figure 7-34: GetFWT function header and footer Programming with Microsoft Visual Basic 2008, Fourth Edition
Completing the btnCalc Control’s Click Event Procedure • Must call GetFwt function from btnCalc’s Click event procedure • Math.Round function: Used to round value to specific number of decimal places • Syntax: Math.Round (value[, digits]) • value: Numeric value to work on • digits: Number of places to right of decimal point Programming with Microsoft Visual Basic 2008, Fourth Edition
Completing the btnCalc Control’s Click Event Procedure (continued) Figure 7-35: Payroll calculations displayed in the interface Programming with Microsoft Visual Basic 2008, Fourth Edition
Lesson C Summary • Use form’s FormClosing event procedure to process code when form is about to be closed • Set Cancel property of FormClosing event procedure’s e parameter to true to prevent form from being closed • Use Math.Round function to round number to specific number of decimal places Programming with Microsoft Visual Basic 2008, Fourth Edition
