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Programming in Visual Basic

Properties and Names:

We had a quick look at properties in Unit One when we set up the Caption property for the labels and command buttons. An important part of the design of a Visual Basic project is to ensure that the forms displayed for the user are well labelled and provide straightforward and validated data entry. For the programmer, the use of meaningful names for controls makes coding easier to understand and debug. Trying to understand and alter something like

If x1 > y1 Then
   Text2 = "Too much"
Else
   z2 = 2
End If

is almost impossible. What are x1, y1 and z2? What are we checking? Where is Text2 situated?

Properties:

For forms, labels and command buttons, the Caption property should always be set. The caption is the text that appears on a label or command button or at the top of a form.

For text boxes, the value that you want to appear as the form is loaded should always be set. In Unit One, we didn't do this and the default values of Text1, Text2, Text3 and Text4 were displayed. This is unsatisfactory for this application - the user would need to delete this text before meaningful values could be entered. Normally, we don't want to anything to be displayedin data entry boxes and would therefore clear the Text property. Occasionally, we would set a default value, for example, setting a VAT box to 17.5%.

Names:

For controlsthat we need to refer to during coding, the name property needs to be set. This enables us to write meaningful programs that are easy to follow and easy to change. There are a number of conventions used by programmers, but control names should always be descriptive and, if naming controls in Visual Basic consists of a standard prefix to describe the type of control, followed by whatever you want to call the object.

For example, in Unit One, we ended up with a form called Form1 (the Order Entry form), a text box called Text1 (the Data Entry box for the Item Name) and a command button called Command1. The following names would meet the standard Visual Basic naming convention:

Control

Control Prefix

Object Name

Complete Name

Form1

frm

Order

frmOrder

Text1

txt

Item

txtItem

Command1

cmd

Calculate

cmdCalculate

N.B. In Visual Basic, object names cannot contain spaces. Normally we use capitalisation for multiple words. For Example, if we wished to name the stock form "Stock Data Entry" we would use the name frmStockDataEntry.

Task 2:

1. Open the project created in Exercise One (File Menu, Open Project, stock.vbp)

2. Click somewhere on the form's background - not on any of the controls.

3. Click on the Caption property and change the caption to 'Order Entry'.

4. Click on the Name property and change the name to frmOrder
N.B. Note the difference between Caption and Name. The caption is a label for the form to help the eventual user understand what this form does. We make this descriptive and spaces are allowed. The Name is the name used by tou, the programmer. It is still descriptive, but just enough to tell us the type of object and an indication of its purpose. No spaces are allowed.

5. Click on the text box Text1. In the properties window, click on the name property and change the name to txtItem. Click on the text property and delete the text 'Text1'. Similarly delete the existing text and change the name properties of Text2, Text3 and Text 4 to txtPrice, txtQuantity and txtTotalPrice respectively.

6. Change the name proerties of the two command buttons to cmdCalculate and cmdExit.

Click on the Save shortcut on the toolbar to save your changes (alternatively choose Save Project from the File menu)

Exercise Two :

1. Open th Project ADDRESS.VBP and make the following changes to its properties:

Default Name/Caption

New Caption

New Name

Notes

Form1

Address Entry

 

 

Text1

 

txtName

Delete existing text

Text2

 

txtAddress1

Delete existing text

Text3

 

txtAddress2

Delete existing text

Text4

 

txtTown

Delete existing text

Text5

 

txtPostCode

Delete existing text

Text6

 

txtPhone

Delete existing text

Exit

 

cmdExit

 

2. Open the Project QUIZ.VBP and make the following changes to its properties:

Default Name/Caption

New Caption

New Name

Notes

Form1

Quiz

frmQuiz

 

Text1

 

txtQuestion

Delete existing text

Text2

 

txtAnswer1

Delete existing text

Text3

 

txtAnswer2

Delete existing text

Text4

 

txtAnswer3

Delete existing text

Check1

 

chkAnswer1

Delete existing caption

Check2

 

chkAnswer2

Delete existing caption

Check3

 

chkAnswer3

Delete existing caption

Exit

 

cmdExit

 

Coding :

You've now finished setting up the properties and names for the controls on our form and we're now able to make it do something!!

We need to write code to instruct the Calculate command button to respond to a mouse click. The program should then look at the values entered into the Price and Quantity text boxes, multiply them together and put the answer in the Total Price text box.

You will write an event procedure called cmd_Calculate_Click. The name of this procedure consists of the object's name (cmdCalculate), the underscore character (_) and the type of event (Click).

Don't worry if this sounds complicated. Visual Basic will automatically create the procedure names for you!!

When is a Text box not a Text box?

All data entry in Visual Basic is treated as text, even if numbers are entered. Unlike older versions of Basic, Visual Basic will sometimes recognise that numbers have been entered and will allow them to be used in calculations.
However, care needs to be taken when making assumptions about numbers entered into text boxes. At a later stage, we will make sure the user isn't allowed to enter letters by mistake in a text box that should contain a number, and that numbers are treated correctly by Visual Basic.

Task Three :

1. Double click on the command button named "cmdCalculate" (caption "Calculate"). The procedure name and End Sub is automatically entered:

Sub cmdCalculate_Click()

End Sub

2. Complete the event procedure by entering the calculation for Total Price:

Sub cmdCalculate_Click()

   txtTotalPrice = txtPrice * txtQuantity

End Sub

This will take the value entered in the Price text box (txtPrice), multiply it (note the *) by the value entered in the Quantity text box (txtQuantity) and place the answer in the Total Price text box (txtTotalPrice).

3. Click on the Start Program shortcut on the toolbar.

4. Type in Headphones for the Item, enter 3.50 for the Price and 2 for the Quantity.

5. Click on the Calculate button and the answer 7 should appear in the Total Price text box. We'll worry about signs and decimal places later.

6. Try entering other values for Price and Quantity and calculating the Total Price.

7. Click on the Stop Program shortcut on the toolbar.

8. We need a better way of stopping the program, one that will be available as part of the form. Double Click on the button called "cmdExit" (Caption "Exit")

9. The programming command to stop the program is End. Enter this into the event procedure for cmdExit_Click:

Sub cmdExit_Click()

   End

End Sub

10. Run the program again using the Start Program shortcut, try various calculations and then click the Exit button to finish.

11. Save your project.

Exercise Three :

1. Change the Caption of the Total Price label to Sub Total. Change the name of txtTotalPrice to txtSubTotal.

2. Create a new label with the caption 'VAT'. Create a new text box withe the name txtVAT and clear its text property.

3. Edit the cmdCaltulate_Click() event procedure to work out the sub total (txtSubTotal=txtPrice*txtQuantity) and an extra line to calculate the VAT (txtVAT=txtSubTotal * 17.5 / 100).

4. Create a new label and text box for 'Price Including VAT' and add an extra line to the cmdCalculate_Click() procedure to calculate a value for this new text box.

Save your work and run your program with various values.

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