Wyo VB Lecture Notes

VB Lecture Notes - Methods

Objective #1: Use a call statement to execute a method from another method.

So far in our study of VB, methods only execute in response to the user performing an event such as clicking the mouse, which executes an object's Click method, or starting the project, which executes the startup form's Load method.

However, it is possible to execute a method with a statement in another method.

For example, if you have a button that calculates the cost of purchasing a certain quantity of donuts such as

    ' computes the cost of purchasing donuts
    Private Sub btnCompute_Click(sender As System.Object, e As System.EventArgs) Handles btnCompute.Click

        ' *********************** declaration statements *******
        Dim numDonuts As Integer = 0                ' # of donuts purchased
        Dim totalCost As Double = 0                 ' total cost of purchases
        Const PRICE_PER_DONUT As Double = 0.75      ' price per donut
        Const TAX_RATE As Double = 1.06             ' PA state sales tax

        ' ********************* obtain user input **************
        numDonuts = Val(txtDonuts.Text)

        ' ********************** calculations ******************
        totalCost = numDonuts * PRICE_PER_DONUT * TAX_RATE

        ' ********************** display the output ************
        lblTotalCost.Text = "$" + Str(totalCost)

        Call btnClear_Click(sender, e)

    End Sub

and a Clear button that clears a textbox such as

Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click
   txtDonuts.Clear()
End Sub

you can use the statement Call btnClear_Click(sender, e) to execute the btnClear_Click method
so that it automatically clears out the textbox when the user clicks the Compute button.

The statement Call btnClear_Click(sender, e) is a call statement. Iit causes VB to call (i.e. execute) the btnClear_Click method
without the user having to click the Clear button!

Having the ability to call methods from other methods in this way makes code more reusable. In this example, the variables sender and e are called parameters. They must be typed in the parentheses since they must be "passed" from the btnCompute_Click method to the btnClear_Click method. We will learn more about passing parameters later in this course. The word Call is optional
in a call statement, so the call statement above could have been typed as

btnClear_Click(sender, e)

Objective #2: Create your own method that is not tied to an event.

You can create a method from scratch so that it can be called over and over again from other methods. This increases the reusability and efficiency of a class and its methods.
To create a method you make up a name for the method and type statements into the body of the method. Here is an example:

' moves player right by 20 pixels
Private Sub MoveRight()
picPlayer.Left = picPlayer.Left + 20
End Sub

The first line of code at the top of a method is called the method header. The method header contains the name of the method as well as a set of parentheses. MoveRight is the name of the method in the example above. The name of the method can be anything the programmer desires. However, you should use logical names for methods that indicate their purpose. You should also capitalizing each word in the name of the method. No prefix is required though for a method's name. The parentheses are empty since it requires no parameters to be passed to it. There are two indented statements in the body of the method above.

At this point in our VB course, methods should be declared as Private rather than Public so the first word in the method header should be Private. The first body statement causes the picture box picPlayer to move right each time the method is called.
To call the MoveRight method illustrated above from within another method such as a button's click method you can use either of the following call statements:

Call MoveRight()

or

MoveRight()
Another example of a method made from scratch is:

Private Sub ChangeColor()
Me.BackColor = Color.Red
End Sub

This method would change the background color of the form to the color red.
You can pass a parameter (also known as an argument) to a method to make it even more flexible. A parameter is like a variable. It has an associated data type (usually Integer or Double) and the word ByVal must be typed in front of it. Parameters appear in the parentheses that follow the name of a method. In the following example, dblPrice is a parameter.

Private Sub DisplayPriceWithTax(ByVal price As Double)
   Dim totalPrice As Double = 0.0
   totalPrice = price + price * 0.06
   MessageBox.Show(totalPrice)
End Sub

The following statement could be used to call the method DisplayPriceWithTax:

   Call DisplayPriceWithTax(10.00)

A message box with the number 11.20 will be displayed since 6% tax added to an item that costs $10 is $11.20

Here is a demo program that uses a few methods written from scratch to animate a player across the screen

Public Class Form1



    Dim playerX As Integer = 10   ' horizontal position of player
    Dim playerY As Integer = 10   ' vertical position of player



    ' draws graphics on screen
    Private Sub Form1_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint
        DrawPlayer(sender, e)
        DrawEnemy(sender, e)
    End Sub



    ' draws player
    Private Sub DrawPlayer(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs)
        e.Graphics.DrawLine(Pens.Black, playerX, playerY, playerX, playerY + 100)
    End Sub



    ' draws enemy
    Private Sub DrawEnemy(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs)
        e.Graphics.DrawLine(Pens.Red, 200, 50, 200, 150)
    End Sub



    ' moves players
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        MoveDiagonal()
        Me.Refresh()
    End Sub



    ' moves player right 10 pixels
    Private Sub MoveRight()
        playerX += 10
    End Sub



    ' moves player down 10 pixels
    Private Sub MoveDown()
        playerY += 10
    End Sub



    ' moves player diagonally down and to the right
    Private Sub MoveDiagonal()
        MoveRight()
        MoveDown()
    End Sub


End Class

In the demo above, the methods MoveRight, MoveDown, MoveDiagonal, DrawPlayer, & DrawEnemy were all created by the programmer. It does not matter in what order these methods are typed into the code window. VB finds the methods when necessary.

The statement MoveDiagonal() is a call statement that causes the MoveDiagonal method to execute when Button1 is clicked. The statement MoveRight() is a call statement that makes the MoveRight method execute from within the MoveDiagonal method. The statement MoveDown() is a similar call statement.

The DrawPlayer and DrawEnemy methods are called to draw the respective characters on the screen. In order for draw commands to work correctly, you must pass two special parameters to those methods. At this point in our VB course, you should just understand that sender and e must be passed as parameters in this situation. You do not have to memorize the code that is inside the parentheses.

The Me.Refresh() statement is necessary in order for VB to call the Form1_Paint method that draws the graphics.

Objective #3: Explain the benefits of using methods.

Programmers should write methods so the methods can be reused in many programs and projects without having to change or modify them. For example, if you create a method with code that computes the Pennsylvania sales tax on a user-inputted value for one program, you can use it for any other business program that your write in the future. The reusability of methods is an important reason and justification for writing methods.
As you gain experience, you will learn what kinds of tasks can be solved with methods that may apply to future programming assignments. You also will learn how to separate the code that does not change from the code that will have to change for future assignments. In that way, during pseudocoding, you will be able to organize the algorithm and your efforts so that you can eventually reuse certain modules of code.
Reusing code also saves time during program testing. If you already thoroughly tested a method, you will not need to test that part of a program in the future.
Also, reusing methods makes programs more consistent with one another and more user-friendly.
Designing a large project so that it uses multiple methods allows you to divide up the programming tasks and logic in a way that multiple programmers can work together. If one programmer can focus on a method named DrawMan and another programmer can focus on the method named DrawBattleship, the whole program can be assembled more quickly and easier in the end when they are put together.
There are many methods and other code segments at vbcode.com and other Web sites that you can purchase or use for free. This saves a lot of time, effort, and sometimes even money.
In summary, here is a list of reasons for using methods:
1. organization of code
2. reusability of code
3. consistency
4. easier to maintain and upgrade a program in the future
5. easier to debug problems
6. allows for programmers to specialize and become good at writing code for specific types of methods
7. saves time not having to test methods a second or third time in many situations
8. saves money by not having to write code that can be obtained for free or inexpensively

Objective #4: Organize the code in your code window.

The order of the multiple methods in a form's code window technically does not matter as far as Visual Basic and the execution of your program is concerned. For example you could have an Exit button's Click method first and the Form1_Load method typed last even though that order would not be logical or as helpful to someone else reading your code.
You can fold a section of your code window that includes multiple methods into a region. This makes it easier to manage and read through long pages of code. For example,

Public Class Form1

#Region "Opening the program"
   Public Sub Form1_Load()
      MessageBox.Show("Welcome")
   End Sub
#End Region

   Public Sub btnExit_Click()
      Application.Exit()
   End Sub

End Class

allows you to hide the Form1_Load method.