Wyo VB Lecture Notes

Objective #1: Use an array to store data.

• An array (also called a vector or a list) is a set of related variables. In fact, all of the variables in the array are referenced by the same name (the name of the array itself). For example, if you wanted to store John Doe's five exam grades, it would be more efficient to use an array instead of 5 separate variables.

  Instead of declaring 5 variables with the statement

      Dim intExam1, intExam2, intExam3, intExam4, intExam5 As Integer
  
  You should declare an array named intExam with the statement
  
      Dim intExam(5) As Integer

  This statement creates one array that has six positions. Each position of the array can store a separate exam value. The size or length of the array intExam is said to be 6 simply because that is how many elements it can hold. This array has 6 elements not 5 because Visual Basic always includes an element with position (subscript) zero. You can declare arrays to be as big as you would like in Visual Basic.

• An array can initialized within its declaration statement by include the initial values in a set of curly braces. In this case, you are not allowed to declare the size of the array so you must leave the parentheses empty. Here is an example:
  
  Dim intExam() As Integer = {77, 90, 73, 80, 91}
  
  
  
• Arrays are useful because they make it easier to program with loops and other structures. They organize data so that it can be manipulated and processed more efficiently. For example, instead of five separate statements that initialize each of John Doe's exam grades to 100 as in:
  
  intExam1 = 100
  intExam2 = 100
  intExam3 = 100
  intExam4 = 100
  intExam5 = 100
  
  It would be easier to initialize them with a For loop as in:
  
  For J = 1 To 5
          intExam(J) = 100
  Next
  
  
• In addition to storing integers in an array (as above), you can declare an array to store strings or any other variable data type as in
  
  Dim strNames() As String = {"Alfred", "Betty", "Chris", "Donna", "Edgar"}
  
  However, you can not store two different data types in the same array. That is you cannot mix integers and strings.
  
  
• The values in an array are called elements.   The 2 in the statement        intExam(2) = 94         is called a subscript or index position. The variable J in the loop example above is called an index variable, meaning that it is being used to represent the subscript.
  
• While arrays can be declared as local variables within a method, it is usually more efficient to declare an array at the top of the form. When you declare any variable at the top of the form, the variable is called a module-level variable rather than a local variable. It is considered to be good style to use a lower case m as a prefix for a module-level variable. So a declaration statement for a module-level array variable might look like this:
  
  Dim mintExamScores(5) As Integer
  
• The length of an array can be determined by using the Length method. The following example shows how the length of an array can be used as the upper bound of a For loop
  
  For J = 1 To mintExamScores.Length()
      MessageBox.Show(mintExamScores(J))
  Next
  
• There are a few static methods of the Array class that are useful with arrays including
  
  The Clear method allows you to set one or more of the elements of an array of integers to zero. For example,
  
  Array.Clear(mintExamScores, 0, 4)
  
  would set all of the elements from index position 0 to index position 4 equal to zero.
  
  The Reverse method reverses the elements in an array. It is used like this    Array.Reverse(mintExamScores)
  
  The BinarySearch method searches an array for a value and returns the index position where the value is found. For example,
  
  Dim intExam() As Integer = {77, 90, 73, 80, 91}
  MessageBox.Show(Array.BinarySearch(intExam, 90)
  
  causes the value 1 to display in the message box since the value 90 is found in index position 1 of the array.
  
  The Sort method sorts an array. The following statement rearranges the elements in mintExamScores so they are in ascending order:
  
  Array.Sort(mintExamScores)
  
  
• Here is a code segment that computes the running total of the elements in an array
  
  Dim intScores() As Integer = {73, 77, 82, 71, 98}
  Dim intSum As Integer = 0
  Dim J As Integer = 0
  
  For J = 0 To intScores.Length() - 1
     intSum = intSum + intScores(J)
  Next
  
  Notice that you must start the loop variable J at 0 since the first subscript position of the array is position zero and you must stop J at the length of the loop minus 1
  
  
• Parallel arrays are simply two separate arrays that are used together in the logic of an algorithm to store associated data. For example, an array of names and an array of scores where the score in position zero of the scores array belongs to the person whose name is found in position zero of the names array and so on. Two parallel arrays are the same size but they do not have to contain elements of the same data type. The names array would be an array of strings and the scores array would be an array of integers.
  

Objective #2 - Create arrays of objects.

• Previously we learned how to create arrays of Integer's, Double's, and String's. We also previously learned how to make an object (e.g. textbox, label, button, picturebox, etc) using code and without double-clicking the VB Toolbox. Well, we can put all of this together to create an array of one kind of object using code. For example, you can create 10 or more "alien" pictureboxes and place them on the screen avoiding the need to create them individually using the mouse. Why would you want to do this? For one, you have more flexibility and control when creating objects with code. Also, it would be very tedious and time consuming to create and edit 30 "alien" picture boxes when you could use a loop in your computer program do the same thing.
  
  
• To declare an array of picture boxes you would use the statement
  
  Dim picFrankenstein(5) As New PictureBox
  
  If you want the array to be module-level, you would use the statement
  
  Dim picFrankenstein(5) As PictureBox
  
  without using the keyword New. Then in the Form_Load method perhaps, you would use the code
  
  For J = 0 To 5
     picFrankenstein(J) = New PictureBox
     picFrankenstein(J).Image = Image.FromFile(System.AppDomain.CurrentDomain.BaseDirectory() +    "..\..\images\frankenstein.jpg")
     picFrankenstein(J).SizeMode = PictureBoxSizeMode.AutoSize
     picFrankenstein(J).Left = Rnd() * 300
     picFrankenstein(J).Top = Rnd() * 300
     Me.Controls.Add(picFrankenstein(J))
  Next
  
  The Me.Controls.Add method must be used to actually place each new picture box onto the current form.

Objective #3: Use an ArrayList to store data.

• An ArrayList is similar to an array. It can also store multiple items and is primarily used when you need to store objects rather than numbers. To declare an ArrayList, you use
  
  Dim list As New ArrayList
  
  In a Form1_KeyDown method, you could use something like
  
  If e.KeyCode = Keys.Space Then
     Dim next As New Label
     next.Top = 50
  next.Height = 50
  Me.Controls.Add(next)
  list.Add(next)
  
  You can also use a For Each loop such as
  
  For Each temp in Me.list
     Label.Top += 5
  Next
  
  and
  
  For Each control in Me.Controls
     If (TypeOF(control) Is Label) Then
        control.Top += 5
     End If
  Next