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| The While...End While Statement |
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- With the For...Next type of programming when it begins the loop, the number of iterations is known.
- If not known explicitly then it is known by a variable that resolves to an integer that sets its ending value.
- Inside the loop,the loop iterates that many times unless exited on a condition that arises.
- There may be situations, however, where it cannot be known in advance exactly how many times to execute the statements and to what value the ending value should be set.
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The While...End While Loop
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For these situations,the While...End While loop is designed. Its general format is given below.
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While condition
...statements
End While
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A While...End While loop does not execute a fixed number of times and it executes as long as a condition test at the beginning of the loop remains True. It could be that the condition is False at the start of the loop, in which case it doesn't get executed at all. Of course, it is important that somewhere inside the loop the condition becomes True; otherwise, the loop never ends. Whereas a For...Next loop presents a condition for ending the loop, a While...End While presents a condition for continuing the loop.
Before a more practical application is presented, take a look at how a While...End While loop can be set up to resemble a For...Next loop and as in a previous illustration an array is loaded with values and a loop displays its contents.
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Sub Get_Fruit (Src As Object, Args As EventArgs)
Dim Fruit() As String = {"apples","oranges","lemons","grapes","beer"}
Dim i As Integer = 0
While i <= Fruit.Length - 1
FruitOut.Text &= Fruit(i) & " "
i += 1
End While
End Sub
<asp:Button Text="Get Fruit" OnClick="Get_Fruit" runat="server"/>
<asp:Label id="FruitOut" EnableViewState="False" runat="server"/>
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Loading an Array from a File
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An array is loaded from an external data store -- a database or file table -- containing information that can be added to, changed, or deleted without have to revise the script that contains the array.
In the example below, two arrays (StatesArray() and CodesArray()) are loaded from an external text file (StateCodes.txt). This is a file of state names and state codes for all 50 states. In the following script these arrays are defined globally for access by other subprograms which perform processing against the arrays.
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<SCRIPT runat="server">
Dim StatesArray(0) As String
Dim CodesArray(0) As String
Sub Page_Load
Dim FileReader As StreamReader
Dim LineIn As String
Dim FieldArray() As String
FileReader = File.OpenText("e:\WebSite\tutorials\vbnet\vbnet04\StateCodes.txt")
LineIn = FileReader.ReadLine()
While LineIn <> ""
FieldArray = Split(LineIn, ",")
StatesArray(UBound(StatesArray)) = FieldArray(0)
CodesArray(UBound(CodesArray)) = FieldArray(1)
LineIn = FileReader.ReadLine()
If LineIn <> "" Then
ReDim Preserve StatesArray(StatesArray.Length)
ReDim Preserve CodesArray(CodesArray.Length)
End If
End While
FileReader.Close()
End Sub
</SCRIPT>
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Using the Arrays
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In this example, the Page_Load subprogram loads the CodesArray and StatesArray arrays, so the current page has these arrays available to it.
Just to put a finishing touch on this discussion,with the arrays,a couple of things can be done.
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Array Lookup
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Since the two arrays have corresponding element values and for that reason they can be used as corresponding arrays for table look-up.
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Sub Get_The_Code (Src As Object, Args As EventArgs)
If State.Text <> "" Then
Dim i As Integer
For i = 0 To UBound(StatesArray)
If StatesArray(i) = State.Text Then
Code.Text = CodesArray(i)
Exit For
End If
Next
End If
End Sub
<b>State: </b><asp:TextBox id="State" Size="12" runat="server"/>
<asp:Button Text="Get Code" OnClick="Get_The_Code" runat="server"/>
<b>Code: </b><asp:Label id="Code" EnableViewState="False" runat="server"/>
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