The For...Next Statement

There are certain processing situations, however, that require a set of statements to be executed repeatedly --until a certain condition is met to iterate over and over again.

You might, for instance,until you find a matching value you need to look through the items in an array, one at a time.

The code to effect this search needs to execute again and again and each time indexing to the next element in the array.

The For...Next Loop

To iterate a set of statements, visual Basic provides this ability with the For...Next statement whose general format is shown below.

For counter = start value To end value

...statements

Next

To keep track of iterations through the enclosed statements,a For...Next loop establishes a counter.

It is given a start value and an end value -- integer values -- to control the number of times the statements are executed and the counter incremented by 1 each time through the loop.

The code below, for example, executes the statements in the For...Next loop 10 times.

Dim i As Integer
For i = 1 To 10

...statements

Next

Variable i is defined as the counter and the counter is initialized to 1 when the loop is first entered. After the statements are executed for the first time and the Next statement is encountered, program control returns to the matching For statement. The counter is incremented by 1 and a test is made to see if the counter has yet reached the end value 10 or not. If not, the statements are executed a second time, and control again returns to the For statement where is tested against the end value and the counter is incremented again. This processing continues until finally the counter reaches the end value and also the loop comes to an end and then, program control "skips over" the loop and continues in sequence with the statement following Next.

In the above example, end value and start value are given literal integer values. More likely, in the script start value, and especially end value, are assigned through variables set elsewhere.

The following script is a simple illustration of a For...Next loop and the user enters an integer value that is used as the ending value for the loop.The loop displays the sequence of integers to the ending value,beginning with 1,

Sub Make_Numbers (Src As Object, Args As EventArgs)

If IsNumeric(EndValue.Text) Then

Dim i As Integer
For i = 1 To EndValue.Text
Numbers.Text &= i & " "
Next

End If

End Sub

<asp:TextBox id="EndValue" Size="1" MaxLength="2" runat="server"/>
<asp:Button Text="Make Numbers" OnClick="Make_Numbers" runat="server"/>
<asp:Label id="Numbers" EnableViewState="False" runat="server"/>

Iterating Arrays

For iterating the elements in an array,the For...Next loop provides an ideal mechanism. To increment through the array's indexes,the counter can be set up, pointing in sequence to each element from the first through the last. Returning to a previous example, the following array contains the first five letters of the Greek alphabet. To display,a loop is set up , in turn, each of these five letters.

Sub Get_Greek (Src As Object, Args As EventArgs)

Dim Letters() As String = {"alpha","beta","gamma","delta","epsilon"}
Dim i As Integer
For i = 0 To Letters.Length - 1
GreekOut.Text &= Letters(i) & " "
Next

End Sub

<asp:Button Text="Get Greek" OnClick="Get_Greek" runat="server"/>
<asp:Label id="GreekOut" EnableViewState="False" runat="server"/>

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