Looping

Without loops, a programmer would need to write the same statements repeatedly. This would make code long, repetitive, and difficult to maintain. A loop lets the computer repeat a task automatically.

For example, you can use loops to:

• Add numbers repeatedly until a total exceeds a chosen value.
• Display a sequence such as 1 to 10 in a list box.
• Process repeated user input.
• Calculate a cumulative sum or running total.

The For...Next loop is used when you know in advance how many times the loop should run. It uses a counter variable that starts at one value and ends at another.

For counter = startNumber To endNumber (Step increment)
    One or more Visual Basic 2015 statements
Next

If you need to stop the loop before it reaches the end, you can use the Exit For statement inside the loop.

Dim counter As Integer
For counter = 1 To 10
    ListBox1.Items.Add(counter)
Next

This program inserts the numbers 1 through 10 into a ListBox.

The following program calculates the running sum of the numbers:

0 + 10 + 20 + 30 + 40 + ... + 100

Dim counter, sum As Integer
For counter = 1 To 100 Step 10
    sum += counter
    ListBox1.Items.Add(sum)
Next

The Step 10 part increases the counter by 10 each time.

The increment in a For...Next loop can also be negative. The following example computes:

1000 - 100 - 95 - 90 - ... - 5

Dim counter, sum As Integer

sum = 1000
For counter = 100 To 5 Step -5
    sum -= counter
    ListBox1.Items.Add(sum)
Next

Here, the counter starts at 100 and decreases by 5 until it reaches 5.

You can terminate a For...Next loop early by using Exit For. The original lesson’s code appears to contain a small syntax issue, but the intended idea is:

Dim n As Integer
For n = 1 To 10
    If n > 6 Then
        Exit For
    Else
        ListBox1.Items.Add(n)
    End If
Next

This program adds values to the ListBox only while n is 6 or below. As soon as n becomes greater than 6, the loop exits.

In Visual Basic 2015, there are several forms of the Do Loop. These loops are especially useful when you do not know מראש how many repetitions will be needed.

Common structures include:

Do While condition
    Block of statements
Loop

Do
    Block of statements
Loop While condition

Do Until condition
    Block of statements
Loop

Do
    Block of statements
Loop Until condition

You can also use Exit Do to escape the loop before the condition is reached.

In this example, the procedure keeps increasing the value of counter by 1 until it becomes greater than 1000.

Do While counter <= 1000
    TextBox1.Text = counter
    counter += 1
Loop

The same logic can also be written like this:

Do
    TextBox1.Text = counter
    counter += 1
Loop Until counter > 1000

Both versions achieve the same result, but they test the condition in different positions.

The following example repeatedly adds numbers and shows the running total in a ListBox. The process stops after 100 repetitions.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim sum, n As Integer

    ListBox1.Items.Add("n" & vbTab & "Sum")
    ListBox1.Items.Add("----------------")
    Do
        n += 1
        sum += n
        ListBox1.Items.Add(n & vbTab & sum)
        If n = 100 Then
            Exit Do
        End If
    Loop

End Sub

The same loop can also be written more directly as:

Do Until n = 10
    n += 1
    sum += n
    ListBox1.Items.Add(n & vbTab & sum)
Loop

This shows how Do Loop structures can be adjusted depending on the style of logic you prefer.

The While...End While loop is very similar to the Do While form. It continues looping as long as the specified condition remains true.

While condition
    Visual Basic 2015 statements
End While

Example 15.3 uses a While loop to generate a running total:

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim sum, n As Integer
    ListBox1.Items.Add("n" & vbTab & "sum")
    ListBox1.Items.Add("----------------")

    While n <> 10
        n += 1
        sum += n
        ListBox1.Items.Add(n & vbTab & sum)
    End While
End Sub

This loop continues until n becomes 10.

Each loop structure is useful in different situations:

• For...Next is best when the number of repetitions is known in advance.
• Do...Loop is useful when repetition depends on a condition that may change dynamically.
• While...End While is useful when you want to repeat actions as long as a condition remains true.