DevTut

# Looping

# For...Next

For...Next loop is used for repeating the same action for a finite number of times. The statements inside the following loop will be executed 11 times. The first time, i will have the value 0, the second time it will have the value 1, the last time it will have the value 10.

For i As Integer = 0 To 10
    'Execute the action
    Console.Writeline(i.ToString)
Next

Any integer expression can be used to parameterize the loop. It is permitted, but not required, for the control variable (in this case i) to also be stated after the Next. It is permitted for the control variable to be declared in advance, rather than within the For statement.

Dim StartIndex As Integer = 3
Dim EndIndex As Integer = 7
Dim i As Integer

For i = StartIndex To EndIndex - 1
    'Execute the action
    Console.Writeline(i.ToString)
Next i

Being able to define the Start and End integers allows loops to be created that directly reference other objects, such as:

For i = 0 to DataGridView1.Rows.Count - 1
    Console.Writeline(DataGridView1.Rows(i).Cells(0).Value.ToString)
Next

This would then loop through every row in DataGridView1 and perform the action of writing the value of Column 1 to the Console. (The -1 is because the first row of the counted rows would be 1, not 0)

It is also possible to define how the control variable must increment.

For i As Integer = 1 To 10 Step 2
    Console.Writeline(i.ToString)
Next

This outputs:

1 3 5 7 9

It is also possible to decrement the control variable (count down).

For i As Integer = 10 To 1 Step -1
    Console.Writeline(i.ToString)
Next

This outputs:

10 9 8 7 6 5 4 3 2 1

You should not attempt to use (read or update) the control variable outside the loop.

# For Each...Next loop for looping through collection of items

You can use a For Each...Next loop to iterate through any IEnumerable type. This includes arrays, lists, and anything else that may be of type IEnumerable or returns an IEnumerable.

An example of looping through a DataTable's Rows property would look like this:

For Each row As DataRow In DataTable1.Rows
    'Each time this loops, row will be the next item out of Rows
    'Here we print the first column's value from the row variable.
    Debug.Print(Row.Item(0))
Next

An important thing to note is that the collection must not be modified while in a For Each loop. Doing so will cause a System.InvalidOperationException with the message:

Collection was modified; enumeration operation may not execute.

# Short Circuiting

Any loop may be terminated or continued early at any point by using the Exit or Continue statements.

Exiting

You can stop any loop by exiting early. To do this, you can use the keyword Exit along with the name of the loop.

Loop Exit Statement
For Exit For
For Each Exit For
Do While Exit Do
While Exit While

Exiting a loop early is a great way to boost performance by only looping the necessary number of times to satisfy the application's needs. Below is example where the loop will exit once it finds the number 2.

Dim Numbers As Integer() = {1,2,3,4,5}
Dim SoughtValue As Integer = 2
Dim SoughtIndex
For Each i In Numbers
    If i = 2 Then 
        SoughtIndex = i
        Exit For
    End If
Next
Debug.Print(SoughtIndex)

Continuing

Along with exiting early, you can also decide that you need to just move on to the next loop iteration. This is easily done by using the Continue statement. Just like Exit, it is proceeded by the loop name.

Loop Continue Statement
For Continue For
For Each Continue For
Do While Continue Do
While Continue While

Here's an example of preventing even numbers from being added to the sum.

Dim Numbers As Integer() = {1,2,3,4,5}
Dim SumOdd As Integer = 0
For Each i In Numbers
    If Numbers(i) \ 2 = 0 Then Continue For
    SumOdd += Numbers(i)
Next

Usage Advice

There are two alternative techniques that can be used instead of using Exit or Continue.

You can declare a new Boolean variable, initializing it to one value and conditionally setting it to the other value inside the loop; you then use a conditional statement (e.g. If) based on that variable to avoid execution of the statements inside the loop in subsequent iterations.

Dim Found As Boolean = False
Dim FoundIndex As Integer
For i As Integer = 0 To N - 1
    If Not Found AndAlso A(i) = SoughtValue Then
        FoundIndex = i
        Found = True
    End If
Next

One of the objections to this technique is that it may be inefficient. For example, if in the above example N is 1000000 and the first element of the array A is equal to SoughtValue, the loop will iterate a further 999999 times without doing anything useful. However, this technique can have the advantage of greater clarity in some cases.

You can use the GoTo statement to jump out of the loop. Note that you cannot use GoTo to jump into a loop.


   Dim FoundIndex As Integer
    For i As Integer = 0 To N - 1
        If A(i) = SoughtValue Then
            FoundIndex = i
            GoTo Found
        End If
    Next
    Debug.Print("Not found")
Found:
    Debug.Print(FoundIndex)

This technique can sometimes be the neatest way to jump out of the loop and avoid one or more statements that are executed just after the natural end of the loop.

You should consider all of the alternatives, and use whichever one best fits your requirements, considering such things as efficiency, speed of writing the code, and readability (thus maintainability).

Do not be put off using GoTo on those occasions when it is the best alternative.

# While loop to iterate while some condition is true

A While loop starts by evaluating a condition. If it is true, the body of the loop is executed. After the body of the loop is executed, the While condition is evaluated again to determine whether to re-execute the body.

Dim iteration As Integer = 1
While iteration <= 10
  Console.Writeline(iteration.ToString() & " ")

  iteration += 1
End While

This outputs:

1 2 3 4 5 6 7 8 9 10

Warning: A While loop can lead to an infinite loop. Consider what would happen if the line of code that increments iteration were removed. In such a case the condition would never be True and the loop would continue indefinitely.

# Do...Loop

Use Do...Loop to repeat a block of statements While or Until a condition is true, checking the condition either at the beginning or at the end of the loop.

Dim x As Integer = 0
Do
    Console.Write(x & " ")
    x += 1
Loop While x < 10

or

Dim x As Integer = 0
Do While x < 10
    Console.Write(x & " ")
    x += 1
Loop

0 1 2 3 4 5 6 7 8 9

Dim x As Integer = 0
Do
    Console.Write(x & " ")
    x += 1
Loop Until x = 10

or

Dim x As Integer = 0
Do Until x = 10
    Console.Write(x & " ")
    x += 1
Loop

0 1 2 3 4 5 6 7 8 9

Continue Do can be used to skip to the next iteration of the loop:

Dim x As Integer = 0
Do While x < 10
    x += 1
    If x Mod 2 = 0 Then
        Continue Do
    End If
    Console.Write(x & " ")
Loop

1 3 5 7 9

You can terminate the loop with Exit Do - note that in this example, the lack of any condition would otherwise cause an infinite loop:

Dim x As Integer = 0
Do
    Console.Write(x & " ")
    x += 1
    If x = 10 Then
        Exit Do
    End If
Loop

0 1 2 3 4 5 6 7 8 9

# Nested Loop

A nested loop is a loop within a loop, an inner loop within the body of an outer one. How this works is that the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of the outer loop triggers the inner loop again. This repeats until the outer loop finishes. a break within either the inner or outer loop would interrupt this process.

The Structure of a For Next nested loop is :

For counter1=startNumber to endNumber (Step increment)

    For counter2=startNumber to endNumber (Step increment)

        One or more VB statements

    Next counter2

Next  counter1

Example :


   For  firstCounter = 1 to 5

      Print “First Loop of ” + firstCounter

    For   secondCounter= 1 to 4

      Print “Second Loop of ” + secondCounter

    Next secondCounter

  Next firstCounter

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