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VB.NET from beginner to advanced programmer Part 9 - Loops Part 1

VB.NET form loop

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#1 Vswe

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Posted 01 November 2009 - 05:20 PM

Welcome to the VB.NET tutorial series: "VB.NET from beginner to advanced programmer" which will take you from the very beginning to be a good programmer. VB.NET is a good first language for new programmers so this 21 part long series is written for completely beginners but it will also works perfectly fine if you already know another programming language.


VB.NET from beginner to advanced programmer





The Loops are block of codes that often will be run more then once at a row. Some of them will run a certain amount of time while others while run depending on the output of a condition. In this Part we will go through 3 of the 5 loops in VB.NET.






The While Loop:


The while loop will continue with its loop as long a condition remains True. The condition is tested at the beginning each time the loop will start a new turn. If the condition isn't met at the beginning the loop won't even run once. The syntax of a while loop looks as following:

While <condition>
     'Code that will be run once per turn
End While


To show a simple example:

Dim X as Integer = 3
While X<300
X*=2
End While

The above loop will loop while X<300, each time in the loop the value of X is doubled. After some turns X will be doubled from 192 to 383. Then when the loop check if the condition "X<300" is met it will get a False value and therefor the loop will end.


You have to be sure you don't create a infinite loop. An infinite loop is a loop which will never end. The result is that the program will never continue and will stop to response. Here's an infinite loop:

Dim X As Integer = 141
While X < 200
    X -= 5
End While

Independening on how many times the loop will loop, the loop won't end. The condition X<200 will always be True since X will only be smaller and smaller for each turn. The program will stop to response.





Do While loop:

This type of loop has only one difference to the While loop more then the syntax. The Do While loop will check the condition at the end instead of in the beginning. This is actually pretty important. Here's the syntax anyway:


Do
     'Code that will be run once per turn
Loop While <condition>


So why do we sometimes want to use the Do While loop? The reason is that sometimes we wants the loop to be run at least once even though the conditions aren't met.

In the examples below I will use some more functionalities in the messagebox, I add a caption and what buttons it should use, I also check what button that was pressed to use it in an if statement:


While Loop
Dim nPoints As Integer
While nPoints >= 15
    nPoints -= 10
 
    If MessageBox.Show("Do you want any points?", "Points=" & nPoints, MessageBoxButtons.YesNo) = Windows.Forms.DialogResult.Yes Then
         nPoints += 40
    End If
 
End While

Do While loop
Dim nPoints As Integer
Do
    nPoints -= 10
 
    If MessageBox.Show("Do you want any points?", "Points=" & nPoints, MessageBoxButtons.YesNo) = Windows.Forms.DialogResult.Yes Then
        nPoints += 40
    End If
 
Loop While nPoints >= 15


Here, in both examples, the loop will continue as long as the user have 15 or more points, the user loses 10 points each time but can gain 40 new if he/she answer yes on the messagebox. But we want the user to be able to play this "game" so then the loop need to loop at least once. The while loop will test the condition in the beginning but since the user starts with 0 points the loop won't start at all. Because of that we use the Do While loop, then the code will run at least once so the "game" can start.




Do Until loop:

The Do Until loop as the Do While loop but with one exception. The Do While loop will continue if the condition are True and stop if the condition are False. The Do Until loop does the opposite, it will continue if the condition are False and stop if the condition are True. It will loop Until the condition are met. The syntax of the Do Until loop and also an example of it:

Syntax:
Do
     'Code that will be run once per turn
Loop Until <condition>

Example:
Dim X As Integer = 312
Do
    X /= 10
Loop Until X = 0

The loop above will loop until X is equals to 0, the it will stop.




That was all for this time, we'll take the two last loops later on in the series.

Edited by Vswe, 21 March 2010 - 02:31 PM.

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#2 Guest_Jordan_*

Guest_Jordan_*
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Posted 02 November 2009 - 05:43 AM

Not bad, +rep!
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