The For Loop in VB.net

By: Issac Viewed: 153244 times  Printer Friendly Format    


We'll use one to add up our 4 numbers, and then discuss the code. Study the following. In fact, create a new Project. Add a button to your new Form. Double click your new button and type the following code for it:

Dim ans As Integer

Dim start As Integer

ans = 0

For start = 1 To 4

ans = ans + start

Next start

MsgBox ans

Run the program and see what happens when you click the button. The number 10 should have been displayed in your message box.

The For loop code

We start by setting up two integer variables. We set one of these to zero. Then we start our loop code. Let's examine that in more detail.

For start = 1 To 4

ans = ans + start

Next start

We start our loop by telling Visual Basic what type of loop we want to use. In this case it is a For loop:

For start = 1 To 4

The next thing you have to do is tell Visual Basic what number you want the loop to start at:

For start = 1 To 4

Here we are saying "Start the loop at the number 1". The variable start can be called anything you like. A popular name to call a start loop variable is the letter i ( i = 1). So what we're doing is setting up a variable - the start of the loop variable - and putting 1 into it;

Next, you have to Tell Visual Basic what number to end the loop on:

For start = 1 To 4

The To word, followed by a number or variable, tells Visual Basic how many times you want the loop to go round and round. We're telling Visual Basic to loop until the “start” variable equals 4

The command that tells Visual basic to grab the next number in the sequence is this:

Next start

When Visual Basic reaches this line, it checks to see what is in the variable “start”. It then adds one to it. In other words, "Get me the next number after the one I've just used."

The next thing that happens is that Visual Basic will return to the word For. It returns because it’s in a loop. It needs to know if it can stop looping. To check to see if it can stop looping, it skips the start = 1 part, and then jumps to your end number. In our case, the end number was 4. Because Next start adds one to whatever is in start, then start is now 2 (It was 1 at the start. The next number after one is 2 isn’t it?)

So if start is now 2, can Visual Basic stop looping? No it can’t. Because we’ve told it to loop until it reaches number 4. It’s only reached number 2, so off it goes on another trip around the loop. When the start is greater than the end number, Visual Basic drops out of the loop and continues on its way.

But remember why we're looping: so that we can execute some code over and over again.

To clarify things, change the above code to this:

Dim start As Integer

For start = 1 To 4

MsgBox("Start = " & start)

Next start

Run the programme, and click your button. What happens? You should have seen this in the message box, one after the other:

Start = 1

Start = 2

Start = 3

Start = 4

Each time round the loop, the code for the message box was executed. You had to click OK four times - start = 1 To 4.



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