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We saw with the For Loop that a specific number of times to loop was coded into it. We said

For startNumber = 1 To 4

We knew that we only wanted to loop four times. But what if we don’t know how many times around the loop we want to go? Later, we’ll be opening text files and reading the data from them. If we used a For loop to get every line of text, we’d have to know before hand how many lines the text file held. A For Loop would not be very efficient in this case.

But a Do Loop would be. With a Do Loop we can use word s like “While” and “Until”. And then we can say, “Go round and round the loop While there’s still text to be read from the file.” An example might make things clearer.

Load the form you created for the last exercise, the one that has two textboxes and a Button and tested your understanding of For loops.

Add another button to the Form. Your form might look something like this:

Double click the new button to open the code window, and then type the following code for the new button:

Dim number as Integer

number = 1

Do While number < 5
MsgBox number
number = number + 1
Loop

When you’ve finished, run the programme and see what happens. The numbers 1 to 4 should have displayed in your message box.

So the Do loop keeps going round and around. And when it gets to the top, it tests the “While” part – Do While number is Less Than 5. It’s really answering a Yes or No question: is the number inside the variable called number Less Than 5? If it is Less Than 5, go round the loop again. If it’s not Less than 5, Visual Basic jumps out of the Loop entirely.

You can add the “While … ” part to the bottom, just after the word “Loop”. Like this:

Do

number = number + 1

Loop While number < 5

Try it and see what difference it makes.

None, right? But there is a difference between the two. With the “While … ” part at the bottom, the code above it gets executed at least once. With the code on the first line after the word “Do”, the code might not get executed at all. After all, the number inside the variable might already be Greater Than 5. If it is, Visual Basic won’t execute the code.

 

Do … Until

You have another choice for Do Loops – Do … Until.

There’s not much difference between the two, but a Do … Until works like this. Change your Loop code to the following:

Do Until number < 5

MsgBox number
number = number + 1

Loop

Run the code and see what happens.

Nothing happened, right? That’s because we “Keep looping UNTIL the number in the variable called number is Less Than 5″ The problem is, the number inside the variable is already Less Than 5. And if the number is Less than 5, then the code won’t execute – because it has already met the end condition.

Change that Less Than sign to a Greater Than sign, and then test your code again. Now what happens?

The numbers 1 to 5 should have displayed. Again, the loop keeps going round and around testing to see if our end condition is met, in this case Is Greater Than 5. If the condition is met, VB breaks out of the Loop; if not, keep going round.

Change the Greater Than sign to Greater Than or Equal to ( >= ), and test it again. It should now print 1 to 4.

The “Until” part can go at the bottom, just after the word Loop. Like this

Do

MsgBox number
number = number + 1

Loop Until number >= 5

To sum up, use a Do Loop if you don’t know what the end number is going to be, otherwise a For Loop might be better.

You’re now going to write a programme that uses a For Loop inside a Do Loop. The programme works out the times table.

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This first type of loop we’ll look at is called a For Loop. It is the most common type of loop you’ll use when you programme. 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 answer As Integer
Dim startNumber As Integer

answer = 0

For startNumber = 1 To 4

answer = answer + startNumber

Next startNumber

MsgBox answer

Run the programme, 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 startNumber = 1 To 4

answer = answer + startNumber

Next startNumber

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 startNumber = 1 To 4

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

For startNumber = 1 To 4

Here we are saying “Start the loop at the number 1″. The variable startNumber 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 startNumber = 1To 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 startNumber variable equals 4

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

Next startNumber

When Visual Basic reaches this line, it checks to see what is in the variable startNumber. 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 startNumber = 1 part, and then jumps to your end number. In our case, the end number was 4. Because Next startNumber adds one to whatever is in startNumber, then startNumber is now 2 (It was 1 at the start. The next number after one is … ?).

So if startNumber 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 startNumber is greater than the end number, Visual Basic drops out of the loop and continues on it’s 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 startNumber As Integer

For startNumber = 1 To 4

MsgBox(“Start Number = ” & startNumber)

Next startNumber

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

Start Number = 1
Start Number = 2
Start Number = 3
Start Number =4

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

Summary

So to sum up:

1. A For loop needs a start position and an end position, and all on the same line
2. A For loop also needs a way to get the next number in the loop
3. A loop without any code to execute looks like this:

For i = startNumber To endNumber

Next i

The above code uses two variables for the start and end numbers. The start number for the loop goes directly into the variable called i. When Visual Basic wants the next number, it just add one to whatever is in the variable i. You could use it like this:

Dim startNumber As Integer
Dim endNumber As Integer
Dim i As Integer

startNumber = 1
endNumber = 4

For i = startNumber To endNumber
Msgbox i
Next i

Change the code for your button to that new code, and test it out. Study the code so that you understand what is going on.

For Loops might not be easier to understand than just typing answer = 1 + 2 + 3 + 4, but they are a lot more powerful if you want to add up a thousand numbers!

Exercise

Put two textboxes on your form. The first box asks users to enter a start position for a For Loop; the second textbox asks user to enter an end position for the For loop. When a button is clicked, the programme will add up the numbers between the start position and the end position. Display the answer in a message box. You can use this For Loop code

For i = startNumber To endNumber

answer = answer + i

Next i

Get the startNumber and endNumber from the textboxes.

 

Exercise

Amend your code to check that the user has entered numbers in the textboxes. You will need an If statement to do this. If there’s nothing in the textboxes, you can halt the programme with this code

Exit Sub

For this exercise, you will be passing whatever is in the textboxes to integer variables. It is these variables you are checking with your If Statement. Because numbers will be entered into the textboxes, remember to convert the text to a value with Val( ).

But the Text property will return a zero if the box is empty. So your If statement will need to check the variables for a value of zero. If it finds a zero, Then you can use the Exit Sub code. The If statement should come first, before the For Loop code.

This has been quite a long section, and you may need a breather! Don’t worry if you don’t understand all that in one sitting. Come back to it, and it will sink in – eventaully! In the next section, we’ll take a look at Do Loops.

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There are three types of loop for us to cover with VB.NET: a For loop, a Do loop, and a While … End While loop. This last one is almost the same as a Do loop, and we won’t be covering it here. But the other two types of loop come in very handy, and a lot of the time you can’t programme effectively without using loops.

 

What is a Loop?

A loop is something that goes round and round and round. If someone told you to move your finger around in a loop, you’d know what to do immediately. In programming, loops go round and round and round, too. In fact, they go round and round until you tell them to stop. You can programme without using loops. But it’s an awful lot easier with them. Consider this.

You want to add up the numbers 1 to 4: 1 + 2 + 3 + 4. You could do it like this

Dim answer As Integer

answer = 1 + 2 + 3 + 4

MsgBox answer

Fairly simple, you think. And not much code, either. But what if you wanted to add up a thousand numbers? Are you really going to type them all out like that? It’s an awful lot of typing. A loop would make life a lot simpler.

But don’t get hung up too much on the name of the Loop. Just remember what they do: go round and round until you tell them to stop.

We’ll discuss the For Loop first.

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