Iteration in Visual Basic

Do and For loops provide use with several possible tools to use for looping in Visual Basic.

Do Loops

The Do loop is used when you do not know how many times you will need to execute a block of code. It is used when you will execute the same block of code repeatedly until something changes to indicate you are done. The Do loop is controlled by a condition.

The For.... Next loop

The FOR.... NEXT loop is used when we need a counter controlled loop, which is when we know how many times we want/need to do something.
 

Format:

For loopVariable = StartValue To StopValue Step StepValue
 
        one or more VB instructions
 
Next loopVariable

For example, a simple For..next loop:

Loop var   Start Value     stop value         Step Value

For intx = 1     To 10         Step 1
        MsgBox (intx)
Next intx

MsgBox("the end")

What is happening?
 
1) The loop variable is initialized to the Start Value.
 
2) The loop variable is compared to the stop value. When the loop variable becomes greater than the stop value the loop has been satisfied and control passes to the statement following the Next intX statement.

There are a couple of ways you can think about whether the statements in side the loop are going to be executed. The statements will be execute

• Until the loop variable becomes greater than the stop value

• Or While the loop control variable is less than or equal to the stop value

3) If the loop variable is not greater than the stop value the instructions inside the loop are executed.
 
4) At the end of the loop the loop variable is incremented by the step value.
 
5) Control passes back to the top of the loop to compare the loop variable to the stop value again.
 
The preceding code will display all the numbers beginning with 1 and ending with 10.

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The step value is optional. If it is left off the value is assumed to be 1. Explicitly stating the loop control variable in the Next statement is optional as well.
 

For intNum= 1     To 10       
        MsgBox (intNum)
Next

MsgBox("the end")

 
It is possible to have a negative step value

 
For intNum= 1     To 0   Step -1      
        MsgBox (intNum)
Next

MsgBox(intNum)

 
When the Step Value is negative the loop is executed until the loop variable becomes less than the Stop Value. Notice the value of intNum after the loop is completed is -1.
 
 
The Start Value and step value can be other values as well
 

For intNum= 10     To 100   Step 10      
        MsgBox (intNum)
Next

MsgBox(intNum)

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You must be careful when constructing For loops as with any kind of loop not to create impossible loops. What do you think will happen with the following loop?

For intNum= 12     To 1   Step 2      
        MsgBox (intNum)
Next

 
What do you think the result of the following loop will be?

 
For intNum= 1    To 10   Step 0     
        MsgBox (intNum)
Next
 
 

You would never consciously set a Step Value up as zero but the Step Value as well as the Initial and Stop values can and often are variables.
 
A common error is to forget or incorrectly initialize the Stop Value as in the following code. What is the result?

intBegin = 1
 
For intNum = intBegin To intStop Step 1
        MsgBox (intNum)
Next intNum

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For……Next loops are used when you know how many times you need to execute a block of instruction. Do loops can be also be used when you know the number of times to repeat an action. However if you use a Do loop instead of a For…Next loop when you know how many times you need to execute a block of code you are responsible for keeping track of how many times you have executed the conditional block of code.