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Iteration (Looping Constructs in VB)Iteration: Groups of statements which are repeatedly

executed until a certain test is satisfied

Carrying out Iteration in VB programming, requires the

use ofLooping Constructs

There are 2 main types ofLooping Constructs:

Determinate Loops: will repeat themselves a known

(specific) number of times (For..Next Loops)

Indeterminate Loops: will repeat themselves an

unknown number of times (Do Loops) [Until; While]

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A group of statement is repeated a specific number of

times

For..Next Loops

Repeat the statements in a loop a specific number oftimes

Each Forstatement has a corresponding Next

statement

For..Next Loop syntax:For Counter/Loop

Index = StartTo End [step step]

Statements (BodyofLoop)

NextCounter/LoopIndex

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The For..Next loop uses the Forand Nextstatements

and a Counter Variable [LoopIndex]

The elements of a For..Next:The Counter/LoopIndex must be a numeric variable,

determines the number of times the statements inside

the loop will be executed

Start and End may be Constants, Variables, Numeric

Property Values, orNumeric Expressions, and further

determine the initialand finalvalue of the counter

The optional word Step may be included, along with the

value to be added to the LoopIndex(positive/negative)

for each iteration of the loop, and if omitted, the default

value is 1 for each increment of the loop

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Dim iLoopIndex As Integer

Dim iMaximum As Integer

iMaximum = Inputbox(Enter the value, Number of

Entries)

ForiLoopIndex = 0 To iMaximum

The statements inside of the loop are indented, and

referred to as the body of the loop

Next iLoopIndex

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ACounter-Controlled Loop generally has 3 elements:

Initialise the Counter

Increment the Counter[step]

Test the Counter to determine when it is time to

Terminate the loop

ForiIndex = 2 To 100 Step 2

will count from 2 to 100 by 2

The statements in the body of the loop will be executed

50 times, with iIndex = 2, 4, 6, .

The program checks forgreater than the test value and

not equal to

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Exiting For..Next Loops

If you enter an Endless Loop, the program executionwill have to be broken manually

Therefore, you will need to enterBreak Time

Ctrl + Break

With For..Next loops, you may need to terminate the

loop before the loop index reaches its final value

VB provides an Exit For statement for this situation

Generally, an Exit Forstatement is part of an If

statement

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ForiLoopIndex = 1 To 10

IftxtInput.Text = Then nothing was entered into the inputtextbox

MsgBox You must enter somethingExit For

End If

.

. statements in the loopNext iLoopIndex

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