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 of Looping Constructs

There are 2 main types of Looping 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]

Determinate LoopsA group of statement is repeated a specific number of times

For..Next Loops

Repeat the statements in a loop a specific number of timesEach For statement has a corresponding Next statement

For..Next Loop syntax:

For Counter/LoopIndex = Start To End [step step]Statements (Body of Loop)

Next Counter/LoopIndex

The For..Next loop uses the For and Next statements 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, or Numeric Expressions, and further

determine the initial and final value 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

Dim iLoopIndex As IntegerDim iMaximum As Integer

iMaximum = Inputbox(“Enter the value”, Number of Entries)

For iLoopIndex = 0 To iMaximum‘The statements inside of the loop are indented, and referred to as ‘the body of the loop

Next iLoopIndex

A Counter-Controlled Loop generally has 3 elements:

Initialise the CounterIncrement the Counter [step]

Test the Counter to determine when it is time to Terminate the loop

For iIndex = 2 To 100 Step 2

will count from 2 to 100 by 2The statements in the body of the loop will be executed 50

times, with iIndex = 2, 4, 6, ….

The program checks for greater than the test value and not equal to

Exiting For..Next Loops

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

Therefore, you will need to enter Break 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 For statement is part of an If statement

For iLoopIndex = 1 To 10 If txtInput.Text = “ ” Then ‘nothing was entered into the input textbox

MsgBox “You must enter something” Exit For

End If……….………. ‘statements in the loop

Next iLoopIndex

Do LoopsA Do..Loop terminates based on a condition that is

specified

Execution of a Do..Loop continues while a condition is True or until a condition is True

The condition can be placed at the Top or at the Bottom of the loop

Align the Do and Loop statements with each other and indent the lines of code to be repeated in the body of the

loop

Checking the condition before entering the loop:

Do {While / Until} Condition‘statements in the loop

LoopTesting for completion at the top of the loop

Also known as a pretest

The statements inside the loop may never be executed if the terminating condition is True, the first time it is tested

iTotal = 0

Do Until iTotal = 0

‘statements in the loop

Loop

Checking the condition after one iteration of the loop:

Do ‘statements in the loop

Loop {While / Until} Condition Testing for completion at the bottom of the loop

Also known as a posttest

The statements inside the loop will always be executed at least once

iTotal = 0

Do

‘statements in the loop

Loop Until iTotal = 0

Private Sub cmdCalculate_Click( )'Given an Account Balance and an Interet Rate, it returns the number'of years it will take to reach $10,000

'Declare the VariablesDim iNumYears As IntegerDim cAccountBal As CurrencyConst sngInterestRate As Single = 0.1

'The Account Balance is taken from the user through an inputboxcAccountBal = InputBox("Enter the Balance in the Account which you hold", "AccountBalance")

'Assign iNumYears a value of 0 (which is the counter for the Loop)iNumYears = 0

'Loop runs until cAccountBal exceeds $10,000Do Until cAccountBal > 10000 'Increases cAccountBal by sngInterestRate cAccountBal = cAccountBal + (cAccountBal * sngInterestRate) 'iNumYears increased by 1 with each execution of the loop iNumYears = iNumYears + 1LooptxtNumYears.Text = ("The number of years it will take is " & iNumYears)

End Sub

Private Sub cmdNesting_Click( )'Gets input from the user and prints a 5 times table from'that input

'Set the counter variablesDim OuterCount As IntegerDim InnerCount As Integer

'Set up the number to be inputted by the userDim FirstNum As Integer

OuterCount = 1FirstNum = InputBox("What is the Value of the First Number", "First Number")

'InnerLoop runs 5 times for every time the OuterLoop runs once

Do While OuterCount <= FirstNum

'This is the beginning of the Inner Loop For InnerCount = 1 To 5 Print InnerCount * OuterCount Next InnerCount 'This is the end of the Inner Loop

Print 'This prints a blank line

OuterCount = OuterCount + 1Loop

End Sub

Private Sub cmdLoop5Times_Click( )'Checks 5 times whether the user's name is of a certain length'(in this case 10 characters in length).'After the 5th iteration of the loop it ends.

'Declare the user input variable and the varible counterDim UserName As String 'Accepts user inputDim NumLoops As Integer 'Used as the loop counter

'Set the value of the counter NumLoopsNumLoops = 0

'Loop runs until the length of UserName is > 10 characters'in length, or the user has entered their name 5 times or more

Do 'UserName gets a value from an inputbox UserName = InputBox("Please Enter Your Name", "Enter Name") txtName.Text = UserName 'Adds 1 to the counter variable NumLoops NumLoops = NumLoops + 1 If (Len(UserName) < 10) Then txtName.Text = "" MsgBox "Please enter a UserName greater than 10 characters in length",vbinformationonly, "User Name Length" End IfLoop Until (Len(UserName) > 10) Or (NumLoops >= 5)

End Sub

Private Sub cmdExitDo_Click( )'Checks 5 times whether the user's name is of a certain length'(in this case 10 characters in length).'After the 5th iteration of the loop it ends by using the'Exit Do statement

'Declare the user input variable and the varible counterDim UserName As String 'Accepts user inputDim NumLoops As Integer 'Used as the loop counter

'Set the value of the counter NumLoopsNumLoops = 0

'Loop runs until the length of UserName is > 10 characters'in length, or the user has entered their name 5 times or more

Do 'UserName gets a value from an inputbox UserName = InputBox("Please Enter Your Name", "Enter Name") txtName.Text = UserName 'Adds 1 to the counter variable NumLoops NumLoops = NumLoops + 1 'If the number of iterations of the loop is greater than 'or equal to 5 then the loop is exited, through the use 'of the Exit Do statement If NumLoops = 5 Then txtName.Text = "" MsgBox "You have been given 5 chances to enter a name of the correct lengthand failed to do so", vbCritical, "User Name Length Information" End If If NumLoops >= 5 Then Exit Do

If (Len(UserName) < 10) Then txtName.Text = "" MsgBox "Please enter a UserName greater than 10 characters in length",vbinformationonly, "User Name Length" End IfLoop Until (Len(UserName) > 10)

End Sub

Example

Code a procedure that prompts the user for a password to login. If the password is not

greater then 5 characters then a message is displayed to the user. The user is allowed 3

attempts to login, if unsuccessful at this point the application shuts down

Example

Code a procedure that checks 5 times whether the username is of a certain length (in this case greater then 10 characters). After the fifth attempt the user is locked out of the

application.