1. 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]

2. Determinate Loops A group of statement is repeated a specific number of times For..Next Loops Repeat the statements in a loop a specific number of times Each 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

3. 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

4. Dim iLoopIndex As Integer Dim 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

5. A Counter-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 For iIndex = 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 for greater than the test value and not equal to

6. 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

7. 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

14. Do Loops A 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

15. Checking the condition before entering the loop: Do {While / Until} Condition ‘statements in the loop Loop Testing 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

16. 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

25. 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

26. 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.