1. Chapter 6 – Repetition 6.1 Do Loops 6.2 For...Next Loops
6.3 List Boxes and Loops

2. Do Loops
Pretest Form of a Do Loop
Posttest Form of a Do Loop

3. 6.1 Do Loops
A loop is one of the most important structures in programming.
Used to repeat a sequence of statements a number of times.
The Do loop repeats a sequence of statements either as long as or until a certain condition is true.

4. Pretext Do Loop Condition is tested, If it is true, the loop is run.
If it is false,
the statements
following the
Loop statement
are executed.
Do While condition
statement(s)
Loop
These statements are inside
the body of the loop and
are run if the condition
above is true.

5. Pseudocode and Flow Chart

6. Example 1 Private Sub btnDisplay_Click(...) _ Handles btnDisplay.Click
'Display the numbers from 1 to 7
Dim num As Integer = 1
Do While num <= 7
lstNumbers.Items.Add(num)
num += 1 'Add 1 to the value of num
Loop
End Sub
Loop Test
Loop Body
Question #1: How many times will the test be done?
Question #2: How many times will the body of the loop execute?

7. Example 1 (cont.) Private Sub btnDisplay_Click(...) _
Handles btnDisplay.Click
'Display the numbers from 1 to 7
Dim num As Integer = 1
Do While num <= 7
lstNumbers.Items.Add(num)
num += 1 'Add 1 to the value of num
Loop
End Sub
Note: A ListBox’s Items property contains a collection (list) of data items. You can programmatically place items in the list using the Add method.

8. Example: Repeat Request as Long as Response in Incorrect
Dim passWord As String = ""
Do While passWord <> "SHAZAM"
passWord = InputBox("What is the password?")
passWord = passWord.ToUpper
Loop
passWord is the loop control variable because the value stored in passWord is what is tested to determine if the loop should continue or stop.

9. Example: Sentinel-Controlled Loop
Dim num As Double = 0
Dim prompt As String = "Enter a nonnegative number. Enter -1 " &
"to terminate entering numbers."
num = CDbl(InputBox(prompt))
Do While num <> -1 '-1 is sentinel value .
Loop
First input precedes the loop
A sentinel value is a special value that is used to decide that a loop should terminate
Each subsequent input is in the last statement of the loop body

10. Example 3 (cont): The full loop body calculating average
The loop body statements do the following:
Count user inputs
Accumulate the sum of numbers together
Input the user’s next input value

11. Counters and Accumulators
A counter is a numeric variable that keeps track of the number of items that have been processed.
An accumulator is a numeric variable that totals numbers.

12. Example 3 (cont): The full loop body calculating average
After the loop is completed, the average can be determined by dividing the sum by the count.

14. Posttest Do Loop Do statement(s) Loop Until condition
Loop is executed once and then the condition
is tested. If it is false, the loop is run again.
If it is true, the statements following the
Loop statement are executed.

15. Example: Repeat Request Until Proper Response
Dim passWord As String = "" Do
passWord = InputBox("What is the password?")
passWord = passWord.ToUpper
Loop Until passWord = "SHAZAM"
This is a BOTTOM-tested loop. In this case the loop body executes first, and then the test is done. So the number of tests EQUALS the number of body-executions.
Unlike a TOP-tested loop, in which the number of tests is always one more than the number of body-executions.

16. Pseudocode and Flowchart

17. Example 5: Form
txtAmount
txtWhen

18. Example 5: Code Private Sub btnCalculate_Click(...) Handles _
Dim balance As Double, numYears As Integer
balance = CDbl(txtAmount.Text)
Do While balance <
balance += 0.06 * balance
numYears += 1
Loop
txtWhen.Text = "In " & numYears &
" years you will have a million dollars."
End Sub
Question 1: what’s the accumulator variable?
Question 2: what’s the counter variable?

19. Example 5: Output