1. Lists, Loops, Validation, and More

2. This chapter covers the Visual Basic .NET looping statements
Do … While
Do … Until
For … Next

3. Input Boxes
Input Boxes Provide a Simple Way to Gather Input Without Placing a Text Box on a Form

4. Format of the InputBox Function
InputBox(Prompt [,Title] [,Default] [,Xpos] [,Ypos])
Prompt - message to the user
Title - text for the box's title bar
Default - default text for user's input
Xpos - X coordinate for the box's position
Ypos - Y coordinate for the box's position
Title and beyond are optional arguments

5. Sample InputBox Usage
userInput = InputBox("Enter the distance.", "Provide a Value", "150")

6. Xpos, Ypos, and Twips
Xpos specifies the distance from the left of the screen to the left side of the box
Ypos, from the top of the screen to the top of the box
Both are specified in twips
One twip is 1/440 inch

7. A Loop Is Part of a Program That Repeats
The Do While Loop
A Loop Is Part of a Program That Repeats

8. Repetition Structure (or Loop)
Visual Basic .NET has three structures for repeating a statement or group of statements
Do While
Do Until
For Next

9. Do While Flowchart The Do While loop
If/While the expression is true, the statement(s) are executed
Expression
statement(s)
True
False

10. Do While Syntax "Do", "While", and "Loop" are new keywords
The statement, or statements are known as the body of the loop
Do While expression
statement(s)
Loop

11. Do While Example
Private Sub btnRunDemo_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnRunDemo.Click
' Demonstrate the Do While loop
Dim count As Integer = 0
Do While count < 10
lstOutput.Items.Add("Hello")
count += 1
Loop
End Sub

12. Infinite Loops
Generally, if the expression is true and, hence the starts executing:
Something with the body of the loop must eventually make the test expression false
Otherwise, the Do While loop will continuously loop forever - called an infinite loop

13. Counters
Variables called counters are frequently used to control Do While loops (see count in the previous example
Counters are invariably initialized before the loop begins (above: Dim count As Integer = 0)
They are also usually modified within the body of the loop (above: count += 1)

14. Pretest vs. Posttest Loops
The preceding Do While loops were written in their pretest syntax
The expression is always tested before the body of the loop is executed
Do While loops also have a posttest form
In these, the body of the loop is always executed first, then the expression is evaluated to check to see if additional iterations are needed

15. Posttest Do While Syntax and Flowchart
statement(s)
Loop While expression
statement(s)
The statement(s) will always be done once, irrespective of the expression used
Expression
True
False

16. Example: Keeping a Running Total
count = 1 ' Initialize the counter
total = 0 ' Initialize total Do
input = InputBox("Enter the sales for day " & _
count.ToString, "Sales Amount Needed")
If input <> "" Then
sales = CDec(input)
total += sales ' Add sales to total
count += 1 ' Increment the counter
End If
Loop While count <= 5

17. The Do Until and For Next Loops
The Do Until Loop Iterates Until Its Test Expression Is True
The For...Next Loop Is Designed to Use a Counter Variable and Iterates a Specific Number of Times

18. Do Until: Pretest and Posttest Forms
Do Until expression
statement(s)
Loop Do
statement(s)
Loop Until expression

19. Pretest Do Until Example
input = InputBox("How many test scores do you want " & _
"to average?", "Enter a Value")
numScores = Val(input)
total = 0
count = 1
Do Until count > numScores
input = InputBox("Enter the value for test score " & _
count.ToString, "Test Score Needed")
total = total + Val(input)
count = count + 1
Loop

20. For … Next Loop, I The syntax is
Where 'For', 'To', and 'Next' are keywords
Other details follow …
For CounterVariable = StartValue To EndValue _[Step]
statement
Next [CounterVariable]

21. For … Next Loop, II CounterVariable is a numeric counter variable
StartValue is the initial value of the counter
EndValue gives the number to test for completion
Step indicates the increment for count at the end of each iteration; it is optional and defaults to 1 if not specified

22. For…Next Flowchart False statement(s) True set counter to StartValue
EndValue?
statement(s)
increment
counter
False
True

23. For…Next Example For count = 1 To 10 square = count ^ 2
str = "The square of " & count.ToString & " is " & _
square.ToString
lstOutput.Items.Add(str)
Next count

24. More on the StepValue
It is optional, if not specified, it defaults to 1
It may be negative, in which case the loop counts downwards
For x = 0 To 100 Step 10
MessageBox.Show("x is now " & x.ToString)
Next x
For x = 10 To 1 Step -1
MessageBox.Show("x is now " & x.ToString)
Next x

25. Exiting a Loop Prematurely
In some situations it is convenient to be able to gracefully end a loop early
Exit Do (used in Do While or Until loops)
Exit For (used in For Next loops)
Will accomplish that task
Since these override the normal loop termination mechanism, they should be used sparingly

26. Exiting a Loop Prematurely, Example
maxNumbers = CInt(InputBox("How many numbers do " & _
"you wish to sum?"))
total = 0
For x = 1 to maxNumbers
input = InputBox("Enter a number.")
If input = "" Then
Exit For
Else
num = CDbl(input)
total += num
End If
Next x
MessageBox.Show("The sum of the numbers is " & total.ToString)

27. When to Use the Do While Loop
Use the Do While loop when you wish the loop to repeat as long as the test expression is true
You can write the Do While loop as a pretest or posttest loop
Pretest Do While loops are ideal when you do not want the loop to iterate if the test expression is false from the beginning
Posttest loops are ideal when you always want the loop to iterate at least once

28. When to Use the Do Until Loop
Use the Do Until loop when you wish the loop to repeat until the test expression is true
You can write the Do Until loop as a pretest or posttest loop
Pretest Do Until loops are ideal when you do not want the loop to iterate if the test expression is true from the beginning
Posttest loops are ideal when you always want the loop to iterate at least once

29. When to Use the For Next Loop
The For...Next loop is a pretest loop that first initializes a counter variable to a starting value
It automatically increments the counter variable at the end of each iteration
The loop repeats as long as the counter variable is not greater than an end value
The For...Next loop is primarily used when the number of required iterations is known

30. Nested Loops
Nested Loops Are Necessary When a Task Performs a Repetitive Operation and That Task Itself Must Be Repeated

31. Nested Loop Example, I For hours = 0 To 24
lblHours.Text = hours.ToString
For minutes = 0 To 59
lblMinutes.Text = minutes.ToString
For seconds = 0 To 59
lblSeconds.Text = seconds.ToString
Next seconds
Next minutes
Next hours