Microsoft Visual Basic 2005: Reloaded Second Edition

1
Microsoft Visual Basic 2005 Reloaded Second
Edition

• Chapter 5
• Repeating Program Instructions

2
Objectives

• Write loops using For...Next and Do...Loop
• Display a dialog box using the InputBox function
  Include a list box and a combo box in an
  interface
• Initialize and update counters and accumulators
• Create a multiline text box that cannot be edited

3
We use loops because computers are really good at
doing the same thing over over over again...

4
The Repetition Structure (loop)

• This is a structure that repeatedly/iteratively
  processes one or more program instructions
  usually until some condition is met
• Pretest loop
• The condition is evaluated before the
  instructions within the loop are processed
• The instructions may never be processed
• Posttest loop
• The condition is evaluated after the instructions
  within the loop are processed
• The instructions are always processed at least
  once

5
Types of loops in VB

• For...Next
• Used when we know how many times the loop needs
  to iterate the stopping condition is the number
  of times the loop needs to run
• Pretest
• Do...Loop
• Used when we dont know how many iterations are
  needed the stopping condition is not a number
• Pretest (while) or posttest (until)
• More general than the ForNext

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ForNext example

• Dim sum As Integer 0
• Dim num As Integer
• For num 1 To 5
• sum num sum
• Next num
• sumLabel.Text Convert.ToString(sum)
• What value is displayed in the label?

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More ForNext examples

• Compute n!
• Ask a user to enter 10 integers, one at a time,
  and calculate their sum their product. Or,
  find the largest in the list the smallest in the
  list.
• Can repeatedly reuse a text box for this input,
  or better to use an input box
• Ask a user to enter two integers and calculate
  the sum (or product) of all the integers between
  the two integers.

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The For...Next Statement
A nice way to display results from each iteration
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The For...Next Statement syntax

• For num startvalue To endvalue Step stepvalue
• sum num sum
• Next num
• Startvalue, endvalue, and stepvalue items
• Control the number of times the loop is processed
• Must evaluate to numeric values
• Can be positive or negative
• If positive, its a countup
• If negative, its a countdown

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What if we want to display something for each
iteration of the loop?

• We need a way of filling up a text box
  incrementally.
• Can do it horizontally 1 2 3 4 5
• Or vertically 1
• 2
• 3
• 4
• 5

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Horizontally

• We must build the output string as we go
• For num 1 To 5
• evenNumsLabel.Text
• evenNumsLabel.Text num.ToString
• Next num

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Vertically

• We must build the output string as we go
• For num 1 To 5
• evenNumsLabel.Text
• evenNumsLabel.Text num.ToString
  ControlChars.NewLine
• Next num

Whats this?
Whats this?
13
All three loops in VB

• While
• entrance controlled - pretest
• Until
• exit controlled - posttest
• For Next
• Counting - pretest

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How they are alike

• They all have the format
• loop control
• loop body
• loop control
• They all cause the statements in the loop body to
  be executed repeatedly
• loop control contains the condition for loop
  termination

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But each type of loop has its own special
features

• Sometimes its appropriate - even correct - to
  use one type over the others
• Often, the choice is yours
• Lets see how they compare with each other...

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While loops - pretest

• Do While condition
• loop body
• Loop
• The condition is checked before the loop body is
  executed.
• It must be initialized outside the loop.
• The loop body must contain an instruction that
  affects the condition.
• Otherwise, the loop will be infinite!

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Pretest

• Dim Number As Integer
• Number 1
• Do While number lt 3
• messageBox.Show (number.ToString)
• number number 1
• Loop

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The pretest Do...Loop
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Until loops - posttest

• Do
• loop body
• Loop Until condition
• The condition is checked after the loop body is
  executed.
• It does not have to initialized outside the loop
• The loop body must contain an instruction that
  affects the condition.
• Otherwise, the loop will be infinite!

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Posttest

• Dim Number As Integer
• number 1
• Do
• messageBox.Show (number.ToString)
• number number 1
• Loop Until number gt 3

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The posttest Do...Loop
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When to use one or the other?

• If you dont want the loop body to be executed
  even once before the condition is checked
• If you want the loop body to be executed at least
  once before the condition is ever checked
• If you know how many times the loop body should
  be executed...

23
Examples
Problem from last time

• Display the evens between 2 and 8 using
• ForNext
• Do While
• Do Until
• Which do you like the most for this application?
  Why?

24
How to check for correct password?

• Which loop to use?
• How many tries will we give the user?
• First, lets do unlimited
• Then three tries

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Read in data from user inside the loop

• Good idea to use input box.
• Processing average of input values involves
• A counter
• An accumulator
• Both must be initialized
• Both must be updated

26
The Sales Express Application
Use input box
Use input box
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The InputBox Function

• This function displays a predefined dialog box
  that contains
• a text message
• an OK button
• a Cancel button
• an input area
• The InputBox function returns
• The users entry if the user clicks OK
• An empty string if the user clicks Cancel or Close

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The InputBox Function
29
Loops and List Boxes
A match made in heaven!
A match made in heaven!
30
Using a List Box in an Interface

• For now, well learn how to create and fill them
• Next time, well use loops to process through
  them
• The ListBox tool creates a ListBox control that
• displays a list of choices for the user
• The SelectionMode property of the control
  controls the number of choices a user can select
• None user can scroll but not select anything
• One user can select one item
• MultiSimple and MultiExtended user can select
  multiple items

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Using a List Box in an Interface
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Adding Items to a List Box

• Items collection
• a collection of the items in a list box
• Collection
• a group of one or more individual objects treated
  as one unit
• Index
• a unique number that identifies an item in a
  collection
• is zero-relative the first item has index of 0
• Add method
• adds an item to the list boxs Items collection
• Sorted property
• Determines if the list box items are sorted
• Sort order is dictionary order

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Adding Items to a List Box
34
Adding Items to a List Box
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Adding Items to a List Box
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SelectedItem and SelectedIndex Properties

• SelectedItem property
• Contains the value of the selected item in the
  list
• If nothing is selected, it contains the empty
  string
• SelectedIndex property
• Contains the index of the selected item in the
  list
• If nothing is selected, it contains the value -1
• Default list box item
• The item that is selected by default when the
  interface first appears

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SelectedItem and SelectedIndex Properties
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SelectedItem and SelectedIndex Properties
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SelectedValueChanged and SelectedIndexChanged
Events

• SelectedValueChanged or SelectedIndexChanged
  events
• occur when a user selects an item in a list box

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SelectedValueChanged and SelectedIndexChanged
Events
41
Modifying the Monthly Payment Calculator
Application
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Modifying the Monthly Payment Calculator
Application (continued)
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Modifying the Monthly Payment Calculator
Application (continued)
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Using a Combo Box in an Interface

• ComboBox tool creates a combo box control
• ComboBox control
• Similar to a list box
• May contain a text field that allows the user to
  type an entry that is not on the list
• List portion may be hidden
• Three styles of combo boxes
• Simple
• DropDown
• DropDownList

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Using a Combo Box in an Interface (continued)
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Using a Combo Box in an Interface (continued)
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Using a Combo Box in an Interface (continued)
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Using a Combo Box in an Interface (continued)

• SelectedItem property contains the value of the
  selected item in the list
• Text property contains the value that appears in
  the text portion of the control (item selected or
  typed in)

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Using a Combo Box in an Interface (continued)
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Using a Combo Box in an Interface (continued)
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Using a Combo Box in an Interface (continued)
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Using a Combo Box in an Interface (continued)
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Programming Example
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Selecting Existing Text in a Text Box

• Windows standard highlight the existing text
  when a text box receives the focus
• SelectAll method selects all text in a text box
• Enter event occurs when the text box receives
  the focus

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Selecting the Existing Text in a Text Box
(continued)
56
Selecting the Existing Text in a Text Box
(continued)
57
Selecting the Existing Text in a Text Box
(continued)
58
Coding a Controls TextChanged Event Procedure

• TextChanged event
• Occurs when a change is made in a controls Text
  property
• Change may be made by user or the program

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Coding a Controls TextChanged Event Procedure
(continued)
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Coding a Controls TextChanged Event Procedure
(continued)