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Visual Basic Looping Statements, Input Boxes, and List Boxes

Learn about Visual Basic looping statements (Do...While, Do...Until, For...Next) and how to use Input Boxes and List Boxes for user input and selection. Explore properties and methods for validation.

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Visual Basic Looping Statements, Input Boxes, and List Boxes

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  1. Week 6 Lists, Loops, Validation, and More

  2. Introduction • This chapter covers the Visual Basic looping statements • Do … While • Do … Until • For … Next • It also discusses the use of • List Boxes • Combo Boxes • As well as presenting some properties and events used for user input validation

  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 (required) • 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 • Square brackets around Title and following arguments indicate these are optional

  5. Sample InputBox Usage strUserInput = InputBox("Enter the distance.", _ "Provide a Value", "150") • If the users clicks OK without entering a value, 150 will be assigned to strUserInput due to the default value

  6. Xpos & Ypos • Xpos specifies the distance from the left of the screen to the left side of the box • Ypos specified the distance from the top of the screen to the top of the box • Both are specified in pixels

  7. List Boxes List Boxes Display a List of Items and Allow the User to Select an Item From the List

  8. The ListBox Control A ListBox control displays a list of items and allows the user to select one or more Drag from Toolbox to create this control on a form

  9. ListBox Items Property • The Items property holds an entire list of values from which the user may choose • The list of values may be established at run time or as part of the form design • To set list values in the form design: • Select the list box in the Design window • View properties & click the Items ellipsis button • This property is a collection, a list of values • Type each value on a separate line

  10. ListBox Items.Count Property • This property returns an integer with the number of entries stored in the Items property • Example of use: • The number of entries in the list can be assigned to an integer variable If lstEmployees.Items.Count = 0 Then MessageBox.Show("The list has no items!") End If numEmployees = lstEmployees.Items.Count

  11. Item Indexing • The Items property values can be accessed from your VB code • Each item value is given a sequential index • The first item has an index of 0 • The second item has an index of 1, etc. • Example: name = lstCustomers.Items(2) ' Access the 3rd item value

  12. ListBox SelectIndex Property • The SelectIndex property returns an integer with the index of the item selected by the user • If no item is selected, the value is set to -1 (an invalid index value) • Can use SelectIndex to determine if an item has been selected by comparing to -1 • Example: If lstLocations.SelectedIndex <> -1 Then location = lstLocations.Items(lstLocations.SelectedIndex) End If

  13. ListBox SelectedItem Property • Instead of using the SelectedIndex property as follows: • The SelectedItem property can be used to retrieve the value of a selected item as follows: If lstMonths.SelectedIndex <> -1 Then month = lstMonths.Items(lstMonths.SelectedIndex) End If If lstMonths.SelectedIndex <> -1 Then month = lstMonths.SelectedItem.ToString) End If

  14. ListBox Sorted Property • Sorted is a boolean property • When set to true, values in the Items property are displayed in alphabetical order • When set to false, values in the Items property are displayed in the order they were added

  15. ListBox Items.Add Method • Items can be added to the end of a ListBox list in your VB code using the Add method • Format is ListBox.Items.Add(Item) • ListBox is the name of the control • Item is a string value to add to the Items property • Example: lstStudents.Items.Add("Sharon")

  16. ListBox Items.Insert Method • Items can be added at a specific position of a ListBox in VB code using the Insert method ListBox.Items.Insert(Index, Item) • Index specifies position where Item is placed • Index is zero based similar to SelectedIndex property • Items that follow are “pushed” down • Example inserting "Jean“ as the 3rd item lstStudents.Items.Insert(2, "Jean")

  17. ListBox Methods to Remove Items • ListBox.Items.RemoveAt(Index) • Removes item at the specified index • ListBox.Items.Remove(Item) • Removes item with value specified by Item • ListBox.Items.Clear() • Removes all items in the Items property • Examples: lstStudents.Items.RemoveAt(2) ‘remove 3rd item lstStudents.Items.Remove(“Jean”) ‘remove item Jean lstStudents.Items.Clear() ‘remove all items

  18. Other ListBox Methods • ListBox.Items.Contains(Item) • Returns true if Item is found in the collection • ListBox.Items.IndexOf(Item) • Returns an integer with the index position of the first occurrence of Item in the collection • Examples: blnFound = lstMonths.Items.Contains(“March”) intIndex = lstMonths.Items.IndexOf(“March”)

  19. The Do While Loop A Loop Is Part of a ProgramThat Repeats

  20. Repetition Structure (or Loop) • Visual Basic has three structures that allow a statement or group of statements to repeat • Do While • Do Until • For...Next

  21. Expression statement(s) True False Do While Flowchart • The Do While loop • If the expression is true, the statement(s)are executed • Expression is thenevaluated again • As long as the expression remains true, the statement(s) continue to be repeated

  22. Do While Syntax • Do, While, and Loop are new keywords • The Do While statement marks the beginning of the loop • The Loop statement marks the end • The statements to repeat are found between these and called the body of the loop Do While expression statement(s) Loop

  23. 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 intCount As Integer = 0 Do While intCount < 10 lstOutput.Items.Add("Hello") intCount = intCount + 1 Loop End Sub Note that programming styledictates the body of theloop be indented for clarity

  24. Infinite Loops • A loop must have some way to end itself • Something within the body of the loop must eventually force the test expression to false • In the previous example • The loop continues to repeat • intCount increases by one for each repetition • Finally intCount is not <10 and the loop ends • If the test expression can never be false, the loop will continue to repeat forever • This is called an infinite loop

  25. Counters • Variables called counters are frequently used to control Do While loops • intCount in previous example is a counter • Counters generally initialized before loop begins Dim intCount As Integer = 0 • Counter must be modified in body of loop intCount = intCount + 1 • The test expression ends the loop when the counter compares to some value

  26. Pretest vs. Posttest Loops • Previous Do While loops are in pretest form • Expression is tested before the body of the loop is executed • The body may not be executed at all • Do While loops also have a posttest form • The body of the loop is executed first • Then the expression is evaluated • Body repeats as long as expression is true • A posttest loop always executes the body of the loop at least once

  27. Posttest Loop Syntax and Flowchart • The statement(s) mustbe executed at leastonce, irrespective of the expression used Do statement(s) Loop While expression statement(s) Expression True False

  28. A Posttest Running Total Loop intCount = 1 ' Initialize the counter decTotal = 0 ' Initialize total Do strInput = InputBox("Enter the sales for day " & _ intCount.ToString, "Sales Amount Needed") If strInput <> "" Then decSales = CDec(strInput) decTotal = decTotal + decSales ' Add sales to total intCount = intCount + 1 ' Increment the counter End If Loop While intCount <= 5

  29. The Do Until andFor Next Loops A 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

  30. Do Until vs. Do While • A Do While loop • Repeats as long as its test expression is true • Ends when its test expression becomes false • A Do Until loop • Repeats as long as its test expression is false • Ends when its test expression becomes true • The Do Until loop has a pretest and posttest form just as a Do While loop

  31. Do Until: Pretest & Posttest Forms • Pretest: • Posttest: Do Until expression statement(s) Loop Do statement(s) Loop Until expression

  32. Do Until Loop – Test Score Average strInput = InputBox("How many test scores do you " _ & “want to average?", "Enter a Value") intNumScores = CInt(strInput) ‘ Store starting values sngTotal = 0 intCount = 1 ‘ Get the test scores Do Until intCount > intNumScores strInput = InputBox("Enter the value for test score " _ & intCount.ToString, "Test Score Needed") sngTotal = sngTotal + CSng(strInput) intCount = intCount + 1 Loop ‘ Calculate the average If intNumScores > 0 then sngAverage = sngTotal / intNumScores Else sngAverage = 0.0 End If

  33. For…Next Loop • Ideal for loops that require a counter • For, To, and Next are keywords • CounterVariable tracks number of iterations • StartValue is initial value of counter • EndValue is counter number of final iteration • Optional Step allows the counter to increment at a value other than 1 at each iteration of the loop For CounterVariable = StartValue To EndValue [Step] statement Next [CounterVariable]

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

  35. For…Next Example • The following code uses a For…Next loop to place the squares of the numbers 1 through 10 in a ListBox For intCount = 1 To 10 intSquare = CInt(intCount ^ 2) strTemp = "The square of " & intCount.ToString _ & “ is “ & intSquare.ToString lstOutput.Items.Add(strTemp) Next intCount

  36. More on the StepValue • It’s optional and if not specified, defaults to 1 • The following loop iterates 11 times with counter values 0, 10, 20, …, 80, 90, 100 • StepValue may be negative, causing the loop to count downward 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

  37. Example: User Specify End Value Dim intCount, intMaxNumbers As Integer Dim dblTotal As Double = 0 Dim dblNum As Double Dim strInput As String intMaxNumbers = CInt(InputBox("How many numbers do " & _ "you wish to sum?")) For intCount = 1 to maxNumbers strInput = InputBox("Enter a number.") dblNum = CDbl(strInput) dblTotal = dblTotal + dblNum Next intCount MessageBox.Show(“Sum of the numbers is " & total.ToString)

  38. When to Use the Do While Loop • Use Do While when the loop should repeat as long as the test expression is true • Can be written as a pretest or posttest loop • A pretest Do While is ideal when the body should not be perfomed for a test expression that is initially false • Posttest loops are ideal when you always want the loop to iterate at least once

  39. When to Use the Do Until Loop • Use Do Until when the loop should repeat as long as the test expression is false • Can be written as a pretest or posttest loop • A pretest Do Until is ideal when the body should not be perfomed for a test expression that is initially true • Posttest loops are ideal when you always want the loop to iterate at least once

  40. When to Use the For Next Loop • The For...Next loop is a pretest loop ideal when a counter is needed • 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 • Used primarily when the number of required iterations is known

  41. Nested Loops A Loop that is Inside Another Loop is Called a Nested Loop

  42. Nested Loops • The body of a loop can contain any type of VB statements including another loop • When a loop is found within the body of another loop, it’s called a nested loop

  43. Nested Loop Example • A clock is an example of a nested loop • Minute hand repeats 60 times for each hour • Second hand repeats 60 times for each minute For intHours = 0 To 24 lblHours.Text = intHours.ToString For intMinutes = 0 To 59 lblMinutes.Text = intMinutes.ToString For intSeconds = 0 To 59 lblSeconds.Text = intSeconds.ToString Next intSeconds Next intMinutes Next intHours

  44. Nested Loop Example Analysis • The innermost loop will iterate 60 times for each iteration of the middle loop • The middle loop will iterate 60 times for each iteration of the outermost loop • 24 iterations of the outermost loop require: • 1,440 iterations of the middle loop • 86,400 iterations of the innermost loop • An inner loop goes through all its iterations for each iteration of the outer loop • Multiply iterations of all loops to get the total iterations of the innermost loop

  45. Multicolumn List Boxes,Checked List Boxesand Combo Boxes A Multicolumn List Box Displays Items in ColumnsA Checked List Box Displays a Check Box Next to Each Item in the List A Combo Box Is Like a List Box Combined With a Text Box

  46. List Box Multicolumn Property • The ListBox has a Multicolumn property • Boolean property with default value of false • If set to true, entries can appear side by side • Below, ColumnWidth is set to 30 • Note the appearance of a horizontal scroll bar in this case

  47. Example For intNumber = 0 To 100 lstNumbers.Items.Add(intNumber) Next

  48. Checked List Box • One item at a time may be selected but many items in a Checked List Box can be checked • The CheckOnClick property determines how items may be checked • False - user clicks item once to select it, again to check it • True - user clicks item only once to both select it and check it

  49. Finding the Status of Checked Items • The GetItemChecked method returns true if the item at Index has been checked CheckedListBox.GetItemChecked(Index) Dim i as Integer Dim intCheckedCities as Integer = 0 For i = 0 to clbCities.Items.Count – 1 If clbCities.GetItemChecked(i) = True Then intCheckedCities += 1 End If Next i MessageBox.Show(“You checked “ & _ intCheckedCities.Tostring() & “ cities.”)

  50. Combo Boxes Similar to List Boxes • Both display a list of items to the user • Both have Items, Items.Count, SelectedIndex, SelectedItem, and Sorted properties • Both have Items.Add, Items.Clear, Items.Remove, and Items.RemoveAt methods • These properties and methods work the same with combo boxes and list boxes

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