ISM 3230 Introduction to Programming

 Chapter 7: Lists, Loops, and Printing

Download Example Programs

List Boxes and Combo Boxes

• List/Combo boxes allow a list of items
• Types include
  • simple combo boxes
  • dropdown combo boxes
  • dropdown lists
• Combo box has style property; list does not
• Combo box has design-time text property; list box has only run-time text property.
• Combo box does not have a MaxLength property; a list box does.

Filling the List

• You can fill the list at design- or run-time
  • Fill at design time if list rarely changes
  • Click List property, type entry, press Ctrl+Enter
  • Make Sorted property true to keep list in order
• Fill at run time if list is volatile
  • Use AddItem method to add items
  • Form: object.AddItem value [,Index]
  • Example: lstFruits.AddItem "Apple"

   

List/Combo Box Methods

• AddItem - Add item to list

• Clear - Delete all items from the list

• Add items to a list with code like this:

  lstSchools.AddItem "Stanford"

  lstSchools.AddItem txtSchoolName.Text

  lstSchools.AddItem cboFirstChoice.Text

• When a user types an entry in a combo box, that entry is not automatically added to the list. Code must be invoked to add the item (on the lost focus event, for example)

  cboIdeaList.AddItem cboIdeaList.Text

List/Combo Box Properties

• Sorted maintains list in order (if true)
• ListIndex holds user-selected item index number
  • begins at zero, -1 if none selected
  • setting the ListIndex property highlights a selection in the list
• ListCount is number of items in list
  • if the list is empty, then the ListCount is zero

 

• List object.List(index) extracts string value of list item
  • Deselect all items in a list by setting the ListIndex property to -1
  • Set the ListIndex property to 0 to select (highlight) the first item in the list
  • Remember that indexes run from 0 to n-1
  • ListCount holds the number of items in a list.
  • ListCount is always one higher than the highest ListIndex.
  • Display one of the items in a list by selecting the list object and referring to the ListIndex value as an index:

    lstBookTypes(iValue)

• Add items to the list in code with:

  lstSchools.AddItem "Purdue"

lstSchools.AddItem "University of San Diego"

• The Sorted property of the list keeps items in alphabetic order.
• To copy a selected list item to a text box:

  txtSelection.text = cboFruit.List(3)

txtSelection.text = cboFruit.List(cboFruit.ListIndex)

• Setting a list index to a value selects—highlights—the list item:

  lstCoffeeTypes.ListIndex = 3 ‘Highlight the 4th item in list

  lstCoffeeTypes.ListIndex = -1 'Deselect all in list

List/Combo box Events

• Change
  • Change event occurs whenever the user types anything into a combo box (styles 0 and 1).
  • Each keystroke generates another change event.
  • A list box does not have a change event, because list boxes do not have text boxes—only fixed lists that the user cannot add to by typing at runtime.
• GotFocus
  • GotFocus event occurs when a control—a list box or combo box for example—receives control.
• LostFocus
  • LostFocus event occurs as the control loses the focus—when the user tabs to another control on the form for example.

Do Loops

• Allow repeated execution of instructions
• Do loop terminates on a condition you specify
• Do loop can test for condition at
  • Top of loop (pretest)
  • Bottom of loop (posttest)
• Loops can continue while a condition is either true or false
  • Go while True: …While condition
  • Go while False: …Until condition
• Be sure to change the loop variable within the loop or it will loop infinitely!

For/Next Loops

• For/Next allows you to repeat series of instructions using a counter (a loop index)
• Form:

• You can loop "backwards" by using a negative increment
• Loop testing is done before executing body
• Do not change the loop control inside the body
• Halt infinite loops by pressing Ctrl+Break
• Exit For allows early For loop exit from within the body, but should be avoided - poor programming practice

• Looping through a list:

  Dim intLoopIndex As Integer

  For intLoopIndex = 0 To lstSchools.ListCount - 1

  ... (body of loop here)...

Next intLoopIndex

• Loop test is for greater than limit, not greater than or equal to limit
• Use For/Next loop when you know the number of times a loop will run
• When loop decrements, the test is for strictly less than the ending value
• The For/Next ending condition may exist before entering the loop. If so, then the loop body is skipped

• Example of an infinite (endless) loop:

  Dim intLoop As Integer

  For intLoop = 0 To 10 Step 1

  intLoop = 1

  Next

Using the MsgBox Function

• MsgBox has a function form (we have used statement form until now)
• MsgBox function returns which key the user pressed of those available
• Use VB intrinsic constants to determine which key was pressed.

MsgBox Values

Important button and icon values:

• Button to Display Intrinsic Constant
• OK - vbOKOnly
• OK and Cancel - vbOKCancel
• Abort, Retry, Ignore - vbAbortRetryIgnore
• Yes, No, Cancel - vbYesNoCancel
• Yes and No - vbYesNo
• Retry and Cancel - vbRetryCancel
• Icons to Display Intrinsic Constant
• Critical Message - vbCritical
• Warning Query - vbQuestion
• Warning Message - vbExclamation
• Information Message - vbInformation

Examples of using instrinsic constants:

intResp = MsgBox(strMess, vbYesNoCancel + vbQuestion, "Do What?")

intResp = MsgBox(strMess, vbCancel + vbCritical, "Big Problems")

intResp = MsgBox(strMess, vbOKOnly + vbInformation, "FYI")

Using String Functions

• Left(str, nchars) - extract left nchars
• Right(str, nchars) - extract right nchars
• Mid(str,start,[nchars]) - get middle chars
• str is any string, nchars is integer, and start is integer number of starting position
• Len(str) returns # chars in string
• Suppose a text box named txtName contains the following:

• Left(txtName.Text, 3) is "Wha" (without the quotation marks)
• Right(txtName.Text, 3) is "ob?" (without the quotation marks)
• Mid(txtName.Text, 6, 5) is "about" (without the quotation marks)
• Mid (txtname.Text, 6) is "about Bob?" (without the quotation marks)

• String functions and looping facilitate the "zooming" feature of many Microsoft list boxes and combo boxes.
• "Zooming" causes a search of the list, refining the search with each additional keystroke the user enters.

Examine the following code to ensure yourself that it mimics the Windows activity of "zooming" to a list box entry in attempting to match what the user types:

Sending Information to the Printer

• Visual Basic’s Print method applies to a Form, a printer object, the Debug window, or a picture box
• Printer.Print — sends output to printer
• Print is a method of the Printer object
• Printing is somewhat difficult, because VB is designed for use with forms, not printers

• You format printer lines just like normal Basic programming does—with commas, print zones, Tab, Spc functions, etc.
• Examples:

  Printer.Print, "Hello World" ‘prints message in 2nd print zone

  Printer.print "Name:"; txtName ‘do not use print zones

  Printer.Print "" ‘print a blank line

• Output is not actually printed until:
  • job is terminated or
  • you go to a new page (use the NewPage method) or
  • you end the entire printing process (use EndDoc method)
• Change font: Printer.Font.Name = "Times New Roman"
• Change point size: Printer.Font.Size = 12
• Go to a new page: Printer.NewPage
• Send end command Printer.EndDoc

Formatting Lines

• Printer.print x, y, z prints in three "zones"
• Printer.print x;y;z ignores zones
• Trailing comma or semicolon causes next print method to continue the line
• Each .print starts on a new line
• Printer.print (alone) prints blank line
• Tab(20) advances to 20th position:

• Spc(17) inserts spaces (17) in line
• Numbers print with one space for sign of #
• Printer.NewPage sends page to printer and goes to new page
• Printer.EndDoc sends page to printer and halts print job

Printing to a Picture Box

• You can use Print method with a picture box to avoid actually printing a result:
• picMyPictureBox.Print "howdy" prints to a picture box

Hands-On Programming Example

Chapter Summary:

OBJECTS		PROPERTIES		METHODS		STATEMENTS		FUNCTIONS
List box		List		AddItem		Do / Loop		Left
Combo box		ListCount		Clear		Exit For		Len
		ListIndex		EndDoc		For / Next		Mid
		Sorted		NewPage				Right
		Style		Print				Spc
				RemoveItem				Tab