CIS 230, Visual Basic

Ch. 7 Notes, page 1

Objective #1: Create and use list boxes and combo boxes.

• To handle information within a VB form, you can use the list box and combo box controls.
• There are three styles of combo boxes: simple combo boxes, dropdown combo boxes, and dropdown lists. No doubt you have already seen all three of these in commercial Windows applications.
  • Combo boxes can provide choices to the user at Run time. Instead of simply displaying information, a combo box allows the user to select one item.
  • Combo boxes also have a Text property which displays on the screen for 2 of the 3 combo box styles.
  • Use the prefix "cbo" when naming combo boxes.
• List boxes can be used to display information on a form.
  • If a list box is too small to display all of its items, VB adds a scroll bar to the list box.
  • Use the prefix "lst" when naming a list box.
• Three events associated with list boxes and combo boxes are particularly useful. You will learn how to use the Change, GotFocus, and LostFocus events in a later chapter.

Objective #2: Enter items into list boxes using the Properties window and the AddItem method.

• There are a couple of ways to input data into a list box.
  • You can type the data for a list box or a combo box into its List property during design time. Be sure to press Control + Enter between items.
  • You can use the AddItem method during Run time in order to add items to the List property. If the name of your list box is lstPlayers then the statement         lstPlayers.AddItem "Mickey Mantle"    would add an item to the end of the list box. You can place an item in a specific position by specifying the position in a statement like,        lstPlayers.AddItem "Mickey Mantle", 4    This method would move the original items in positions 4 and beyond back one place.    

Objective #3: Determine which item in a list is selected.

• When a user selects an item in a list box, the index number (that is, position) of that item is stored in the ListIndex property of the list box or a combo box. If no list item is selected by the user, the ListIndex property is set to -1. You can then display that item which was selected by the user somewhere else in your project, in a label perhaps.

Objective #4: Use the ListCount property to determine the number of items in a list.

• To display the number of items that are contained in a list box or a combo box, you can use the ListCount property. The statement   lblNumPlayers = lstPlayers.ListCount        would display the number of items in the list box lstPlayers in the label lblNumPlayers.

Objective #5: Display a selected item from a list.

• If you simply need to display an item from the list box that is in a specific position, you can use the List property of the list box. For example, if "Yogi Berra" is in the third position of the list box then the statement
     lblCatcher = lstPlayers.List(3)         would place his name in the label lblCatcher. Note that the index position of the string "Yogi Berra" is 2 since the first item of a list box is stored in index position 0.

Objective #6: Differentiate among the available types of combo boxes.

• Unlike list boxes, combo boxes have a Style property which determines which of three styles the combo box is to be:
  • dropdown combo - allows the user to see the Text property and to select an item from a dropdown list of individual items.
  • simple combo box - does not allow the user to see the Text property and only allows the user to enter text into the text box area at Run time.
  • dropdown list - does not allow the user to see the Text property but does allow the user to select from a dropdown list of individual items.

Objective #7: Use Do/Loops and For/Next statements to iterate through a loop.

• Programmers make much use of repetition statements. One type of repetition statement is a loop. There are two kinds of loops, indeterminate loops and determinant loops. Do/Loops are commonly used indeterminate loops in VB and For/Next loops are determinant ones.
• Do/Loops are indeterminate because they repeat a sequence of statements until a given condition is met. How many times the loop will iterate (i.e. repeat) cannot be determined by the programmer at Design time. The condition is specified in a control expression that is placed at the "top of the loop" (i.e. on its first line) or at the "bottom of the loop" (i.e. on its last line.) If the control expression is located at the top of the loop then the loop can be called a pretest. Otherwise, it is a posttest. Pretests are also called top-checking loops and posttests are bottom-checking loops.
  
  • One typical format of a pretest is:
    
    Do Until (control expression)
            statements
    Loop
    
    where the control expression is any condition that evaluates to True or False. If the the expression is False, then the computer skips the remaining statements of the loop and the program continues with the first statement after the loop. Any number of statements can be placed (and should be indented) in the loop.
  • A typical format of a posttest is:
    
    Do
            statements
    Loop Until (control expression)
    
  • In both examples above the keyword Until can be replaced by While to change the logic of the algorithm.
  • Boolean variables used as flags are very useful in indeterminate loop control expressions. A Boolean variable can only hold the value of True or False. By setting a Boolean variable blnDone to True, a programmer could exit the loop such as
    
    blnDone = False
    
    Do Until blnDone
    statements
    Loop
    
  • Be careful not to write infinite (or endless) loops. Indeterminate loops can be infinite if the control expression never causes the loop to terminate.
    
• For/Next loops on the other hand are determinant loops. A programmer can tell how many iterations a For loop will have at design time. A loop index (sometimes called a loop variable) can be used to step through the loop a predetermined number of times.
  • An example of a For/Next loop is:
    
    For j = 1 To 10
    statements
    Next j
    
    in which case the loop would iterate 10 times exactly. Note that j's final value will be 11.
  • A For/Next loop can use of a step value. In the example,
    
    For m = 1 To 100 Step 20
    statements
    Next m
    
    The loop would iterate 5 times with m taking on the values 1, 21, 41, 61, and 81. While m's final value would be 101, the loop would not iterate when m equals 101.
  • Do not attempt to change the value of the loop index within the statements of the loop. This is bad programming style and will cause errors.
  • If you wish to break out of a loop in some situation, you may use an   Exit For    statement inside the loop. This will cause the program control to move to the first line of the program outside of the loop.
    

Objective #8: Use the MsgBox function to determine the button pressed by the user.

• Message boxes can be used to display more than one button. In fact, you can determine which button the user pressed.
• The syntax for using the MsgBox function is        MsgBox(Prompt [, Buttons] [,Title])        where Prompt is the prompt string the the user reads in the message box. Buttons refers to a combination of Visual Basic buttons that appear in the message box. Title refers to the caption on the message box. The function returns an integer value between 1 and 7 indicating which button was pressed. See the charts on p. 262 for details of using the MsgBox function.

Objective #9: Use the string functions Left, Right, and Mid to refer to part of a string and use the Len function to count the number of characters in a string.

• There are a number of string functions that can be used in VB to format strings nicely for output or display. For example, if a person enters their name as "John Doe", you may wish at a later point in the program to display the name "Mr. Doe". You could use string concatenation along with the Right function to do this.
• The Left, Right, and Mid string functions each return a selected portion of a string.
  • The Left function returns a specific number of characters from the left end of a string variable, a string literal, or a text box.
    • The general form of the Left function is Left(StringExpression, NumberOfCharacters)
    • Examples:
      • Left("John", 2)     returns "Jo"     ' note that "John" is a string literal
      • Left(strName, 5)     returns "Sally" if the string variable strName = "Sally Fields"
      • Left(txtEntry, 3)    returns "113" if the Text property of the textbox, txtEntry is "113,456"
  • The Right function returns a specific number of characters from the right end of a string expression.
    • The general form of the Right function is Right(StringExpression, NumberOfCharacters)
    • Examples:
      • Right("John", 3)    returns "ohn"
      • Right(strName, 5)    returns "ields" if the string variable strName = "Sally Fields"
      • Right(txtEntry, 3)    returns "456" if the Text property of the textbox, txtEntry is "113,456"
  • The Mid function returns a substring that begins with a particular character of the original string and ends with a specified spot.
    • The general form of the Mid function is Mid(String Expression, StartPosition, NumberOfCharacters). The third argument NumberOfCharacters is optional. If it is not specified then the whole substring formed by the characters after the StartPosition is returned.
    • Examples:
      • Mid("John", 3, 1)    returns the letter "h"
      • Mid(strName, 5, 3)    returns " Fi" (a total of 3 characters with the first one being a space) if the string variable strName = "Sally Fields"
      • Mid(txtEntry, 3)    returns "3,456"
  • The Len function returns the length of a string expression as an integer. All of the characters in the string expression are counted as part of the length including spaces and punctuation symbols.
    • Examples:
      • Length("Mississippi") = 11
      • Length(strName) = 12 where strName = "Sally Fields"
      • Length("Bill" & "Gates") = 9
      • Length(Right("Visual", 3) & "Basic") = 8
  • The SelStart and SelLength properties of text boxes can be useful with the string formatting functions. When the focus is placed into a text box, the user can select any number of characters in the string expression of its Text property. The SelStart property of a text box can set or return the position of the first selected character. The SelLength property can set or return the number of selected characters.
    • Examples:
      • This event subroutine causes the text within the text box named txtExample to be selected as soon as it receives the focus:
        
        Private Sub txtExample_GotFocus( )
            With txtExample
                .SelStart = 0
                .SelLength = Len(.Text)
            End With
        End Sub

Objective #10: Send information to the printer using the Print method.

• The Print method allows you to print text on a form, on the Printer object, or in the Debug window.
• Each time that you use the Printer.Print method, your output is added to the Printer object which is created by VB. When the print job ends or when it receives an EndDoc or NewPage method, VB sends the current contents of the Printer object to the printer.
• The NewPage method actually sends the current page to the printer and clears the printer object in the memory of the computer so you can begin a new page. The EndDoc method sends the current page to the printer and terminates the whole printer job. Within code the statements would look like        Printer.NewPage         and     Printer.EndDoc    since they are methods.

Objective #11: Control the format of printing using commas, semicolons, the Tab function, and the Spc function.

• Choose a fixed-pitch font such as Courier, if you want every character of your printed output to take the same amount of space. Otherwise, you may be using a proportional font.
• A VB output page has preset tab settings with five columns per line. Each column is a print zone. A comma placed in a string of output is used to advance a single print zone. Therefore a comma works similarly to a tab in word-processing or typing.
  • Example:    Printer.Print "Name", "Social Security Number"    causes Name to appear in the first print zone and Social Security Number in the second print zone.
  • The only way to change the size of a print zone is to change the font size. The width of a print zone is 14 characters, based on the average size of a character for the font being used. So more than 14 characters could fit in a print zone if you used a proportional font and included many i's and t's. Sometimes, the items you print could overflow the print zone. If the number of print zones exceeds the number for a single line, the output will word wrap to the next line on the printed page. To change the Font property of the Printer object, you must use statements like    Printer.Font.Name = "Arial"    and     Printer.Font.Size = 12    
• If you wish two string expressions to be printed next to each other you can use a semicolon in the output. For example, Printer.Print "Vis";"ual"     will cause the string "Visual" to be printed on the printer.
• To print a blank line, simply use the statement       Printer.Print
• The Tab function can be used to specify the column position in which you wish to precisely print a string expression. For example, the statement        Printer.Print Tab(10); "Hello"        prints the string hello where the 'H' in the 10th column of the line.
• The Spc function can be used to specify a number of spaces between several printed items. For example, the statement   Printer.Print "John";Spc(10);"Doe"    causes 10 blank spaces to appear between "John" and "Doe".
• It is possible to print string expressions next to each other with statements like      Printer.Print strFirst & strLast     which would print "JohnDoe" if strFirst = "John" and strLast = "Doe". But VB automatically adds a space in front of a numerical value for the sign (positive or negative). Therefore, the statement    Printer.Print "123" & 456    would print "123 456" since space is allowed for the sign of the numerical value 456.

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