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McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved.
Lists, Loops, and Printing
Chapter 7
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-2
Objectives (1 of 2)
• Create and use list boxes and combo boxes.
• Differentiate among the available types of combo boxes.
• Enter items into list boxes using the Items collection in the Properties window.
• Add and remove items in a list at run time.
• Determine which item in a list is selected.
• Use the Items.Count property to determine the number of items in a list.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-3
Objectives (2 of 2)
• Display a selected item from a list.
• Use Do/Loops and For/Next statements to iterate through a loop.
• Terminate a loop with the Exit statement.
• Skip to the next iteration of a loop by using the Continue statement.
• Send information to the printer or the Print Preview window using the PrintDocument class.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-4
Filling a List — Design Time
Click ellipses button to open
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-5
ListBoxes and ComboBoxes (1 of 2)
• Have most of the same properties and operate in a similar fashion
•An exception is that a combo box control has a DropDownStyle property
• Provide the user with a list of items to select from
• Various styles — choose based on
•Space available
•Need to select from an existing list
•Need to add to a list
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-6
List Boxes and ComboBoxes (2 of2)
Various Styles of List and Combo boxes
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-7
The Items Collection
• List of items in a ListBox or ComboBox is a collection.
• VB Collections are objects that have properties and methods that allow
• Adding items• Removing items• Referring to individual elements• Counting items• Clearing the collection
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-8
Filling a List/Using the Properties Window
• Design time in Properties window• Items property
• Click on ellipses to open StringCollection Editor.
• Type list items, end each line withEnter key.
• Run time methods• Items.Add
--OR--
• Items.Insert
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-9
Using the Items.Add Method
• Use to add new items to the list at run time.
• General Form
• ExamplesObject.Items.Add(ItemValue)
SchoolsListBox.Items.Add("Harvard")SchoolsListBox.Items.Add("Stanford")SchoolsListBox.Items.Add(schoolsTextBox.Text)MajorsComboBox.Items.Add(majorsComboBox.Text)MajorsComboBox.Items.Add(majorString)
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-10
Using the Items.Insert Method
• Use to add new items to the list at run time in a specific location (index position) in the collection.
• General Form
• ExamplesSchoolsListBox.Items.Insert(0, "Harvard")MajorsComboBox.Items.Insert(1, MajorsComboBox.Text)
Object.Items.Insert(IndexPosition, ItemValue)
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-11
The SelectedIndex Property
• Index number of currently selected item is stored in the SelectedIndex property.
• If no list item is selected, SelectedIndex property is negative 1 (-1).
• Use to select an item in list or deselect all items in code.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-12
The Items.Count Property
• to determine number of items in the listRemember: Items.Count is always one more than the highest possible SelectedIndex, because indexes begin with 0For example, if there are five items in a list:
Items.Count = 5 ANDHighest Index = 4
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-13
Referencing the Items Collection
• Use the index of the item to reference a specific item in the collection.
• Remember that the index is zero based, so the first item in the list is index position zero.
SchoolsListBox.Items(5) = "University of California"MajorLabel.Text = MajorsComboBox.Items(IndexInteger)SelectedMajorLabel.Text = MajorsComboBox.Items(MajorsComboBox.SelectedIndex)SelectedMajorLabel.Text = MajorsComboBox.Text
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-14
Removing an Item from a List
• Use the Items.RemoveAt method to remove an item by index from the list and the Items.Remove method to remove by specifying the text.
• General Form
• ExamplesNamesListBox.Items.RemoveAt(0)' Remove the item in position IndexInteger.SchoolsComboBox.Items.RemoveAt(IndexInteger)CoffeeComboBox.Items.RemoveAt(CoffeeComboBox.SelectedIndex)
Object.Items.RemoveAt(IndexPosition)
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-15
The Items.Remove Method
• Use the Items.Remove method to remove an item by specifying the text.
• General Form
• Examples
NamesListBox.Items.Remove("My School")SchoolsComboBox.Items.Remove(SchoolTextBox.Text)' Next line removes the currently selected item.CoffeeComboBox.Items.Remove(CoffeeComboBox.Text)
Object.Items.Remove(TextString)
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-16
Clearing a List
• Use the Items.Clear method to clear all items and empty a combo box or list box.
• General Form
• Examples
SchoolsListBox.Items.Clear( )MajorsComboBox.Items.Clear( )
Object.Items.Clear( )
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-17
List Box and Combo Box Events
• In the Editor window, select the control name in the Class Namelist (at the top-left of the window), drop down the Method Name list, and select the event for which you want to write code or double-click the event name in the Properties window after clicking the Events button.
• The Editor will create the procedure header for you.
• TextChanged Event
• Occurs when user types text into combo box
• List box does not have TextChanged Event.
• Enter Event (control receives focus) — an Enter event fires when a user tabs from control to control.
• Leave Event (control loses focus) — a Leave event triggers as user tabs between controls.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-18
Do/Loops
• A loop repeats a series of instructions.
• An iteration is a single execution of the statement(s) in the loop.
• Used when the exact number of iterations is unknown
• A Do/Loop terminates based on a specified condition.
• Execution of the loop continues while a condition is True or until a condition is True.
• The condition can be placed at the top or the bottom of the loop.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-19
The Do and Loop Statements — General Form
Do {While |Until} condition
' Statements in loop.
Loop
--OR--
Do
' Statements in loop.
Loop {While | Until} condition
Top of Loop Condition,
Pretest/Entry test
Bottom of Loop
Condition,Posttest/ Exit
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-20
Pretest vs. Posttest
• Pretest — loop may never be executed since tested BEFORE running.
Do While … Loop
Do Until … Loop
• Posttest — loop will always be executed at least once.
Do … Loop While
Do … Loop Until
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-21
Pretest vs. Posttest Diagram
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-22
The Boolean Data Type Revisited
• Can help when searching a list for a specific value
• Boolean variable is always in one of two states: True or False.
• When a particular situation occurs, set Boolean variable to True.
• Use a loop to check for True
• Many programmers refer to Boolean variables as switches or flags.
• Switches have two states — on or off.
• Flags are considered either up or down.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-23
For/Next Loops
• Used to repeat statements in a loop a specific number of times
• Uses a numeric counter variable, called Loop Index, which is tested to determine the number of times the statements inside the loop will execute
• Loop Index is incremented at the bottom of the loop on each iteration.
• Step value can be included to specify the incrementing amount to increment Loop Index, step can be a negative number.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-24
The For and Next Statements — General Form
For LoopIndex [As DataType] = InitialValue To TestValue [Step Increment]
' Statements in loop.Next [LoopIndex]
A For/Next loop can handle all three elements of a counter-controlled loop.
Initialize the counter.
Increment the counter.
Test the counter to determine when it is time to terminate the loop.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-25
For/Next Loop Diagram
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-26
Exiting Loops
• In some situations, you may need to exit the loop prematurely.
• Click on the form’s close box or use the VB menu bar or toolbar to stop the program; or Ctrl+Break.
• Use the Exit For statement inside the loop structure.
• Generally, the Exit For statement is part of an If statement.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-27
Making Entries Appear Selected
• When a user tabs into a text box that already has an entry, the user-friendly approach is to select the text.
• If a text box fails validation, select the text.
• Selecting the entry in a Text Box• Use the SelectAll method
• Good location is in the text box’s Enter event
• Selecting an entry in a List Box• Set the SelectedIndex property to make a single item in a
list box appear selected.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-28
Sending Information to the Printer
• Components appear in the Printing tab of the toolbox.
• Most professional programmers use a separate utility program to format printer reports.
• Several companies sell utilities that do a nice job designing and printing reports.
• VB Professional Edition and Enterprise Edition include Crystal Reports for creating reports from database files.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-29
The PrintDocument Component
• Appears in the Component Tray
• Execute the Print method to start printing.
• The code belongs in the Click event procedure for the Print button or menu item that can be selected to begin printing.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-30
Setting Up the Print Output
• PrintPage event is fired once for each page to be printed, and is referred to as callback.
• BeginPrint and EndPrint are also fired at the beginning and end of the printing.
• PrintPage event includes the argument e as System.Drawing.Printing.PrintPageEventArgs.
• Properties of the PrintPageEventArgs are useful for handling page margins and sending strings of text to the page.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-31
The Graphics Page
• Set up graphics page in memory and then the page is sent to the printer.
• Can contain strings of text and graphic elements
• Specify the exact X and Y coordinates of each element to be printed on the page.
X coordinate is the horizontal distance from across the page; the Y coordinate is the vertical distance from the top of the page.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-32
Using the DrawString Method
• Used to send a line of text to the graphics page
• Belongs to the Graphics object of the PrintPageEventArgs argument
• Is an overloaded method so there are several forms for calling the method
• Arguments for the DrawString method include:• What to print
• What font and color to print in
• Where to print
• Set up the Font and X/Y coordinates to be used before executing the DrawString method.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-33
The DrawString Method
General Form
Examples
DrawString(StringToPrint, Font, Brush, Xcoordinate, Ycoordinate)
e.Graphics.DrawString(printLineString, printFont, Brushes.Black, _ horizontalPrintLocationSingle, verticalPrintLocationSingle)e.Graphics.DrawString("My text string", myFont, Brushes.Black, _ 100.0, 100.0)e.Graphics.DrawString(nameTextBox.Text, New Font("Arial", 10), _ Brushes.Red, leftMarginSingle, currentLineSingle)
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-34
Setting the X and Y Coordinates
• For each print line, specify X and Y coordinates.
• Create variables declared as Single data type to set the X and Y values.
Dim HorizontalPrintLocationSingle As SingleDim VerticalPrintLocationSingle As Single
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-35
PrintPageEventArgs
• PrintPageEventArgs argument has several useful properties that are used to determine the present settings.–MarginBounds–PageBounds–PageSettings
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-36
Printing the Contents of a List Box
• Techniques for printing, a loop, and the list box properties can be combined to send the contents of a list box to the printer
• Use the Items.Count property as the number of iterations to make.
• Items collection allows printing out the actual values from the list.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-37
Aligning Decimal Columns
• It is important to align the decimal points of numeric data.
• Proportional fonts make aligning decimal points tricky.
• Format each number to be printed and measure the length of the formatted string.
• Declare an object as a SizeF Structure which has a Width property.
• Use MeasureString method of the Graphics class to determine the width of a formatted string in pixels.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-38
Aligning Decimal ColumnsCode Example (1 of 2)
' SizeF structure for font size info. Dim FontSizeF As New SizeF( ) ' Set X for left-aligned column. HorizontalPrintLocationSingle = 200 ' Set ending position for right-aligned column. ColumnEndSingle = 500 ' Format the number. FormattedOutputString= AmountDecimal.ToString("C") ' Calculate the X position of the amount. ' Measure string in this font. FontSizeF= e.Graphics.MeasureString(formattedOutputString, _ PrintFont)
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-39
Aligning Decimal ColumnsCode Example (2 of 2)
' SizeF structure for font size info (cont). ' Subtract width of string from the column position. ColumnXSingle = ColumnEndSingle - FontSizeF.Width ' Set up the line--each element separately. e.Graphics.DrawString("The Amount = ", PrintFont, _ Brushes.Black, HorizontalPrintLocationSingle, _ VerticalPrintLocationSingle) e.Graphics.DrawString(FormattedOutputString, printFont, _ Brushes.Black, ColumnXSingle, VerticalPrintLocationSingle) ' Increment line for next line. VerticalPrintLocationSingle += LineHeightSingle
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-40
Displaying a Print Preview
• The PrintPreviewDialog component is the key to print preview.
• Add PrintPreviewDialog component to form.• Appears in the Component Tray
• Default name is fine
• Assign in code the same PrintDocument object you are using for printing.
• Execute the ShowDialog method of the PrintPreviewDialog component.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-41
PrintPreviewDialog Component
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-42
The Using Block
• System resources such as fonts can be access inside a Using block.
• Variables that are declared in a Using block are only accessible within that block.
• The advantage of declaring a variable inside a Using block is that system resources are released as soon as the block terminates.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-43
Printing Multiple Pages
• The PrintDocument’s PrintPage event fires once for each page.
• Set the HasMorePages property of the PrintPageEventArgs argument to True to print more than one page.
McGraw-Hill © 2009 The McGraw-Hill Companies, Inc. All rights reserved. 7-44
Using Static Variables
• Static local variables retain their value for the life of the project.
• Can be useful for• Running totals
• Running counts
• Boolean switches
• Storing current page number/count when printing multiple pages