mcgraw-hill © 2009 the mcgraw-hill companies, inc. all rights reserved. lists, loops, and printing...

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.


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.


Filling a List — Design Time

Click ellipses button to open


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


List Boxes and ComboBoxes (2 of2)

Various Styles of List and Combo boxes


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


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


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)


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)


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.


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


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


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)


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)


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( )


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.


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.


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


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


Pretest vs. Posttest Diagram


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.


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.


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.


For/Next Loop Diagram


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.


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.


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.


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.


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.


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.


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.


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)


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


PrintPageEventArgs

• PrintPageEventArgs argument has several useful properties that are used to determine the present settings.–MarginBounds–PageBounds–PageSettings


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.


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.


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)


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


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.


PrintPreviewDialog Component


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.


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.


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

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