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Microsoft Visual Basic 2008 CHAPTER NINE Using Arrays and File Handling
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Microsoft Visual Basic 2008
CHAPTER NINE
Using Arraysand File Handling
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Chapter 9: Using Arrays and File Handling 2
Objectives
►Initialize an array
►Initialize an array with default values
►Access array elements using a loop
►Use ReDim [ Preserve ] to resize an array
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Chapter 9: Using Arrays and File Handling 3
Objectives
►Determine the number of elements in an array using the Length property
►Use the For Each loop (read-only iteration)
►Initialize two-dimensional arrays
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Chapter 9: Using Arrays and File Handling 4
Objectives
►Read a text file
►Write to a text file
►Calculate depreciation
►Use multiple Form objects
►Access Variable objects on other forms
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Chapter 9: Using Arrays and File Handling 5
Introduction
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Chapter 9: Using Arrays and File Handling 6
Introduction to Arrays
►An array is a collection of data of the same type. It is implemented in VisualBasic as an object.
►Each individual item in array that contains a value is called an element
►Arrays provide access to data by using a numeric index, or subscript, to identify each element in the array
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Chapter 9: Using Arrays and File Handling 7
Declaring/Initializing an Array
300 is the upper bound of the array. This array actually contains 301 elements (zero-based indexing)
9 Example Array Declarations
Dim names(1) as String
names(0) = “Jack”
names(1) = “Marge”
Dim names( ) as String = {"Jack", "Marge"}
Dim names as String( )
names = New String() {"Jack", "Marge"}
-------------------------------------------------------------------------------------------------
MsgBox(names(0), , "First Name")
MsgBox(names(1), , "Second Name")
MsgBox("Array size for names is " & names.Length, , "Array Size")
Chapter 9: Using Arrays and File Handling 8
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Chapter 9: Using Arrays and File Handling 9
Default Data Types
Data Type Default Value
Any numeric data type 0
Reference data type Null
Boolean data type False
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Chapter 9: Using Arrays and File Handling 10
Initializing an Array
►Parallel arrays store related data in two or more arrays
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Chapter 9: Using Arrays and File Handling 11
Accessing Array Elements Using a Loop
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Introduction to Arrays
► The Visual Basic compiler determines if each subscript is within the boundaries set when you initialized the array (bounds checking). If a subscript is out of range an IndexOutOfRangeException exception is generated.
► An array may be declared using a constant value representing the upper-bound index of the array
► Every array in Visual Basic is considered dynamic, which means that you can resize the array at run time
► The ReDim statement assigns a new array size to the specified array variable ReDim arrayX(50)
• All data in the array will be lost
► If you want to preserve the existing data you can use the keyword Preserve
• Ex: ReDim Preserve arrayX(100)
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Chapter 9: Using Arrays and File Handling 13
Using the Length Property
►The Length property of an array contains the number of elements in an array (1 less than upper bound)
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Chapter 9: Using Arrays and File Handling 14
Using Arrays
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Chapter 9: Using Arrays and File Handling 15
The For Each Loop
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The For Each Loop
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Scope of Arrays
►The scope of an array declared within a procedure is local to that procedure, but an array can be declared as a class level variable
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Chapter 9: Using Arrays and File Handling 18
Passing an Array [ pass a reference… ]
►An array can be passed as an argument to a Sub procedure or a Function procedure ( book page 666 )
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Chapter 9: Using Arrays and File Handling 19
Sorting an Array [ Use Array class ]
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Chapter 9: Using Arrays and File Handling 20
Searching an Array
► Searching each element in an array is called a sequential search
► The BinarySearch method searches a sorted array for a value using a binary search algorithm• The binary search algorithm searches an array by repeatedly
dividing the search interval in half
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Chapter 9: Using Arrays and File Handling 21
Creating a Two-Dimensional Array
► A two-dimensional array holds data that is arranged in rows and columns
► A two-dimensional array can also be described as an “array of arrays”.
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Chapter 9: Using Arrays and File Handling 22
Creating a Two-Dimensional Array
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Chapter 9: Using Arrays and File Handling 23
File Handling
►To process data more efficiently, many developers use text files (or binary files) to store and access information for use within an application
►Text files have an extension that ends in .txt
►A simple text file is called a sequential file (actually it is usually called an ascii text file)
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Reading a Text File
►To open a text file, you need an object available in the System.IO namespace called a StreamReader
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Chapter 9: Using Arrays and File Handling 25
File Processing Classes
►System.IO namespace File Processing Classes
►File [ open, close, exists, … ]
►BinaryReader [ binary files ]►BinaryWriter
►StreamReader [ text files ]►StreamWriter
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Reading a Text File
►To determine whether the end of the file has been reached, use the Peek procedure of the StreamReader object
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Reading a Text File
► Open the code editing window by clicking the View Code button on the Solution Explorer toolbar. Click inside the frmDepreciation_Load event
► Initialize the variables.
► Create a StreamReader object
► Open the text file and assign the reference to the StreamReader object
Declare the StreamReader object by typing Dim objReader As IO. And an IntelliSense window opens. Select StreamReader. Press ENTER. Finish declaring the rest of the variable names
► Verify that the inventory.txt data file is available
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Chapter 9: Using Arrays and File Handling 28
Reading a Text File
Dim objReader as IO.StreamReader
If IO.File.Exists(“e:\inventory.txt”) Then
objReader = IO.File.OpenText(“e:\inventory.txt”)
Else
MsgBox(“The file is not available.”, , “Error”)
Me.close()
End If
Dim ObjReader as New IO.StreamReader( “…” )
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Chapter 9: Using Arrays and File Handling 29
Reading a Text File
►To read each line of the text file:
insert a Do While loop that continues until the Peek procedure returns the value of -1.
Specify that the ReadLine( ) procedure reads each line of the text file. Use the variable intCount to determine the index of each array element
►After the data file has been read, close the file.
Insert an Else statement that informs the user if the file cannot be opened and closes the application
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Reading a Text File
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Chapter 9: Using Arrays and File Handling 31
Writing to a Text File
►Writing to a text file is similar to reading a text file.
The System.IO namespace also includes the StreamWriter class which is used to write a stream of text to a file
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Writing to a Text File
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Chapter 9: Using Arrays and File Handling 33
Computing Depreciation
►Depreciation is the decrease in property value and the reduction in the balance sheet value of a company asset to reflect its loss of value through age and wear and tear
►The simplest and most common, straight-line depreciation, is calculated by dividing the purchase or acquisition price of an asset by the total productive years the asset can reasonably be expected to benefit the company, which is called the life of the asset
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Chapter 9: Using Arrays and File Handling 34
Computing Depreciation
►The double-declining balance depreciation method is like the straight-line method doubled
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Chapter 9: Using Arrays and File Handling 35
Using Multiple Form Objects
►In the Solution Explorer, right-click the project file name Depreciation. Point to Add on the shortcut menu, and then point to New Item on the submenu
►Click New Item. In the Add New Item dialog box, click Windows Form in the Templates area, and then type frmDisplayInventory.vb in the Name text box
►Click the Add button in the Add New Item dialog box. A second Form object opens in the Visual Basic 2008 window named frmDisplayInventory.vb.
In the Properties window, change the Text property of the frmDisplayInventory object to Sorted Inventory Listing
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Using Multiple Form Objects
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Startup Objects►Every application begins executing a project by displaying
the object designated as the Startup object
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Chapter 9: Using Arrays and File Handling 38
Creating an Instance of a Windows Form Object
►To display a second or subsequent form, the initial step in displaying the form is to create an instance of the Windows Form object
►When creating multiple Windows Form objects, Visual Basic allows you to generate two types of forms: modal and modeless
• A modal form retains the input focus while open [ ShowDialog( ) ]
• A modeless form allows you to switch the input focus to another window [ Show( ) ]
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Creating an Instance of a Windows Form Object
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Chapter 9: Using Arrays and File Handling 40
Accessing Variables on Other Forms
►You control the availability of a variable by specifying its access level, or access specifier
9 Application Class
►If you have an application with multiple forms (windows), you can exit the “application” and close all the open forms (windows) by using the Exit method of the Application Class.
►Application.Exit( )
Chapter 9: Using Arrays and File Handling 41
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Program Design
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Designing the Program Processing Objects
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Designing the Program Processing Objects
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Designing the Program Processing Objects
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Summary
►Initialize an array
►Initialize an array with default values
►Access array elements using a loop
►Use ReDim to resize an array
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Chapter 9: Using Arrays and File Handling 48
Summary
►Determine the number of elements in an array using the Length command
►Use the For Each loop
►Initialize two-dimensional arrays
►Read a text file
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Chapter 9: Using Arrays and File Handling 49
Summary
►Write to a text file
►Calculate depreciation
►Use multiple Form objects
►Access Variable objects on other forms
Microsoft Visual Basic 2008
CHAPTER 9 COMPLETE
Using Arraysand File Handling
