1. Chapter 10: Structures and Sequential Access Files

2. Previewing the CD Collection Application
Keeps track of a person’s CD collection
Saves each CD’s name, artist’s name, and price
Uses a sequential access file named CDs.txt
Can add information to or remove information from a file
Figure 10-1 CD information added to the list box
Figure 10-2 Contents of the CDs.txt file
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3. Lesson A Objectives After studying Lesson A, you should be able to:
Create a structure
Declare and use a structure variable
Pass a structure variable to a procedure
Create an array of structure variables
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4. Structures Structure statement Structure (or user-defined data type)
Enables you to create your own data types
Used to group related items of different data types into one unit
Typically appears in a form’s Declaration section
Structure (or user-defined data type)
A data type created with the Structure statement
Member variables
Variables, constants, or procedures declared within the structure declaration
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5. Structures (cont.) Programming with Microsoft Visual Basic 2012
Figure 10-3 Syntax and an example of the Structure statement
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6. Declaring and Using a Structure Variable
Structure variables
Declared using the structure data type
Example: Dim hourly As Employee
hourly is a variable declared with the Employee structure type
To access the member variable in code, use structureVariableName.memberVariableName
Example: hourly.dblPay = 26
Member variables are used like scalar variables
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7. Declaring and Using a Structure Variable (cont.)
Figure 10-4 Syntax and examples of declaring a structure variable
Figure 10-5 Examples of using a member variable
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8. Passing a Structure Variable to a Procedure
Application for the sales manager at Norbert Pool & Spa Depot
Allows the user to enter the length, width, and depth
Calculates the gallons of water needed to fill a pool
Advantages of using a structure to group dimensions:
Three inputs are stored in one structure variable
You pass a single structure variable to a procedure instead of three scalar variables
Your code is structured in a more readable form
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9. Passing a Structure Variable to a Procedure (cont.)
Figure 10-6 Interface showing the required number of gallons
Figure 10-7 Code for the Norbert Pool & Spa Depot application (without a structure)
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10. Passing a Structure Variable to a Procedure (cont.)
Figure 10-8 Code for the Norbert Pool & Spa Depot application (with a structure)
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11. Creating an Array of Structure Variables
Three ways to manage pairs of ID-price data:
Two parallel one-dimensional arrays
One two-dimensional array (tabular format)
A one-dimensional array of structure variables
Each structure variable will contain:
A String variable for the ID
An Integer variable for the price
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12. Creating an Array of Structure Variables (cont.)
Figure 10-9 Problem specification for the Treasures Gift Shoppe
Figure Interface for the Treasures Gift Shoppe application
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13. Creating an Array of Structure Variables (cont.)
Figure Code for the Treasures Gift Shoppe application (without a structure)
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14. Creating an Array of Structure Variables (cont.)
Figure Syntax and examples of referring to member variables in an array
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15. Creating an Array of Structure Variables (cont.)
Figure Code for the Treasures Gift Shoppe application (with a structure)
Figure Interface showing the price for product ID FE15
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16. Lesson A Summary To create a structure (user-defined data type):
Use the Structure statement
In most applications, you enter the Structure statement in the form’s Declarations section
To declare a structure variable, use the following syntax:
{Dim | Private} structureVariableName As structureName
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17. Lesson A Summary (cont.)
To refer to a member within a structure variable, use the following syntax:
structureVariableName.memberVariableName
To create an array of structure variables:
Declare the array using the structure as the data type
To refer to a member within a structure variable stored in an array, use the following syntax:
ArrayName(subscript).memberVariableName
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18. Lesson B Objectives After studying Lesson B, you should be able to:
Open and close a sequential access file
Write data to a sequential access file
Read data from a sequential access file
Determine whether a sequential access file exists
Test for the end of a sequential access file
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19. Sequential Access Files
Reading a file means getting data from a file
Writing to a file means sending data to a file
Output files store application output
An application uses data in input files
Sequential access files
Composed of lines of text that are both read and written in consecutive order
Also called text files
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20. Writing Data to a Sequential Access File
Stream of characters
A sequence of characters
StreamWriter object
Used to write a stream of characters to a sequential access file
Must declare the StreamWriter variable
The Game Show Contestants application uses the StreamWriter variable
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21. Writing Data to a Sequential Access File (cont.)
Figure Syntax and an example of declaring a StreamWriter variable
Figure Interface for the Game Show Contestants application
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22. Writing Data to a Sequential Access File (cont.)
CreateText method
Used to open a new sequential file
AppendText method
Used to open a file that exists and add data to it
Figure Syntax and examples of the CreateText and AppendText methods
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23. Writing Data to a Sequential Access File (cont.)
Write method
Leaves the cursor at the end of the last character
WriteLine method
Moves the cursor to the next line
Figure Syntax and examples of the Write and WriteLine methods
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24. Closing an Output Sequential Access File
Close method
Used to close an output sequential access file
All open files must be closed before being opened again
Figure Syntax and an example of closing an output sequential access file
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25. Reading Data from a Sequential Access File
StreamReader object
Used to read data from a sequential access file
Must declare the StreamReader variable
OpenText method
Used to open a sequential access file for input
You can use this method to automatically create a StreamReader object
Exists method
Used to determine if a file exists
Returns True if a file exists, otherwise False
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26. Reading Data from a Sequential Access File (cont.)
Figure Syntax and an example of declaring a StreamReader variable
Figure Syntax and an example of the OpenText method
Figure Syntax and an example of the Exists method
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27. Reading Data from a Sequential Access File (cont.)
Figure Additional code entered in the procedure
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28. Reading Data from a Sequential Access File (cont.)
Line
A sequence (stream) of characters followed by the newline character
ReadLine method
Used to read the contents of a file, one line at a time
Returns the String value containing data in the current line
Returns only data, not including the newline character
Peek method
Determines whether a file contains another character to read
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29. Reading Data from a Sequential Access File (cont.)
Figure Syntax and an example of the ReadLine method
Figure Syntax and an example of the Peek method
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30. Closing an Input Sequential Access File
Close method
Used to close input sequential access files
Figure Syntax and an example of closing an input sequential access file
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31. Closing an Input Sequential Access File (cont.)
Figure Click event procedures for the btnWrite and btnRead controls (continues)
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32. Closing an Input Sequential Access File (cont.)
(continued)
Figure Click event procedures for the btnWrite and btnRead controls
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33. Closing an Input Sequential Access File (cont.)
Figure Five contestant names listed in the Contestants box
Figure Nine contestant names listed in the list box
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34. Lesson B Summary To write data to a sequential access file:
Declare a StreamWriter variable and then use either the CreateText method or the AppendText method to open a sequential access file
Assign the method’s return value to the StreamWriter variable
Use either the Write method or the WriteLine method to write the data to the file
Close the file using the Close method
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35. Lesson B Summary (cont.)
To read data from a sequential access file:
Declare a StreamReader variable
Use the Exists method to determine whether the sequential access file exists
If the file exists, use the OpenText method to open the file
Assign the method’s return value to the StreamReader variable
Use the ReadLine and Peek methods to read the data from the file
Close the file using the Close method
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36. Lesson B Summary (cont.)
To determine whether a sequential access file exists:
Use the Exists method using the syntax IO.File.Exists(fileName)
The method returns the Boolean value True if the file exists; otherwise, it returns the Boolean value False
To determine whether the end of a sequential access file has been reached:
Use the Peek method using the syntax streamReaderVariableName.Peek
The method returns the number –1 when the end of the file has been reached; otherwise, it returns the next character in the file
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37. Lesson C Objectives After studying Lesson C, you should be able to:
Add an item to a list box while an application is running
Align columns of information
Remove an item from a list box while an application is running
Save list box items in a sequential access file
Write records to a sequential access file
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38. Coding the CD Collection Application
Figure Interface for the CD Collection application
Figure TOE chart for the CD Collection application
Figure CDs.txt window
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39. Coding the Form’s Load Event Procedure
Figure Pseudocode for the form’s Load event procedure
Figure Additional comment and code entered in the Load event procedure
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40. Coding the Form’s Load Event Procedure (cont.)
Figure Contents of the CDs.txt file shown in the list box
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41. Coding the btnAdd_Click Procedure
Figure Pseudocode for the btnAdd_Click procedure
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42. Aligning Columns of Information
PadLeft and PadRight methods
Used to pad strings with characters
These methods can be used to align text in a list box or text written to a sequential access file
Strings.Space method
Used to include a specific number of space characters in a string
Syntax: Strings.Space(number)
number is an integer representing the number of spaces to include
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43. Aligning Columns of Information (cont.)
Figure Examples of aligning columns of information
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44. Aligning Columns of Information (cont.)
Figure CD information added to the list box
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45. Coding the btnRemove_Click Procedure
Figure Pseudocode for the btnRemove_Click procedure
Figure Syntax and examples of the Items collection’s Remove and RemoveAt methods
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46. Coding the Form’s FormClosing Event Procedure
Figure Pseudocode for the form’s FormClosing event procedure
Figure CD information saved in the CDs.txt file
Figure Current contents of the CDs.txt file
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47. Coding the Form’s FormClosing Event Procedure (cont.)
Figure Code for the CD Collection application (continues)
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48. Coding the Form’s FormClosing Event Procedure (cont.)
(continued)
Figure Code for the CD Collection application (continues)
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49. Coding the Form’s FormClosing Event Procedure (cont.)
(continued)
Figure Code for the CD Collection application (continues)
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50. Coding the Form’s FormClosing Event Procedure (cont.)
(continued)
Figure Code for the CD Collection application
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51. Lesson C Summary To align columns of information:
Use the PadLeft and PadRight methods
To align a column of numbers by the decimal point:
Format each number in the column to ensure that each has the same number of digits to the right of the decimal point, and then use the PadLeft method to right-align the numbers
To include a specific number of spaces in a string:
Use the Strings.Space method using the syntax Strings.Space(number) in which number is an integer that represents the number of spaces to include
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52. Lesson C Summary To remove an item from a list box:
Use either the Items collection’s Remove method or its RemoveAt method
The Remove method’s syntax is object.Items.Remove(item) where item is the value of the item you want to remove
The RemoveAt method’s syntax is object.Items.RemoveAt(index) where index is the index of the item you want removed
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