1. Microsoft Visual Basic 2005: Reloaded Second Edition
Chapter 9
Structures and Sequential Access Files

2. Objectives After studying this chapter, you should be able to:
Create a structure
Declare and manipulate a structure variable
Create an array of structure variables
Write information to a sequential access file
Align the text written to a sequential access file
Microsoft Visual Basic 2005: Reloaded, Second Edition

3. Objectives (continued)
Read information from a sequential access file
Determine whether a file exists
Code the FormClosing event
Prevent a form from closing
Microsoft Visual Basic 2005: Reloaded, Second Edition

4. Structures Structure statement: creates your own data type
User-defined data type (or structure): data types created by using the Structure statement
Member variables: variables defined within a structure
Structure can include member variables of:
Any of the standard data types
Other structure types
Microsoft Visual Basic 2005: Reloaded, Second Edition

5. Structures (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

6. Using a Structure to Declare a Variable
Structure variables: variables declared using a structure
Refer to an entire structure in code using its name
Refer to a member variable using the structure name with the dot operator and the member variable name
Member variables can be used like any other variables
Structures are used to group related items into one unit
Microsoft Visual Basic 2005: Reloaded, Second Edition

7. Using a Structure to Declare a Variable (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

8. Using a Structure to Declare a Variable (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

9. Passing a Structure Variable to a Procedure
Microsoft Visual Basic 2005: Reloaded, Second Edition

10. Passing a Structure Variable to a Procedure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

11. Passing a Structure Variable to a Procedure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

12. Passing a Structure Variable to a Procedure (continued)
Sample application using a structure:
Less code to pass a structure variable to a procedure
Stores all of the data in a single unit
Microsoft Visual Basic 2005: Reloaded, Second Edition

13. Passing a Structure Variable to a Procedure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

14. Passing a Structure Variable to a Procedure (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

15. Creating an Array of Structure Variables
Use a one-dimensional array of structure variables instead of two-dimensional or parallel one-dimensional arrays
Refer to a member variable in an array element by:
arrayname(subscript).memberVariableName
Microsoft Visual Basic 2005: Reloaded, Second Edition

16. Creating an Array of Structure Variables (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

17. Creating an Array of Structure Variables (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

18. Creating an Array of Structure Variables (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

19. Creating an Array of Structure Variables (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

20. Creating an Array of Structure Variables (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

21. File Types Reading a file: getting information from a file
Writing to a file: sending information to a file
Output files: files to which information is written
Input files: files that are read by the computer
Three types of files in Visual Basic:
Sequential
Random
Binary
Microsoft Visual Basic 2005: Reloaded, Second Edition

22. File Types (continued)
Sequential file: data is accessed sequentially, in consecutive order from the beginning to the end
Random file: data can be accessed in consecutive or random order
Binary file: data is accessed by its byte location in the file
Microsoft Visual Basic 2005: Reloaded, Second Edition

23. Sequential Access Files
Sequential access file: often called a text file because it is composed of lines of text
Data must be read sequentially from beginning to end
Microsoft Visual Basic 2005: Reloaded, Second Edition

24. Sequential Access Files (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

25. Writing Information to a Sequential Access File
WriteAllText method:
Writes information to a sequential access file
File argument: contains the name of the file to be used
Text argument: contains the data to be written
Append argument: if True, data is added to end of existing file; otherwise all existing data is erased
Microsoft Visual Basic 2005: Reloaded, Second Edition

26. Writing Information to a Sequential Access File (continued)
My feature: exposes a set of commonly used objects to the programmer
My.Computer object: allows access to objects and methods to manipulate files
Computer uses a file pointer to trace where the next character will be read from or written to a file
Strings.Space method: writes a specific number of spaces to a file
Microsoft Visual Basic 2005: Reloaded, Second Edition

27. Writing Information to a Sequential Access File (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

28. Aligning Columns of Information in a Sequential Access File
PadLeft, PadRight methods:
Pads a string with a specified number of characters based on current length of the string being padded
Aligns columns of information written to a sequential access file
Aligns columns of numeric information by the decimal point
Microsoft Visual Basic 2005: Reloaded, Second Edition

29. Aligning Columns of Information in a Sequential Access File (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

30. Aligning Columns of Information in a Sequential Access File (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

31. Reading Information from a Sequential Access File
ReadAllText method: reads information stored in a sequential access file
File argument: contains the name of the sequential access file
Microsoft Visual Basic 2005: Reloaded, Second Edition

32. Reading Information from a Sequential Access File (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

33. Determining Whether a File Exists
An attempt to read a file that does not exist will cause an error
FileExists method: determines if the file exists before attempting to read it
File argument: contains the name of the file
Microsoft Visual Basic 2005: Reloaded, Second Edition

34. Determining Whether a File Exists (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

35. The FormClosing Event FormClosing event:
Occurs when a form is about to be closed by the program code or by the user
Allows you to trap the closing action and take any necessary actions such as saving data
Can be used to cancel the close action
Set e.Cancel = True to cancel the closing action
Microsoft Visual Basic 2005: Reloaded, Second Edition

36. The FormClosing Event (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

37. The Friends Application
Microsoft Visual Basic 2005: Reloaded, Second Edition

38. The Friends Application (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

39. The Friends Application (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

40. The Friends Application (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

41. The Friends Application (continued)
Microsoft Visual Basic 2005: Reloaded, Second Edition

42. Programming Tutorial
Microsoft Visual Basic 2005: Reloaded, Second Edition

43. Programming Example
Microsoft Visual Basic 2005: Reloaded, Second Edition

44. Summary
Structure statement: defines a user-defined data type or structure
Structure variable contains one or more member variables
Structure variable can be passed to procedures
Create a one-dimensional array of structure variables
Use the dot member access operator to access the structure member variables
Microsoft Visual Basic 2005: Reloaded, Second Edition

45. Summary (continued) Application can read from and write to a file
Sequential access files are always accessed in consecutive (sequential) order from beginning to end
ReadAllText method: reads text from a sequential access file
WriteAllText method: writes text to a sequential access file
Use Strings.Space, PadLeft, and PadRight methods to align data in a sequential access file
Microsoft Visual Basic 2005: Reloaded, Second Edition

46. Summary (continued)
FileExists method: determines if a file exists before trying to read from it
FormClosing event: occurs when the form is about to be closed
Use the FormClosing event to save data or to prevent a form from being closed
Microsoft Visual Basic 2005: Reloaded, Second Edition