Microsoft Visual Basic 2010: Reloaded Fourth Edition

1. Microsoft Visual Basic 2010: ReloadedFourth Edition Chapter Eleven Structures and Sequential Files

2. Objectives After studying this chapter, 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 • Write data to a sequential access file • Close a sequential access file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

3. Objectives (cont'd.) • Read data from a sequential access file • Use the Exists and Peek methods • Align columns of information • Code the FormClosing event procedure • Write and read records • Use the Split function Microsoft Visual Basic 2010: Reloaded, Fourth Edition

4. Structures • Structure statement: used to create your own data type • User-defined data types (or structures): data types created by using the Structure statement • Member variables: variables defined within a structure • Structure can include member variables of: • Any standard data types • Other structure types • Used to group related items into one unit • Usually declared in the form’s Declarations section Microsoft Visual Basic 2010: Reloaded, Fourth Edition

5. Structures (cont'd.) Figure 11-1: How to define a structure Microsoft Visual Basic 2010: Reloaded, Fourth Edition

6. Declaring a Structure Variable • Structure statement only creates the data type • Structure variables: variables declared using a structure Figure 11-2: How to declare a structure variable Microsoft Visual Basic 2010: Reloaded, Fourth Edition

7. Declaring a Structure Variable (cont'd.) • 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 Microsoft Visual Basic 2010: Reloaded, Fourth Edition

8. Declaring a Structure Variable (cont'd.) Figure 11-3: How to use a member variable Microsoft Visual Basic 2010: Reloaded, Fourth Edition

9. Passing a Structure Variable to a Procedure • Sample application without using a structure: Figure 11-4: Sample run of the Willow Pools application Microsoft Visual Basic 2010: Reloaded, Fourth Edition

10. Figure 11-5: Code for the Willow Pools application (without a structure) Microsoft Visual Basic 2010: Reloaded, Fourth Edition

11. Figure 11-6: Code for the Willow Pools application (with a structure) Microsoft Visual Basic 2010: Reloaded, Fourth Edition

12. Passing a Structure Variable to a Procedure (cont'd.) • When you pass a structure variable to a procedure, all of its member variables are automatically passed • Sample application using a structure: • Uses less code to pass a structure variable to a procedure • Stores all of the data in a single unit Microsoft Visual Basic 2010: Reloaded, Fourth Edition

13. Creating an Array of Structure Variables • Use a one-dimensional array of structure variables • Each element in the array is a structure variable Figure 11-7: Sample run of the Treasures Gift Shop application Microsoft Visual Basic 2010: Reloaded, Fourth Edition

14. Figure 11-8: Partial code for the Treasures Gift Shop application Microsoft Visual Basic 2010: Reloaded, Fourth Edition

15. Creating an Array of Structure Variables (cont’d.) Figure 11-8: Partial code for the Treasures Gift Shop application (cont’d.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition

16. 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 • Sequential access files (text files): composed of text that are both read and written sequentially, one line at a time Microsoft Visual Basic 2010: Reloaded, Fourth Edition

17. 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 • Use IO.File methods to manipulate the file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

18. Writing Data to a Sequential Access File (cont'd.) Figure 11-9: How to declare a StreamWriter variable Microsoft Visual Basic 2010: Reloaded, Fourth Edition

19. Writing Data to a Sequential Access File (cont'd.) • CreateText method: opens a sequential access file for output • Creates a new, empty file to which data can be written • If the named file already exists, its contents are erased before writing to it • AppendText method: opens a sequential access file for append • New data is written after any existing data in the file • If the named file does not exist, it is created for you Microsoft Visual Basic 2010: Reloaded, Fourth Edition

20. Figure 11-10: How to create a StreamWriter object Microsoft Visual Basic 2010: Reloaded, Fourth Edition

21. Writing Data to a Sequential Access File (cont'd.) • After opening a file for output or append, you can begin writing data to it • Write method: writes a line of data to the file • WriteLine method: writes a line of data to the file and writes a newline character after the data Microsoft Visual Basic 2010: Reloaded, Fourth Edition

22. Writing Data to a Sequential Access File (cont'd.) Figure 11-11: How to write to a sequential access file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

23. Closing an Output Sequential Access File • Close method: used to close an output sequential access file • Ensures that the data is saved • Makes the file available for use elsewhere in the application • Uses a StreamWriter variable to refer to the file in code Microsoft Visual Basic 2010: Reloaded, Fourth Edition

24. Closing an Output Sequential Access File (cont’d.) Figure 11-12: How to close an output sequential access file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

25. Reading Data from a Sequential Access File • StreamReader object: used to read data from a sequential access file • Must first declare a StreamReader variable • OpenText method: used to open a sequential access file for input • Automatically creates a StreamReader object • Arguments: • fileName: the filename and optionally the path to the file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

26. Reading Data from a Sequential Access File (cont'd.) Figure 11-13: How to declare a StreamReader variable Microsoft Visual Basic 2010: Reloaded, Fourth Edition

27. Reading Data from a Sequential Access File (cont'd.) Figure 11-14: How to create a StreamReader object Microsoft Visual Basic 2010: Reloaded, Fourth Edition

28. Reading Data from a Sequential Access File (cont'd.) • If an input file cannot be located at run time, an error occurs • You can avoid the error by determining if the file exists before trying to open it • Exists method: returns a Boolean value of True if a file exists; otherwise returns a Boolean value of False Microsoft Visual Basic 2010: Reloaded, Fourth Edition

29. Reading Data from a Sequential Access File (cont'd.) Figure 11-15: How to determine whether a sequential access file exists Microsoft Visual Basic 2010: Reloaded, Fourth Edition

30. Reading Data from a Sequential Access File (cont'd.) • 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 • Does not include the newline character at the end of the line • Uses the file associated with the StreamReader variable Microsoft Visual Basic 2010: Reloaded, Fourth Edition

31. Reading Data from a Sequential Access File (cont'd.) Figure 11-16: How to read data from a sequential access file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

32. Reading Data from a Sequential Access File (cont'd.) • Peek method: “peeks” into the file to determine whether the file contains another character to read • If there is another character, Peek returns the character • If there are no more characters, Peek returns a value of -1 Microsoft Visual Basic 2010: Reloaded, Fourth Edition

33. Reading Data from a Sequential Access File (cont'd.) Figure 11-17: How to use the Peek method Microsoft Visual Basic 2010: Reloaded, Fourth Edition

34. Closing an Input Sequential Access File • Use the Close method to close an input sequential access file when you are finished using it • Makes the file available for use elsewhere in the application Figure 11-18: How to close an input sequential access file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

35. The Game Show Contestants Application • Sample application: used to record the names of game show contestants in a sequential access file Figure 11-19: Sample run of the Game Show Contestants application Microsoft Visual Basic 2010: Reloaded, Fourth Edition

36. Figure 11-20: Partial code for the Game Show Contestants application Microsoft Visual Basic 2010: Reloaded, Fourth Edition

37. The Game Show Contestants Application (cont’d.) Figure 11-20: Partial code for the Game Show Contestants application (cont’d.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition

38. Aligning Columns of Information • PadLeft, PadRight methods: • Pads a string with a specified number of characters based on current length of the string being padded • Used to align columns of information written to a sequential access file • The default pad character is the space character • Align columns of numeric information by the decimal point Microsoft Visual Basic 2010: Reloaded, Fourth Edition

39. Figure 11-22: How to align columns of information Microsoft Visual Basic 2010: Reloaded, Fourth Edition

40. Aligning Columns of Information (cont'd.) Figure 11-22: How to align columns of information (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition

41. 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 • Form may be closed by Me.Close() statement or by user clicking the Close button on title bar Microsoft Visual Basic 2010: Reloaded, Fourth Edition

42. The FormClosing Event (cont'd.) Figure 11-23: How to use the FormClosing event procedure Microsoft Visual Basic 2010: Reloaded, Fourth Edition

43. The FormClosing Event (cont'd.) Figure 11-23: How to use the FormClosing event procedure (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition

44. Writing and Reading Records • Field: a single item of information about a person, place, or thing • Record: a group of related fields that contain all of the necessary data about a specific person, place, or thing • If a record contains more than one field, the fields are separated by a delimiter character Microsoft Visual Basic 2010: Reloaded, Fourth Edition

45. Figure 11-24: How to write records to a sequential access file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

46. Writing and Reading Records (cont’d.) • Split function: used with ReadLine to divide a line of text from a file into fields • Assigns each field to an element in an array • Must specify the delimiter that was used in the file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

47. Writing and Reading Records (cont’d.) Figure 11-25: How to read records from a sequential access file Microsoft Visual Basic 2010: Reloaded, Fourth Edition

48. Writing and Reading Records (cont’d.) Figure 11-25: How to read records from a sequential access file (cont’d.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition

49. Programming Tutorial 1 • Modifying the Concentration Game Application • Use Concentration Game application from Ch. 8 Figure 11-26: Contents of three of the four sequential access files Microsoft Visual Basic 2010: Reloaded, Fourth Edition

50. Programming Tutorial 2 • Creating the CD Collection Application Figure 11-30: MainForm for the CD Collection application Microsoft Visual Basic 2010: Reloaded, Fourth Edition