Managing CD Collections in Visual Basic 2010: Structures and Sequential Access Files

1. Programming with Microsoft Visual Basic 2010 5th Edition Chapter Ten Structures and Sequential Access Files

2. Previewing the CD Collection Application • CD Collection application • Keeps track of person’s CD collection • Saves each CD’s name, artist’s name, and price • Uses sequential access file named CDs.txt • Can add to or remove information from file • Open the CD.exe file

3. Previewing the CD Collection Application (cont’d.) Figure 10-1 CD information added to the list box

4. Previewing the CD Collection Application (cont’d.) Figure 10-2 Contents of the CDs.txt file

5. 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

6. Structures • Structure statement • Enables you to create your own data types • Used to group related items of different data types into one unit • Typically appears in form’s Declaration section • Structure (or user-defined data type) • Data type created with Structure statement • Member variables • Variables, constants, or procedures declared within structure declaration

7. Structures (cont’d.) Figure 10-3 Syntax and an example of the Structure statement

8. Declaring and Using a Structure Variable • Structure variables: Declared using structure • Structure is data type for variable • Example: Dim hourly As Employee • hourly is variable declared with Employee structure type • Accessing member variable in code • Use structureVariableName. memberVariableName • Example: hourly.dblPay = 26 • Member variables are used like scalar variables

9. Declaring and Using a Structure Variable (cont’d.) Figure 10-4 Syntax and examples of declaring a structure variable

10. Figure 10-5 Examples of using a member variable

11. Passing a Structure Variable to a Procedure • Application for sales manager at Willow Pools • Allows user to enter length, width, and depth • Calculates gallons of water to fill pool • Advantages of using structure to group dimensions • Three inputs are stored in one structure variable • You pass single structure variable to procedure instead of three scalar variables • Your code is structured in more readable form

12. Figure 10-7 Code for the Willow Pools application (without a structure)

13. Figure 10-8 Code for the Willow Pools application (with a structure)

14. Lesson A Summary • Structures: User-defined data types • Structure members can be variables, constants, or procedures • Refer to member within structure variable using structureVariableName.memberVariableName • To create an array of structure variables: • Declare array using structure as data type • Refer to member within structure variable stored in an array using: arrayName(subscript).memberVariableName

15. 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

16. Sequential Access Files • Reading a file: Getting data from a file • Writing to a file: Sending data to a file • Output files: Store application output • Input files: Application uses data in these files • Sequential access files • Composed of lines of text that are both read and written in consecutive order • Also called text files

17. Writing Data to a Sequential Access File • Stream of characters • Sequence of characters • StreamWriter object • Used to write stream of characters to sequential access file • Must declare StreamWriter variable • Game Show Contestants application • Uses StreamWriter variable

18. Writing Data to a Sequential Access File (cont’d.) Figure 10-15 Syntax and an example of declaring a StreamWriter variable

19. Figure 10-17 Syntax and examples of the CreateText and AppendText methods

20. Figure 10-18 Syntax and examples of the Write and WriteLine methods

21. Closing an Output Sequential Access File • Close method • Used to close an output sequential access file Figure 10-19 Syntax and an example of closing an output sequential access file

22. Reading Data from a Sequential Access File • StreamReader object • Used to read data from sequential access file • Must declare StreamReader variable • OpenText method • Used to open sequential access file for input • Can use this method to automatically create StreamReader object • Exists method • Used to determine if file exists • Returns True if file exists, otherwise False

23. Figure 10-21 Syntax and an example of declaring a StreamReader variable

24. Figure 10-22 Syntax and an example of the OpenText method

25. Figure 10-23 Syntax and an example of the Exists method

26. Figure 10-24 Additional code entered in the procedure

27. Reading Data from a Sequential Access File (cont’d.) • Line: Sequence (stream) of characters followed by newline character • ReadLine method • Used to read contents of file, one line at a time • Returns String value containing data in current line • Returns only data, not including newline character • Peek method • Determines whether file contains another character to read

28. Figure 10-25 Syntax and an example of the ReadLine method

29. Figure 10-26 Syntax and an example of the Peek method

30. Closing an Input Sequential Access File • Close method • Used to close input sequential access files Figure 10-27 Syntax and an example of closing an input sequential access file