Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Ten Structures and Sequential Access Files

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 Programming with Microsoft Visual Basic 2008, Fourth Edition2

Previewing the CD Collection Application (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition3 Figure 10-1: CD information added to the list box

Previewing the CD Collection Application (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition4 Figure 10-2: Contents of the CDs.txt file

Lesson A Objectives After studying Lesson A, you should be able to: Create a structure Declare a structure variable Pass a structure variable to a procedure Create and manipulate a one-dimensional array of structures Programming with Microsoft Visual Basic 2008, Fourth Edition5

Creating a Structure 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 Programming with Microsoft Visual Basic 2008, Fourth Edition6

Creating a Structure (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition7 Figure 10-3: Syntax and an example of the Structure statement

Declaring and Using a Structure Variable Structure variables: Declared using structure –Structure is data type for variable Example: Dim manager As Employee –manager is variable declared with Employee structure type Accessing member variable in code: –Use structureVariableName. memberName –Example: manager.dblSalary = 59000D Member variables are used like scalar variables Programming with Microsoft Visual Basic 2008, Fourth Edition8

Declaring and Using a Structure Variable (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition9 Figure 10-4: Syntax and examples of declaring a structure variable

Declaring and Using a Structure Variable (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition10 Figure 10-5: Examples of using a member variable

Passing a Structure Variable to a Procedure Application for sales manager at Willows Pool: –Allows user to enter length, width, and depth –Calculates volume of 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 Programming with Microsoft Visual Basic 2008, Fourth Edition11

Programming with Microsoft Visual Basic 2008, Fourth Edition12 Figure 10-7: Code for the Willow Pools application (without a structure) Passing a Structure Variable to a Procedure (continued)

Programming with Microsoft Visual Basic 2008, Fourth Edition13 Figure 10-8: Code for the Willow Pools application (with a structure) Passing a Structure Variable to a Procedure (continued)

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) –One-dimensional array of structure variables Structure variable will contain: –String variable for ID –Integer variable for price Programming with Microsoft Visual Basic 2008, Fourth Edition14

Programming with Microsoft Visual Basic 2008, Fourth Edition15 Figure 10-9: Code for the Treasures Gift Shop application (without a structure) Creating an Array of Structure Variables (continued)

Programming with Microsoft Visual Basic 2008, Fourth Edition16 Figure 10-10: Names of some of the member variables in the priceList array

Lesson A Summary Structures are user-defined data types Structure members can be variables, constants, or procedures Refer to member within structure variable using structureVariableName.memberName Element in array of structure variables is structure variable Refer to member within structure variable stored in an array using: arrayName(subscript).memberName Programming with Microsoft Visual Basic 2008, Fourth Edition17

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 Programming with Microsoft Visual Basic 2008, Fourth Edition18

Sequential Access Files Reading a file: Getting data from a file Writing to a file: Sending data to a file Output files: Files to which information is written Input files: Files that are read by a computer Sequential access files: Files composed of lines of text that are both read and written sequentially Programming with Microsoft Visual Basic 2008, Fourth Edition19

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 sample application uses StreamWriter variable Programming with Microsoft Visual Basic 2008, Fourth Edition20

Writing Data to a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition21 Figure 10-15: Syntax and an example of declaring a StreamWriter variable

Writing Data to a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition22 Figure 10-17: Syntax and examples of creating a StreamWriter object by opening a file

Writing Data to a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition23 Figure 10-18: Syntax and examples of writing data to a sequential access file

Writing Data to a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition24 Figure 10-19: Names contained in the contestants.txt file

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 Programming with Microsoft Visual Basic 2008, Fourth Edition25

Reading Data From a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition26 Figure 10-21: Syntax and an example of creating a StreamReader object by opening a file

Reading Data From a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition27 Figure 10-22: Syntax and an example of the Exists method

Reading Data From a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition28 Figure 10-23: Code entered in the selection structure’s false path

Reading Data From a Sequential Access File (continued) 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 Programming with Microsoft Visual Basic 2008, Fourth Edition29

Reading Data From a Sequential Access File (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition30 Figure 10-24: Syntax and examples of reading data from a sequential access file

Programming with Microsoft Visual Basic 2008, Fourth Edition31 Figure 10-25: Code entered in the btnWrite and btnRead controls’ Click event procedures Reading Data From a Sequential Access File (continued)

Programming with Microsoft Visual Basic 2008, Fourth Edition32 Figure 10-25: Code entered in the btnWrite and btnRead controls’ Click event procedures (continued) Reading Data From a Sequential Access File (continued)

Programming with Microsoft Visual Basic 2008, Fourth Edition33 Figure 10-26: Contestant names appear in the Contestants box Reading Data From a Sequential Access File (continued)

Lesson B Summary Sequential access file: Stores data items in consecutive order (sequentially) Use StreamWriter variable to write data to sequential access file Use StreamReader variable to read data from sequential access file Use Exists method to determine if file exists Use Peek method to determine whether end of sequential access file has been reached Programming with Microsoft Visual Basic 2008, Fourth Edition34

Lesson C Objectives After studying Lesson C, you should be able to: Fill a list box with values stored in a sequential access file 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 Programming with Microsoft Visual Basic 2008, Fourth Edition35

Coding the CD Collection Application Programming with Microsoft Visual Basic 2008, Fourth Edition36 Figure 10-28: User interface for the CD Collection application

Coding the CD Collection Application (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition37 Figure 10-29: TOE chart for the CD Collection application

Coding the CD Collection Application (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition38 Figure 10-30: CDs.txt window showing the contents of the sequential access file

Coding the Form’s Load Event Procedure Programming with Microsoft Visual Basic 2008, Fourth Edition39 Figure 10-31: Pseudocode for the form’s Load event procedure

Coding the Form’s Load Event Procedure (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition40 Figure 10-32: Additional comment and code entered in the form’s Load event procedure

Coding the Form’s Load Event Procedure (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition41 Figure 10-33: The contents of the CDs.txt file appear in the list box

Coding the btnAdd Control’s Click Event Procedure Programming with Microsoft Visual Basic 2008, Fourth Edition42 Figure 10-34: Pseudocode for the btnAdd control’s Click event procedure

43Programming with Microsoft Visual Basic 2005, Third Edition Aligning Columns of Information PadLeft and PadRight methods: Used to pad strings with characters –These methods can be used to align text in list box or text written to sequential access file Strings.Space method: Used to include specific number of space characters in string Syntax: Strings.Space(number) –number: Integer representing number of spaces to include

Coding the btnRemove Control’s Click Event Procedure Main task: Allow user to remove selected line from list box control RemoveAt method: Removes list box item at specified index Programming with Microsoft Visual Basic 2008, Fourth Edition44

Coding the btnRemove Control’s Click Event Procedure (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition45 Figure 10-37: Pseudocode for the btnRemove control’s Click event procedure

Coding the Form’s FormClosing Event Procedure Programming with Microsoft Visual Basic 2008, Fourth Edition46 Figure 10-38: Pseudocode for the form’s FormClosing event procedure

Lesson C Summary Use Add method of list box to add an item to list Use PadLeft method to right-align string, or to align column of numbers by decimal point Use PadRight method to left-align string Use Strings.Space method to include specific number of spaces in string Use RemoveAt method to remove item at specific index from listbox Programming with Microsoft Visual Basic 2008, Fourth Edition47