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

Programming with Microsoft Visual Basic 2008

Fourth Edition

Chapter TenStructures 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 Edition 2

Previewing the CD Collection Application (continued)


Figure 10-1: CD information added to the list box

Previewing the CD Collection Application (continued)


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


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


Creating a Structure (continued)


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


Declaring and Using a Structure Variable (continued)


Figure 10-4: Syntax and examples of declaring a structure variable

Declaring and Using a Structure Variable (continued)


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



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

Passing a Structure Variable to a Procedure (continued)


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



Figure 10-9: Code for the Treasures Gift Shop application (without a structure)

Creating an Array of Structure Variables (continued)

Creating an Array of Structure Variables (continued)


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


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


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


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


Writing Data to a Sequential Access File (continued)


Figure 10-15: Syntax and an example of declaring a StreamWriter variable



Figure 10-17: Syntax and examples of creating a StreamWriter object by opening a file



Figure 10-18: Syntax and examples of writing data to a sequential access file



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



(continued)


Figure 10-21: Syntax and an example of creating a StreamReader object by opening a file


(continued)


Figure 10-22: Syntax and an example of the Exists method


(continued)


Figure 10-23: Code entered in the selection structure’s false path


(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



(continued)


Figure 10-24: Syntax and examples of reading data from a sequential access file


Figure 10-25: Code entered in the btnWrite and btnRead controls’ Click event procedures


(continued)


Figure 10-25: Code entered in the btnWrite and btnRead controls’ Click event procedures (continued)


(continued)


Figure 10-26: Contestant names appear in the Contestants box


(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


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


Coding the CD Collection Application


Figure 10-28: User interface for the CD Collection application

Coding the CD Collection Application (continued)


Figure 10-29: TOE chart for the CD Collection application

Coding the CD Collection Application (continued)


Figure 10-30: CDs.txt window showing the contents of the sequential access file

Coding the Form’s Load Event Procedure


Figure 10-31: Pseudocode for the form’s Load event procedure

Coding the Form’s Load Event Procedure (continued)


Figure 10-32: Additional comment and code entered in the form’s Load event procedure

Coding the Form’s Load Event Procedure (continued)


Figure 10-33: The contents of the CDs.txt file appear in the list box

Coding the btnAdd Control’s Click Event Procedure


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


Coding the btnRemove Control’s Click Event Procedure (continued)


Figure 10-37: Pseudocode for the btnRemove control’s Click event procedure

Coding the Form’s FormClosing Event Procedure


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 Edition Chapter Ten Structures and Sequential...

Documents

Transcript of Programming with Microsoft Visual Basic 2008 Fourth Edition Chapter Ten Structures and Sequential...