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

Microsoft Visual Basic 2005: Reloaded, Second Edition2 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 Edition3 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 Edition4 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 Edition5 Structures (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition6 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 Edition7 Using a Structure to Declare a Variable (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition8 Using a Structure to Declare a Variable (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition9 Passing a Structure Variable to a Procedure

Microsoft Visual Basic 2005: Reloaded, Second Edition10 Passing a Structure Variable to a Procedure (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition11 Passing a Structure Variable to a Procedure (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition12 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 Edition13 Passing a Structure Variable to a Procedure (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition14 Passing a Structure Variable to a Procedure (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition15 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 Edition16 Creating an Array of Structure Variables (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition17 Creating an Array of Structure Variables (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition18 Creating an Array of Structure Variables (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition19 Creating an Array of Structure Variables (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition20 Creating an Array of Structure Variables (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition21 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 Edition22 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 Edition23 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 Edition24 Sequential Access Files (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition25 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 Edition26 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 Edition27 Writing Information to a Sequential Access File (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition28 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 Edition29 Aligning Columns of Information in a Sequential Access File (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition30 Aligning Columns of Information in a Sequential Access File (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition31 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 Edition32 Reading Information from a Sequential Access File (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition33 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 Edition34 Determining Whether a File Exists (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition35 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 Edition36 The FormClosing Event (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition37 The Friends Application

Microsoft Visual Basic 2005: Reloaded, Second Edition38 The Friends Application (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition39 The Friends Application (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition40 The Friends Application (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition41 The Friends Application (continued)

Microsoft Visual Basic 2005: Reloaded, Second Edition42 Programming Tutorial

Microsoft Visual Basic 2005: Reloaded, Second Edition43 Programming Example

Microsoft Visual Basic 2005: Reloaded, Second Edition44 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 Edition45 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 Edition46 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