Microsoft Visual Basic 2005: Reloaded Second Edition

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

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 Edition

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 Edition

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 Edition

Structures (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

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 Edition

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

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

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

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 Edition

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

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 Edition

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

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 Edition

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 Edition

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 Edition

Sequential Access Files (continued) Microsoft Visual Basic 2005: Reloaded, Second Edition

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 Edition

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 Edition

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

Aligning Columns of Information in a Sequential Access File • PadLeft, PadRightmethods: • 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 Edition

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

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 Edition

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

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 Edition

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

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 Edition

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

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

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

Programming Tutorial Microsoft Visual Basic 2005: Reloaded, Second Edition

Programming Example Microsoft Visual Basic 2005: Reloaded, Second Edition

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 Edition

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 Edition

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 Microsoft Visual Basic 2005: Reloaded, Second Edition

Microsoft Visual Basic 2005: Reloaded Second Edition