outline

File Handling

• Is a method of handling external data
• Data is kept in FILES on the disk and a program can read and write information from or to these files.
• The programmer is responsible for controlling the flow of this information

Sequential Access Files

• These work like a cassette tape or VHS tape. You start at the beginning of the tape and listen or watch from beginning to end (if it is a good tape or movie)
• This method can be slow if there is a large number of records
• Records can have a variable length
• This type of file access is useful when a flexible structure is more important than quick access

The OPEN Statement

• Syntax: OPEN filename FOR method AS # filenumber
• The filename is any valid DOS filename
• Caution in allowing the user to create the file names

Methods for the OPEN Statement

• INPUT: reads information from an existing file (retrieving existing file for input)
• OUTPUT: creates a new file, adds information to it, overwrites with new information to an existing file
• APPEND: adds information to the end of an existing file

Choosing a method

• Usually OUTPUT is used only when creating a new file as it will destroy existing files with the same name
• APPEND is generally used for output

The filenumber

• Must be an integer, usually 1 or 2
• This is unique for each file opened - it tells what file number you want to access
• Keep this simple

The WRITE # statement

• This is used to write to a file
• Before you can write to the file, you must have opened the file via OPEN (Output/Append)

The Syntax

• WRITE # FileNumber, Value(s)
• The FileNumber must match the file number on the OPEN statement
• The Value(s) would be the direct value or variables separated by commas

The INPUT # statement

• This is used to get data from a file and the file must have been opened via the OPEN statement (Input)

The INPUT Syntax

• INPUT # FileNumber, variable
• Where the FileNumber is a valid, open file and variables separated by commas

The CLOSE statement

• This statement is used to close the file and release its filenumber
• The syntax: CLOSE # FileNumber

The EOF function

• EOF (end of file) is a Boolean function - it is either TRUE or FALSE
• Is TRUE if the program has reached the end of the file and is usually used in DO LOOPS with file handling

Syntax and Example

• EOF (FileNumber)
• Example:
     DO
       …..program code
     LOOP UNTIL EOF(1)

NOT operator

• This sets a Boolean to its opposite value and is commonly used for check boxes and Boolean properties
• Example:
  Text1.FontUnderlined = NOT Text1.FontUnderlined
• If Font Underlined is true, it will become false, if false it will become true

SETFOCUS method

• This method is used in conjunction with an object and that object receives the focus.
• Example:
     Text1.SetFocus
• By doing this the insertion cursor will appear in the text box.
• Any subsequent key presses will be processed by the text box

On Error Resume Next

• This is used when you anticipate an error in a procedure, especially in file handling
• When an error occurs, the program skips to the next line
• This is only good within the procedure where it is defined