Files (Copy)

Introduction to Files

• Computer programs often need to store and retrieve data persistently.
• This is accomplished using files, which serve as external storage units for data.
• Each file is uniquely identified by a filename.
• This section primarily discusses text files, which contain sequences of characters.
• Text files may include an end-of-line (EOL) character, enabling structured reading and writing.

File Operations in Pseudocode

• File handling in pseudocode involves specific statements to open, read, write, append, and close files.

Opening a File

• Before reading from or writing to a file, it must be opened:

  OPEN <file identifier> FOR <file mode>
  

• Files can be opened in three modes:
  1. READ – Allows reading data from the file.
  2. WRITE – Writes new data to the file, replacing any existing content.
  3. APPEND – Adds new data at the end of the file without altering existing content.

Reading from a File

• Once a file is opened in READ mode, data can be read line by line using:

  READFILE <file identifier>, <variable>
  

• The retrieved line of text is stored in the variable.

Writing to a File

• A file opened in WRITE or APPEND mode can be written to line by line:

  WRITEFILE <file identifier>, <variable>
  

• Only STRING data type is allowed for reading and writing operations.

End-of-File (EOF) Function

• EOF function is used to determine whether the end of a file has been reached.
• It returns:
  • TRUE – When the end of the file is reached.
  • FALSE – If there is still more content to read.

  EOF(<file identifier>)
  

Closing a File

• Once all file operations are complete, it must be closed:

  CLOSEFILE <file identifier>
  

• This ensures that the file is properly saved and no longer occupies system resources.

Example: Writing to and Reading from a File

• Consider a file named myText.txt.
• The following pseudocode demonstrates writing user input to the file and then reading it.

Writing to a File (Example)

DECLARE textLn : STRING
DECLARE myFile : STRING

myFile ← "myText.txt"

OPEN myFile FOR WRITE

REPEAT
    OUTPUT "Please enter a line of text"
    INPUT textLn
    WRITEFILE myFile, textLn
UNTIL textLn = ""

CLOSEFILE myFile

Explanation

• The file is opened in WRITE mode.
• The program prompts the user to enter a line of text.
• Each line is written to the file using WRITEFILE.
• The loop continues until the user provides an empty input.
• The file is closed after writing.

Reading from a File (Example)

DECLARE textLn : STRING
DECLARE myFile : STRING

myFile ← "myText.txt"

OPEN myFile FOR READ

WHILE NOT EOF(myFile)
    READFILE myFile, textLn
    OUTPUT textLn
ENDWHILE

CLOSEFILE myFile

Explanation

• The file is opened in READ mode.
• A loop reads each line until the EOF is reached.
• Each line is displayed using OUTPUT.
• Finally, the file is closed.

File Modes Explained in Detail

Common File Handling Errors

• File Not Found Error: Occurs when trying to open a file that does not exist.
• Permission Denied Error: When attempting to write to a file without proper permissions.
• End-of-File (EOF) Unexpectedly Reached: Occurs when attempting to read past the last line of a file.
• Corrupted File Data: Can occur if the file is not properly closed before the program terminates.

Practical Applications of File Handling

• Configuration Files: Programs often store user preferences in text files.
• Log Files: Systems maintain logs to track events, errors, and user activity.
• Data Storage: Simple databases use text files to maintain records.
• User Input and Output: Programs can store user-provided text for later retrieval.

Advanced File Handling Concepts

Binary Files vs. Text Files

• Text Files:
  • Store data as readable text characters.
  • Includes newline characters for structured reading.
  • Easily modified using text editors.
• Binary Files:
  • Store data in machine-readable format (e.g., images, videos).
  • More efficient for storage and processing.
  • Not human-readable.

File Organization Methods

1. Serial File Organization:
   • Data is stored sequentially in the order it was added.
   • Example: Logging events in an activity log.
2. Sequential File Organization:
   • Data is stored in a specific order, usually based on a key.
   • Example: A sorted list of student records.
3. Random File Organization:
   • Data is stored at arbitrary locations, typically using hashing.
   • Example: Database storage for fast lookup.

File Access Methods

1. Sequential Access:
   • Reads records one by one from the beginning.
   • Suitable for log files or reports.
2. Direct Access (Random Access):
   • Allows jumping to a specific record without reading all previous records.
   • Useful for databases.

Key Takeaways

• Files provide persistent storage for programs.
• Text files store sequences of characters and support structured reading and writing.
• Pseudocode operations for handling files include OPEN, READ, WRITE, APPEND, and CLOSE.
• The EOF function ensures proper reading of file content.
• Error handling is crucial to avoid issues like file corruption.
• Different file organization and access methods cater to varied data storage needs.