What is the primary difference between opening a file for WRITE versus opening for APPEND?

Opening for WRITE deletes all existing content and starts from the beginning of the file, whereas APPEND keeps existing content and places the pointer at the end to add new data. Use WRITE for new files and APPEND for adding to logs.

When should a programmer use the READ mode instead of WRITE mode?

READ mode should be used when the program only needs to fetch data without modifying the file. This is safer because it prevents accidental overwriting of data and allows multiple processes to potentially read the file simultaneously.

How does sequential file access differ from random access (conceptually)?

Sequential access requires reading data in order from the beginning to the end, much like a cassette tape. You cannot skip to the middle without passing through all preceding data, which is why EOF loops are so common in this context.

What is a common consequence of forgetting to use the CLOSEFILE command?

Forgetting to close a file can lead to data loss because the system might not 'flush' the final bits of data from the buffer to the disk. Additionally, the file may remain 'locked' by the OS, preventing other programs from accessing it.

Why is it an error to attempt to READ from a file opened in WRITE mode?

File modes are specific permissions granted by the OS; WRITE mode only grants permission to send data out. Attempting to read would violate the access protocol established when the file was opened, usually resulting in a runtime error.

What happens if a program tries to open a non-existent file for READING?

The program will typically throw a 'File Not Found' exception or error. To prevent a crash, programmers should use exception handling (try/except) to catch this error and provide a user-friendly message.

Define the 'EOF' function and its purpose.

EOF stands for End Of File. It is a boolean function that returns True when the file pointer reaches the end of the file, signaling that no more data is available to be read.

What is a 'File Handle' or 'File Identifier'?

It is a variable or reference used within the program to represent an open file. Instead of using the full file path (like 'C:/data/test.txt') in every command, the program uses this shorter identifier to perform operations.

What does the term 'Persistence' mean in the context of file handling?

Persistence refers to data that survives after the program that created it has closed. Files provide persistence by storing data on physical hardware like a hard drive or SSD.

Why is a WHILE loop preferred over a FOR loop when reading a file?

A WHILE loop is preferred because the exact number of lines in a file is often unknown at runtime. The WHILE loop can continue dynamically until the EOF condition is met, whereas a FOR loop usually requires a pre-defined count.

Revision Notes

AS-Level

Cambridge International Examinations

Computer Science

10. Data Types & Structures

File handling

File Handling

Summary

File handling is a fundamental programming concept that enables the persistence of data by allowing programs to interact with external storage. Unlike data stored in variables which is lost when a program terminates, file handling allows for the creation, retrieval, and modification of data stored in text files on a disk.

1. Definition & Core Concepts

File Handling refers to the set of programming techniques used to manage data stored in external files rather than volatile memory (RAM). This process is essential for any application that needs to save user progress, logs, or configuration settings between sessions.

A File Handle or identifier is typically used within code to represent the connection between the program and the physical file on the storage device. This abstraction allows the programmer to perform operations like reading or writing without managing low-level hardware interactions.

Diagram showing data flow between volatile RAM and persistent Disk storage via Read and Write operations.

2. File Access Modes

READ Mode: This mode is used when the program only needs to retrieve information from an existing file. If the file does not exist when attempting to open it in this mode, the program will typically trigger an error or exception.

WRITE Mode: This mode is used to send data to a file, creating a new file if it doesn't exist or completely overwriting the existing content if it does. It is a destructive operation because any previous data in the file is lost immediately upon opening.

APPEND Mode: This mode allows data to be added to the end of an existing file without removing the current content. It is the preferred method for logging events or building lists over time where historical data must be preserved.

3. The File Handling Lifecycle

The first step is Opening the File, where the program requests access from the operating system and specifies the intended mode (Read, Write, or Append). This establishes a stream through which data can flow between the application and the storage medium.

Once open, the program performs Processing, which involves reading lines into variables or writing strings/values into the file. This is often done within loops to handle multiple records or lines of text sequentially.

The final, critical step is Closing the File, which releases the system resource and ensures all buffered data is physically written to the disk. Failing to close a file can lead to data corruption or prevent other programs from accessing that file.

4. End of File (EOF) Logic

5. Key Distinctions

6. Exam Strategy & Tips

File Handling

Summary

1. Definition & Core Concepts

Diagram showing data flow between volatile RAM and persistent Disk storage via Read and Write operations.

2. File Access Modes

3. The File Handling Lifecycle

4. End of File (EOF) Logic

The EOF (End Of File) function is a boolean check used to determine if the 'read pointer' has reached the end of the data stream. It returns true when there is no more data left to read, serving as the primary termination condition for file-reading loops.

A common pattern is the WHILE NOT EOF(FileIdentifier) loop, which ensures that the program processes every line in the file exactly once. This prevents 'index out of bounds' style errors that would occur if the program tried to read data that doesn't exist.

5. Key Distinctions

It is vital to distinguish between Write and Append modes to avoid accidental data loss. While both involve outputting data to a file, Write starts with a blank slate, whereas Append preserves existing data.

Feature	READ Mode	WRITE Mode	APPEND Mode
File Existence	Must exist	Created if missing	Created if missing
Existing Content	Preserved	Overwritten (Deleted)	Preserved
Pointer Position	Start of file	Start of file	End of file

6. Exam Strategy & Tips

When writing pseudocode or code, always verify that every OPENFILE statement has a corresponding CLOSEFILE statement. Examiners often look for this to ensure the student understands resource management and data integrity.

Always use a loop combined with an EOF check when reading files of unknown length. Hard-coding the number of lines to read is a common mistake that makes the program fragile and likely to fail if the input data changes.

Incorporate Exception Handling (try/catch blocks) when dealing with files. This demonstrates an understanding of defensive programming, as it allows the program to handle 'File Not Found' errors gracefully instead of crashing.