File Handling

Computer Science· 20 min read

1. Core File Lifecycle: Open, Process, Close★★☆☆☆⏱ 5 min

All file operations follow a standard, three-step lifecycle: open the file to get access, perform your read or write operations, then close the file when done. Closing files is critical to free system resources and ensure all in-memory data is written to permanent storage.

📘 Definition

File Mode

A setting that specifies what operations can be performed on an open file, and how the file content is interpreted.

Example:

Common modes: read, write, append, text, binary

📐 Worked Example

Write CIE standard pseudocode to open a text file scores.txt in read mode, output the first line to the console, then close the file.

1
Declare a file handle to reference the open file, and a string variable to store the read line:

DECLARE FileHandle : FILE
DECLARE FirstLine : STRING

3
Open the file in read-only mode:

OPENFILE "scores.txt" FOR READ AS FileHandle

5
Read the first line, output it, then close the file:

READLINE FileHandle, FirstLine
OUTPUT FirstLine
CLOSEFILE FileHandle

2. Text vs Binary File Modes★★★☆☆⏱ 6 min

Text files store data as sequences of human-readable characters, encoded with standards like ASCII or UTF-8. Binary files store raw binary data directly, which is more compact and preserves the exact size of original data types.

📐 Worked Example

Compare how the integer 123 is stored in a text file vs a 4-byte integer binary file.

1
In a text file: The integer is converted to three separate characters '1', '2', '3'. This takes 3 bytes of storage, and opening the file in a text editor displays the readable text 123.
2
In a binary file: The raw 4-byte binary value of the integer 123 is stored directly. This takes 4 bytes of storage, and opening the file in a text editor shows unreadable non-printing characters.

3. Common File Operations★★★☆☆⏱ 7 min

Beyond basic single-line reads, you will often need to process an entire file line-by-line, append new data to an existing file, or search for a specific entry. The EOF (end of file) marker is used to detect when you have reached the end of the file content.

📐 Worked Example

Write pseudocode to count the total number of lines in a text file called data.txt.

1
Initialize a counter to 0, and open the file for reading:

DECLARE Count : INTEGER ← 0
DECLARE CurrentLine : STRING
OPENFILE "data.txt" FOR READ

3
Loop through the file, reading lines until the end of file is reached, incrementing the counter each iteration:

WHILE NOT EOF("data.txt") DO
  READLINE "data.txt", CurrentLine
  Count ← Count + 1
ENDWHILE

5
Close the file and output the final count:

CLOSEFILE "data.txt"
OUTPUT "Total lines: " + Count

✓ Quick check

Test your understanding:

Which mode do you use to add new data to the end of an existing file without deleting its current content?
- Read mode
- Write mode
- Append mode
- Binary mode
Reveal answer
2 —
Correct! Append mode adds new data to the end of the file, while write mode overwrites the entire existing file.

4. File Error Handling★★★★☆⏱ 6 min

File operations often fail at runtime for common reasons: the file does not exist, you have incorrect permissions, or the storage device is full. Good programming practice requires checking for these errors and handling them gracefully instead of crashing.

📘 Definition

File Not Found Error

A common runtime error that occurs when a program tries to open a file that does not exist in the specified path.

📐 Worked Example

Modify the line counting code above to handle the case where data.txt does not exist.

1
After opening the file, add a check to confirm the file exists before processing:

OPENFILE "data.txt" FOR READ
IF NOT FILEEXISTS "data.txt" THEN
  OUTPUT "Error: Input file not found"
  STOP
ENDIF

3
The rest of the line counting code remains unchanged, but the program outputs a clear error message to the user instead of crashing unexpectedly.

5. Common Pitfalls

Wrong move:

Forgetting to close a file after opening it

Why:

Leaves system resources allocated, and may result in data not being saved to disk

Correct move:

Always add an explicit CLOSEFILE statement after finishing work on any open file

Wrong move:

Using write mode when adding data to an existing file

Why:

Write mode overwrites the entire existing file, erasing all original content before adding new data

Correct move:

Use append mode when you want to add new data to the end of an existing file

Wrong move:

Not checking for EOF before reading the next entry

Why:

Leads to reading garbage data after the end of the file, or runtime errors during execution

Correct move:

Always wrap repeated read operations in a WHILE NOT EOF() loop to stop at the end of content

Wrong move:

Calculating text file size the same way as binary

Why:

Text files store data as variable-length characters, while binary files use fixed size per data type

Correct move:

For text files, count characters to get size; for binary, multiply number of records by fixed record size

Wrong move:

Not handling missing files in read mode

Why:

Examiners expect robust error handling, and will deduct marks for unhandled errors

Correct move:

Always add a FILEEXISTS check after opening a file in read mode

6. Quick Reference Cheatsheet

Operation	CIE Pseudocode	Common Use Case
Open read-only	OPENFILE "f.txt" FOR READ	Read existing file
Open write	OPENFILE "f.txt" FOR WRITE	Create/overwrite file
Open append	OPENFILE "f.txt" FOR APPEND	Add data to end of file
Read line	READLINE handle, lineVar	Read one line of text
Write line	WRITELINE handle, data	Write one line of output
Check end of file	NOT EOF(handle)	Loop through all content
Check file exists	FILEEXISTS "filename"	Handle missing file error
Close file	CLOSEFILE handle	Save changes, free resources

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 12
Text file search question
2023 · 13
Binary file record write
2024 · 12
File error handling question

What's Next

File handling is a core foundational skill for both the theory and practical papers of CIE A-Level Computer Science. It is regularly tested in multiple-choice theory questions and forms a core part of most programming practical tasks, so mastering the full file lifecycle is critical for scoring full marks. Once you are comfortable with sequential file handling, you can move on to more advanced storage concepts that build on this foundation. You can also practice error handling more broadly to improve your programming robustness for exams.