File Handling (Cambridge (CIE) A Level Computer Science): Revision Note

File operations

What are file-processing operations?

• File-processing involves interacting with external files stored on secondary storage (e.g. hard drives, SSDs)

• In programming, files must be properly opened, read/written, and closed to ensure data is handled correctly

• Different types of file access are also used

Common file operations

Types of file access

Programming file-operations

Pseudocode

Basic file operations

OPEN filename FOR mode      // mode: READ, WRITE, APPEND
READ record FROM filename   // Read one record
WRITE record TO filename    // Write one record
CLOSE filename              // Always close the file

Serial access (e.g. logging events)

• Data is written one after another (no specific order)

• Often used with APPEND mode

Example: Writing logs serially

TYPE LogRecord
    DECLARE message : STRING
    DECLARE timestamp : STRING
ENDTYPE

DECLARE log : LogRecord

SET log.message = "System started"
SET log.timestamp = "12:00"

OPEN "logfile.txt" FOR APPEND
WRITE log TO "logfile.txt"
CLOSE "logfile.txt"

Sequential access (e.g. reading through all records)

• Records are read in order from start to end

• Common for reports or summaries

Example: Reading student grades sequentially

TYPE Student
    DECLARE name : STRING
    DECLARE grade : INTEGER
ENDTYPE

DECLARE student : Student

OPEN "students.txt" FOR READ

WHILE NOT EOF("students.txt")
    READ student FROM "students.txt"
    OUTPUT student.name, student.grade
ENDWHILE

CLOSE "students.txt"

Random access (e.g. accessing specific records by key)

• Accesses records directly using a key (e.g. record number or unique ID)

• Requires indexed files or fixed-length records

Example: Update a student’s grade by record position

TYPE Student
    DECLARE name : STRING
    DECLARE grade : INTEGER
ENDTYPE

DECLARE student : Student
DECLARE recordNumber : INTEGER

SET recordNumber = 5    // go directly to the 5th record

OPEN "students.txt" FOR RANDOM
SEEK "students.txt", recordNumber
READ student FROM "students.txt"

SET student.grade = 90  // update grade

SEEK "students.txt", recordNumber
WRITE student TO "students.txt"
CLOSE "students.txt"

Summary

Python

Basic file operations

file = open("filename.txt", "r" or "w" or "a")  # Open for read, write, or append
line = file.readline()                         # Read one line
file.write(data)                               # Write one line
file.close()                                   # Close the file

Serial access (e.g. logging events)

Example: Appending log entries to a file (serial write)

from datetime import datetime

message = "System started"
timestamp = datetime.now().strftime("%H:%M")

log = f"{timestamp} - {message}\n"

with open("logfile.txt", "a") as file:
    file.write(log)

Sequential access (e.g. reading through all records)

Example: Reading through each student line by line

with open("students.txt", "r") as file:
    for line in file:
        name, grade = line.strip().split(",")
        print(f"Name: {name}, Grade: {grade}")

Random access (e.g. accessing specific records by key)

Example: Update a student’s grade by index (record number)

record_number = 4  # 5th record (0-indexed)

with open("students.txt", "r") as file:
    records = file.readlines()

name, grade = records[record_number].strip().split(",")
records[record_number] = f"{name},90\n"  # Update grade to 90

with open("students.txt", "w") as file:
    file.writelines(records)

Summary

Java

Basic file operations

BufferedReader reader = new BufferedReader(new FileReader("filename.txt"));
String line = reader.readLine();
reader.close();

BufferedWriter writer = new BufferedWriter(new FileWriter("filename.txt", true)); // true = append
writer.write("Some text");
writer.newLine();
writer.close();

Serial access (e.g. logging events)

Example: Appending log entries to a file (serial write)

import java.io.*;
import java.time.LocalTime;
import java.time.format.DateTimeFormatter;

public class SerialAccessExample {
    public static void main(String[] args) throws IOException {
        String message = "System started";
        String timestamp = LocalTime.now().format(DateTimeFormatter.ofPattern("HH:mm"));
        String log = timestamp + " - " + message;

        BufferedWriter writer = new BufferedWriter(new FileWriter("logfile.txt", true));
        writer.write(log);
        writer.newLine();
        writer.close();
    }
}

Sequential access (e.g. reading through all records)

Example: Reading through each student line by line

import java.io.*;

public class SequentialAccessExample {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new FileReader("students.txt"));
        String line;

        while ((line = reader.readLine()) != null) {
            String[] parts = line.split(",");
            String name = parts[0];
            String grade = parts[1];
            System.out.println("Name: " + name + ", Grade: " + grade);
        }

        reader.close();
    }
}

Random access (e.g. accessing specific records by key)

Example: Update a student’s grade by index (record number)

import java.io.*;
import java.util.*;

public class RandomAccessExample {
    public static void main(String[] args) throws IOException {
        List<String> records = new ArrayList<>();
        int recordNumber = 4; // 5th line (0-indexed)

        BufferedReader reader = new BufferedReader(new FileReader("students.txt"));
        String line;
        while ((line = reader.readLine()) != null) {
            records.add(line);
        }
        reader.close();

        // Update the grade
        String[] parts = records.get(recordNumber).split(",");
        String updatedRecord = parts[0] + ",90";
        records.set(recordNumber, updatedRecord);

        // Overwrite the file
        BufferedWriter writer = new BufferedWriter(new FileWriter("students.txt"));
        for (String record : records) {
            writer.write(record);
            writer.newLine();
        }
        writer.close();
    }
}

Summary